At Last the Circular / Bridge Amplifier
As we have mentioned in previous issues, the use of two floating power supplies makes this amplifier inscrutable to far too many tube fanciers. In the murky understanding of how this push-pull amplifier topology works hides the deception that this topology must be Class A in nature, that the output stage could only work if both output devices provide a current path to the other.
Contrary to the dictums (dicta for those I the know) from the advertising department, ignorance of the actual functioning of a circuit not an excuse to bestow magical attributes on the circuit. The Circular / Bridge Amplifier is just as much a push-pull amplifier as the Stereo 70. It too obeys the laws of physics. It too can only deliver twice its idle current into a load before leaving Class A operation. It too can be biased down to Class B or even Class C operation.
Examine the absurdity of claiming that an amplifier puts 60 watts of Class A power independent of the idle current. If we assume that Class A operation can be independent of idle current, why not bias the output stage down to 1 mA? And then we will have a wonderful paradox: a cold Class A amplifier. For that is the assumption being made when we are told that this push-pull amplifier must be Class A by its very nature and it is no less insane than claiming since the bottle the wine is in is very attractive, the wine must be delectable.
One last time, Class A means that at no time while reproducing the output signal do any of the output devices turn off and stop conducting current. Imagine a tug of war: if one side lets go, the war is over. If one leg of a push-pull amplifier stops conducting, the amplifier has left Class A operation. Now, just what would be the minimum requirements for a true Class A, OTL amplifier that put 60 watts RMS? The first step, how much peak current is needed to put out 60 watts RMS into an 8 ohm load? The math: