VK-4 LOW-VOLTAGE AUDIO SYSTEM


     The System represents an original approach to constructing hi-fi audio equipment and possesses the following merits. Firstly, it is able to run from a single 13-14V power supply, that ensures its using practically everywhere, including such vast areas as battery-powered and car audio electronics. Secondly, it is a complete versatile system which can accept at its input a signal both from the linear output of any source equipment (tuner, tape or CD player etc.) and from a moving-magnet (MM) or moving-coil (MC) heads. All that needs to be done is to select by a single rotary switch the necessary sensitivity: 10, 20, 50, 100, 200, 500, 1000mV for linear inputs (with a 2kOhm input impedance per each 10mV) and 0,25mV/10-Ohm or 5mV/47kOhm for MC- and MM-head correspondingly. The System's first output (standard 500mV) may be used for feeding an external power amplifier, while the second output can directly drive a 4-Ohm loudspeaker, delivering a 10W RMS rated and 15W RMS maximum non-distorted electrical power.
     The signal transformation within the Audio System includes a low-noise preamplification and accurate RIAA equalization (for MC and MM signals), obtaining a normalized 100mV RMS level, an effective blameless noise reduction, bass and treble tone controls with maximum ±10dB (at 63Hz and 16kHz) and evenly variable boost and cut, an active gain and balance control, and, at last, an exceptionally low-distortion power amplification. The System therefore provides not only high-fidelity reproduction of an ideal audio program, it gives a dramatic improvement in sounding of old, "non-ideal" recordings.
Its circuit diagram is depicted below (click to rotate and show in detail).



     The third and most important feature of the System is its non-traditional circuitry based on the Inverting Audio Amplifier configuration. Every such an 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 and in this respect it outperforms any op-amplifier.
     Stability of the block's direct currents and output DC voltage is good, two necessary reference voltages being common for all the blocks of the System. Open-loop gain (more than 3000 in the whole audio 20Hz-20kHz range and still 160 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 cascade topology ensures an excellent dynamic characteristic (slew rate of the output is more than 20V/µs).
     The configuration is free from common-mode problems and practically all distortion occurs at the very output which itself is remarkably linear as it operates in pure class-A. The used feedback effectively reduces the distortion to negligible low levels (the measured by the VK-1,2 instruments typical figures of THD are below 0,001% at any frequency within 20Hz-20kHz, all that in the inverting amplifier with a closed-loop gain of 5 and output voltage of 2V RMS applied to a 2kOhm load resistor). Given that the Audio System’s preamplifier handles signals not exceeding 500mV, its THD lies in the region of 0,0002%.
     Noise performance of the amplifying block is determined by the input transistor and can be easily optimized for a concrete source impedance and feedback network. When using a BC546B transistor under typical working conditions (0,3mA collector current and 0,8V collector-emitter voltage), the generated unweighted input noise voltage is 0,28µV over the audio 20Hz-20kHz band. Total signal-to-noise ratio of the whole Audio System is -86dB (unweighted, at maximum gain) for a 500mV sensitivity.
     The MM-head preamplifier uses the shunt-feedback RIAA equalization and has an accurate frequency and excellent transient characteristics that provides an unusual clarity and naturalness of reproduction of vinyl disc recordings. Seemingly poor signal-to-noise ratio (-64dB, unweighted) isn’t here the problem to worry about, as due to the RIAA equalization the noise has a falling content of most perceptible high frequency components and always is swamped by inherent noise of the recordings. A cardinal solution of the latter, really serious problem gives the subsequent noise reduction with its marked (12dB) improvement of signal-to-noise ratio.
     Although the MC-head preamplifier copes with signal levels of twenty times lower (about 0,25mV RMS) than those of the above preamplifier, the total unweighted signal-to-noise ratio the Audio System has from this input is -70dB. Such a high figure is explained by the exceptionally low MC-head internal impedance (3,3-Ohm for the Ortofon MC10), low necessary loading (10-Ohm) and also by the ubiquitous amplifying block whose input transistor is represented here as a parallel connection of four identical devices supplied with raised currents. This modification allows to obtain a low equivalent noise-generating base spreading resistance (16,7-Ohm) and unbelievable dynamics of the preamplifier (slew rate of 100V/µs).
     The listed above advantages are inherent to the amplifying block the System is built on. The only snag of its performance is the possibility of using it exclusively as the inverting amplifier, but in most parts of the Audio System just such a configuration is desirable. To apply it in the noise reducer and power amplifier was a puzzling task indeed and required novel circuit solutions.
     Originally designed and built around the Inverting Amplifier, the voltage-controlled second-order low-pass filter automatically and continuously varies its cut-off frequency (between 2kHz and 32kHz) in response to the level and frequency content of the input signal (maximum 100mV), thus performing noise reduction. The noise reducer also comprises a very effective control path which produces the control voltage for the filter according to a special non-traditional concept. All this gives not only a 12dB noise reduction with widely variable threshold level, but makes this process practically ideal, free from any kind of noise modulation and other unpleasant effects. An eminent feature of the noise reducing filter is its low THD and internally generated noise as when its bandwidth is maximum (32kHz) and when it is minimum (2kHz). In the first case, THD measured at the maximum 100mV RMS operation input voltage level is less than 0,001% at all audio frequencies, in the second case - less than 0,1% below 2kHz. The figures of unweighted noise are correspondingly -90dB and -80dB.
     The Audio System power amplifier uses a pair of the amplifying blocks each of which is added by the output four-transistor stage, a 4-Ohm load being directly switched between the two outputs - unbalance of the bridge doesn’t exceed 30mV.
See the below schematics, detailed circuitry being obtained by clicking:







As in the previous cases, the Inverting Amplifier proves here to be unsurpassed in providing the magically high performance everywhere it is used. Applying a 6,3V RMS output voltage swing to the load provides a 10W RMS output power and produces THD of less than 0,01% at any audio frequency (typically 0,003% at 1kHz and 0,007% at 20kHz, all this being achieved by using a single 14V supply! A 7,8V RMS output starts clipping (1%THD) which is softly indicated by the amplifier overload circuit even when the clipping occurs during a single period of the highest audio frequency. The corresponding power is 15W RMS.
     The amplifier works at frequencies up to 200kHz (0,08%THD at a 10W output power) that confirms its excellent dynamic characteristic (slew rate of 30V/µs). It also is insensitive to shunting the load by a 4.7µF capacitor and exhibits the low distortion as well (0,043% at 20kHz, see the above screenshot). The amplifier withstands short-circuiting of its output terminals and sounds astonishingly clear even when delivering a 30W output power, i.e. running all the time in conditions of deep but soft clipping; in spite of this, its single heat sink (125×38×25mm), common for both stereo channels, has a temperature not exceeding +60°C.
     A novel, highly effective quiescent current control is used in each half of this bridge power amplifier and acts independently. It retains within 10% the set current value (about 50mA) even if deliberate heating of the involved in this control transistors is carried out by a soldering iron. The control circuit is elegantly simple, it might be recommended for use in any power amplifier.
     An attractive feature of the used amplifying block is a low cost of the components each block consists of. The components are widespread and accessible for everyone, they also can be replaced by their various equivalents without any worsening of the specified characteristics. The Audio System was initially assembled in the form of operating breadboard constructions which then were thoroughly tested, adjusted and ultimately have been brought to a high degree of perfection.
     The System’s complete prototype is built on two printed circuit boards, the board of each stereo channel (size 190×125mm) containing all parts of the System - from its most sensitive MC-head preamplifier to power amplifier with overload indication. These double sided copper clad boards are attached to the front and rear chassis, their component sides face each other inside the firm construction which is installed into a standard enclosure (200×180×65mm). The front chassis carries the Sistem’s control potentiometers and switches, the rear one – input and output jacks and one common heat sink.
     Layouts of the upper and lower boards are mutually inversed and slightly different to be suitable for the mounted components. The PCBs’ pictures are shown below in scale of 1:1, their clicking produces more detailed animated images of various color performance. The recommended for these boards polyester film capacitors – WIMA MKS2 (pin spacing 5mm, voltage 63V), metal film resistors – 0,25W.








     This compact Audio System is very light (1,5kg), but produces the impression of a really magic box because of its paradoxical capability to drive at a deafening power big loudspeaker systems, its unusual clarity of reproduction and unbelievable selectivity in removing the noise background from a program material, the material itself (music, voice) remaining after that absolutely intact. Most dramatically the improvement in sounding is perceived when reproducing via the System vinyl disc recordings of slow piano music in which the inherent to this type of recordings noise is particularly annoying. To the ideal noise conditions can be brought and compact disc recordings, particularly those with disappointingly low signal-to-noise ratio, it’s only necessary to set a proper threshold level of the noise reduction.
     The recently conducted virtual testing of the System has confirmed the impeccability of circuit solutions taken fifteen years ago and implemented at that time in this complete audio device. Its circuitry is excellently simulated in several programs, all the obtained in Multisim 10 simulation data fully coincide with the results of real measurements carried out earlier. The represented below are characteristics of the Low-voltage Preamplifier and Low-Voltage Power Amplifier which were tested separately.
     But the Low-Voltage System is first of all an audio device and the below objective characteristics must be supplemented with the System's subjective testing. Here I would like to offer such a possibility based on the listening comparisons the human ear is able to make with 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 propose 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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