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.