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Problem with oscillation on receiver audio stages

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If that is what you found, the 50R is not the problem itself, it must be conducting signal from it's ground end back into the amplifier. You have a feedback loop somewhere and it appears the 50R is in series with it. Can you try a different ground point and see if that fixes it.

Brian.
 
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    neazoi

    neazoi

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betwixt said:
If that is what you found, the 50R is not the problem itself, it must be conducting signal from it's ground end back into the amplifier. You have a feedback loop somewhere and it appears the 50R is in series with it. Can you try a different ground point and see if that fixes it.

Brian.
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Well, I just figured this out. It was not the 50R. It was because the inductor (molded choke) was placed very close to the mixer body, 1-2 mm on top, just not touching. When I bent the leads of the choke to place it 5-10mm away of it, there was no oscillation problem anymore.
Have you ever encountered a similar situation?

Oh, and I had to follow your guideline, one bjt preamp followed by the lm317 with a gain of just 20. Not trying to achieve the full gain with just the lm317 in gigh-gain mode. This also gave the best sound (more bass).
 

LM386 not LM317 I hope!

I'm not sure why moving the inductor worked, I can only assume it was either coupling audio through it or having a voltage magnetically coupled to it. As you can see, when high gain is involved, layout becomes critical, especially when dealing with microvolt signals.

Brian.
 
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betwixt said:
LM386 not LM317 I hope!

I'm not sure why moving the inductor worked, I can only assume it was either coupling audio through it or having a voltage magnetically coupled to it. As you can see, when high gain is involved, layout becomes critical, especially when dealing with microvolt signals.

Brian.
Click to expand...

Yes LM386...
It seems that the good old trick worked. Finger touching components, or touching with a metal object has helped me to debug such issues in the past as well.
Also, the 50R does not play a role in sensitivity, believe it or not.
 

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