Hi.
I did some mods on Studiomaster 24-2 mixer. Now I have 3-4MHz cca 1V oscillation on everything I touch with scope.
I could really use some ideas how to get rid of oscillation.
The following was done:
1. recapped 20+yrs old AL caps with new (better?) Panasonic caps.
2. changed most opamps from TL072 to 5532 - all 5532's are decoupled with 100nF rail-rail, right at IC pins (used IC sockets and inserted 100nF cap between legs 4-8 inside IC socket. (I didn't want to decouple rail-ground, to avoid injecting rail noise into ground). Originally there was no 100nF decoupling for TL072's, per board just 2 rail-ground AL caps 47uF per board.
3. Tweaked opamp compensation caps for better square wave response.
4. made stronger power supply to cover 5532's higher power requirements.
Each channel was tested individually before putting in back in, and all were stable individually. Compensation on channel is such, that 10000Hz square test wave looks almost perfectly square on chan output. No overshoot or ringing on channels individually. 1000Hz square test is perfect square.
If I disconnect cca 8 channels, oscillation disappears. If I add channels back one by one, at one point it gradually starts oscillating, first arround 4-5 MHz @ cca 100mV, and then more channels I add, oscillations moves to arround 3MHz @ cca 1V level.
Each channel has 2 opamps. Most problematic are channels, where both 072 were replaced with 5532.
I changed power supply back to original one, and oscillation remains, but a bit different frequency and level.
If I change enough opamps back to TL072, oscillation is gone.
So I figure, problem is in 5532's. But they are all decoupled rail-rail with 100nF multilayer caps. Any ideas what more need to be done to tame it?
Any suggestion is most welcome.
...
I did some mods on Studiomaster 24-2 mixer. Now I have 3-4MHz cca 1V oscillation on everything I touch with scope.
I could really use some ideas how to get rid of oscillation.
The following was done:
1. recapped 20+yrs old AL caps with new (better?) Panasonic caps.
2. changed most opamps from TL072 to 5532 - all 5532's are decoupled with 100nF rail-rail, right at IC pins (used IC sockets and inserted 100nF cap between legs 4-8 inside IC socket. (I didn't want to decouple rail-ground, to avoid injecting rail noise into ground). Originally there was no 100nF decoupling for TL072's, per board just 2 rail-ground AL caps 47uF per board.
3. Tweaked opamp compensation caps for better square wave response.
4. made stronger power supply to cover 5532's higher power requirements.
Each channel was tested individually before putting in back in, and all were stable individually. Compensation on channel is such, that 10000Hz square test wave looks almost perfectly square on chan output. No overshoot or ringing on channels individually. 1000Hz square test is perfect square.
If I disconnect cca 8 channels, oscillation disappears. If I add channels back one by one, at one point it gradually starts oscillating, first arround 4-5 MHz @ cca 100mV, and then more channels I add, oscillations moves to arround 3MHz @ cca 1V level.
Each channel has 2 opamps. Most problematic are channels, where both 072 were replaced with 5532.
I changed power supply back to original one, and oscillation remains, but a bit different frequency and level.
If I change enough opamps back to TL072, oscillation is gone.
So I figure, problem is in 5532's. But they are all decoupled rail-rail with 100nF multilayer caps. Any ideas what more need to be done to tame it?
Any suggestion is most welcome.
...