Heavy Amplifier Circuit Oscillation Happened

[Stonestreecty]

TL;DR

Heavy oscillation occurred within my amplifier circuit when there was no AC coupling capacitor between voltage amplification stage (MC1458) and output power stage, I don't know why.

Hi, all
I am experimenting with audio amplifiers for a while now. I learned 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 100 W 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:

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 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 1Ohm) 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).

 

[Baluncore]

Did it sound like a series of pops?
What was the frequency of oscillation?
Were your power supplies stable?
I would look for an oscillator involving C1, where the phase shift is partly canceled by C2.

 

[tech99]

It looks to me that without the coupling capacitor the system is entirely DC coupled. This means that there is no low frequency limitation, and so that very low frequency oscillation is possible perhaps as a result of common impedance coupling via the power supply or the connecting leads. It is better to restrict the LF (and HF) response of an amplifier to what is actually required.

 

[Baluncore]

I have built this circuit few minutes ago on breadboard:

That may explain the problem. Breadboards have high inductance, resistive connections and poor grounds. You will encounter many unexpected problems with circuits built on breadboards.

Another point. The op-amp has JFET inputs with very very low input current. There is little point in your Rbias of 47k, you can throw it out. Only high input current bi-polar op-amps needed that resistor. Op-amp offset voltage will swamp the input bias current error, and any offset will disappear in the feedback loop.