Voltage amplifier stage burning constantly - 10W class AB - diyAudio
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Old 30th October 2014, 04:52 PM   #1
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Default Voltage amplifier stage burning constantly - 10W class AB

Hello gentlemans, i'm having some issues with this amplifier. I've a stereo sound system with it, but one of the sides keep burning the VAS transistor(Q1) and i really don't have a clue why. It only happens when i'm using the amplifier.

If you guys have some sugestions for me about what to check, i'd really appreciate.

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Old 30th October 2014, 06:18 PM   #2
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It may oscillate heavily in reality and possibly burn the transistor. There's no current limiting and Q1 may be driven too hard by the input stage. You may want to add a 22R ~ 100R "degeneration" resistor between the supply and the emitter of Q1. In addition, you can increase C1, the "miller compensation" capacitor, up to values like 150pF.

Do you have an oscilliscope to measure the output?
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Old 30th October 2014, 06:24 PM   #3
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YEs i might find one for testing. But why only one side? the other one work as a charm. It could be something with the VBE multiplier? as it's the only adjustable part conected to the VAS section.

I'll try to make some changes, but about replace the capacitor to higher values seems very odd to me, since the other side has this value and works fine
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Old 30th October 2014, 06:40 PM   #4
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Other things you may check is the PCB itself, no dirt and flux left, just to be sure. When there are no obvious differences, it should work, I agree.

Increasing the C1 capacitor and adjusting the outputstage bias current to a minimum (I know, worse sound, but it's to test) you can figure out if the VBE multiplier works as intended for that side. Q8 is probably not thermally coupled to the output devices as it's TO92 and that may be a cause for the output to 'run away' when things start to heat up. If Q1 stops blowing up by minimizing bias current and increasing C1, it's very likely the circuit is on the edge of stability. The other side may just be stable enough not to blow up.

My first guess would be to thoroughly check the PCB for any dirt between pads / traces, even flux residue with tin particles may form a resistive connection where there should not be one. This assumes you etched and built the amp yourself.

After that, I would then increase C1 to rule out oscillation.

Edit: Another little check: Be sure the current source emitter resistor (at the bottom) is actually 330 ohms and not 33 ohms by accident (or another value).
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Last edited by MagicBox; 30th October 2014 at 06:52 PM.
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Old 30th October 2014, 07:53 PM   #5
AndrewT is offline AndrewT  Scotland
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There are no resistors feeding current to the driver transistors.
These are usually wired across the Base emitter of the output transistor.

Because the output transistor has substantially constant Vbe, then the B to E resistor passes substantially constant current.
Usually the driver transistor is set to pass around 1mA to 10mA. These result in values varying from 620r to 62r
If you are using To92 aim for the lower current, 1 or 2 mA
To126 aim for 3mA to 5mA
To220 >6mA, some designers go very much higher.

Have you looked at esp's p3a?
regards Andrew T.
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