The VK-6 Live Sound audio system is a complete device for the ultimate sound reproduction. It includes a phono-preamplifier bringing the MM-cartridge signals to a sufficient level and performing their accurate RIAA-equalization, a normalizing amplifier – subsonic filter, an automatic noise reducer built around the ideally working second-order low-pass filter, bass and treble tone controls providing monotonous boost which is variable in accurate steps reaching 2dB at frequencies 63Hz, 16kHz, and at last an active volume and balance control.
     The processed audio signal then comes to a 100W power amplifier which produces distortion of less than 0,001% on 4Ω load and 0,0005% on 8Ω at any frequency within 20Hz-20kHz, this amplifier having an original output stage and dynamic quiescent current stabilization.
     The noise reducer can be excluded from the signal path, the only its control is setting the noise reduction threshold being different for tape, vinyl disc and noisy CD. In all these cases, the filter provides an effective (up to 15dB) noise reduction, leaving the musical program absolutely untouched, particularly its weakest high-frequency components.
     The main feature distinguishing the VK-6 system from others is its original circuitry based on the so-called audio inverting amplifier. This amplifying block has a uniform, very transparent and consistent structure (4 transistors, 4 resistors and 3 capacitors) that causes minimum degradation of the signal passing through it (see the schematics below).

Fig.1. Audio inverting amplifier.

     The circuit open-loop gain (3700 at 1kHz, falling to 3000 at 30kHz and still 200 at 1MHz) is ample for applying very great amounts of negative feedback without any fear of HF instability - the block's single amplifying stage has reliable immunity to that and its cascode topology ensures an excellent dynamic characteristic (slew rate of the output is 20V/µs). The configuration is free from common-mode problems, and practically all distortion occurs at the very output which itself is remarkably linear because the output transistor operates in pure class A.
     The used feedback effectively reduces the contributed distortion to the levels not exceeding 0,001% at any frequency within the audio 20Hz-20kHz range and even up to 30kHz. This negative feedback is exclusively of shunt type and comparing the input and output signals takes place very accurately at one point, all that explains the phenomenon of unprecedentedly natural sounding the VK-6 audio system demonstrates, particularly when reproducing the vinyl recordings with rich “live” content.
     Unlike that, most audio circuits built around operational amplifiers, even of the best types (NE5534, AD797), employ them in non-inverting configuration when the input and feedback signals are applied to various points – to the bases of the input differential pair. Just this non-ideal comparing seems to be responsible for loss of subtle but very important details of the reproduced musical panorama, the ultra-low measured distortion being unable here to improve the matter.
     The VK-6 Live Sound audio system represents therefore a rare example of matching the excellent objective characteristics of audio equipment with the strong, even magical subjective impression it produces.
     Its full circuit diagram is depicted in Fig.2, detailed circuitry being obtained by clicking.

Fig.2. VK-6 Live Sound preamplifier.

     There are three selectable inputs – two linear (500mV/100kOhm and 1V/200kOhm) and one for MM-cartridge signals (5mV/47kOhm). The necessary for vinyl disc reproduction RIAA characteristic is formed by a network C13, R17, C14, R18 in the first stage of the preamplifier. The second stage carries out the stereo channel balancing, it also can be configured as a rumble filter suppressing dangerous vibrations in the loudspeakers. The second stage output is brought to the level within 0÷100mV to ensure optimal operation of the following noise reducer stage.
     This inverting, second-order low-pass RC-filter automatically and continuously varies its cut-off frequency in response to the signal amplitude and its frequency content. When the signal is small in amplitude and has a poor high frequency content, the filter restricts the bandwidth to 2kHz, thus reducing the unwanted noise during quiet musical passages or in pause. If the signal becomes increased in amplitude and rich of high frequency components, it can mask the noise and the filter cut-off frequency is automatically pushed up to 30kHz, thus leaving the audio signal practically intact.
     Frequency varying JFETs Q13, Q14 work in conditions which guarantee the filter distortion of below 0,001%. They are controlled by a special circuit containing an amplifier (U1, U2), detector and the noise reduction threshold level regulator. All that make the filter operation practically ideal, free from any kind of noise modulation and other unpleasant effects.
     The next is a bass and treble regulating stage which accomplishes the tone control in equal accurate steps reaching 2dB at 63Hz and 16kHz. Thanks to capacitors' switching, the obtained amplitude characteristics (see the corresponding graph below) are ideally frequency dependent, they are easily reproducible to know exactly what bass or treble lift is chosen (maximum +8dB). There is another version of the VK-6 preamplifier, with the traditional Baxandall tone control where two potentiometers are used.
     The final link in the signal transformation chain is an active volume control allowing to achieve minimum noise and distortion. The preamplifier requires a single +24V power supply, all its stages use two common reference voltages derived from VCC with the help of a simple network D1, D2, R10, R20, R30, C55. This provides a good DC stability of each amplifying block (stage).
     Various kinds of analysis (AC, noise, distortion) of the separate stages and the whole preamplifier are placed below, they are obtained by the circuit computer simulation in the Multisim 10 program, these characteristics being close to those measured by real instruments.
     The VK-6 preamplifier printed circuit board is represented in Fig.3 (190mmx68mm, one channel). The picture clicking produces more detailed animated images of various color performance. The recommended for this board polyester film capacitors – WIMA MKS2 (pin spacing 5mm, voltage 63V), metal film resistors – 0,25W. To get a stereo preamplifier, two identical boards should be combined in a stacked construction and being divided by a screen.

Fig.3. The VK-6 preamplifier PCB (scale 1:1).

     Subjective testing is most demonstrative when making listening comparisons and the human ear has a unique sensitivity for that. I've prepared a LP rip sample recorded from my vinyl disk with rich "live" content via the system’s phono MM-preamplifier, this sample can be downloaded from here.
     For comparison, I offer also to download a typical rip of the same song, made with the help of an ordinary audio equipment. Alternate playing of these two audio files from the same chosen point in Sound Forge 9 allows to clearly hear the difference in sounding - not in level or tone balance, but just in "live" content whose maximum the system easily retrieves. Of course, all should be reproduced via decent audio equipment and acoustics to enjoy the sound in full degree.
     In the same manner, by comparison, can be evaluated and the work of the system's noise reducer, the downloadable here are clean record and noisy record (title "Duet Besedina and Taranenko - I Like"). The track was originally recorded from my vinyl disk to cassette tape by using a Denon DRW-750A cassette deck and then played via the system and converted to the digital form twice – with the noise reducer switched on and off, producing the above two files.
     More music of various genres, recorded via the System's phono stage, can be found at www.onlyliveaudio.com.
     For more detailed information please contact   Vladimir Katkov,     email:   vlad@vkaudiotest.co.uk

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