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Solution to oscillating preamp issue

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smuel

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Hi,

I've built a guitar amp using the schematics on this site **broken link removed**

It is mostly working fine, but when i flick the 'bright' switch, the opamp goes into oscillation. I've bypassed both power pins to ground with 0.1uF ceramics, and I've tried it with the power amp disconnected, so it's definetely got something to do with the opamp.

Could this be due to the lack of sheilding from the power supply? There is about 15cm between the preamp board and the PSU, but i haven't got any sheilding.

Also, when i turn both the volume, and master all the way down, there is a 'pop' then a quite hum. As soon as either of them is turned up again, there is a 'pop' then everything works fine.

Any Ideas?

Thanks a lot

Sam
 

Re: Oscillating Preamp

maybe you should first check and retrace step by step from input to output, w the diagram also check for solder bridges, bad solders and the likes, proper polarity orientation of dides and caps.
i've built things from that place too.
and they also have a section to help you toublrshoot your projects.
they wont help you solve specific problems thou but maybe you cantry
 

Re: Oscillating Preamp

Do you have PCB from Rod Elliot or you have done it yourself? I have build 3 of this Preamp + Amp on my boards and they work all OK. What OP Amp do you use? If you use the OPA2134 you must put 10uF + 100nF from each power pins to ground nearest possible to op-amp and also 10uF + 100nF between + and - power pin directly on op-amp. I preferd the TL072 - it is not so quiet like OPA but which guitar amp is death quiet.

If you want I can sand you my PCB lay-out for preamp and power amp.

Cheers
 

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