I've the following amplifier circuit:
The two BDX77 are actually a single darlington BDX53C (I do not have the spice model for it), and the same for the two BDX78 (a single BDX54C).
It sounds quite well actually, even if it is not an usual topology for audio amplifiers (it is only didactic and temporary built on a breadboard). But it has a DC offset on the 4ohm load (a 10W speaker) that produces a higher current on the upper darlington transistor than the current of the lower one, with a consequent DC current through the load and a overall unbalancing in power consumption.
I thought that coupling the two emitter followers (the two darlington-connected BD139 before the power stage) in AC mode with the final stage would have solved. In effect, with the circuit below, the current balancing is ok (almost the same current through the BDX53 and BDX54), but the sound quality is largely degraded than with DC coupling: noise, distorsion of any type, clipping, some strange oscillation. I used an electrolitic capacitor (tried 100uF, 470uF, 1000uF). What I'm doing wrong (apart the bad amplifier topology, I know... ) ? it is about the electrolitic nature of the capacitor or that kind of coupling is completely wrong ?