Criteria

Audio power amplifier for home recording studio must be some special! But what it should do? The dozens of epithets are spinning around, but paradoxically, it is much better if they are not, because the correct audio power amplifier does not have to add anything by itself.

"Transparent" - that is the only criteria of the quality required to make the audio amplifier work in the home recording studio.

Minimalism for creativity

Take a look at the article "Reflections on the creation of the proper audio power amplifier". No mystery. Just let's not complicate things unnecessarily, and do only what is necessary. In music, there are only seven notes, but will you make a masterpiece or not - it depends on how you play them.

To solve the problem, we will use the principles of minimalism. Which means, we will not use anything in its construction, which has no real practical value. At least for this particular task: ensuring sound in a home recording studio.

In its simplest form, the technical conditions for studio audio amplifier reduces to the choosing an output power and matching amplifier with speaker system. It is hard to make the other parameters insufficient.

Practice shows that the power from 30W to 60 - 80W is enough for a studio amplifier.

As for the operational requirements, they are switchable loudness, volume control and the input selector. Although the input selector can be eliminated if this amplifier and speaker system does not intend to actively use the other sound sources.

Another important detail. This amplifier will probably be used only in your own recording studio, in your house. Therefore, it is not necessary to make a good-looking and suitable case for this power amplifier.  All you need is a volume control.

This means that the number of functional units may be less by one position. It means that the case for the amplifier may not even exist, you can just make a structural rigidity of such components as the heat sink and power transformer, or use the case from the old amplifier.

For example, my studio amplifier has a case from the amplifier "Brig 001", it works great and will work even for many years, satisfying with my studio needs on 100%.

Volume control

My switchable tone-compensated audio volume control I borrowed from the same power amplifier "Brig 001", slightly modifying it.

Choosing the denominations, I was focusing on the immutability of the tonal balance in the volume regulation in specific conditions, as I hear it. Therefore, the value of R1 and C2 are a little bit different from the calculated ones.

Power supply unit

The power supply has no specific features, so I did not draw it. It is an ordinary power transformer with capacity more than 100W and two separate rectifiers, separately for each channel of the stereo amplifier.

An important point: classic power amplifier circuits have the output signal, which seriesly passes through the rectifier’s capacity. Therefore, pay attention to this filter.

Passing characteristics of the power filter’s capacitors are imperfect. For different frequencies of all the audio range, the capacitors have different resistance. Therefore,

• firstly it is a bad idea to use one rectifier for two channels of a stereo amplifier, since on this section, the currents of both channel will sum, which can cause the turbid sound

• secondly, the quality of power supply filter’s capacitors depends mostly on the quality of the sound. That is why, there is a general rule that the capacitors with good performance in the top of audio range must be set in parallel with the high-capacity capacitors. Better with lavsan or polypropylene insulation, but not ceramic.

So we should make two rectifiers, one for each channel, and set the schottky diodes in them.

Transformer plays a minor role, all you need is enough power. Approximately 1.5 times more than the estimated total output power of both amplifier’s channels. Two transformers - even better, but it is not so far minimalistic.

Audio power amplifier circuit

Audio power amplifier circuit is a controversial issue, but we will solve it in the same pragmatic, minimalistic style. We have a wonderful chip TDA7293 (or TDA7294). By the sum of characteristics, it is almost the same as the power amplifier LM386.

Do not listen to the skeptics and snobs who think that you cannot get enough quality of sound for such money. Just make the audio amplifier, and work with it. You will find out that in fact, correctly assembled power amplifier TDA7293 sounds great. It adds nothing, has a high specification and microdynamics.

On the first look, the circuit of the TDA7293 is not much different from the standard inclusion recommended by the manufacturer. But there are some differences, and they are important.

• The circuit excludes all ceramic capacitors. Instead of them, we have well-working metal-foil capacitors K73-17 or CL21. Or the polypropylene ones - CBB21 or K78-2? Which are even better/

• We have removed all unnecessary coupling capacitors. On the signal’s path there is only one input capacitor C1 1m - 4,7m CL21 or CBB21.

• We have excluded the capacitor in NFB (negative feedback). Even in such inclusion we can neglect the bias voltage. But the fact, that the amplifier ceases being a second-order filter at the same time, is a very significant advantage, claiming on the level of High End audio amplifiers.

To match the speaker, our power amplifier has a combined feedback R5R6R9R11. I am guided by load resistance. This means that the speaker system - 3,5om, and the output resistance of the power amplifier - 3,5om.

More details about the matching I wrote here. In contrast to the adaptive matching of the power amplifier and speaker system, combined feedback, the output resistance of the audio amplifier is not affected by the load resistance. But in general, it gives very good results for the sound, almost repeating the load characteristic of a tube amplifier.

That is all.

By the way, even in this power amplifier I made a  manage of "Standby" mode and "Mute" by the circuit on transistor VT1. This circuit, in contradistinction to the standard one, allows you to disable the chip neatly, smoothly and quickly, without waiting for the start of the power supply capacitors’ discharge. If you need to delay when you turn on, the series with a diode VD1 you should include a resistor, and the capacity of C6 may need to be increased.

Design

Correctly assembled power amplifier is a huge subject, but not as scary as it seems. And the most important secrets of a properly assembled audio amplifier are the wires.

It is very simple.

The most important thing is not to make ground loops and not to create the longitudinal interference for the input signal of power amplifier. This means that all ground findings of the circuit should converge at a single point, and not to be duplicated through the case, conductive parts and components, etc.

Pay some extra attention to the grounding point of the input circuit. The fact is that the bipolar power of the power amplifier creates additional problems with the definition of the point of the grounding. A longitudinal interference from the power supply’s current’s ripple and the output current of the power amplifier can be manifested on only 1mm of the ground bus and can be attached to the input. The intensity and the asymmetry of the interference depends on the level of the output signal.

A nonoptimality of the "ground" point in the amplifier with negative feedback current can be observed on the oscilloscope screen by moving the connection point of the current R11 sensor with the earth bus. Just do not forget to turn off the amplifier before soldering the resistor to a new location.

And yet we must bear in mind that the parts of the general "ground" for the two stereo channels also add garbage to the sound.

This situation may create mud and slime in the sound, and can ruin the sound of the nicest circuit ever. More correct is a circuit of a power amplifier with a single polar power supply and separated "grounds," or two monoblocks. But this is a topic for another article.

So, All we need is to connect all this.

If someone does not know there is a ‘-‘ of power on the chip’s body, not ‘ground’. So I recommend you to put the chip through the isolation pads. The radiators themselves are from 500sm² to 750sm² per chip.

I collected the whole circuit, using surface mount with a mounting bracket with petals. This gives a greater flexibility in the choice of details, in making some changes in the circuit or some experiments.

Now that’s it.

That is the minimalism.

Sergey Shevgota