thermal runaway problem

[Dan2]

please help me figure this out....

i have an amp with 2 sets of inputs - one set goes through a chip which controls volume, bass, treble etc. the other set of inputs has a normal preamp (for microphones). these 2 inputs join at a stereo opamp which then goes to the power amp.

about a year back i replaced the stereo opamp with 2 mono opamps (one inverting, the other non-inverting) so i could bridge both channels of the amp. this has worked fine for ages.

yesterday i took the amp circuit out 'cos i want to use it as a subwoofer amplifier. i connected the inputs to the inputs of the opamps (bypassing bass, treble, volume controls) and the thermal runaway started.

so i hooked the chip back up to the opamps - it was better but thermal runaway was still there.

then i hooked the amp back up exactly how it was - still thermal runaway.

then i took the sub and hooked it up to one channel (not bridged) and it is stable now.

what could i have done wrong??? it was stable (bridged) and now its not. i have got a schematic so i will post it later.

[nikwal]

are you sure the root cause is thermal?.. maybe more like self oscillating?? I'm not sure I understand what you mean but if it is oscillating maybe a resistor in series with the input will make it a bit stable?).. and of couse you could try a coil on the output too and also resistor in series with a capacitor to earth.. and so on.. but with bass, I guess you need very high bias,if it 's now only going to be used with bass

[xiphmont]

The first thing I thought of was oscillation too. This is probably a question you can't answer without an oscilloscope.

[Dan2]

okay well luckily i fixed it - it was a resistor i unsoldered and forgot to put back - but i would still like to figure out how this amp biases itself (there is nothing on the heatsink except for the output transistors)

i am still trying to figure out how to put the schematic up - if i compress it so i can attach it then you cant see anything.

i did cut the pic up and posted it like this on another thread:

cheap GemSound amp

[AndrewT]

CFP output stage usually monitors the driver or pre-driver junction temperatures.

[Dan2]

sorry i have no idea what you posted andrew.

[sreten]

Originally posted by Dan2
sorry i have no idea what you posted andrew.

Hi,

A complementary follower pair (CFP) output stage does not monitor
the power transistors or main heatsink temperature for biasing.

The biasing is set against the driver stage transistor temperatures.

/sreten.