semiconductor amplifier designed to drive an
8 ohm load would usually deliver only half its
normal power into a 16 ohms, and might be
damaged if operated with 4 ohms. Also, with
an output transformer provided that it is
correctly loaded, the amplifier input sensitivity
without feedback is the same whatever the
value of load impedance; and by taking the
negative feedback connection from a fixed
point on the secondary winding the sensitivity
with feedback can be made similarly
independent of load impedance: in other
words, the number of decibels of feedback
and therefore the reduction of distortion,
damping factor and so on, are the same
whatever the load. So there is something to be
said for having an output transformer.
Perhaps enough has been said to suggest at
least that the advantages are not entirely on
the side of semiconductors, and that points
can be made in favour of tubes, concerning
both performance and convenience in use.
Semiconductors may produce un-welcome
effects on over driving, so difficult to avoid in
practise; and not only the output stage, but
also low level stages are involved in these.
Tubes have a distict advantage in operation
with reactive loads, and are easier to cool.
Even the need for an output transformer is not
quite such an unmitigated drawback as it may
sometimes seem.
These may be some of the reasons why a
substantial part of the audio amplifier market
has stayed with tubes during the “transistor
era”, and why there has recently been such a
remarkable “Tube Renaissance”.
Tube Renaissance 10 - 5
