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werfi 31st January 2010 07:11 PM

P3A bias setting
I finished my first P3A amp few weeks ago. I did testing as described at 60-80W Power Amplifier and set the bias, the voltage across R13 and R14 to 50mV .

Now I have finished balanced input stage on separate board and mounted and stuff onto chassis. I checked the bias again to make sure it was okay. Now the voltage drop was 100mV. I tried to set it back to 50mV, but even with the trimmer pot at maximum resistance the voltage drop is still 70mV.

The pcb is my own design. The driver transistors (including Q4) are mounted to same heatsink with power transistors, I'm not sure if this is okay.

Amp sounds good and is working, only problem is that bias setting.

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