Problem with RF buzz and noise when using LM3886 and Nokia 6303

[Split841]

Hello. I am currently working on amplifier which is using the LM3886 chip. It was working fine, until (unfortunately) I placed it on an aluminium plate and turn it on. Since then, there is one problem in only one specific case. When i choose my mobile phone (Nokia 6303), there is a pretty loud buzz and also some RF hum. In other cases (notebook plugged in, mp3 plugged in, CD player plugged in) it is OK. Some guys, which know a lot about amplifiers told me, that there can be problem with capacitors in the circuit, that they may have been destroyed when i unfortunately shortened it on aluminium plate, but i have checked them and they are allright. The second idea was, that the LM3886 could be slightly damaged (something to do with internal compensation ), but i am not so sure. I measured it and it sounds pretty good, there is no noise or distortion or whatsoever (and i also measured it and it really amplifies 18 times). To complete the analysis, the circuit was working fine and the sound was without noise and buzz, even when the mobile phone was the source of input signal. And also, with the max volume on the phone, the speaker was a lot louder, then it is now. Could it really be that the chip (LM3886) is really destroyed?

Thank you all in advance for your replies

Sorry for my english, if there are some mistakes

 

[alexan_e]

Re: LM3886 buzz problems

It can be the mobile that introduces noise to the amplifier, I have the same problem from my PC speakers when I use the mobile phone near it.

Alex

 

[Orson Cart]

Re: LM3886 buzz problems

C2 & C4 can act as small Rf antennas and pick up RF signal and inject into the (-) pin where it is demodulated and amplified and the noises get to the output. also long tracks to the input pins will pick up same. Regards, Orson Cart.