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Old 24th April 2007, 12:49 PM   #1
felix42 is offline felix42  Germany
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Join Date: May 2006
Default 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!
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Old 24th April 2007, 01:50 PM   #2
jaycee is offline jaycee  United Kingdom
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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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Old 24th April 2007, 04:13 PM   #3
felix42 is offline felix42  Germany
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I'm using MJL4281/4302 and BD140/139 for drivers.
The bias transistor is reasonably close to the driver, yes.
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Old 24th April 2007, 09:55 PM   #4
jaycee is offline jaycee  United Kingdom
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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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Old 25th April 2007, 07:21 AM   #5
felix42 is offline felix42  Germany
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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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