Once again about the love of good sound or again about the “sounding iron”

Recently, quite a lot of information has appeared in our magazine about all kinds of “sound” problems, terms, characteristics … So, readers really need it (or not? …). In particular, from the feedback received, it became clear that many readers would like to see not general, but “specific” recommendations for assembling amplifiers, making speakers …

Many will say that for this there is a magazine “Radio” or other publications like it, but they often describe either too simple or too complex things (a note is the subjective opinion of the author …). And computer scientists are often just too lazy to understand sophisticated schemes, and the rule “yes, it’s easier for me to buy …” comes into play.

Only now, the purchase does not always please with its quality or reliability. So, it’s time to start again with a soldering iron (or promise beer to a neighbor for making the amplifier described here …). Well, “forward and with the song” ..?

HI-FI, but not from felt boots

In one of the letters, I met a request to bring a simple volume and tone control circuit, but such that there were no double variable resistors. It’s no secret that their quality and durability “leaves much to be desired …”, if not to put it simply. For example, in expensive speakers there are balance controls, loudness (real, and not with the help of a couple of details). Are you jealous?

There is a solution to the problem, and it was invented a long time ago, and now the necessary microcircuit can be obtained almost everywhere (or ask someone to bring it, in the end). There are already many such microcircuits, and interested readers can search for something similar on their own. And here is the amplifier circuit, delve into it.

In this case, all adjustments are performed by the TDA1524A microcircuit. There are few “strapping” details, and the scheme of its inclusion is generally simple, but meanwhile it is a stereophonic electronic volume control, bass and treble tone, as well as stereo balance. In addition, she knows how to “loudspeaker” the sound.

Thanks to the efforts of the developers, all frequency-dependent functions are performed by only four external capacitors. When adjusting, there are no “rustles” and “clicks”, which are well known to many music lovers, because the adjusting resistors are included not in the “signal” circuits, but in the circuits for generating control voltages.

Smoothing capacitors provide a smooth variation of these voltages even when using “used” resistors. There is also no problem of mismatching the parameters of “paired” resistors – Philips guarantees that the mismatch between the channels will be no more than 2.5 dB (in reality it is no more than 0.5 dB) – after all, the adjustment is performed with the same constant voltage.

Loudness is switched on and off with switch S1. The power supply is allowed to “walk” in the range from 7.5 to 16.5 volts. Volume control range (transmission coefficient) – from –80 to +21.5 dB.

Philips gives rather large values ​​of the harmonic distortion (no more than 0.3%), but those who measured it say that in reality it is not so great. In addition, the use of voltage dividers at the output allows better use of the dynamic range of the TDA1524A and a decrease in the relative noise level by about 10 dB.

For especially sophisticated readers – a table. If they are really experts, they themselves will understand what it is about.

frequency Hz	Tone control depth, db
	blockage	rise
twenty	-25.1	+18.8
100	-11.7	+11.0
10,000	-12.1	+12.3
20,000	-16.9	+17.1

And also – the microcircuit has an internal voltage regulator with a ripple suppression factor of 50 dB. From the output of the regulating microcircuit, the signal goes to the input of the power amplifier – the TDA1555Q microcircuit.

Well, this is a separate song! It is a pleasure to connect TDA1555Q. Only two input capacitors are required. At the same time, the microcircuit contains four 11-watt amplifiers with an output current of up to 4 A, two of which are inverting, and the other two are non-inverting. The gains of all amplifiers are set the same and equal to 10 (20 dB), the modes of all stages are also set only by internal elements.

Thanks to this, a simple connection of the inputs of inverting and non-inverting amplifiers results in a pair of bridge amplifiers, the load to which, even with a unipolar power supply, can be connected directly – directly to the outputs of the microcircuit, without bulky and expensive electrolytic isolation capacitors. Another “pluses” of this inclusion are the output power of 22 watts per channel at a load of 4 ohms when the circuit is powered from 12 volts, as well as a significant decrease in the level of even harmonics.

Other small advantages are also hidden inside the microcircuit – built-in protection of current outputs and against static electricity, protection against overheating, against polarity reversal of supply voltages, the ability to switch to standby mode (for this, pin 14 is disconnected from the power bus and hung in the “air”), internal voltage stabilizer with a ripple suppression coefficient of at least 48 dB … Is it not enough for you?

With a supply voltage of 15 Volts, the power at a 4 Ohm load is about 15 watts with a relative harmonic level of no more than 0.1%. If the supply voltage is increased to 18 Volts, then about 23 Watts of power will “go” into the 4 Ohm load (with the same harmonic distortion).

Another feature of this microcircuit is the built-in nonlinear distortion detector. This detector performs the function of comparing the input signal with an appropriately scaled output signal. If there is a difference of 2–3% between them, then a current begins to flow at the output of the detector. If you connect the output of the distortion detector to the input of an electronic volume control (this is done by closing the switch), you get an automatic distortion limiting system that will automatically reduce the gain level of the volume control if the output signal of the power amplifier differs from the input signal (for example, in the case of an uncontrolled or sharp increase in the level signal to the limiting threshold of TDA1555Q, in a simple way – in case of amplifier overload).

The difference in the amount of distortion in the “soft” and “normal” clipping modes can be up to 10 times! Such a mode in professional equipment is called “SOFT CLIPPING”, while the sound of “solid-state” amplifiers is practically indistinguishable from tube amplifiers. Naturally, such an amplifier can work not only with a computer, but also with other “sound sources”.

What does the audio amplifier power from?

To power such an amplifier, a sufficiently powerful rectifier is required. The circuit is the simplest: a transformer, a diode bridge and a large electrolytic capacitor. It is not necessary to stabilize its output voltage with a special circuit. On this occasion, the letters contained questions and even accusations of insufficient competence. To clarify the situation, an excerpt from the technical description of another powerful ULF microcircuit is given, but this does not change the essence of the matter:

… A regulated supplay is not usually used for the power …