Moddeed Cambridge Audio 340A SE buzz problem

[mikesnowdon]

Its definitely a possibility, I remember the first time I dis-assembled the amp I had trouble removing the Vol and balance pot so I could have damaged the trace or solder joint. I'll check it very soon.

At the moment my hunch is that the input selector i/c got fried when I did the first mods; I mistakenly fitted a local decoupling cap the wrong way on the op-amp. The supply to the op-amp also powers the selector i/c so damage could have been caused by the wrong orientation of the cap. I remember getting a loud buzz on the first power up, this occurred 3 times but eventually went away by itself. Then later I noticed the faint buzzing sound so I opened up the amp and found the local supply de-coupling cap on the negative rail was badly swollen. I replaced them with the original caps and the buzz was still there so some damage was caused elsewhere in the circuit.

I have a new i/c on order which I hope to receive in the next week or so. In the meantime I'll check those ground traces on the pots.
Mike.

[audio1st]

My new input selector ic arrived today. When I get time in the next few days get it in. Lets hope that cures my buzzing!

Hi Mike,
If your amp still buzzes with the input shorted to ground after the input selector, I don't see how it can be the problem?
Turning the volume to zero is grounding the input. This must mean the problem is between the volume control and the LM3886 chip IMHO..
Check the resistance to ground both sides of R81 and R82 with vol at zero and amp turned off. Should read 0 and 470r..

Barry.

[mikesnowdon]

Your right. Will check those resistors as you suggest.

I never thought of it that way, I could even have a faulty 3886 perhaps?

[Stream]

Mike, can you describe your problem once again?

Connect your source to amp, pause it, set your volume to 12'o clock, and use headphones. Do you hear audible buzzing? I can hear something "dirty" after 12' and it's defenetly audible at 14-15...

P.s. I think it's just poor PCB design...

[mikesnowdon]

The thing is I tried that already and the buzz is there even with no source and the volume at zero, on headphones and speakers. I have listened to the amp late at night with the volume right down before I modded it and I never noticed any buzzing before. It definitely a fault somewhere I'm sure. I'll have one more go at solving it and if I cant I'll just bin the PCB alltogeter and get some chipamp.com PCBs and a good diy pre plus 'Dantimax.dk' relay input selector and re-do the whole thing. It will probably sound better too.

[mikesnowdon]

Guess what guys?.....


I found the problem!!!!

The local supply decoupling cap on the negative rail of the input selector was in wrong polarity. I had a look at the service manual and traced back the PCB track and spotted the problem. I hadn't seen it before but the trace goes the opposite direction meaning the caps both 'face' the same way as you look from above. There is still a little buzz but its so quiet you have to put the speaker in your ear to hear it! I just have 'normal' amp noise now, much less than before.

As a celebration I added loads of nice parts to the amp and did a bit of modding.

Thanks for all the suggestions and helpful ideas.

[Stream]

Gratz! )))
Time to close this topic and forget about this buzzzzzing