Instruments
     Unlike an audio device which serves our hearing and can be evaluated rather subjectively, each measurement instrument has its strongly definite, objective characteristics. While a well-designed, good sounding audio brings satisfaction first of all to our heart and soul, a well-designed measurement device brings satisfaction to our mind, this feeling being particularly understandable for a professional who constantly use such devices.
     I began my engineering career as a designer of general-purpose test equipment, but then became more and more involved in the so-called audio measurements which cope with AC signals having frequencies within at least 5Hz-100kHz. This widened frequency range allows to analyze more deeply what is going on in the standard 20Hz-20kHz range of really reproduced sounds which can be heard by our ears.
     It's impossible at all to build an audio device without making any measurements. First of all, the DC conditions of a concrete audio circuit should be checked and, maybe, adjusted to provide its proper operation in the AC conditions, when performing amplification, filtering etc.
     To obtain the circuit AC characteristics, a sine-wave signal of a certain amplitude and frequency should be applied to its input, the output signal being then analyzed with the help of an oscilloscope and proper measurement instruments. The output component of the same frequency as input, compared with this input in amplitude or phase, gives the circuit transfer characteristic at this frequency. The output components of other appearing frequencies (2, 3, 4, 5... times higher than the input frequency) give in their sum the signal distortion. The output contains also a wideband component - noise which is generated within the circuit by its active and passive components.
     Audio circuits are usually analyzed at several frequencies evenly distributed within the whole audio 20Hz-20kHz range (for example, 31,5; 63; 125; 250; ... 8000; 16000Hz). The obtained data allow then to plot the main AC characteristics as functions of the input frequency.
     To conduct all the above measurements, an audio oscillator is necessary. It produces the input signal for the circuit under test, this signal own distortion and noise must be as low as only possible, for correct sensitive measurements. Another important instrument, a distortion meter, helps to analyze the circuit output signal. It removes the signal main component of the fundamental frequency, leaving the residual, distortion and noise, intact for further oscilloscope monitoring, spectrum analysis and measuring by a RMS millivoltmeter.
     The described here VK-1,2 instruments represent a set for the ultimate distortion measurements.
VK-1 AUDIO OSCILLATOR
Distortion of this oscillator is so deeply buried under its noise, that only a precision spectrum analysis can extract it from there. Any harmonic of distortion doesn't exceed -130dB at any audio frequency, the oscillator output noise being at the same time about -110dB in a 20Hz-100kHz noise measurement bandwidth. The used two-loops system of amplitude stabilization contributes nothing to this noise, its main task is to maintain the output amplitude very stable (0,3%) both statically and dynamically. There is a standard control of the output signal level and an original convenient control of its frequency.
VK-2 DISTORTION METER
In this distortion meter, a classical method of measuring distortion is employed - to remove the signal main component of the fundamental frequency and to properly amplify the residual containing the unchanged distortion and noise. All this is performed by the meter's automatic rejection filter which in 4sec tunes itself to the fundamental frequency, ensuring its -130dB suppression and the residual's +80dB amplification. The rejection filter operates in the whole 20Hz-20kHz frequency range, its internally generated distortion and noise are correspondingly -130dB and 3uV (referred to the input, within 20Hz-100kHz).
AUDIO UNIVERSAL METER
The whole measurement laboratory housed inside two compact devices. Its two oscillators (sweep and precision with 0,0001-0,0002% THD), two fully automatic filters (rejection and selective), two millivoltmeters (broadband two-channel and fast wide-range) and also a frequency meter and a unique self-calibrating analogue-to-digital converter allow to carry out accurately and automatically all conceivable measurements and tests in audio equipment even of the highest class. The described here VK-1 oscillator, VK-2 distortion meter and RMS-millivoltmeter are separately built parts of the universal meter.
VK-8 RMS CONVERTER
A millivoltmeter which uses the most true, thermal method of converting broadband AC signals of arbitrary waveform to a DC equivalent. Its key element, an optocoupler's lamp, is alternately supplied with the AC and DC currents being correspondingly proportional to the unknown AC voltage and a regulated DC voltage. The lamp heating produces light which acts upon a photoresistor, the converter's automatic system maintaining its resistance always the same. Therefore, the produced DC voltage can serve an exact (0,3%) RMS equivalent of the input AC voltage lying within 0,1-1V and having frequency range of 10Hz-1MHz.

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