Violectric HPA V281 User Manual page 26

reduced by 12 dB (a fourth), with 0,5 V instead of 2 V input as the result.
This 0,5 V is again amplified by 4.5, then equalling 2.25 V. Now the
volume control can be turned over almost the entire range.
2nd example::
The (pre-) amplifier provides 1 V, whereas the headphone requires 20 V
to release 120 dB of sound pressure.
With the volume control fully clockwise, the V281 would provide 4.5 V at
+14 dB gain only - much too low for the headphone. By means of PRE-
GAIN, input level can be boosted by 12 dB (four times), resulting in
effective 4 V input voltage instead of 1 V. These are again multiplied by
4.5, now equalling 18 V. This is still not enough, but far closer to the
optimum value.
Why does frequency bandwidth limiting make sense ?
In signal processing, sound is represented by AC voltages. Sound is
audible - for young people - from about 20 to 20000 Hz. The elder the
listener, the less he will hear high frequencies in particular.
In order to transmit these frequencies at optimum quality, the frequency
response of an amplifier should be as wide and as "flat" as possible. At
the low end of the scale, this limit is represented by DC, as there is no
frequency lower than zero. In upward direction, the limit can be set to
practically any frequency, but the higher, the more susceptible the
device becomes concerning electro-magnetic interference. This is not
audible in the first place, but may interfere with the useful signal and
then become evident. Therefore, unrestricted frequency response
attests thoughtlessness rather than remarkable engineering skill.
Why is a good volume pot essential ?
A volume potentiometer is a mechanical control element, which can be
obtained on the market at any low price. Meanwhile it is often replaced
by electronic circuitry, exhibiting essential disadvantages concerning
dynamic range, noise and distortion.
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