Need help suppressing LM3876 noise

[madiger]

I have 400Hz signal running into the amp, but the output is extremely noisy at the top peaks (bottoms are fine - see green signal). How do i fix this? I appreciate your help

NOTE: the sine wave is not a perfect sine because it is the product of a square wave i converted to a sine with a bandpass filter.

 

[scanman]

madiger,

Looks like it could be some sort of high frequency ringing due to feedback coupling.
May be related to the bandpass filter if it is high Q.
Check all decoupling networks on the power amp and if possible inject a known clean signal from function generator to verify.

Regards,
Scanman

 

[madiger]

Looks like it could be some sort of high frequency ringing due to feedback coupling.
May be related to the bandpass filter if it is high Q.
Check all decoupling networks on the power amp and if possible inject a known clean signal from function generator to verify.

Thanks, I tried feeding it a clean 400hz sine wave from the signal generator, but i get the same effect. It definitely has something to do with my amp configuration. It's just strange because yesterday it was working fine, but today, after i installed the bandpass filter, it started acting up.

 

[keith1200rs]

Maybe you should show your circuit.

Keith

 

[madiger]

The circuit i'm using is similarly matched to the circuit on page 6 of the LM3876 datasheet (minus all the optional components). i think it may be a problem with the board i'm using. I just tried the same circuit on another board and it worked fine.

 

[scanman]

Are you using those plug in proto-boards ? They can be problematic in their ability to create unexpected capacitive inductive effects, especially at high frequencies.

Scanman