P3A bias problems & drivers getting hot
 

Hi everybody..

I just finished one channel of a P3A amp (my own PCB) and have some problems trying to set the bias:

I turned the pot until i measured the recommended 50mV across the two 0.33 resistors, then turned it back because the voltage i measured increased rapidly.. The voltage remains unstable no matter how careful i adjust the trimpot, it's either increasing (pretty fast) or steadily decreasing but i cannot get it to stay around 50mV.

While trying to set the bias i noticed that the output drivers (Q5, Q6. not heatsinked) are getting quite hot while Q6 and Q8 stay cool.

Anyone any idea what's wrong?

Thanks in advance!

Probably oscillation of some kind. What transistors are you using?

Where is the bias transistor mounted? it should be close to one of the drivers - not the outputs, due to a CFP output stage being used.

I'm using MJL4281/4302 and BD140/139 for drivers.
The bias transistor is reasonably close to the driver, yes.

Hm one thing I usually suggest - do you only have the 100nF capacitors on the power rails on your pcb ? Try adding 220-470uF near the outputs.

This is why I don't like the CFP - they oscillate if you so much as look at them wrong.

Does the amp work otherwise? Do you have a 'scope to check for oscillations?

Well, i fixed the problem: Seems i checked against the wrong package drawing for the output transistors all the time. Quite embarassing!