large dynamic range of the audio signal; and
the other is that the loading is not always
resistive. It is under these (usually uninten-
tional) wrong conditions that differences may
show up.
Let us consider the over driving first. Owing
to continual improvements in recording and
playback technique, the possible dynamic
range of music signals- from either disc or
tape - is greater now than it used to be. As a
tentative estimate, it appears that the loadest
passage of a modern disc recording maybe
40dB above the average sound level. Now it
may be said that amplifiers in a high fidelity
system ought theoretically to be able to
reproduce the loudest of loud bursts without
distortion. However, to allow for 40dB above
50mW - not a very high listening level - a
power capability of 500W would be required;
and further developments may make the
figure even greater. One seems to hear a cry
of “where is it all going to end?” Anyway,
when setting up an amplifier system one
adjusts the gain to give the prefered average
sound level. One has no way in advance of
knowing in advance whether there is an
exceptionally loud passage coming that will
over drive the amplifier. Bursts in excess of
30dB above the average are quite rare.
If we accept, then, that occasional over
driving is virtually inevitable, how will the
amplifier behave? We now come to the first
possible reasons why tubes and semiconduc-
tors may “sound different”.
Presented with an over large signal, tubes
merely clip the peaks, delivering a flat-topped
waveform while the over driving is taking
place. The limiting may occur at the grid as
the circuit resistances are commonly such as
to prevent it from being driven more than
slightly positive, or it may be the results of
coalescence of characteristic curves at lower
voltages. The ear is surprisingly tolerant of
such clipping when it occurs only on these
occasional load bursts.
The semiconductors used in audio amplifiers
are virtually always bipolar transistors, either
discrete or integrated. They require base
Tube Renaissance 10 - 2