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Old 3rd November 2012, 12:46 PM   #1
diyAudio Member
Join Date: Oct 2012
Default AV400 Bias adjust and Transistor burning accidents

Hello community!
This is my first contribution and question. I'm no professional, I have no electronics education and do this just for fun. Let me present my project:
I found instructions in the web for a circruit that is called AV400, February 17. 2004.
I made 3 PCBs and my plan is to make a amplifier with separate bass, just like in car audio.

Sadly this is not the first time I blew MOSFETs. Careless bias adjustment bears some danger. But the Transistors die because of overheat, too, without the amplifier producing much power. I matched them, so this is not the problem.
In the instructions is stated, that bias is turned to the point where the meter, connected to the output-resistor that has the most voltage (one of the 8) reads some 18 mV.
When this is the case, the Transistors produce too much heat. I get not much output power (my small car bass is for 100 W RMS only and it does not produce pressure like in my car).
Has anyone experience with this circuit? I don't have good loudspeakers here and I don't hear difference between 5 mV and 18 mV on the output resistors adjusting bias.
Is it possible to get 400 W out of that? The free state voltage of my transformer does not really drop much...
What did I do wrong! The circuit is posted many times in the web and I think its high quality work!
I'd be happy if someone shared experiences with this thing!
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Old 3rd November 2012, 02:40 PM   #2
jaycee is offline jaycee  United Kingdom
diyAudio Member
Join Date: Apr 2004
Location: Norwich, UK
With all that wiring all over the place, I would expect your amp is oscillating. Do you have an oscilloscope to test that?
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Old 3rd November 2012, 03:05 PM   #3
diyAudio Member
Join Date: Oct 2012
Thanks for helping Jaycee!
I have this old 4-channel oszi. Problem is that: this circuit has signal ground and power ground connected by a 10 Ohm resistor. When I connect the oszilloscope directly, this resistor goes up in smoke. Don't know, what ground has my computers output signal... So I connect the oscilloskope not directly - I connect it by a 4,2 KOhm resistor...
OK, but I could use an mp3-player which would bring no ground problems...
The amplifier boards have capacitors in the supply lines - so no oscilation here, right?
Do you mean the MOSFETs connecting cables?
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