Join Date: May 2005
Location: Colorado Springs
Originally Posted by event horizon
Ok, im not doubting you
Q9 & 10 are effectively emitter followers though, if you mount them on the same heatsink as the output transistors surely they can't compensate for quiescent on the output stage?
Perhaps i'm having a stupid attack due to a few beers
My apologies if this is so...
Actually , the thermal ~stability comes from D1,2,3,4, because the cross-over bias is dependent on VAS current which is a function of the diff-amp current, and one diode drop is across the current source(s) emitter resistor.
I'm not fond of completely symmetric front ends because there is so much bias dependency from one stage to the next and requires VAS emitter resistors. And according to some simulations I've done, there is no huge advantage in distortion that you would expect. And we know that symmetry cancels even order harmonics, which are "pleasant sounding", vs odd harmonics which are not. Harley Davidson sell a lot of motorbikes because the engine "defect" is very musical.
Years ago, Sinclair modules used a single resistor for cross-over bias and depended on the thermal compensation from the VAS current source, and that was a BJT circuit. Now, they blew up a lot because the output transistors were tiny and the system grounding critical, prone to oscillations. But they sounded great when they worked. That was my "previous life".