oscillation at high gain in Auditronics 110a channel

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plumsolly

Well-known member
Joined
Nov 3, 2007
Messages
969
Location
Denver, Colorado
I noticed yesterday that a channel on my console goes into oscillation when it is set to mic input and at it's highest gain setting or it is set to line input and is at the highest or second-to-highest gain setting. The fader must also be turned up almost to the top of it's throw. Engaging the low-pass makes it go away. I can't really hear the oscillation, but it pins the meters. Here is a schematic: Contrary to the schematic, the chips are all 5534's and they all have 150pf caps in parallel with their feedback resistors. I replaced these caps and resistors and all the power supply bypass caps. I also replaced all the chips. If anyone has any ideas where to look, it would be appreciated. Thanks, Ben
 
plumsolly said:
Contrary to the schematic, the chips are all 5534's and they all have 150pf caps in parallel with their feedback resistors. I replaced these caps and resistors and all the power supply bypass caps. I also replaced all the chips. If anyone has any ideas where to look, it would be appreciated. Thanks, Ben
Has the compensation been modded too (22pF cap between pins 1 & 8)?
 
abbey road d enfer said:
plumsolly said:
Contrary to the schematic, the chips are all 5534's and they all have 150pf caps in parallel with their feedback resistors. I replaced these caps and resistors and all the power supply bypass caps. I also replaced all the chips. If anyone has any ideas where to look, it would be appreciated. Thanks, Ben
Has the compensation been modded too (22pF cap between pins 1 & 8)?
22pF compensation @ pins 5 & 8 for slew rate 6V/us to make the amp stable at unitygain might not be the OPs problem.
Is the load of the fader still in circuit or substituted by a fixed resistor ?
 
Is there any input plugged in, when this oscillation occurs?  If no, and it doesn't oscillate with a valid input, you can mitigate with some input capacitance to ground, or just don't turn it up with no input.

If it oscillates with a valid input plugged in, then disregard this response. 

JR
 
Thanks for all your responses. I tracked it down to a layout problem with a modification I did. Moving some wires helped and using shielded cable at at a particular spot eliminated it. Now to go back and do this to the rest of the channels...  :(
Thanks again and best, Ben
 

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