Given those lucky few who managed to construct and still enjoy their very own, why are germanium transistor based audio power amplifiers seem like an “undiscovered country” in the hi-fi world?
By: Ringo Bones
Even though during the mid to late 1990s, hi-fi equipment
manufacturers have already managed to produced the “holy grail” of the budget
conscious audiophile – i.e. solid state power amplifiers of either silicon
transistor or MOSFET based that can rival the sound quality of single ended
zero feed back triode audio power amplifiers based on either the 300B or the
2A3 vacuum tube – while priced competitively at between 500 to 1,000 US dollars
each. Yet unknown to most audiophiles, a type of power transistor – namely of
the germanium type – can even approach closer to the sound of a zero feedback
SET audio power amp than either silicon or MOSFET types. But why aren’t hi-fi
audio power amps or even integrated amplifiers based on germanium transistors
flooding the hi-fi market these days?
To those electronic enthusiasts fortunate enough to dabble
with germanium transistors, these types of transistors are very notorious for
their over-the-place variability. Even though they are the first ones to be
mass produced for consumer electronics use, germanium transistors are somewhat
difficult to manufacture and not very stable. Germanium transistors are very
hard to produce with consistent parameter quality on a large scale – as in
widely varying gain, leakage, noise and overall tone – even germanium
transistors manufactured from the same batch.
The inherently widely varying parameters in germanium
transistors means resistor values selected for AC / DC biasing, Q-point
operation, feedback and stability that works for one circuit may not
necessarily work in another similarly designed circuit even though both use
germanium transistors from the same batch. This means resistor values must be
“tweaked” – i.e. slightly varied higher or lower in order for a stereo pair of
a germanium transistor based audio power amplifier will achieve the same
consistent tonal quality.
And during much of the 1960s, even then commonly available
germanium output power transistors – like the now extremely rare AD 149 PNP
germanium output transistor which can produced 10 watts in a single-ended
configuration if properly heat sinked – remains under utilized by electronic
enthusiasts of the day because back then heat sinking was often inadequately
specified in published audio power amplifier designs of this sort. Back then,
specification sheets for germanium transistor audio power amplifier designs
were not totally reasonable and many marginal designs with inadequately heat
sinked germanium output transistors that can only safely handle 500-milliwatts
boasted 120-watt peak-to-peak power ratings.
Assuming if you are lucky enough to find “truthful”
specification sheets and application notes for germanium transistors these
days, it is safe to conclude that it is a more superior semiconductor in
comparison to silicon transistors – as in silicon bipolar junction transistors
and MOSFETs. Not just on subjective sound quality terms because germanium
transistors conduct better than their silicon counterparts because germanium
transistors have inherently higher electron mobility, smaller band-gap and
requires lower impurities to dope into P-type. Most of this parameters probably
explains why a germanium transistor based audio power amplifier based on the AD
149 PNP output transistor that is properly heat sinked to produce a healthy 10
watts in single-ended configuration can easily perform with a sound quality
much closer to that of a zero-feedback single ended triode (SET) vacuum tube
power amplifier based on the iconic 300B or 2A3 tube in comparison to its
silicon based bipolar junction transistor and / or MOSFET counterpart. Just
think how better the 1970s era Naim NAP 250 integrated amplifier could have
sound if audio engineers at Naim discovered a way to design a germanium
transistor based audio power amplifier able to produce 35-watts RMS.
