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Old 12th September 2002, 07:58 PM   #11
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Join Date: Mar 2001
Yeah, always put the bias pot Base to Emitter. Why not
try using a Mosfet like an IRF610 or so for the bias device?
That way the gain will only need to be about 2 to get
the correct value, instead of 12, and the bias adjustment
will be a lot less sensitive.

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Old 14th September 2002, 06:15 AM   #12
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Old 20th September 2002, 10:35 PM   #13
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I made those changes, and the amp simply did not worked, it has shown 25V dc output level and de bias adjust wasn't adjusting anything...... I'm about to give up on this.

And this time I've matched the fets!
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Old 6th October 2002, 09:32 PM   #14
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Default I found the main problem!

The main problem of the amp is the bias adjust stage, The amp worked well using a single trimpot to adjust the bias, so can anyone help-me design a good bias adjust for that amp, the suggestions of nelson pass did not worked.....
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Old 7th October 2002, 11:51 AM   #15
sangram is offline sangram  India
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I see another possible issue here:

The addition of the second pair of transistors is what is causing the problem. Remove them first. Maybe they're 'overloading' the bias stage. I would change from the BC 546 and implement probably a Darlington pair, to maybe add to what Nelson said, and increase the sensitivity and enhance the ability of the bias stage to drive two output stages.

The other is the transistors themselves. They should be rated to take full rail voltage, not just one half of it, in an ideal situation. When one half of the output is locked into conduction and will show full voltage across the other half.

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