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Old 25th September 2012, 10:41 AM   #11
jaycee is offline jaycee  United Kingdom
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The circuit you show isn't complete, but good enough to see whats going on.

The big problem with this arrangement is that there is gain in the output stage. This is difficult to stabilise and frequently causes oscillation. That is probably why you have had blown output transistors. There's also no emitter resistors so I expect the amp has gone into thermal runaway at times too.

I agree with the other posts here - if you are looking for a replacement amp to fit in a limited amount of PCB space, you should use a chip amp such as LM3886. You're not going to be able to build anything discrete that is as compact and reliable as those.
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Old 26th September 2012, 01:22 AM   #12
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Quote:
Originally Posted by jcx View Post
monolithic audio power amp chips - check the chip amp forum - LM3886, LM4780, TDA7293 are all pretty complete amps, have many project articles - sometimes known as "Gain Clones"

most have output protection, thermal overload protection circuits built in too
Hi guys,

To answer your earlier posts.

1. +/-12V supplies for op amp and DAC are from 7812 and 7912 regulators.
2. I've simplified the amplifier to include the op amp inputs. The DAC outputs a sinusoidal wave 0 to 10V/0 to -10V, direct coupled. For a 4 ohm load, the max current is 1.5A and through R60 is sensed from the output.
3. The damper circuit samples the output and when the voice coil stops, generates opposite and decaying wave. Fed back to + input of op amp. Capacitor coupled to prevent DC.
4. As for the output circuit with gain, I find similarity to LH0101.
5. Am I correct to say that the op amp just buffers the DAC output. The quasi-amplifier is just a voltage to current converter?
6. Is is possible to remove the quasi-amp gain and bring it outside the op amp + quasi-amp like the Walt Jung composite op amp + buffer? My problem is how to integrate the various resistors at the op amp inputs.
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