Low frequency preamplifiers. Putting together a tube pre-amplifier

Jan 15

According to the tradition that has already developed for oneself, once a year you need to solder something worthwhile, new and useful, and since sound sickness for which they had not yet come up with a name and, accordingly, medicines, they did not heal, I wanted to do something like that related to sound. There is a normal amplifier, acoustics too… .Oh! pre with a tone control is not enough! Well, it began. See below. To be honest, it all started about a year ago. The scheme was chosen, the parts were purchased, but suddenly, as often happens, all the zeal and desire disappeared somewhere. I put all the documentation and accessories in the building of the future and froze the project until better times. These times came with the onset of cold weather. And then let's go point by point.

1- Choosing a scheme preamplifier

The most difficult theoretical part is to choose a scheme that combines high repeatability and quality of the result obtained. From multi-band equalizers and timbre block circuits on ready-made, specially sharpened microcircuits for this, they dissuaded on the forum, saying that this is a GE and is not at all suitable for obtaining high-quality sound... I also tried just such a preamplifier circuit with a tone control

Preamplifier circuit on TL072

In general, it's not bad, and for most amplifiers assembled on popular microcircuits, such as TDAxxxx, this will be enough. The HF and LF control is quite in a wide range, it is not the worst option in terms of noise, and the simplicity in manufacturing is captivating, but you want to get a result above average, so we are looking further.

I looked at the Solntsev preamplifier. The scheme has been known for a long time, it is not difficult to assemble and configure, and in terms of the ratio of good / bad reviews, the good ones outweigh great advantage... However, a person is such a harmful creature who always wants more. I didn't want to use Soviet components from the last century. You can assemble Solntsev using modern imported components instead of domestic ones, and people collect, so why not try? ...

The next task was to choose a tone control circuit. Active, passive, on operational amplifiers, there are many options, but you need to choose one. Again, exploring the forums, I came across a discussion of the Matyushkin tone control. A passive tone control, in which, apart from resistors and capacitors, there are no more elements, but according to reviews, such a correctly calculated TB produced some kind of special sound, very pleasant and different from other RTs.

I started to "smoke" how to dock the Matyushkin tone control with the Solntsev preamplifier and wandered into the cxem.net forum where I came across the topic of a high-quality Nataly preamplifier. This preamplifier uses just a bunch of PUs similar to Solntsevsky and RT Matyushkin. Spent a few days reading the topic, which at that time was about 90 pages, but the time spent was worth it. As a result, I came to the decision to make this particular preamplifier!

2 - Adjusting the preamplifier circuit for yourself.

The original Natalie preamplifier circuit and the ready-made printed circuit boards available for it did not suit me for a number of reasons. First, the original has a +/- 15V bi-level power supply for the op amp and +/- 30V for the rest. Well, this is half the trouble, there connect the op-amp power resistor to the +/- 30 bus and, instead of 30, give 15V a second. The main thing that prompted me to change the circuit and the board is the dimensions of the existing case, and according to estimates with those boards that are available on the forum and tested, I can not fit into the dimensions of the box. There is only one way out - to simplify the circuit a little and throw away unnecessary parts in order to reduce the PCB dimensions, and this should also facilitate the layout of the board.

This is the original circuit

Nataly preamplifier circuit

And this is mine, a little simplified

Pre-amplifier circuit

The main differences are:

1-removed several electrolytes for power supply, replaced them with larger capacitors.

2 - cut out the bypass of the tone control from the circuit, and adjust the balance

3 - and the third change - also cut out the loudness block at the preamp output.

These changes made it possible to slightly reduce the size of the printed circuit board, which was enough for the normal installation of the PCB in the PC case.

This is how I tried on all the boards printed on paper.

Preamplifier layout

It turned out that the finished device consists of 7 separate boards, or blocks. Below I will dwell on each block in more detail and try not to repeat what I wrote in a series of articles about this preamplifier under the heading "In the process of work"

3 - Complete description of the pre-amplifier

3.1 - Pre-amplifier board

Preamplifier seal

I'll start with the pre-amplifier board. No matter how much I would like to shove other opamps here, but from my sad experience I will say - save your time and nerves, and put what you need, but you need OPA134 or their dual version OPA132. Unfortunately, at the time of ordering, these op amps were not available in the online store, and I ordered NE5534, which, by the way, is better than OPA in terms of overload capacity. How much I fiddled with them later, when I began to tune the pre in endless and unsuccessful attempts to get rid of the constant at the preamplifier output. I even installed 100 Ohm multiturn trimmers, instead of resistors R9-R10, R30-R31, marked with *. At the output of the op-amp, it turns out to set 0, and at the output of the buffer, -100 - -150mV also remains. It does not seem to affect the ear and sound, it does not introduce any distortions, and there is no hum characteristic of a constant voltage, but these millivolts should not be!

The victims of these experiments were headphones, one ear of which bravely died in the process of tuning the preamplifier. I eliminated excitation in one channel, closed the input of the opera to ground through a capacitor, soldered a capacitor of several pf, I don’t remember where, I’m looking at the oscilloscope, the excitement disappeared. I unsolder the capacitor, thereby opening the input and without bothering to poke the oscilloscope into the buffer output, I connect the headphones. There is something strange, there is sound in one channel, in the other something farted and fell silent ... I looked with an oscilloscope, and there it was excited with an amplitude of 10 volts, which mercilessly killed a small defenseless headphone speaker. The reason for this was the same capacitor that eliminated excitation with a closed input, but amplified it many times with an open one. In general, I toiled, toiled, and in the end there was nothing left but to remove these NE5534s and order OPA134.

I stuck OPAashki into the panels, turned on the power and, with trembling hands, I touch the buffer output with the oscilloscope probe, and the oscilloscope beam remained in the same position! Maybe the microcircuits are defective and don't amplify anything at all? I increase the sensitivity of the oscillator, and I see that the constant is still there, but it is at the level of several mV. And what about the output of the op-amp? The output is a little more, but with the help of trimmers it is reduced to zero.

Hence the conclusion. Guys, you don't need to put in the diagram details that are not intended for this purpose. Perhaps in another circuit the same NE5534 will behave even better than OPA, but here OPA is needed from inexpensive opamp.

3.2 - Matyushkin tone control board

Matyushkin tone control circuit

Why Matyushkin? Again, there are several reasons. Well, first of all, the original Nataly preamplifier contains this tone block. Secondly, the rather large dimensions of the board are compensated by the simplicity of assembly and the absence of any adjustment; it is enough to simply select the denominations of the parts as accurately as possible. Thirdly, my personal opinion is that any electronic enhancer, which are active tone controls, brings its own additional bad buns, and the passive tone block is devoid of this drawback. And the fourth reason is frequency response form Matyushkin tone control, which differs from other RTs. I wanted to hear with my own ears and compare with other timbre blocks.

RT Matyushkin's board

The board for the RT also had to be drawn anew with a reduction in size. And besides, I did not find RT Matyushkin's seal on the network with switching to the RES47 relays I have. Here I did not change anything, except for a resistor that sets the depth of the HF adjustment. In the original, there is a 4.7kΩ trimmer resistor, but instead of it I soldered a regular, 4.7kΩ constant resistor. The control, as I said, is organized on the RES47 relay.

3.3 - control and indication board

As they say, a bad head does not give rest to his hands. The fixed buttons are small in size, stick LEDs to them to show which relay is currently on, it wouldn't be a big deal, but no! The latched switches are somehow not interesting (it's good that it didn't occur to me to do touch control), and the LEDs look rustic. It is necessary to make digital indication and non-fixed switching, and better with one button. Write the firmware? Ha! It’s a trifle when you can do it ... damn it, I don’t know how. Then there is only one way out - logic chips Made in USSR-Russia. I will not go into details and describe the operation algorithm of these microcircuits, I did it as best I could in the article "Nataly preamplifier - part 2. Control of the tone control relay and indication", which I recommend for reading to everyone interested in this type of control.

PU control unit diagram

So this is the diagram of this small fee, although it could consist of only eight elements S1-S4 and HL1-HL4. In general, the switching of the PT relay occurs cyclically, i.e. the relays on the tone block board are switched on and off alternately, and at the same time the indicator reading changes from 0 to 4. "0" corresponds to a sort of disabled tone control and then the bass rise increases 1-2-3. There are a lot, very, very much on the bottom three! Compared with the only factory amplifier Vega 10U-120S I have, the number 4 on the indicator will be about the same by ear as if you turn the bass adjustment on Vega to the maximum and additionally turn on loudness. So bass lovers can collect the fourth part of RT Matyushkin, corresponding maximum level LF and enjoy life. Well, and tweak the treble with a variable as in ordinary timbre blocks.

Control and display unit board

Two more buttons switch the preamplifier inputs and the point / bump signal level indication mode. You can also call redundant function but what to do, show-off more valuable than money... And of course, I could not help but make a signal level indicator, because when the LEDs blink beautifully, it looks more interesting.

Signal level indicator on LM3915

The indicator is assembled according to a scheme tested by many on the LM3915 MC, one per channel. And since in the dimensions of the board, again, I was limited, and the entire area of ​​the main board was occupied by parts for switches, and the central part of the LED block was forced to make a kind of two-story composite board.

Signal level indicator board on LM3915

LM3915 microcircuits and their entire strapping on a small board are connected to the main board with a pin connector.

3.4 - power supply board

Where does the power supply begin? That's right - from the transformer! But using satellite receiver as a case for a preamplifier, dictated its own conditions for choosing a transformer in a power supply, since the height of the case is only about 4 cm and you can't put any transformer there. Fortunately, a disassembled intercom was found at work, fortunately with a TP-30 transformer.

Preamplifier transformer

An excellent transformer, easily disassembled and accordingly easily rewound under required voltage, and most importantly in terms of height, as if it was created specifically for my body. The power of the transformer is about 30 watts, which is enough to use this trance in a preamplifier.

I rewound it under the required voltage, assembled it using epoxy resin as usual, apparently guessed well with the ratio of resin and hardener, and after assembly the transformer does not produce a sound.

Preamplifier power supply

For the pre, you had to get three different voltages: +/- 15v for powering the preamplifier board, 9v for powering the relay and indication board, and 5v for the sound card. For each voltage, I wound a separate winding and installed three diode bridges.

Preamplifier power supply circuit

I love the stabilized voltage, so I made a stabilized power supply for the LM317 / LM337 to power the preamplifier. To fine-tune the output voltage in each arm for LMok, I installed multi-turn trimmers. At the output, for additional smoothing, 1 Ohm resistors were soldered. A relay on the display board rested against one of the LMoks, so she moved to live on reverse side boards.

LM317 power supply for preamplifier

I also made a 5v stabilizer using LM317 according to the standard scheme, but without a trimmer, but with a conventional constant resistor, since there are additional stabilizers on the DAC board.

9 Volt made it even easier by using the 7809 microcircuit as a stabilizer. Here, the presence of noise will not affect the sound in any way and you can simplify the circuit, but stabilization is mandatory for the stable operation of logic microcircuits

Next in line >>>

3.5 - feeUSBsound card onPCM 2704

Sound card on PCM2704

A series of articles about "pin building" on the datagore prompted me to try building a USB for myself. sound card... This card is a digital-to-analog converter, i.e. when connecting this board to a computer, it is determined as sound device... Incoming digital signal goes to the board through USB cable, and at the exit we get the usual, familiar to our ears sound signal... I chose for repetition the simplest circuit on the PCM2704 chip in order to listen to whether such a sound device actually plays better than the sound card installed in the computer.

Scheme USB sound cards on PCM2704

Before that, I listened to all amplifiers and headphones through the Creative Audigy2 PCI card and was very pleased with it. I will skip the assembly point, after all, it's not specifically about assembling the DAC, but about synopsis sound card as part of a pre-amplifier. I can say that the result exceeded my expectations. Indeed, the sound made by this little card turned out to be better than sound with Audigy 2 and even more so built into motherboard chip. In the course of assembling the preamplifier, I was forced to switch back to "in-computer" sound due to the impossibility USB enable, and what a wadded and washed-out sound comes from the built-in chip. No transparency and airiness, as if you drew a drawing with a pencil, and then lightly rubbed all the lines with your finger. It seems that there is a bass, and high, but everything is somehow not so and not natural.

Now, as regards directly USB installation sound card into the preamplifier housing. In the beginning, I didn't even plan to put it in the pre-case, but after thinking and estimating that one and a half meters of a cheap signal cable from the preamplifier to the amplifier would be better than one and a half meters of the "preamplifier-amplifier" cable + the same amount from the "sound-audio-pre" as it would be in the case of using a sound card in the form in which it was, that is, in a separate case. Therefore, I placed the sound card board in the preamplifier case, thereby reducing the length of the “sound card-preamplifier” cable from one and a half meters to 10 centimeters. The food was planned to be made, not from USB input, but from the preamplifier power supply unit, since in theory, the quality of power from a separate transformer source should be better than that that goes with computer USB entrance. In fact, I did not notice the difference either with my ears or with an oscilloscope. And the five-volt power supply rail of the power supply remained in the air without being used. The sound is powered in the same way - from USB, besides, this has one big advantage - you don't need to turn on the preamplifier every time you want to listen to music through headphones.

So, I advise everyone to assemble at least such a simple sound card, you will be very pleased with the result. Or buy a ready-made one if you don't have the skills to assemble digital devices.

3.6 - volume control board and high frequencies

Volume and treble controls board

The smallest board of the entire device, not of particular interest. There are only two parts installed on it - a variable resistor for adjusting the volume, and a variable for adjusting high frequencies. From this board there are two loops of wires, one, a volume control loop, to the input selector board. The second HF control loop goes to the tone control board. There is nothing more to write about this board.

3.7 - input selector board

Input selector board

And the last part of the preamplifier is the input selector board, although it can hardly be called that, it still has only 2 inputs. The board has three connectors: 2 double tulips and a mini jack. Switching occurs through the RES 47 relay, also installed on this board. In the absence of power on the relay, the contacts coming from the sound card with the contacts of the input of the preamplifier board are closed, when power is applied to the relay, this circuit breaks and the contacts of the input of the preamplifier with the audio input "tulip" are closed. That is, the board has the ability to switch only two inputs, or the sound goes with a sound card built into the PU case, or from an external source through “tulip” connectors. Another double "tulip" is designed to output the signal from the preamplifier, and the mini-jack is rigidly connected to the output of the sound card. You can connect another amplifier to it, which will receive a "clean" signal not decorated with a pre-amplifier, or, as in my case, I use this output from a sound card to connect headphones.

4 - setting the pre-amplifier

By by and large there is only one part of the preamp that needs tuning, and that part is the preamp board itself. For normal operation of the circuit, you need to set the quiescent current of the output transistors and this is done by selecting the resistance of the resistors R9-R10, R30-R31 in (the original circuit is 51 Ohm). For this circuit, the recommended quiescent current is 20-22mA, which corresponds to a voltage drop of 300-350mV across resistors R20, R21, R40, R42 with a nominal value of 15 ohms. Calculating the quiescent current is very simple, for this you need to divide the voltage drop across these resistors by their resistance. 300: 15 = 20, i.e. with a voltage drop across resistors R20, R21, R40, R42 - 300mV, our quiescent current will be 20mA. One important point where some beginner soldering workers make a mistake. The voltage drop across the resistors is measured by connecting the voltmeter probes of one terminal of the resistor relative to the other terminal of the same resistor, and not the common wire. An obvious thing, but out of habit, you can connect one pin to a resistor, and the other to a common one, and get a very surprising result. If your voltage drop is outside the range of 300-350 mV, then depending on the deviation up or down, you need to change the value of the resistors R9-R10, R30-R31. To increase the current, you need to increase the resistance of the resistors, and to decrease it, respectively, solder resistors with a lower resistance. In general, to reduce troubles with the selection of these resistors, you can proceed as follows - solder in place of constant resistors, multi-turn trimmer resistors 100 Ohm and easily adjust and change the quiescent current at your discretion.

Setting the quiescent current of the preamplifier

The board does not provide for the installation of such resistors, but since only 2 out of 3 trimmer pins are used for adjustment, we just solder the middle leg of such a resistor to one of the extreme ones, and solder it in place of the permanent one. In the future, for the final setting of the quiescent current, you can measure the resistance on the trimmer and already with high precision select a constant resistor of the required resistance.

Now you need to see the presence of a constant at the output of each buffer and all 4 operational amplifiers... With proper assembly and use of exactly those components that are needed, it should be several mV, no more than 5-10 mV. If you see several tens of mV there, it means either you have something wrongly soldered somewhere, or you have mistakenly soldered a resistor of the wrong value, or somewhere there is an excitation, and an oscilloscope will be needed to search for it. In case you have trimming resistors installed, you can try to set "0" by selecting the resistance of these two resistors, for example R9 and R10 for the first buffer. There will be a slight imbalance in the resistance of the resistors in the positive and negative legs, but there will be a stable zero at the output of the op-amp and the buffer. It should be remembered that a change in the resistance of these resistors leads to a change in the quiescent current, therefore, I advise you to connect two voltmeters, or a voltmeter + oscilloscope and observe their readings. So that the voltage drop does not go beyond the recommended limits, and the constant is close to zero. I forgot to say that all these adjustments need to be done with the preamplifier input closed.

To search for the excitement, you need to look at the signal shape at all sorts of points. Depending on the point on the diagram to which you will connect the oscilloscope, it should be straight line, without different "hedgehogs" characteristic of arousal. In my case, such a "hedgehog", i.e. a 0.5V signal in a form resembling a sinusoid of several megahertz was on the emitter of the VT3 transistor, this problem was easily solved by soldering a 20pF capacitor between the base and the collector of this transistor. I did not find excitation in the other three buffers.

Checking the square wave on the preamplifier

At the output, we should see clear rectangles, but if there is some kind of nasty, we are looking for an error.

About mistakes. Parts should be chosen very carefully, and each part should be additionally checked before installation. Again the case from personal experience... Everything works, the meander is good, I connect it to the generator and I see that after 7 kHz there is a clear blockage. After a careful examination, which took a lot of time, I discovered that instead of a 10pF capacitor, which stands between the 2 and 6 legs of the op-amp and serves to eliminate possible excitation at high frequencies (several MHz), I have a 100pF capacitor, which cut off everything above 7kHz. I replaced it with the desired one, at 10pF and the frequency response became uniform.

As for the relay control board and indication. Not everything is so smooth and clear here. Firstly, I was unpleasantly surprised by the quality of domestic parts, half of which turned out to be defective. Secondly, those who seem to behave like workers are completely incomprehensible. Either they work every other time, or they work in an algorithm known only to them. Let me explain exactly what I mean.

Let's take the K176IE4 microcircuit. When the power is turned on, only known reason then 0, then 1 lights up on the screen. When it turns on with a single one, everything is normal, the modes of the tone block correspond to the number on the indicator, i.e. 0 - minimum bass, 3 - maximum. When it turns on with zero, the minimum is already at 3, and the maximum is at 2. It turns out that the K561IE9A counter counts everything correctly, but IE4 turns it on. In addition to this, they sometimes slip false positives, i.e. I press the button once, and the number from 1 jumps to 3 or even 0.

The same is with the K155TM2, which controls the input selector and switching signal level modes. Two switches, I assembled exactly the same scheme, as a result, one switch works like a clock, the other needs to be pressed 5 times to make it work. How can this be? ... They solder another micra, it doesn't want to switch anything at all. In general, I soldered using the scientific poke method, I don't remember which legs of several pF capacitors, and now it seems like the switching is stable. I will not designate these capacitors on the diagram, so as not to be misleading, assemble according to the standard switching scheme, and there you will be guided by the circumstances.

5. - Layout of land

I was afraid of this moment based on personal experience, because usually at this stage problems arise with the correct wiring of the ground and connecting the common wire. A clear sign of incorrect wiring is a characteristic hum, indicating that an earth loop has formed somewhere, or other irregularities. In the case of the preamplifier, I went the other way, to make it not as beautiful and that there were fewer wires, but more correctly. And I ended up getting positive result... There is no background, even with the volume knob turned to the maximum, no hum from the wrong ground is also observed, in general the result exceeded my expectations.

Preamplifier Ground Wiring

How did I connect the common wires ... It's very simple. I brought everything to one point, and this point turned out to be a board for volume and treble controls. For example, in the power supply of the preamplifier board, the plus and minus wires were soldered to the PU board itself, and the common wire to the controller board, and then from the RG and HF board I soldered a short wiring to the common track of the PU board. He did the same with other common wiring, the numerous tentacles of the electric octopus, they go from the regulation board to all the rest.

Pre-amplifier block diagram

Tried to draw a block diagram of it all. I hope I didn’t confuse anything, and it turned out more or less clear.

6. Housing.

The case, as I already said, came up remarkably well from the Odissey satellite receiver. He bribed me with his large window, which displayed the clock, channel number and other information, as well as the size of the case. Similar in size enclosures from DVD players are much lower, and besides, they have a compartment for loading a disc, which entails rework front panel, in the same case, nothing was needed to redo. For the final adjustment, I just had to drill two holes in the "face" for attaching the volume and treble controls, and paint over unnecessary inscriptions. I used the paint as usual - an aerosol from a car shop. The matte black color exactly matched the color of the panel, so there was no need to paint the entire panel, the work was reduced to accurate painting over the inscriptions and installing aluminum handles.

Preamplifier front panel

Volume and tone controls

I went to some tricks when installing the input selector board. In a standard way it was not possible to establish it, tk. the tone control board interfered, and I had no choice but to screw it upside down and additionally tighten it with a plastic clip.

Input selector board

All boards are secured through plastic grommets. A screw is screwed into the sleeve (or spacer), a hole is drilled in the board along the outer diameter of the sleeve, the whole thing is attracted from above with a nut, and the board is reliably isolated from contact with the case.

Insulator for the board from the case

You can also see that small L-shaped radiators cut from an aluminum plate are screwed to the transistors on the preamplifier board. The heatsinks are not big at all, but the temperature of the transistors has dropped significantly.

For reliability, I filled all the wires soldered to the boards with hot melt glue.

I put a cardboard gasket under the power supply board, just in case.

Isolation pad for power supply board

Although there is a margin of a few mm between the board and the case, I made additional control insulation for reinsurance. Still, there is a power switch on the board and accidentally get on metal case contact with 220V there is no special desire.

As a result, it turned out, as in the saying "In cramped quarters, but not offended." Everything is heap, everything is tight, but nothing interferes.

Pre-amplifier layout

The payment of the ZV card feels like a king, there are still a couple of free centimeters around it! In order to reduce possible interference from the transformer, I closed it with a metal cover. And even during the tests, it turned out that the 9-volt stabilizer is very hot. I had to screw a small radiator to it.

7. - conclusion.

Preamplifier housing

Such is not a small article turned out, but the work was also not small, and what I want to say in conclusion. Do you want to be honest? Made another toy! Yes, it glows and winks, but the sound became, as it were, brighter and it became possible to adjust the high and low frequencies, yes, in fact, the Matyushkin tone control somehow decorates the sound in its own way, in a special way, but in general, some kind of cardinal improvement , from which you want to jump to the ceiling, unfortunately not ... The sound has become more interesting, but nothing more. Do not think that I speak badly about the scheme or discourage you from repeating, in any case! If you are a real radio amateur with "sound sick", then you will get a lot of pleasure from the very process of assembling the device, and I myself do not almost regret the time and effort spent, because in the end, a fairly high-quality thing appeared in my arsenal that allows you to enrich the sound and customize it to your preferences. I will not hide the fact that after assembling the preamplifier, I listen to music not directly through the sound card, but through this preamplifier. I just want to say that my auditory receptors could not make me squeal for joy. Perhaps the acoustics are not the same, perhaps the amplifier, perhaps the ears. By the way, about the amplifier, I have connected this one before only to a hybrid on field workers, I will need to connect it to my beloved purely tube amplifier on G807 and listen to what he has to say about this bundle.

Collected before!

In general, friends! Here are some ready-made seals that I personally checked. I want to warn you about the control board, it may differ slightly from the circuit, because has been modified many times.

Solder, try, experiment, perhaps - this is exactly what you were looking for! Do not listen to anyone, including me, because each of you has your own tastes and preferences, as they say in taste and color ... I hope the article was useful and will give some of you a starting kick for assembling this preamplifier.

Low frequency amplifiers (ULF) are used to convert weak signals predominantly audio range in more powerful signals acceptable for direct perception through electrodynamic or other sound emitters.

Note that high-frequency amplifiers up to frequencies of 10 ... 100 MHz are built according to similar schemes, all the difference most often comes down to the fact that the capacitance values ​​of the capacitors of such amplifiers decrease as many times as the frequency of the high-frequency signal exceeds the frequency of the low-frequency one.

Simple single transistor amplifier

The simplest ULF, made according to the scheme with a common emitter, is shown in Fig. 1. A telephone capsule is used as a load. The permissible supply voltage for this amplifier is 3 ... 12 V.

The value of the bias resistor R1 (tens of kΩ) should be determined experimentally, since its optimal value depends on the amplifier supply voltage, the resistance of the telephone capsule, and the transmission coefficient a specific instance transistor.

Rice. 1. Scheme of a simple ULF on one transistor + capacitor and resistor.

For selection initial value resistor R1, it should be taken into account that its value should be approximately one hundred or more times higher than the resistance included in the load circuit. To select a bias resistor, it is recommended to connect in series a fixed resistor with a resistance of 20 ... 30 kΩ and variable resistance 100 ... 1000 kOhm, after which, by feeding a small amplitude audio signal to the amplifier input, for example, from a tape recorder or a player, by turning the knob variable resistor achieve the best signal quality at its highest volume.

The value of the capacitance of the transition capacitor C1 (Fig. 1) can be in the range from 1 to 100 μF: than more magnitude this capacity, the lower frequencies the ULF can amplify. To master the technique of amplification low frequencies it is recommended to experiment with the selection of the nominal values ​​of the elements and the operating modes of the amplifiers (Fig. 1 - 4).

Improved Single Transistor Amplifier Options

Complicated and improved in comparison with the circuit in fig. 1 amplifier circuits are shown in Fig. 2 and 3. In the diagram in fig. 2, the amplification stage additionally contains a chain of frequency-dependent negative feedback (resistor R2 and capacitor C2), which improves the signal quality.

Rice. 2. Scheme of a single-transistor ULF with a frequency-dependent negative feedback circuit.

Rice. 3. A single transistor amplifier with a divider for supplying bias voltage to the base of the transistor.

Rice. 4. Single transistor amplifier with automatic bias setting for the transistor base.

In the diagram in fig. 3, the bias to the base of the transistor is set more "rigidly" with the help of a divider, which improves the quality of the amplifier when its operating conditions change. The "automatic" setting of the bias based on the amplifying transistor is used in the circuit in Fig. 4.

Two-stage transistor amplifier

By connecting in series two simplest amplification stages (Fig. 1), you can get a two-stage ULF (Fig. 5). The gain of such an amplifier is equal to the product of the gains of the individual stages. However, it is not easy to obtain a large sustained gain by subsequently increasing the number of stages: the amplifier is likely to self-excite.

Rice. 5. Scheme of a simple two-stage bass amplifier.

New developments of low-frequency amplifiers, whose circuits are often cited in the pages of magazines in recent years, are aimed at achieving a minimum total harmonic distortion, increasing output power, expanding the frequency band to be amplified, etc.

At the same time, when setting up various devices and conducting experiments often requires a simple ULF, which can be assembled in a few minutes. Such an amplifier should contain a minimum number of deficient elements and operate over a wide range of supply voltage and load resistance variations.

ULF circuit on field-effect and silicon transistors

A diagram of a simple LF power amplifier with direct connection between the stages is shown in Fig. 6 [Rl 3 / 00-14]. The input impedance of the amplifier is determined by the value of the potentiometer R1 and can vary from hundreds of ohms to tens of megohms. The output of the amplifier can be connected to a load with resistance from 2 ... 4 to 64 Ohm and higher.

With a high-resistance load, the KT315 transistor can be used as VT2. The amplifier is operational in the range of supply voltages from 3 to 15 V, although its acceptable performance remains even when the supply voltage is reduced to 0.6 V.

The capacitance of the C1 capacitor can be selected in the range from 1 to 100 μF. In the latter case (C1 = 100 μF), the ULF can operate in the frequency range from 50 Hz to 200 kHz and above.

Rice. 6. Scheme simple amplifier low frequency on two transistors.

The amplitude of the ULF input signal should not exceed 0.5 ... 0.7 V. The output power of the amplifier can vary from tens of mW to units of W, depending on the load resistance and the magnitude of the supply voltage.

Tuning the amplifier consists in the selection of resistors R2 and R3. With their help, the voltage at the drain of the transistor VT1 is set, equal to 50 ... 60% of the voltage of the power source. Transistor VT2 must be installed on a heat sink plate (heat sink).

Direct-coupled tracked ULF

In fig. 7 shows a diagram of another seemingly simple ULF with direct connections between the stages. This kind of connection improves frequency characteristics amplifier in the low-frequency region, the circuit as a whole is simplified.

Rice. 7. Schematic diagram three-stage ULF with direct connection between the stages.

At the same time, amplifier tuning is complicated by the fact that each amplifier impedance has to be selected individually. Roughly the ratio of resistors R2 and R3, R3 and R4, R4 and R BF should be in the range (30 ... 50) to 1. Resistor R1 should be 0.1 ... 2 kOhm. Calculation of the amplifier shown in Fig. 7 can be found in the literature, for example [P 9 / 70-60].

Cascade ULF circuits on bipolar transistors

In fig. 8 and 9 show diagrams of cascode ULF bipolar transistors. Such amplifiers have a fairly high gain Ku. The amplifier in Fig. 8 has Ku = 5 in the frequency range from 30 Hz to 120 kHz [MK 2 / 86-15]. ULF according to the scheme in Fig. 9 with a harmonic coefficient of less than 1% has a gain of 100 [RL 3 / 99-10].

Rice. 8. Cascade ULF on two transistors with gain = 5.

Rice. 9. Cascade ULF on two transistors with gain = 100.

Economical ULF on three transistors

For portable electronic equipment important parameter is the efficiency of ULF. The diagram of such an ULF is shown in Fig. 10 [RL 3 / 00-14]. Here, a cascade connection of a field-effect transistor VT1 and a bipolar transistor VT3 is used, and the transistor VT2 is turned on in such a way that it stabilizes the operating point VT1 and VT3.

With an increase in the input voltage, this transistor shunts the emitter-base transition VT3 and reduces the value of the current flowing through the transistors VT1 and VT3.

Rice. 10. Scheme of a simple economical bass amplifier on three transistors.

As in the above circuit (see Fig. 6), the input impedance of this ULF can be set in the range from tens of ohms to tens of megohms. A telephone capsule was used as a load, for example, TK-67 or TM-2V. The telephone capsule, which is connected by means of a plug, can simultaneously serve as a power switch for the circuit.

The supply voltage of the ULF is from 1.5 to 15 V, although the device remains operational even when the supply voltage drops to 0.6 V. In the supply voltage range of 2 ... 15 V, the current consumed by the amplifier is described by the expression:

1 (μA) = 52 + 13 * (Upit) * (Upit),

where Usup is the supply voltage in Volts (V).

If you turn off the transistor VT2, the current consumed by the device increases by an order of magnitude.

Two-stage ULF with direct connection between stages

Examples of ULF with direct connections and a minimum selection of the operating mode are the circuits shown in Fig. 11 - 14. They have high gain and good stability.

Rice. 11. Simple two-stage ULF for a microphone (low noise, high KU).

Rice. 12. Two-stage amplifier of low frequency on transistors KT315.

Rice. 13. Two-stage low-frequency amplifier on KT315 transistors - option 2.

The microphone amplifier (Fig. 11) is characterized by a low level of intrinsic noise and high gain [MK 5/83-XIV]. An electrodynamic type microphone is used as a VM1 microphone.

A telephone capsule can also act as a microphone. Stabilization of the operating point (initial bias based on the input transistor) of the amplifiers in Fig. 11 - 13 is carried out due to the voltage drop across the emitter resistance of the second amplification stage.

Rice. 14. Two-stage ULF with a field-effect transistor.

The amplifier (Fig. 14), which has a high input impedance (about 1 MΩ), is made on a field-effect transistor VT1 (source follower) and bipolar - VT2 (with a common one).

Low frequency cascade amplifier on field-effect transistors, also having a high input impedance, is shown in Fig. 15.

Rice. 15. circuit of a simple two-stage ULF on two field-effect transistors.

ULF circuits for working with a low-ohm load

Typical ULF designed to operate on a low-impedance load and having output power tens of mW and more are shown in Fig. 16, 17.

Rice. sixteen. Simple ULF for operation with low impedance load switching.

The VA1 electrodynamic head can be connected to the amplifier output, as shown in Fig. 16, or in the diagonal of the bridge (Fig. 17). If the power source is made of two series-connected batteries (accumulators), the right output of the BA1 head according to the scheme can be connected directly to their midpoint, without capacitors СЗ, С4.

Rice. 17. Low-frequency amplifier circuit with the inclusion of a low-impedance load in the diagonal of the bridge.

If you need a circuit of a simple tube ULF, then such an amplifier can be assembled even on one lamp, look at our electronics website in the appropriate section.

Literature: Shustov M.A. Practical Circuitry (Book 1), 2003.

Corrections in the publication: in fig. 16 and 17, instead of diode D9, a chain of diodes is installed.

- The neighbor started knocking on the battery. Made the music louder so I couldn't hear it.
(From the folklore of audiophiles).

The epigraph is ironic, but an audiophile is not at all necessarily “sick in the head” with the face of Josh Ernest at a briefing on relations with the Russian Federation, who is “rushing” because the neighbors are “happy”. Someone wants to listen to serious music at home as in a hall. The quality of the equipment for this needs such that for fans of decibels of loudness, as such, it simply does not fit where sane people have a mind, but for the latter, it goes beyond the mind from the prices of suitable amplifiers (UMZCH, audio frequency power amplifier). And someone along the way has a desire to join useful and exciting areas of activity - sound reproduction technology and electronics in general. Which in the century digital technologies inextricably linked and can become highly profitable and prestigious profession... The optimal first step in all respects in this matter is to make an amplifier with your own hands: it is UMZCH that allows, with initial training based on school physics, on the same table to go from the simplest structures for half an evening (which, nevertheless, “sing” well) to the most complex units through which a good rock band will also play with pleasure. The purpose of this publication is to highlight the first steps of this path for beginners and, possibly, to communicate something new to the experienced.

The simplest

So, first let's try to make an audio amplifier that just works. In order to thoroughly delve into sound engineering, you will have to gradually master quite a lot of theoretical material and not forget to enrich your knowledge base as you progress. But any “cleverness” is assimilated easier when you see and feel how it works “in hardware”. In this article, further, too, the theory will not do - in what you need to know at first and what can be explained without formulas and graphs. In the meantime, it will be enough to be able to and use a multitester.

Note: If you have not soldered the electronics before, please note that its components must not be overheated! Soldering iron - up to 40 W (better than 25 W), maximum allowable time soldering without interruption - 10 s. The soldered lead for the heat sink is held 0.5-3 cm from the soldering point on the side of the device case with medical tweezers. Acidic and other active fluxes must not be used! Solder - POS-61.

On the left in Fig.- the simplest UMZCH, "which just works." It can be assembled on both germanium and silicon transistors.

On this crumb it is convenient to master the basics of setting up the UMZCH with direct connections between the cascades, which give the clearest sound:

• Before turning on the power for the first time, turn off the load (speaker);
• Instead of R1, we solder a chain of a 33 kΩ constant resistor and a 270 kΩ variable (potentiometer) resistor, i.e. first approx. four times smaller, and the second approx. twice the denomination against the initial one according to the scheme;
• We supply power and, rotating the potentiometer slider, at the point indicated by the cross, set the specified collector current VT1;
• We remove the power supply, solder the temporary resistors and measure their total resistance;
• As R1, we put a resistor of the nominal value from the standard row closest to the measured one;
• We replace R3 with a constant 470 Ohm chain + 3.3 kOhm potentiometer;
• The same as in PP. 3-5, including setting the voltage equal to half the supply voltage.

Point a, from where the signal is taken to the load is the so-called. midpoint of the amplifier. In UMZCH with unipolar power supply, half of its value is set in it, and in UMZCH in bipolar power supply - zero relative to the common wire. This is called amplifier balance adjustment. In unipolar UMZCH with capacitive decoupling of the load, it is not necessary to disconnect it during setup, but it is better to get used to doing it reflexively: an unbalanced 2-pole amplifier with a connected load can burn its own powerful and expensive output transistors, or even "new, good" and very expensive powerful speaker.

Note: components that require selection when setting up the device in the layout are indicated on the diagrams either by an asterisk (*) or by an apostrophe (’).

In the center in the same fig.- a simple UMZCH on transistors, which already develops a power of up to 4-6 W at a load of 4 ohms. Although it works, like the previous one, in the so-called. class AB1, not intended for Hi-Fi sound recording, but if you replace a pair of such class D amplifiers (see below) in cheap Chinese computer speakers, their sound is noticeably improved. Here we learn one more trick: powerful output transistors must be installed on radiators. Components requiring additional cooling are circled in dashed lines in the diagrams; true, not always; sometimes - with an indication of the required dissipative area of ​​the heat sink. The adjustment of this UMZCH is balancing using R2.

On the right in Fig.- not yet a 350 W monster (as shown at the beginning of the article), but already quite a solid beast: a simple 100 W transistor amplifier. You can listen to music through it, but not Hi-Fi, the class of work is AB2. However, to sound the site for a picnic or meeting on outdoors, school assembly or small trading floor it is quite usable. An amateur rock group, having such an UMZCH for an instrument, can perform successfully.

In this UMZCH, 2 more tricks are manifested: firstly, in a very powerful amplifiers the swing stage of the powerful output also needs to be cooled, so VT3 is placed on a radiator from 100 sq. see For output VT4 and VT5 radiators from 400 sq. see Secondly, UMZCH with bipolar power supply are not balanced at all without load. Either one or the other output transistor goes into cutoff, and the conjugate transistor goes into saturation. Then, on full tension power surges during balancing can damage the output transistors. Therefore, for balancing (R6, you guessed it?), The amplifier is powered from +/– 24 V, and instead of the load, a wire resistor of 100… 200 Ohm is included. By the way, the squiggles in some of the resistors on the diagram are Roman numerals, indicating their required heat dissipation power.

Note: a power source for this UMZCH needs a power of 600 watts. Smoothing filter capacitors - from 6800 uF to 160 V. Parallel electrolytic capacitors PI switches on ceramic 0.01 μF each to prevent self-excitation at ultra audio frequencies ah, capable of instantly burning the output transistors.

On the field workers

On the trail. rice. - another option for a fairly powerful UMZCH (30 W, and at a supply voltage of 35 V - 60 W) on powerful field-effect transistors:

The sound from it already pulls the requirements for Hi-Fi entry level(if, of course, the UMZCH works on the acc. Acustic systems, AC). Powerful field workers do not require high power for swinging, therefore there is no pre-power cascade. Even powerful field-effect transistors do not burn out the speakers under any malfunctions - they themselves burn out faster. It is also unpleasant, but still cheaper than changing an expensive bass head of a loudspeaker (GG). Balancing and, in general, adjustment of this UMZCH is not required. It has only one drawback, like a design for beginners: powerful field-effect transistors are much more expensive than bipolar ones for an amplifier with the same parameters. Requirements for IP - similar to the previous one. occasion, but its power is needed from 450 watts. Radiators - from 200 sq. cm.

Note: no need to build powerful UMZCH on field-effect transistors for switching power supplies, for example. computer. When trying to "drive" them into active mode required for the UMZCH, they either simply burn out, or the sound is weak, but in terms of quality "none". The same applies to powerful high-voltage bipolar transistors e.g. from the line scan of old TVs.

Straight up

If you have already taken the first steps, then it will be quite natural to want to build UMZCH class Hi-Fi, without going too deep into the theoretical jungle. To do this, you will have to expand the instrument park - you need an oscilloscope, an audio frequency generator (GZCH) and an AC millivoltmeter with the ability to measure the DC component. The prototype for repetition is better to take UMZCH E. Gumeli, described in detail in "Radio" No. 1 for 1989. For its construction, you will need a few inexpensive available components, but the quality meets very high requirements: power up to 60 W, bandwidth 20-20,000 Hz, non-uniformity of frequency response 2 dB, coefficient of nonlinear distortion (THD) 0.01%, self-noise level –86 dB. However, it is quite difficult to adjust the Gumeli amplifier; if you can handle it, you can take on any other. However, some of the currently known circumstances greatly simplify the establishment of this UMZCH, see below. Bearing in mind this and the fact that not everyone manages to get into the archives of "Radio", it would be appropriate to repeat the main points.

Schemes of a simple high-quality UMZCH

Schemes UMZCH Gumeli and the specification for them are given in the illustration. Output transistor radiators - from 250 sq. see for UMZCH in fig. 1 and from 150 sq. see for option according to fig. 3 (original numbering). The transistors of the pre-output stage (KT814 / KT815) are installed on radiators bent from aluminum plates 75x35 mm with a thickness of 3 mm. It is not worth replacing KT814 / KT815 with KT626 / KT961, the sound does not noticeably improve, but the establishment is seriously hampered.

This UMZCH is very critical to power supply, installation topology and general, therefore, it needs to be adjusted in a constructively finished form and only with a standard power source. When trying to supply power from a stabilized power supply, the output transistors burn out immediately. Therefore, in Fig. given drawings of original printed circuit boards and setup instructions. To them we can add that, firstly, if at the first turn-on the "excitement" is noticeable, they are struggling with it, changing the inductance L1. Secondly, the leads of the parts installed on the boards should be no longer than 10 mm. Thirdly, it is extremely undesirable to change the installation topology, but if it is really necessary, there must be a frame shield on the side of the conductors (an earth loop, highlighted in color in the figure), and the power supply paths must go outside it.

Note: gaps in the tracks to which the bases of powerful transistors are connected - technological, for adjustment, after which they are soldered with drops of solder.

The establishment of this UMZCH is much simplified, and the risk of encountering "excitement" in the process of use is reduced to zero if:

• Minimize interconnect wiring by placing boards on heat sinks of power transistors.
• Completely abandon the connectors inside, performing the entire installation only by soldering. Then there will be no need for R12, R13 in powerful version or R10 R11 in a less powerful one (in the diagrams they are dotted).
• Use for indoor installation an oxygen-free copper audio wire of minimum length.

When these conditions are met, there are no problems with the initiation, and the establishment of the UMZCH is reduced to the routine procedure described in Fig.

Sound wires

Audio piping is not an idle invention. The need for their application is currently undeniable. In copper with an admixture of oxygen, the thinnest oxide film is formed on the faces of the metal crystallites. Metal oxides are semiconductors and, if the current in the wire is weak without a constant component, its shape is distorted. In theory, the distortions on the myriads of crystallites should compensate for each other, but the smallest amount (it seems, due to quantum uncertainties) remains. Enough to be noticed by discerning listeners in the background the purest sound modern UMZCH.

Manufacturers and traders, without a twinge of conscience, slip ordinary electrical copper instead of oxygen-free - it is impossible to distinguish one from the other by eye. However, there is a field of application where counterfeiting does not go unambiguously: cable twisted pair for computer networks... Putting the grid with long segments "left-handed", it will either not start at all, or will be constantly buggy. Dispersion of impulses, you know.

The author, when they were just talking about audio lines, realized that, in principle, this was not idle chatter, especially since oxygen-free wires had long been used in special purpose equipment by that time, with which he was well familiar by his occupation. Then I took and replaced the standard cord of my TDS-7 headphones with a homemade one made of "vitukha" with flexible stranded wires. The sound, by ear, has steadily improved for loop-through analog tracks, i.e. on the path from studio microphone to disc that has never been digitized anywhere. Recordings on vinyl made using DMM technology (Direct Meta lMastering, direct metal deposition) sounded especially brightly. After that, interconnect editing of all home audio was converted to "vitush". Then the improvement in sound began to be noted by completely random people, indifferent to music and not forewarned in advance.

How to make interconnect wires from twisted pair, see next. video.

Video: do-it-yourself twisted pair interconnect wires

Unfortunately, the flexible "vitukha" soon disappeared from the market - it did not hold well in crimped connectors. However, for the information of readers, flexible "military" wire MGTF and MGTFE (shielded) is made only from oxygen-free copper. Counterfeiting is impossible, because on ordinary copper, tape fluoroplastic insulation creeps out rather quickly. MGTF is now widely sold and is much cheaper than branded, with a guarantee, audio wires. It has only one drawback: it cannot be done colored, but this can be corrected with tags. There are also oxygen-free winding wires, see below.

Theoretical interlude

As you can see, already at the very beginning of mastering sound technology, we had to face the concept of Hi-Fi (High Fidelity), high fidelity of sound reproduction. Hi-Fi are different levels which are ranked next. main parameters:

1. Band of reproducible frequencies.
2. Dynamic range is the ratio in decibels (dB) of the maximum (peak) output power to the noise floor.
3. Intrinsic noise level in dB.
4. The coefficient of nonlinear distortion (THD) at the nominal (long-term) output power. THD at peak power is taken as 1% or 2%, depending on the measurement technique.
5. Irregularities of the amplitude-frequency characteristic (AFC) in the reproducible frequency band. For speakers - separately at low (LF, 20-300 Hz), medium (MF, 300-5000 Hz) and high (HF, 5000-20,000 Hz) sound frequencies.

Note: attitude absolute levels any values ​​of I in (dB) is defined as P (dB) = 20lg (I1 / I2). If I1

You need to know all the subtleties and nuances of Hi-Fi when designing and building speakers, and as for a home-made Hi-Fi UMZCH for the home, before moving on to such, you need to clearly understand the requirements for their power required to sound a given room. dynamic range (dynamics), noise floor and THD. To achieve from the UMZCH a frequency band of 20-20,000 Hz with a blockage at the edges of 3 dB and an uneven frequency response at the midrange of 2 dB on a modern element base is not very difficult.

Volume

The power of the UMZCH is not an end in itself, it should provide the optimal volume of sound reproduction in a given room. It can be determined by curves of equal loudness, see fig. Natural noise in residential premises is not quieter than 20 dB; 20 dB is a forest wilderness in complete calm. A loudness level of 20 dB relative to the threshold of audibility is the threshold of intelligibility - a whisper can still be discerned, but music is perceived only as a fact of its presence. An experienced musician can tell which instrument is playing, but which one is not.

40 dB - the normal noise of a well-insulated city apartment in a quiet area or a country house - represents the threshold of intelligibility. Music from the threshold of intelligibility to the threshold of intelligibility can be listened to in the presence of deep correction of the frequency response, especially in the bass. To do this, the MUTE function is introduced into modern UMZCH (mute, mutation, not mutation!), Including, respectively. corrective circuits in UMZCH.

90 dB is the loudness level of a symphony orchestra in a very good concert hall. 110 dB can be given by an expanded orchestra in a hall with unique acoustics, of which there are no more than 10 in the world, this is the threshold of perception: sounds are perceived louder still as discernible in meaning with an effort of will, but already annoying noise. The loudness zone in residential premises of 20-110 dB is the zone of complete audibility, and 40-90 dB is the zone of best audibility, in which untrained and inexperienced listeners fully perceive the meaning of the sound. If, of course, it is in it.

Power

Calculating the power of the equipment for a given loudness in the listening area is perhaps the main and most difficult task of electroacoustics. For yourself, in conditions, it is better to go from acoustic systems (AC): calculate their power using a simplified method, and take the nominal (long-term) power of the UMZCH equal to the peak (musical) speaker. In this case, the UMZCH will not add noticeably its distortions to those of the speakers, they are already the main source of nonlinearity in the sound path. But you should not make the UMZCH too powerful: in this case, the level of its own noise may turn out to be higher than the audibility threshold, because it is calculated from the voltage level of the output signal at maximum power. If it is very simple to consider, then for a room in an ordinary apartment or house and a speaker with normal characteristic sensitivity (sound output), you can take a trace. values ​​of the optimal power UMZCH:

• Up to 8 sq. m - 15-20 W.
• 8-12 sq. m - 20-30 W.
• 12-26 sq. m - 30-50 W.
• 26-50 sq. m - 50-60 W.
• 50-70 sq. m - 60-100 W.
• 70-100 sq. m - 100-150 W.
• 100-120 sq. m - 150-200 W.
• More than 120 sq. m - determined by calculation according to the data of acoustic measurements on site.

Dynamics

The dynamic range of the UMZCH is determined by the curves of equal loudness and threshold values ​​for different degrees of perception:

1. Symphonic music and jazz with symphonic accompaniment - 90 dB (110 dB - 20 dB) ideal, 70 dB (90 dB - 20 dB) acceptable. Sound with dynamics of 80-85 dB in a city apartment cannot be distinguished from ideal by any expert.
2. Other serious music genres - excellent 75 dB, 80 dB above the roof.
3. Pops of any kind and soundtracks for films - 66 dB for the eyes is enough, tk. These opuses are already compressed in levels up to 66 dB and even up to 40 dB during recording, so that you can listen on anything.

The dynamic range of the UMZCH, correctly selected for a given room, is considered equal to its own noise level, taken with a + sign, this is the so-called. signal-to-noise ratio.

KNI

Nonlinear distortion (NI) UMZCH is the components of the output signal spectrum, which were not in the input signal. Theoretically, it is best to "push" the NI to the level of its own noise, but technically it is very difficult to implement. In practice, they take into account the so-called. masking effect: at volume levels below approx. 30 dB, the range of frequencies perceived by the human ear is narrowed, as is the ability to distinguish sounds by frequency. Musicians hear notes, but find it difficult to assess the timbre of the sound. In people without a musical ear, the masking effect is observed already at 45-40 dB of loudness. Therefore, an UMZCH with a THD of 0.1% (–60 dB of a loudness level of 110 dB) will be assessed as a Hi-Fi by an ordinary listener, and with an THD of 0.01% (–80 dB) it can be considered not distorting sound.

Lamps

The latter statement, perhaps, will cause rejection, even furious, among the adherents of tube circuitry: they say, only lamps give real sound, and not just any, but individual types of octal. Calm down, gentlemen - a special tube sound is not a fiction. The reason is fundamentally different spectra of distortions in electronic tubes and transistors. Which, in turn, are due to the fact that the flow of electrons in the lamp moves in a vacuum and quantum effects do not appear in it. A transistor is a quantum device, where minority charge carriers (electrons and holes) move in the crystal, which is generally impossible without quantum effects. Therefore, the spectrum of tube distortions is short and clean: only harmonics up to the 3rd - 4th are clearly visible in it, and there are very few combination components (the sums and differences of the frequencies of the input signal and their harmonics). Therefore, at the time of vacuum circuitry, SOI was called the harmonic coefficient (CH). In transistors, the spectrum of distortions (if they are measurable, the reservation is random, see below) can be traced up to the 15th and higher components, and there are more than enough combination frequencies in it.

At the beginning of solid-state electronics, the designers of transistor UMZCH took for them the usual "tube" THD in 1-2%; sound with a tube distortion spectrum of this magnitude is perceived by ordinary listeners as pure. By the way, the very concept of Hi-Fi did not exist at that time. It turned out - they sound dull and dull. In the process of development of transistor technology, an understanding of what Hi-Fi is and what is needed for it was developed.

Currently, the growing pains of transistor technology have been successfully overcome and the side frequencies at the output of a good UMZCH are hardly captured by special measurement methods. And the lamp circuitry can be considered as having passed into the category of art. Its basis can be anything, why can't electronics go there? An analogy with photography is appropriate here. No one can deny that a modern digital mirror gives a picture that is immeasurably clearer, more detailed, deep in the range of brightness and color than a plywood box with an accordion. But someone with the coolest Nikon "clicks pictures" like "this is my fat cat got drunk like a bastard and is sleeping with his paws out," and someone with Smena-8M takes a picture on Svem's b / w film, in front of which people crowd at a prestigious exhibition.

Note: and calm down again - it's not all bad. Today, low-power lamp UMZCHs have at least one application, and not of the least importance, for which they are technically necessary.

Experienced stand

Many audio lovers, having barely learned how to solder, immediately "go to the lamps." This is by no means blameworthy, on the contrary. Interest in the origins is always justified and useful, and electronics have become such on lamps. The first computers were vacuum tubes, and the onboard electronic equipment of the first spacecraft was also vacuum tubes: transistors were already there, but they could not withstand extraterrestrial radiation. By the way, then tube ... microcircuits were also created under the strictest confidence! On microlamps with a cold cathode. The only known mention of them in open sources is in the rare book of Mitrofanov and Pickersgil "Modern receiving and amplifying tubes".

But enough lyrics, to the point. For those who like to tinker with the lamps in fig. - a circuit of a bench lamp UMZCH designed specifically for experiments: SA1 switches the operating mode of the output lamp, and SA2 switches the supply voltage. The circuit is well known in the Russian Federation, a slight revision has affected only the output transformer: now it is possible not only to "drive" the native 6P7S in different modes, but also to select the switching coefficient of the screen grid in the ultralinear mode for other lamps; for the vast majority of output pentodes and beam tetrodes, it is either 0.22-0.25, or 0.42-0.45. See below for the manufacture of the output transformer.

For guitarists and rockers

This is the case when you can't do without lamps. As you know, the electric guitar became a full-fledged solo instrument after the pre-amplified signal from the pickup was passed through a special attachment - a fuser - which deliberately distorted its spectrum. Without this, the sound of the string was too harsh and short, because the electromagnetic pickup reacts only to the modes of its mechanical vibrations in the plane of the instrument deck.

Soon an unpleasant circumstance came to light: the sound of an electric guitar with a fuser acquires full strength and brightness only at high volumes. This is especially true for guitars with a humbucker pickup, which gives the most "evil" sound. But what about a beginner who is forced to rehearse at home? Do not go to the hall to perform, not knowing exactly how the instrument will sound there. And just rock lovers want to listen to their favorite things in full juice, and rockers are generally decent and non-conflict people. At least those who are interested in rock music, and not outrageous entourage.

So, it turned out that the fatal sound appears at loudness levels that are acceptable for living quarters, if the UMZCH is tube. The reason is the specific interaction of the signal spectrum from the fuser with the clean and short spectrum of tube harmonics. Here again an analogy is appropriate: a b / w photo can be much more expressive than a color one, because leaves only outline and light for viewing.

Those who need a tube amplifier not for experiments, but due to technical necessity, have no time to master the intricacies of tube electronics, they are carried away by others. UMZCH in this case, it is better to do transformerless. More precisely, with a single-ended matching output transformer operating without permanent bias. This approach greatly simplifies and speeds up the manufacture of the most complex and critical unit of the lamp UMZCH.

"Transformerless" tube output stage UMZCH and pre-amplifiers to it

On the right in Fig. a diagram of the transformerless output stage of the UMZCH tube is given, and on the left are the options for a pre-amplifier for it. Above - with a tone control according to the classic Baksandal scheme, which provides a fairly deep adjustment, but introduces small phase distortions into the signal, which can be significant when the UMZCH operates on a 2-way speaker. Below is a preamplifier with a simpler tone control that does not distort the signal.

But back to the “tip”. In a number of foreign sources, this scheme is considered a revelation, however, identical to it, with the exception of the capacity of electrolytic capacitors, is found in the Soviet "Handbook of a radio amateur" in 1966. A thick book of 1060 pages. Then there was no Internet and databases on disks.

In the same place, on the right in the figure, the shortcomings of this scheme are briefly but clearly described. Improved, from the same source, given on the next page. rice. on right. In it, the screen grid L2 is powered from the midpoint of the anode rectifier (the anode winding of the power transformer is symmetrical), and the screen grid L1 is fed through the load. If, instead of high-impedance speakers, you turn on a matching transformer with conventional speakers, as in the previous. circuit, output power approx. 12 W because the active resistance of the primary winding of the transformer is much less than 800 ohms. THD of this power stage with transformer output - approx. 0.5%

How to make a transformer?

The main enemies of the quality of a powerful signal LF (sound) transformer are the stray magnetic field, the lines of force of which are closed, bypassing the magnetic circuit (core), eddy currents in the magnetic circuit (Foucault currents) and, to a lesser extent, magnetostriction in the core. Because of this phenomenon, a casually assembled transformer "sings", buzzes or beeps. Foucault currents are fought by reducing the thickness of the plates of the magnetic circuit and additionally insulating them with varnish during assembly. For output transformers, the optimal plate thickness is 0.15 mm, the maximum allowable is 0.25 mm. It is not necessary to take thinner plates for the output transformer: the filling factor of the core (central core of the magnetic circuit) with steel will fall, the cross-section of the magnetic circuit will have to be increased to obtain the specified power, which will only increase the distortion and losses in it.

In the core of a sound transformer operating with constant bias (for example, the anode current of a single-ended output stage), there must be a small (determined by calculation) non-magnetic gap. The presence of a non-magnetic gap, on the one hand, reduces signal distortion from constant bias; on the other hand, in a conventional magnetic circuit it increases the stray field and requires a larger core section. Therefore, the non-magnetic gap must be expected to be optimal and performed as accurately as possible.

For transformers operating with magnetization, the optimal type of core is made of Shp plates (perforated), pos. 1 in fig. In them, a non-magnetic gap is formed during core punching and is therefore stable; its value is indicated in the passport for the plates or measured with a set of probes. The scattering field is minimal, because the side branches, through which the magnetic flux is closed, are solid. Cores of transformers are often assembled from Shp plates without magnetization, because Shp plates are made of high quality transformer steel. In this case, the core is assembled with an overlap (the plates are placed with a notch in one direction or the other), and its cross-section is increased by 10% against the calculated one.

It is better to wind transformers without magnetization on USH cores (reduced height with widened windows), pos. 2. In them, a decrease in the stray field is achieved by reducing the length of the magnetic path. Since the USH plates are more accessible than the Shp ones, the cores of transformers with magnetization are often recruited from them. Then the assembly of the core is carried out in close-up: a package of W-plates is assembled, a strip of non-conductive non-magnetic material with a thickness equal to the value of the non-magnetic gap is put, covered with a yoke from a package of jumpers and pulled together with a clip.

Note:"Sound" signal magnetic circuits of the ShLM type for output transformers of high-quality tube amplifiers are of little use, they have a large stray field.

At pos. 3 is a diagram of the dimensions of the core for calculating the transformer, at pos. 4 the structure of the winding frame, and in pos. 5 - patterns of its parts. As for the transformer for the "transformerless" output stage, it is better to do it on the ShLMme over the lid, because bias is negligible (bias current is equal to screen grid current). The main task here is to make the windings as compact as possible in order to reduce the stray field; their active resistance will still turn out to be much less than 800 ohms. The more free space left in the windows, the better the transformer turned out. Therefore, the windings wind turn to turn (if there is no winding machine, this is terrible) from the thinnest wire possible, the stacking factor of the anode winding for the mechanical calculation of the transformer is 0.6. The winding wire is of PETV or PEMM brands, they have an oxygen-free core. You do not need to take PETV-2 or PEMM-2, they have an increased outer diameter due to double varnishing and the scattering field will be larger. The primary winding is wound first, because it is its scattering field that most of all affects the sound.

The iron for this transformer must be looked for with holes in the corners of the plates and clamping brackets (see the figure on the right), since "For complete happiness" the assembly of the magnetic circuit is carried out in the next. order (of course, the windings with leads and external insulation should already be on the frame):

1. Prepare half-diluted acrylic varnish or, in the old fashioned way, shellac;
2. Plates with jumpers are quickly varnished on one side and as soon as possible, without pressing hard, put into the frame. The first plate is placed with the varnished side inward, the next - with the unlacquered side to the varnished first, etc .;
3. When the frame window is full, staples are applied and bolted tightly;
4. After 1-3 minutes, when the squeezing of the varnish from the gaps apparently stops, the plates are added again until the window is filled;
5. Repeat paragraphs. 2-4, until the window is tightly packed with steel;
6. The core is pulled tight again and dried on a battery, etc. 3-5 days.

The core assembled using this technology has very good plate insulation and steel filling. Magnetostriction loss is not detected at all. But keep in mind - for the cores of their permalloy, this technique is not applicable, because from strong mechanical influences the magnetic properties of permalloy are irreversibly deteriorated!

On microcircuits

UMZCH on integrated circuits (ICs) are made most often by those who are satisfied with the sound quality up to average Hi-Fi, but are more attracted by the cheapness, speed, ease of assembly and the complete absence of any setup procedures that require special knowledge. Quite simply, an amplifier based on microcircuits is the best option for dummies. The classics of the genre here - UMZCH on the IC TDA2004, standing on the series, God forbid, memory, for 20 years, on the left in Fig. Power - up to 12 W per channel, supply voltage - 3-18 V unipolar. Radiator area - from 200 sq. see for maximum power. Advantage - the ability to work on a very low-impedance, up to 1.6 Ohm, load, which allows you to remove the full power when powered by a 12 V on-board network, and 7-8 W - with a 6-volt power supply, for example, on a motorcycle. However, the output of TDA2004 in class B is non-complementary (on transistors of the same conductivity), so the sound is definitely not Hi-Fi: THD 1%, dynamics 45 dB.

The more modern TDA7261 gives no better sound, but more powerful, up to 25 W, tk. the upper limit of the supply voltage has been increased to 25 V. The lower, 4.5 V, still allows power supply from 6 V of the onboard network, i.e. The TDA7261 can be launched from almost all onboard networks, except for aircraft 27 V. With the help of attached components (strapping, on the right in the figure), the TDA7261 can work in mutation mode and with the St-By function (Stand By, wait), which transfers the UMZCH to the mode of minimum power consumption when there is no input signal for a certain time. Conveniences cost money, so for a stereo you will need a pair of TDA7261 with radiators from 250 sq. see for each.

Note: If you are attracted by amplifiers with the St-By function, keep in mind that you should not expect speakers wider than 66 dB from them.

"Supereconomic" in terms of power supply TDA7482, on the left in the figure, working in the so-called. class D. Such UMZCH is sometimes called digital amplifiers, which is incorrect. For real digitization, samples of the level are removed from the analog signal with a sampling frequency not less than twice the highest of the reproduced frequencies, the value of each sample is recorded with a noise-immune code and stored for further use. UMZCH class D - impulse. In them, the analog is directly converted into a sequence of high frequency pulse width modulated (PWM) pulses, which is fed to the speaker through a low pass filter (LPF).

Class D sound with Hi-Fi has nothing to do: THD of 2% and dynamics of 55 dB for class D UMZCH are considered very good indicators. And TDA7482 here, I must say, is not an optimal choice: other firms specializing in class D produce UMZCH ICs cheaper and requiring less strapping, for example, D-UMZCH of the Paxx series, on the right in Fig.

Of the TDAs, it should be noted the 4-channel TDA7385, see fig., On which you can assemble a good amplifier for speakers up to average Hi-Fi inclusive, with a frequency division into 2 bands or for a system with a subwoofer. Defiltering of LF and MF-HF in either case is done at the input on a weak signal, which simplifies the design of the filters and allows deeper separation of the bands. And if the acoustics are subwoofer, then 2 channels of the TDA7385 can be allocated for the sub-ULF bridge circuit (see below), and the remaining 2 can be used for the MF-HF.

UMZCH for subwoofer

The subwoofer, which can be translated as “sub-bass” or, literally, “pre-bass” reproduces frequencies up to 150-200 Hz, in this range human ears are practically unable to determine the direction to the sound source. In speakers with a subwoofer, the “subwoofer” speaker is placed in the hotel's acoustic design, this is the subwoofer itself. The subwoofer is placed, in principle, as it is more convenient, and the stereo effect is provided by separate mid-high-frequency channels with their own small-sized speakers, the acoustic design of which is not particularly demanding. Experts agree that it is still better to listen to stereo with full channel separation, but subwoofer systems significantly save money or labor on the bass path and facilitate the placement of acoustics in small rooms, which is why they are popular with consumers with ordinary hearing and not particularly demanding ones.

The "leakage" of the midrange-high frequency into the subwoofer, and from it into the air, greatly spoils the stereo, but if you abruptly "cut off" the subbass, which, by the way, is very difficult and expensive, then a very unpleasant sound jump effect will appear. Therefore, the channels are filtered twice in subwoofer systems. At the input, the MF-HF with bass tails are allocated with electric filters, which do not overload the MF-HF path, but provide a smooth transition to the sub-bass. Bass with midrange "tails" are combined and fed to a separate UMZCH for the subwoofer. The midrange is filtered, so as not to spoil the stereo, in the subwoofer it is already acoustically: the subwoofer is placed, for example, in the partition between the resonator chambers of the subwoofer, which does not let the midrange out, see on the right in Fig.

A number of specific requirements are imposed on the UMZCH for a subwoofer, of which the "teapots" consider the greatest possible power to be the main one. This is completely wrong, if, say, the calculation of acoustics for a room gave a peak power W for one speaker, then the power of the subwoofer needs 0.8 (2W) or 1.6W. For example, if speakers S-30 are suitable for a room, then a subwoofer is needed 1.6x30 = 48 watts.

It is much more important to ensure the absence of phase and transient distortions: if they go, there will definitely be a sound jump. As for THD, it is permissible up to 1%. Intrinsic bass distortions of this level are not audible (see curves of equal loudness), and the “tails” of their spectrum in the best audible midrange area will not get out of the subwoofer.

To avoid phase and transient distortions, an amplifier for a subwoofer is built according to the so-called. bridge circuit: outputs of 2 identical UMZCH turn on oppositely through the speaker; the signals to the inputs are applied in antiphase. The absence of phase and transient distortion in the bridge circuit is due to the complete electrical symmetry of the output signal paths. The identity of the amplifiers forming the bridge arms is ensured by the use of paired UMZCH on ICs, made on one crystal; this is perhaps the only case when an on-chip amplifier is better than a discrete one.

Note: the power of the bridge UMZCH does not double, as some think, it is determined by the supply voltage.

An example of a bridge UMZCH circuit for a subwoofer in a room up to 20 sq. m (without input filters) on the TDA2030 IC is given in Fig. left. Additional filtering of the midrange is carried out by the R5C3 and R'5C'3 circuits. Radiator area TDA2030 - from 400 sq. cm. Bridge UMZCH with an open output have an unpleasant feature: when the bridge is unbalanced, a constant component appears in the load current that can damage the speaker, and the protection circuits on the subbases often fail, turning off the speaker when not needed. Therefore, it is better to protect the expensive “oak” bass head with non-polar batteries of electrolytic capacitors (highlighted in color, and a diagram of one battery is given in the inset.

A little about acoustics

The acoustic design of the subwoofer is a special topic, but since a drawing is given here, then explanations are also needed. Case material - MDF 24 mm. The resonator tubes are made of sufficiently durable non-ringing plastic, for example, polyethylene. The inner diameter of the pipes is 60 mm, the projections inward are 113 mm in the large chamber and 61 in the small one. For a specific loudspeaker head, the subwoofer will have to be reconfigured for the best bass and, at the same time, for the least impact on the stereo effect. To tune the pipes, they take a deliberately greater length and, pushing in and out, achieve the required sound. The protrusions of the pipes outward do not affect the sound, they are then cut off. The tuning of the pipes is interdependent, so you have to tinker.

Headphone Amplifier

A headphone amplifier is made by hand most often for 2 reasons. The first one is for listening "on the go", i.e. outside the home, when the audio output of the player or smartphone is not enough to swing "buttons" or "mugs". The second is for high-end home headphones. Hi-Fi UMZCH for an ordinary living room is needed with dynamics up to 70-75 dB, but the dynamic range of the best modern stereo headphones exceeds 100 dB. An amplifier with such dynamics is more expensive than some cars, and its power will be from 200 W per channel, which is too much for an ordinary apartment: listening at a power that is too low against the rated power spoils the sound, see above. Therefore, it makes sense to make a low-power, but with good dynamics, a separate amplifier specifically for headphones: prices for household UMZCH with such a makeweight are clearly absurdly overstated.

A diagram of the simplest transistor headphone amplifier is given in pos. 1 fig. Sound - except for Chinese "buttons", works in class B. It also does not differ in efficiency - 13-mm lithium batteries last 3-4 hours at full volume. At pos. 2 - TDA classic for on-the-go headphones. The sound, however, gives quite decent, up to average Hi-Fi, depending on the parameters of the digitization of the track. There are innumerable amateur improvements to the TDA7050 strapping, but no one has yet achieved the transition of sound to the next level of class: the "mikruha" itself does not allow. TDA7057 (pos. 3) is simply more functional, you can connect a volume control on a conventional, not dual, potentiometer.

UMZCH for headphones on the TDA7350 (pos. 4) is already designed to build good individual acoustics. It is on this IC that headphone amplifiers are assembled in most household UMZCHs of the middle and high class. UMZCH for headphones on the KA2206B (pos. 5) is already considered professional: its maximum power of 2.3 W is also enough to pump such serious isodynamic "mugs" as TDS-7 and TDS-15.

HIGH QUALITY PRE-AMPLIFIER DIAGRAM

At the turn of 2004 and 2005, there is a natural desire to build amplifiers on a modern element base, using the advanced achievements of world electronic technology.
I bring to your attention a high quality preamplifier based on EL2125.
Basic materials are FREE, DIYers are free to use them to repeat in their own designs.
WHY EL2125?
An excellent chip, according to its characteristics, it makes it almost the 2nd place in the top ten op-amps according to model reviews in 2004.
Of course, this is not the AD8099 (first place in the world, Intel's 2004 Innovation Award), but the EL2125 has already appeared on the CIS market and it is quite possible to get it, especially for those who live in capital and large cities.
HOW GOOD THE EL2125 FEATURES, JUDGE FOR YOURSELF:

Ability to work on loads up to - 500 Ohm
Working frequency range up to - 180 MHz
Supply voltage - ± 4.5 ... ± 16.5 V.
Total Harmonic Distortion - less than 0.001%
Slew rate of output signal - 190 V / µs
Noise level - 0.86 nV / vHz (better than AD8099!!!)

The EL2125 retail price is usually $ 3 per unit, not very cheap but worth it.
Most often, EL2125 is found in a case of the SO - 8 type (prepare micro nozzles for soldering irons).
I should note that I would add to the list of characteristics such as - "amazing musicality". This indicator cannot be measured with instruments and expressed in numbers, it is felt only by ear.

1. As an amplifier for telephones with a wide impedance range:

2. As a high-quality preamplifier for power amplifiers with bipolar power supply (in the range from ± 22 to ± 35 V.) and sensitivity of 20 ... 26 dB:

This op-amp unwittingly suggests itself into a more serious pre-amplifier, created on the basis of the Solntsev amplifier and described on the Soldering Iron website:
The amplifier uses double variable resistors R11 and R17 of any type of group B, R1 and R21 of any type of group B or A. As a tone-compensated volume control (R21), a variable resistor of 100 kΩ (with a tap from the middle) can be used. Transistors can be replaced with KT3107I, KT313B, KT361V, K (VT1, VT4) and KT312V, KT315V (the rest). Replacing OA K574UD1 with OA of other types is not recommended. With a significant level of the constant component (in rare cases), a capacitor with a capacity of 2.2 - 5 microfarads must be installed at point A.

The described preamplifier is connected to a AF power amplifier with an input impedance of at least 10 kOhm. With a significant increase in Kg, this PU can also be loaded on UMZCH with Rin up to 2 kOhm (which is highly undesirable), in such cases (if Rin of your UMZCH is less than 10 kOhm), you just need to power up the output stage again (a copy of the VT1-VT2- VT3-VT4-R4-R5-R6-R7, connect to output DA2), connect resistors R23 and R24 in the same way as resistors R2 and R3, although in this case the noise level may increase. And if Rin of your UMZCH is greater than or equal to 100 kΩ, then it is recommended to use K574UD1A (B) as an operational amplifier DA2, this will reduce the level of distortion and noise.

Possible changes in the scheme (improving):
- To exclude the P2K switches (very unreliable in operation) from the path of the sound signal, it is recommended to exclude the SA1 switch from the circuit (together with the resistors R8, R9), and transfer the SA2 switch to the last stage by short-circuiting the resistor R23 (the resistors R13, R14 are excluded from the diagram).

Preamplifier circuit:

It will also not be useless to use this op-amp in a universal pre-amplifier capable of also performing the function of a headphone amplifier. The schematic diagram is shown below:

Emitter followers VT1-VT2 unload the op-amp output, and then a local feedback circuit follows, which further reduces non-linear distortions. Resistors R19 and R20 set the quiescent current of the preamplifier window stage, similar to power amplifiers, within 7-12 mA. In this regard, the last stage must be installed on a small heat sink

The page was prepared based on materials from the site http://yooree.narod.ru and http://cxem.net

©

The word "preamp" is used in different ways by different manufacturers, marketers, and users. It is one of the most widely interpreted terms when discussing audio equipment; if you ask for a "preamplifier" you can also ask for "furniture". Nobody will know exactly what you want. Let's see what a preamp is?

Why do I need a preamplifier and do I need one?

A preamplifier is a “preamplifier,” and as the name suggests, it prepares the signal coming from the source or microphones for further amplification. There are a number of reasons to buy:

Whether or not you need a preamplifier.

When you plug your DAC or microphone directly into an amplifier, how does it sound?

• Is this signal enough?
• Is it balanced?
• Clean?

If this is not the case, then you probably need to buy a preamp.

By the way, a good separate preamplifier gives less pickup, interference and other noise than, for example, a complete
amplifier. Whenever the signal is amplified, the goal is to keep the signal-to-noise ratio as
possible in the best quality. This makes a lot of sense because pickup and noise from the preamplifier can cause non-linear sound when the signal is amplified. In order to avoid introducing additional noise to the preamplifier, it should be placed in a separate block and as close as possible to the signal source like this.

The preamplifier is the part of the amplifier. This means the preamplifier will allow you to hook up many sources such as a CD tuner or DAC.

The preamplifier allows you to change the volume and possibly change the treble and bass parameters.

By the way, 90% of preamplifiers have a phono stage, which you need to connect a turntable.

Finally, one of the reasons to buy a preamp is to switch multiple signals.

All combined systems require pre-amplification.

There is also a multichannel preamplifier that combines the signals for you and creates a single output for the amplifier. The multichannel preamplifier also allows you to adjust the equalizer and power of each signal, depending on yours.

An amplifier can be divided into two main parts, a pre-amplifier and a power amplifier.

Amplifier

One way to get better sound quality was to separate the two sections of the amplifier. By separating the preamplifier and power amplifier, you could design a dedicated power supply to drive the electronics with finer signals without interference from the noisy power amplifier circuits. In some cases, even the power is split otherwise to reduce noise in the preamplifier.

Preamplifiers can also be "passive". They do not require power, since the components (mainly
switch and volume control) are operated directly from your sources (). In theory, this is the best way, but in practice they have quite a few drawbacks, but a passive preamp is a relatively rare type.

When we talk about a pre-amplifier, we usually mean the pre-amplifier in a separate block. Such a pre-amplifier is housed in a separate box, and it contains many knobs for controlling the power amplifier to control the acoustics and switch.

The preamplifier can also be built as an instrument, pedal, rack unit, mixer, sound card, or a variety of other forms; and the preamplifier can also be the input stage of each head amplifier.

Not every preamplifier can drive a power amplifier effectively. Others can be designed to increase the signal level to drive the input.

Some preamps have gain control, while others have a fixed gain. Either way, they usually have a volume knob that just passively turns the overall signal level at the very end of the preamp circuit. Also in the preamplifier there may be a timbre, which may include something like an equalizer control. Some people want a lot of tonal changes and EQ control, other people want absolute.

Find your preamplifier!

Tell us about your sound system, audio-video equipment, construction, setup, etc.on the .

Send by e-mail: [email protected] text, photo, diagrams with notes on the, if you do not know where to start, how to write, then write, we will help you, we will send you a list of ready-made interview questions.

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