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Old 29th May 2003, 09:56 AM   #1
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Join Date: May 2003
Location: Cambridge, UK
Default Grounding buzzing amps in general

Hi,

I built a 4-channel LM3886 amp using my own design circuit board and a standard app note non-inverting circuit from National with zobel and 0.7 uH inductor. I had a terrible buzz (more harmonics than a simple hummmmm) for weeks and searching this forum led me to a 95% solution. Somebody had a problem with exactly the same symptoms as mine and the solution for their problem mostly worked for me. THANKS MOAMPS. The thread is here. Now my actively crossed-over dipole speakers are listenable and sound fantastic.

My grounding scheme is the same as recommended by moamps in that thread and has the amp ground, RCA input grounds, speaker grounds, and transformer center tap all meeting at the 0V of the filter caps, then a single wire goes from there to the case.

There is still a little quiet hum that is audible from about 3 feet away and the tweets hiss and crackle very quietly and you can hear that from about 5 feet away. You don't notice it with music, but it's there when there is a very very quiet passage or between tracks.

Also, one of the amps ( the one that is only a few centimeters from the input jack) whines, whistles, and pops loudly at a frequency of about 3 Hz unless RCA cable connected to the outside of the case (the cable doesn't need to be connected to anything/ if a cable is there, the chip is 95% silent).

Also, I've notices that if I move the ground wire going between the RCAs to the filter caps, the hum gets louder the closer the ground wire is to the transformer, and also changes when it is at different angles to the transformer.

I've noticed that some published circuit boards, say from Rod Elliot or Elektor,have connections ON THE AMP BOARD for input ground and speaker ground. My speaker grounds and input grounds never go anywhere near the amp board and only meet up at the filter caps. This is great for neatness because it saves several wires snaking around the case. I was just wondering if I was asking for trouble with ground loops and oscillation by doing this.

Any comments or suggestions? Thanks.
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