I'm trying to finish off my current mic preamp project which seems to be great, apart from the fact that I am suffering oscillation in the highest gain settings...
The circuit and current progress are posted here:
http://www.groupdiy.com/index.php?topic=16537&start=15
The preamp sounds great until around the highest quarter of the gain, at which point I get that nasty swirling noise....
As Samuel suggested, I have looked over the grounding layout, and I have now made a single ground track (on veroboard) so that all components connected to ground effectively join at the same point (a la star-ground).
I have also looked over the board layout to see where the output could potentially be leaking back towards the input, and have re-routed some circuitry.
I am using a star-ground for the chassis, PSU, circuit board, etc. and have also lifted the earth at the output of the circuit board (left the shield connected only at the output XLR).
Finally, I've also tried adding another three 22pF caps in parallel to C4 (low-pass feedback cap) to reduce the bandwidth to try to combat oscillation. I could add the same to the first opamp to get 12dB/Octave roll-off, but I'm not sure how much this would help.
But! It's still oscillating! :evil:
I'm not sure what to try now? Should I try grounding the unused veroboard tracks? How about the unused opamp pins? I have only ever done that with digital stuff in the past....
Does anyone have any further suggestions regarding re-routing the circuit, or any other ideas? I've laid it logically (in my opinion!) and am a bit lost for any more ideas.
Thanks for your help,
Roddy
The circuit and current progress are posted here:
http://www.groupdiy.com/index.php?topic=16537&start=15
The preamp sounds great until around the highest quarter of the gain, at which point I get that nasty swirling noise....
As Samuel suggested, I have looked over the grounding layout, and I have now made a single ground track (on veroboard) so that all components connected to ground effectively join at the same point (a la star-ground).
I have also looked over the board layout to see where the output could potentially be leaking back towards the input, and have re-routed some circuitry.
I am using a star-ground for the chassis, PSU, circuit board, etc. and have also lifted the earth at the output of the circuit board (left the shield connected only at the output XLR).
Finally, I've also tried adding another three 22pF caps in parallel to C4 (low-pass feedback cap) to reduce the bandwidth to try to combat oscillation. I could add the same to the first opamp to get 12dB/Octave roll-off, but I'm not sure how much this would help.
But! It's still oscillating! :evil:
I'm not sure what to try now? Should I try grounding the unused veroboard tracks? How about the unused opamp pins? I have only ever done that with digital stuff in the past....
Does anyone have any further suggestions regarding re-routing the circuit, or any other ideas? I've laid it logically (in my opinion!) and am a bit lost for any more ideas.
Thanks for your help,
Roddy