Created on 10/14/2019 10:11 AM.
Last updated on 16.02.2020 16:08.
Author: A. Dolinin.
When I wanted to know the opinion about my articles, I tried to go to the conference, namely, to the “Other” section. Basically, the opinion of the “voices from the people” was positive, many ventured and assembled amplifiers, despite little experience in this matter. I wanted to please the readers somehow. At the same time I will try to answer some questions. Here, read and enjoy!
In the article “Once again about the love of good sound” an amplifier was described on two microcircuits – a microcircuit – a volume / balance / timbre control and a microcircuit of a powerful (relatively) output amplifier. The prototype for it was the scheme developed by N. Sukhov. The difference from the original circuit is in the remote input switch – it is not very relevant for computer acoustics.
The comments of the users who assembled this amplifier sounded something like this: “Something is too much noise …” Yes, at present, the TDA1524A, for example, and the K174UN10 do not suit everyone in terms of noise and distortion. Did you know that you can use the LM1036 chip instead? The result will be much better, and the details are the same as in the previous case.
Many more would like to assemble a system with a subwoofer, but do not know how to do it correctly. In this regard, we can say the following: if the speakers are of sufficient size, made not of plastic, but of chipboard (well, at least like the decrepit S-30), then at a reasonable sound volume and they can provide a sufficient number of “lows” (note, a lot also depends on the amplifier!).
It also doesn’t hurt to check if the speakers are “phased” correctly. This requires a phonogram with a large number of low-frequency components in the audio signal (some kind of “Joy of a metallurgist” … sorry, “metalworker”).
Turn on playback, remember how the low frequencies sound, and then REPLACE the wires at one of the speakers. If the low frequencies become less (as if someone ate them), it means that your speakers were phased correctly, and you need to return everything to its previous state. But if there are much more “lower classes” – you can only be glad and not touch anything else. It’s not for nothing that in a decent technique they mark wires and terminals on speakers and amplifiers – specifically to avoid such problems.
And for those who still want to try to create the “2.1 system” on their own – good news! The PC2350 microcircuit is specially designed for you, manufactured by Princeton Technology Corporation.
She is able to regulate the stereo signal at the high and low frequencies (in the sense of the high and low frequencies) with a depth of adjustment of +/- 10 dB at frequencies of 4 kHz and 50 Hz. It also contains a BUILT-IN adder and an active Sullen-Key low-pass filter of the second order, the slope of the frequency response of such a filter is 12 dB / octave, which form the signal for the subwoofer.
The cutoff frequency of the subwoofer channel is 130 Hz, if desired, it can be changed by proportional change in the value of the capacitance of the capacitors connected to the terminals C1, C2. The microcircuit has low distortion (Kg <0.01% at the outputs of the right and left channels and Kg <0.2% at the output of the subwoofer channel).
It also has very little noise: -87dBA. Resistors regulating the volume should be of group “A”, tone controls should be of group “B” (for those who do not know: these letters characterize the dependence of resistance on the angle of rotation of the resistor slider). The supply voltage can range from 3 to 8.5 volts. All that remains is to find this clever microcircuit on the market or write it by mail … if possible.
And here is another design of the “type 2.1” amplifier. It was published in the magazine “Radio” (yes, there you can find relatively simple circuits …) No. 5 for 2003, page 15. It uses the familiar TDA1554Q microcircuit (the same as the TDA1555Q, only without a distortion detector) … Two inverting amplifiers of this microcircuit are used to “sound” small-sized acoustic systems that act as satellites BA1, BA2.
At the inputs of their channels, passive volume and HF tone controls are installed (only on the HF “rise”). In the subwoofer channel, two non-inverting amplifiers from the microcircuit are used, and the BA3 subwoofer speaker is connected via a bridge circuit.
The signal for driving the subwoofer amplifier is removed from the phase-inverted stage at T1, which provides for the possibility of phase inversion and a sensitivity control to ensure the matching of the subwoofer and satellites in terms of volume.
An adder and a low-pass filter with a cutoff frequency of about 100 Hz here exist in the form of a “side effect” of elements BA1, BA2, C13. The circuit provides the possibility of phase inversion (S2.1, S2.2) and the VR3 sensitivity control, which help to match the sound of the satellites and the subwoofer. The maximum output power of the satellites is indicated as 2×6 W, for the subwoofer – 22 W. If you are not afraid to experiment – try to collect.
And yet it hums …
Many “amp builders” have complained that it is difficult to get rid of the ac hum. And this is where some of the subtleties manifest themselves that are little known to ordinary users who rarely hold a soldering iron in their hands …
For example, to ensure high quality sound reproduction, the power supply must have good transient immunity and low RFI. To increase noise immunity, additional mains filters are used, a transformer and mains filters are installed in the magnetic shield, blocking capacitors are connected between the terminals of the secondary windings and the common wire.
Rectifier diodes and capacitors are recommended to be screened – for example, placed in the same screen as the power transformer.
The quality of the power supply for amplifiers is also characterized by the ability to maintain the output voltage at a constant level when the power consumed by the amplifier changes, the level of low-frequency ripple of the output voltage and other parameters. Maintaining the output voltages in unregulated power supplies is ensured by the correct choice of the power of the mains transformer.
For example, pop amplifiers are most often operated at a power close to the maximum, therefore, the power of their mains transformer usually exceeds the output power of the amplifier by 2 … 3 times.
In everyday, “household” amplifying devices, a transformer is often used, the power of which is equal to or even less than the maximum power of the UMZCH, since the amplifier power is selected based on the undistorted reproduction of the loudness peaks of a musical work, while the average power consumption is low. However, the use of a low-power transformer leads to the appearance of infra-low-frequency oscillations of the supply voltage, which is not good for high-quality sound reproduction.
For example, for the amplifier described in the article “Once again about love …”, a standard transformer of the TN-7 type is well suited. It has two 6.3 V output windings, providing a load current of up to 3.3 A. Secondary windings are necessary …