Electronic – AC coupling with

acamplifieraudiocapacitor

I've the following amplifier circuit:

DC couplet power stage

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 ?

thank you.

AC coupled power stage

Best Answer

This seems like a few basically reasonable ideas taken to excess. The individual gain stages may be OK, but three of them simply strung together like that will multiply the non-linearities.

Apparently you discovered darlingtons somewhere, but this is overkill. They are going to add more distortion, but you don't have any feedback to correct things like that except that of the individual stages. You don't need both Q4 and Q5, especially considering the 1 kΩ pulldown on their output. That will be rather assymetric. Even with a single transistor, you have a lot lower impedance pulling up than down. There is also nothing keeping the DC operating point near zero except the open loop bias point of the last gain stage.

What are you trying to do, build a overall amp, a preamp, or a power amp? If you just want a overall amp, I'd replace the whole mess left of the diodes with a opamp. That will be easier to bias with a DC operating point of 0 and will provide lower impedance and more symmetric drive to the power stage. So you don't need a opamp to work on 30 V, give the power stage a small gain, like 2.

Given the impedance reduction you want from the power stage, you don't need three(!) transistors in each leg. With a opamp driving the power stage, you'd start at a lower impedance in the first place, and with even a little global feedback the effective output impedance will go down even more.

Note that this thing will have fairly high and somewhat unpredictable quiescient current. The efficiency will be pretty poor. This topology can't drive the output that close to the power rails, so there will be even less efficiency for the maximum output power. The last two transistors (Q10, Q11) are going to get toasty. They'd better be power transistors on a good heat sink, possibly with forced air cooling.

Since it's not clear what your overall point is, so it's hard to give concrete recommendations. What are you trying to accomplish by not just buying a audio amp?