Violectric HPA V90, User Manual

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Why does PRE-GAIN make sense ?
Two extreme examples (with the HPA V90 at 8dB gain, volume control
set to full):
1st example:
The (pre-)amplifier provides 4V output voltage, whereas the headphone
requires only 2V for 100dB sound pressure level.
With the control fully turned up, the V90 would deliver 10V output at 8dB
gain. Therefore the volume control would have to be operated very
carefully in order to avoid hearing damage. Moreover, any interference at
the input should be avoided since it would be "unforgivingly" amplified as
well. With PRE-GAIN, the input level can be reduced by 12dB (a fourth),
with 1V instead of 4V as the result. This 1V is again amplified by 2.5, then
equalling 2.5V. Now the volume control can be turned over almost the
entire range.
2nd example::
The (pre-)amplifier provides 1V, whereas the headphone requires 20V to
release 100dB of sound pressure.
With the volume control fully clockwise, the V90 would provide 2.5V at
8dB gain only - much to low for the headphone. By means of PRE-GAIN,
input level can be boosted by 12dB (four times), resulting in effective 4V.
These are again multiplied by 2.5, now equalling 10V. This is still not
enough, but far closer to the optimum value: The headphone achieves
114dB sound pressure level.
The HPA V90 is DC-coupled, i.e. it can transduce DC voltage as well.
For good measure, a low-cut filter can be inserted by setting the
corresponding jumpers. After input buffering, the signals are forwarded
to a simple 6dB-per-octave filter stage which can be set to two
characteristics: AC (12 Hz), or DC (0 Hz).
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.