Push-Pull Amplifier and Sziklai Pair Questions

DaveGS

Joined Apr 22, 2022
14
Just because it was posted is no reason to believe it is correct. Consider that the diodes are supposed to be compensating for th temperature gain changes in those power transistors. Are you sure that the original device had failed?? What leads you to think that it had failed???
 

DaveGS

Joined Apr 22, 2022
14
I mistakenly thought that CR18 had failed because when I tested it on the diode position with my VOM it only showed about 1.7volts. Later I tested it on my Atlas DCA55 it showed 2.038 volts.
 

MisterBill2

Joined Jan 23, 2018
18,176
So you know that the forward drop varies with current. That relationship is very not-linear, especially at very low forward currents.
What I am aware of is that the idling current in Marantz receivers is more subject to changing than in some others. I have a 2230 that the output transistors run a lot hotter than is healthy. So far I have not adjusted it, just put it back in the box. I have another system that functions perfectly, so why bother just now.
 

DaveGS

Joined Apr 22, 2022
14
So you know that the forward drop varies with current. That relationship is very not-linear, especially at very low forward currents.
What I am aware of is that the idling current in Marantz receivers is more subject to changing than in some others. I have a 2230 that the output transistors run a lot hotter than is healthy. So far I have not adjusted it, just put it back in the box. I have another system that functions perfectly, so why bother just now.
 

DaveGS

Joined Apr 22, 2022
14
Yes, I am aware of this behavior and its one of the reasons that Marantz thermally coupled the diode pack to the main heat sink.
 

MisterBill2

Joined Jan 23, 2018
18,176
All those capacitors are doing is directly coupling the input signal (to the final amplifier stage) to the load.

This is the exact same circuit:

View attachment 288815

So for small signals, you are just driving the load with the prior stage. Larger signals start will deliver power from the output stage.

There will still be distortion. How much depends on how well your next-to-last stage can drive the load. But with a 100 kΩ load, do you really need that output stage at all? Anything much above 10 Hz is going to be driven from the prior stage and as long as it can deliver 100 µA of current, the final stage will never be asked to do anything.

My guess is that if you look closely at your simulations, you will discover that the output transistors are never being turned on. To turn either one of them on would require that there be at least 0.6 V across the capacitor and to get that would involve enough current that the voltage across the load would exceed the supply voltage. That would be especially true at higher frequency signals, so sufficiently low-frequency signals, if big enough, might turn on the output stage transistors a little bit.

See how much distortion you get if you lower the load resistance to something like 100 Ω.

Then look into a Vbe-multiplier circuit and how it can be used to largely eliminate crossover distortion, as well as provide thermal compensation if mounted to the same heatsink as the output stage transistors.
It might also be that those capacitors are preventing a high frequency oscillation that was causing what sounds like crossover distortion. That is one mechanism where reducing the high frequency response could eliminate cross over distortion.
 
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