| felix42 |
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! |
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| jaycee |
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. |
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| felix42 |
I'm using MJL4281/4302 and BD140/139 for drivers.
The bias transistor is reasonably close to the driver, yes. |
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| jaycee |
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? |
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| felix42 |
| Well, i fixed the problem: Seems i checked against the wrong package drawing for the output transistors all the time. Quite embarassing! |
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