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Old 6th March 2007, 01:22 PM   #1
kubeek is offline kubeek  Czech Republic
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Default Amp oscillation

Hello,

I have again built this amp, but this one is oscillating, probably due to long wiring to the output transistors and to power source.

Could someone please suggest how to stop the oscillations?

Thanks
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Old 6th March 2007, 01:48 PM   #2
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Long wiring to the gates is bad, have you included gate resistors? Try and make the wires shorter.

What decoupling have you included on the driver board and is there any on the output board/devices?
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Old 6th March 2007, 02:00 PM   #3
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Have you used a Zobel at the output?
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Old 6th March 2007, 05:23 PM   #4
kubeek is offline kubeek  Czech Republic
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Yes I included the gate resistors, I will start rewiring it.

If by decoupling you mean capacitors on power rails, then yes I used the capacitors showed on the diagram.

About the zobel: I tried to use it on one channel, don´t know the values right now, but after powering up with the zobel the channel got blown...
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Old 6th March 2007, 06:12 PM   #5
kubeek is offline kubeek  Czech Republic
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The zobel was 1ohm and 10nF.
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Old 6th March 2007, 06:38 PM   #6
MikeB is offline MikeB  Germany
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It should be more like 10ohms+100nf. 1ohm is too low.
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Old 6th March 2007, 07:02 PM   #7
kubeek is offline kubeek  Czech Republic
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OK, but if I rewire the whole thing tomorrow and the oscillations don´t stop, what should I do next?
Add a capacitor(s) somewhere? Increase gate resistors? Use more decoupling?
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Old 6th March 2007, 07:24 PM   #8
Coffee is offline Coffee  Czech Republic
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you can try to use bigger c24 a c25 ,but the high frequency gain margin will be lower.

the best solution is to remake it using shorter wirings to the output transistors.
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Old 9th March 2007, 06:37 PM   #9
kubeek is offline kubeek  Czech Republic
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I rewired the amp and it is stable when nothing is connected, but it starts oscillating at cca 5Mhz, ~0.5V when I connect bare speaker cable to the output.
The cable is about 3m long and has 200pF capacitance.

Any help to this?
The feedback resistor is paralleled with 220pF to decrase bandwidth above 20khz, so why does it behave like this?
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Old 9th March 2007, 06:42 PM   #10
MikeB is offline MikeB  Germany
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Quote:
Originally posted by kubeek
The feedback resistor is paralleled with 220pF to decrase bandwidth above 20khz, so why does it behave like this?
The expcected behaviour with such a large cap in paralell to the feedbackresistor is heavy oscillation. By decreasing closedloop bandwidth you increase the amount of feedback for higher reqs, destabilizing the amp.
You can use this cap in any size only if the amp is unitygainstable.

Reduce this cap below 22pF or skip it.
Dou have a zobel at the output? (100nf+10ohm) Without it many amplifiers start to oscillate when speakercable is connected.

Mike
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