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In Reply to: RE: Odd hum issue - suggestions ?? posted by rogerh113 on May 30, 2017 at 15:41:04
Yes. My limited understand is that the signal ground (neutral) is best only connected to the ground in one spot. I gather to avoid ground loops, etc. There is an existing connection going from the neutral on the main board to a lug on the chassis at the rear of the amp. I removed it and then checked the resistance between the board neutral and chassis (ground) and it was infinite (no second connection).
As it turns out, the problem seems to have been fixed by connecting the signal ground (netural) at the big power caps to the chassis. I tried connecting the ground lug to the board neutral with a large jumper (assume the wire inadequate to handle the 'load'. No reduction of hum. Connected the main board netural lug directly to the ground lug at the front of the amp where the ground wire is attached - no improvement. Connecting the large cap neutrals to the chassis just eliminated the hum. The wires from the cap neutral lugs to the main board neutral are quite large, so it does not seem they are deficient and were causing the problem.
Hooked up a signal generator, and both sides seem to be working nicely, especially without the hum to get in the way. Just need to calibrate the amplifier boards, and hopefully will be ready to go.
Thanks very much for the tips and suggestions. A bit of support really helps when tackling a problem that seems to be somewhat obscure at times..
regards -- Roger
First, ground the faceplate to the chassis.
Second, I would not directly run a wire between the circuit ground (power supply caps) and the chassis- that will open the amplifier to ground loops. Instead, a simple way to do that would be to put a 50 ohm resistor between the chassis and power supply ground, so that a ground loop current can't form but the circuit is still floating at ground potential.
A more sophisticated method would be to also install 2 rectifiers in parallel but in opposite directions to each other and then in parallel with the resistor. This will prevent the chassis from going too far afield should there be some weird problem like a component elsewhere in the system introducing line voltage to the circuit ground of the amp. The diodes will cause a fuse to blow somewhere if things are rated correctly.
I think you will find that this approach is actually quieter than what you've got going on right now.
The Mind has No Firewall~ U.S. Army War College.
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