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Old 21st November 2009, 11:04 AM   #1
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Default Smouldering Amp, Help pls

I'm building an amp with this circuit, but if I solder the 10 Ohm at the output filter. This resistor got very hot and burned.

Could u pls help.
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Old 21st November 2009, 11:31 AM   #2
Dr_EM is offline Dr_EM  United Kingdom
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That is part of the Zobel network, if it is getting hot, one can assume there is a lot of high frequency oscillation at the output. Do you have any photos of your layout? I am not familiar with the amplifier however.
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Old 21st November 2009, 11:41 AM   #3
Mooly is offline Mooly  United Kingdom
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Agreed, the amp is oscillating at HF.
Layout will be very critical with this kind of circuit. You really need a 'scope to see what's going on.
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Old 21st November 2009, 12:14 PM   #4
jaycee is offline jaycee  United Kingdom
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The circuit is straight from the STK3042 data sheet. The circuit will be oscillating.
I also suspect you are not using the original transistors specified by the datasheet... or you are using cheap copies of them. The circuit's compensation would most likely need to be altered in this case.
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Old 21st November 2009, 09:08 PM   #5
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The capacitor becomes a short ciruit at very high frequencies and passes all the power to the 10 ohm resistor and cooks it.

Normal audio has little long term high frequencies so doesnt upset the zobel network.

It sounds like what is happening is the output is overshooting and oscilating.
This means you need some negative feedback in the system somewhere to compensate.

I have had this trouble in a couple of my designs and a 220pf feeding from a collector to a base has killed the oscilation stone dead.

I also found the problem in a quasi setup and again a 220pf was required from a collector a to a base to fix it.

If these caps are already in your amp then it is likely there is feedback somewhere else that is not quite right.
Murton-Pike Systems PCBCAD51 pcb design software.

Last edited by nigelwright7557; 21st November 2009 at 09:11 PM.
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