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Oscillation existed in my audio amplifier circuit?

Alaxleia

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Hi, all
I am experimenting with audio amplifiers according to this content for a while now. And I want to learn how to do an operational amplifier with discrete components and understand all its section and sub-circuits recently. Then I decided to just skip the hard part and use operational amplifier and output power stage for an audio amplifier to work as desired, instead of troubling with discrete components only. The goal from my earliest experimentation was to create around 100W audio amplifier but the things went wrong as soon as applying DC to whole amplifier (no AC for now).
I have built this circuit few minutes ago on breadboard:
amplifier circuit.jpg

When I checked only output power stage with VBE multiplier (without MC1458 op-amp) everything worked just fine. But when I connected op-amp to it directly (no AC coupling - switch position B), as I increased Vce of VBE multiplier just a tiny bit, then the current through both upper and lower output power quadrants suddenly increased to 2A or more and stayed there or it was swinging like there was some kind of heavy oscillation within amplifier. As soon as I added AC coupling capacitor (switch position A), the darn thing worked as expected when Vce of VBE multiplier was changed. As it was increased with potentiometer P2 slightly also the quiescent current through power stage increased slightly - as expected.

Why did such heavy oscillation occurred within my amplifier circuit when there was no AC coupling capacitor between voltage amplification stage (MC1458) and output power stage (upper and lower Sziklai pair with paralleled power transistors)?

By the way, C3, C4, C5 and C6 represent AC bypass to avoid distortion when AC is applied to such circuit. At the collector and/or emitters of power transistors Q4, Q5, Q6 and Q7 there are no low resistance resistors for improving thermal stability of those because I have no low resistance (lower than 1 Ohm) high power resistor. Also, I have replaced MC1458 with another one (same part), but the results were similar (talking about example with no AC coupling capacitor).
 

bravomail

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maybe post this to DIY section? and there are DIY audio forums outside of this site.
 

KSTR

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MC1458 running on +-23V rails (spec is +-18V absolute max, but there is MC1558 which can take +22V absolute max)?
Feedback loop not closed for DC in switch A position?
No emitter resistors, especially in a CFP design?

Please read up on some beginner's literature on amplifier design and then probably make yourself familiar with circuit simulators (LTspice, TINA-TI, etc) before frying things ;-)
 

Larry B. Larabee

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Why did such heavy oscillation occurred within my amplifier circuit when there was no AC coupling capacitor between voltage amplification stage (MC1458) and output power stage
The opamp gain is transfererred to next stage. Ya, why no Re? No Cf for Rf1? You're way off, definitely try a simulation first.
 
Last edited:

tomchr

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The opamp needs to have feedback at DC, so the circuit won't work with the switch in position A.

I suggest simulating the loop gain of the amp. You'll likely find that the circuit has close to zero phase margin, thus, is unstable. For some simulation hints see here: https://neurochrome.com/pages/stability

As other have pointed out, you may get better help in the DIY section or on more DIY-minded forums such as DIY Audio.

Tom
 
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