Project: Simple 60 Watt Power Amplifier with 2N3055 Transistor
The first version of the amp shown here uses a single power supply and capacitor coupled speaker. It also uses quasi-complementary symmetry for the output stage. Note the really sneaky way the Class-A driver amp's collector load is bootstrapped !
Quasi-complementary symmetry was a scheme used in the days when PNP power transistors were expensive and useless. If you wanted any sort of voltage and current rating, you had to use NPN devices. The quasi-complementary output stage used a (discrete) Darlington for the positive side, and a complementary pair for the negative (i.e. a PNP driver coupled to an NPN power transistor).
Almost all amps of the era from which this circuit originated used the 2N3055 power transistor - this was the pre-eminent power transistor (NPN of course), and there were no vaguely equivalent PNP devices for less than about 5 times the price, and even these were highly inferior. As a result, the quasi-complementary output was very common, until decent PNP power devices became more readily available. Immediately, just about everyone started using NPN and PNP Darlington coupled devices for the output stages (as shown for Q3 and Q4) - the funny part is that it was demonstrated back in the mid 1970's that the full Darlington connection actually sounds (or at least measures) worse than quasi-complementary stages. Read full details:http://sound.westhost.com/project12.htm
