RIAA pre amp oscilates. - Page 2 - diyAudio
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Old 23rd April 2012, 01:01 AM   #11
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Quote:
Originally Posted by grufti View Post
MM cartridges are often about 1k. You can check specs on the ortofon website for example.
The pre amp oscillates at about 10k input resistance.
If 1k is typical of a cartridge then there should be no problem.

I wouldnt expect it to perform very well with a floating input.
There is clearly some sort of coupling back into the input as I can see 400mV of noise on the input. If I short the input it stops oscilating.

I had a look at the layout and there is an opamp out track and input track running in parallel. It was just laid out to be as tight as possible.
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Old 23rd April 2012, 01:06 AM   #12
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Here is the layout.
Click the image to open in full size.
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Old 23rd April 2012, 02:02 AM   #13
kaos is offline kaos  United States
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Yeah the layout’s buggered, especially the ground. With a external transducer the (cartridge) ground needs to be close to the first stage inputs and starred to a common point with the output ground and supply bypass ground. It would’ve helped if the op-amps were rotated 90 degrees to make it easier to get a direct path from the cartridge to the inputs as needed. Unfortunately at this point you’re going to be stuck applying band-aids to get it to work oscillation free.
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Old 23rd April 2012, 02:07 AM   #14
kevinkr is offline kevinkr  United States
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Yeah, got to agree with KAOS - you'd be better starting over with a little guidance from the crowd.
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Old 24th April 2012, 09:46 PM   #15
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Quote:
Originally Posted by kaos View Post
Yeah the layout’s buggered, especially the ground. With a external transducer the (cartridge) ground needs to be close to the first stage inputs and starred to a common point with the output ground and supply bypass ground. It would’ve helped if the op-amps were rotated 90 degrees to make it easier to get a direct path from the cartridge to the inputs as needed. Unfortunately at this point you’re going to be stuck applying band-aids to get it to work oscillation free.
I am greatly embarassed to admit I left out a very small capacitor by accident in the feedback loop, the 4n7.
The circuit works a treat now with NE5532 and tl072.
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