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Old 8th February 2013, 02:20 PM   #1
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Default what about some help over here!!!

Hi folks!, i found an old guitar amplifier, with a transformer and pre, so i build an output stage for it with TIP142/147....


the thing is, it works but with a subtle annoying distortion, can somebody help me get rid of it????

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Old 8th February 2013, 09:25 PM   #2
JMFahey is offline JMFahey  Argentina
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Location: Buenos Aires - Argentina
That subtle but annoying distortion is called crossover distortion.
You'll have to mod your simple amp.
Did you built on PCB, perfboard or what?
What +/- voltages do you have in your power supply?
What kind of heatsinks are you using?
A couple pictures would help.
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Old 8th February 2013, 09:25 PM   #3
jaycee is offline jaycee  United Kingdom
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it's an awful circuit... im guessing it has crossover distortion due to the lousy diode biasing
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Old 8th February 2013, 10:02 PM   #4
DUG is offline DUG  Canada
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Input transistor biasing...I would not be surprised if it was in saturation.
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
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Old 8th February 2013, 10:55 PM   #5
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
Angel, they will probably move this to the interument amp section, so look there if it disappears here.

Your circuit is very basic and simple, a crude amplifier. If you play nothing but overdrive tones, you might not notice some distortion, but it does show up[ on clean tones. I agree with the others, it is probably crossover distortion you hear. Because those power transistors are "darlington" transistors, they are inside actually two transistors wired together in one part. But that also means you need more bias than single transistors. I see only two 1N4007 diodes for bias, and this circuit probably needs more like 3 or even four. How about three and a fourth with a parallel resistor. Or just design a more sophisticateed bias circuit.
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Old 10th February 2013, 12:15 PM   #6
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Join Date: Jun 2010
Default Solved!

Alright, the problem was my own dumbness, i put a 100nf cap instead of the 100pf between B and C of TIP41. change it and the problem was gone,

well,i love simple things, and i have to say that this amp works very well. very clear and loud for a 50 years old pre and speaker, thanks for the quick answers!
and, why that cap was making that distortion???

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