Amp oscillation

kubeek · 6th March 2007, 01:22 PM

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

richie00boy · 6th March 2007, 01:48 PM

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?

Tube_Dude · 6th March 2007, 02:00 PM

Have you used a Zobel at the output?

kubeek · 6th March 2007, 05:23 PM

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...

kubeek · 6th March 2007, 06:12 PM

The zobel was 1ohm and 10nF.

MikeB · 6th March 2007, 06:38 PM

It should be more like 10ohms+100nf. 1ohm is too low.

kubeek · 6th March 2007, 07:02 PM

OK, but if I rewire the whole thing tomorrow and the oscillations donÂ´t stop, what should I do next?
Add a capacitor(s) somewhere? Increase gate resistors? Use more decoupling?

Coffee · 6th March 2007, 07:24 PM

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.

kubeek · 9th March 2007, 06:37 PM

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?

MikeB · 9th March 2007, 06:42 PM

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

6th March 2007, 01:22 PM	#1
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	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

6th March 2007, 01:48 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	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? __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

6th March 2007, 02:00 PM	#3
Tube_Dude diyAudio Member Join Date: Mar 2002 Location: Aveiro-Portugal	Have you used a Zobel at the output? __________________ Jorge

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6th March 2007, 05:23 PM	#4
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	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...

6th March 2007, 06:12 PM	#5
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	The zobel was 1ohm and 10nF.

6th March 2007, 06:38 PM	#6
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	It should be more like 10ohms+100nf. 1ohm is too low.

6th March 2007, 07:02 PM	#7
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	OK, but if I rewire the whole thing tomorrow and the oscillations donÂ´t stop, what should I do next? Add a capacitor(s) somewhere? Increase gate resistors? Use more decoupling?

6th March 2007, 07:24 PM	#8
Coffee diyAudio Member Join Date: Apr 2006 Location: Klatovy,CZ, partial USA, CA,	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.

9th March 2007, 06:37 PM	#9
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	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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