Audio
     This site doesn't specialize in offering DIY (do it yourself) audio projects. The task is more delicate - to give some recommendations on the choice of electronic circuitry best suitable for handling audio signals and to describe some methods of testing the complete audio components.
     Everyone building these components, an amplifier for example, dreams first of all to get good sound this amplifier will reproduce, of course together with a source LP or CD player and output loudspeakers. The latter components must be of a maximum possible class, otherwise even the ideal amplifier will be unable to provide the desired good sound.
     There is a great variety of the amplifier schematics, both in types (valve, bipolar or field-effect transistor, hybrid) and particularly within each type, so it's very difficult to make the right choice from the very beginning. You may try therefore several circuit configurations, each time assembling them on a breadboard, conducting listening expertise and finally taking the decision what schematics best suites to your concrete acoustics and source player (LP or CD).
     More fundamental approach would include making various measurements in order to optimize the chosen amplifier circuit's performance (both objectively and subjectively), but this work is much more time consuming and requires a set of standard test instruments.
     And, at last, the ultimate task of audio designing is to understand how does a certain circuit configuration and its characteristics influence the output sound while the corresponding audio signal passes through this circuit. The necessary for that measurements of distortion, noise etc should be extremely sensitive and listening tests are sophisticated and very numerous. Therefore, I was forced to build a special measurement equipment for this studying whose main conclusions have helped me then to develop a special circuitry being optimal for handling audio signals. Most important results of this hard work are represented here.
AUDIO INVERTING AMPLIFIER
A building block of my audio. This compact and transparent configuration (4 transistors, 4 resistors and 3 capacitors) features an open loop gain of about 4000 and closed-loop distortion of less than 0,001% in the whole 20Hz-20kHz frequency range. The circuit has a single amplifying stage and is absolutely DC and HF stable. Its non-differential input, pure A-class output and use with the exclusively parallel negative feedback ensure its very delicate handling both the weakest and the most complex audio signals, in this respect it outperforms the best operational amplifiers.
VK-4 LOW-VOLTAGE AUDIO SYSTEM
The system demonstrates all the possible applications of the audio inverting amplifier and the art of combining the blocks into the whole. There are MC- and MM-head preamplifiers, a linear normalizing preamplifier, bass and treble tone controls, a universal noise reducer and, at last, an original 15W power amplifier with soft clipping and overload indication. Common for all these stages is very low distortion (less than 0,005%) and a single +13V supply voltage. Sounding of this small (200x180x65mm) box is magical indeed.
VK-5 POWER AMPLIFIER
This amplifier doesn't boast of a huge power. Its purpose is reliable clean power amplification, it delivers a 50W power into 4ohm loudspeakers with less than 0,001% distortion in the whole 20Hz-20kHz audio range. All that is achieved by using a special VAS and output stage circuit configuration. The amplifier has an excellent output driving capability, its output impedance is less than 0,03-Ohm, slew rate of the output - 20V/us. The safe operation is guaranteed by the built-in overload indication and protection. The modified VK-5K at all sets the record - 0.000008% THD at 1kHz and 0.00003% THD at 16kHz (100W-4ohm)!
VK-6 LIVE-SOUND AUDIO SYSTEM
A combination of the VK-5 power amplifier and a preamplifier built exclusively on the audio inverting amplifier circuit. The preamplifier includes MC- and MM-phono stages, a linear amplifying-normalizing stage, active stepped bass and treble tone controls and, at last, a universal noise reducer similar to the VK-7. The preamplifier distortion is less than 0,001% in the whole 20Hz-20kHz frequency range. The internal power supply produces +35V,-35V for the power amplifier and a single +24V for the preamplifier.
VK-7 NOISE REDUCER
A universal remedy for curing both noisy recordings and noisy audio equipment. Built around the audio inverting amplifier, this simple automatic low-pass filter carries out an effective (12dB) and very selective noise reduction, leaving the signal's content and, most importantly, its dynamics absolutely intact (the contributed distortion and noise are correspondingly 0,002% and -90dB). In this respect the filter outperforms the best digital software-based noise reducers. Sound samples demonstrating the filter operation are enclosed.

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