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It is currently 21 Apr 2018, 08:40

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PostPosted: 10 Apr 2018, 04:11 
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Joined: 19 Mar 2018, 02:57
Posts: 2
Hello everyone! :headphones:
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:
Image
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).


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PostPosted: 10 Apr 2018, 21:09 
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Joined: 04 Jun 2008, 20:59
Posts: 3607
Location: Arizona, USA
Hi, The first thing I would do is put a scope on it and actually see what is going on. Then you can figure out how to actually correct it. I see no reason for the coupling capacitor to fix it...unless... it is introducing a slight phase shift and that is sufficient to return the amp to a stable condition. It might be oscillating at a very low frequency...like 0.5HZ or so and not be audible or easy to detect. That might draw considerable power. I had that occur in a tube amp with a bad output transformer. Thing got very hot and didn't sound good. The scope showed a sub 1 hz oscillation at nearly full power.

Good listening
Bruce

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