Can you help with my Buzz??

[saurus]

I made some modifications to my P3A, and now it has a nasty buzz that it didn't have before.

The buzz (+120Hz) is of moderate volume, comes from both channels, and is constant regardless of input volume. The music (on top of the buzz) sounds fine. With no interconnects connected, there is no buzz. When the preamp is attached with interconnects, there is still no buzz until the preamp is turned on, then it buzzes. The preamp has no earth ground, and it does not buzz with the same cables and a diffrent amp. The amp has a floating star ground that is tied to the chassis, and an earth ground on the chassis at the AC in. Removing the star-to-chassis connection does not stop the buzz. The input jacks are isolated from the chassis.

All I did to the amp was upgrade the transistors (BD139/140 to MJE15032/33 and MJL3821/1302 to MJL4281/4302) along with new emitter resisters, etc. as per Rod Elliot's P3A changes. I also added a mains filter that has a new earth connection at the AC in. The only other change I made was to route the input shield grounds straight to the star point instead of to the amp boards, but the wires are bundled with the amp grounds, so there is no loop.

I searched the forum but I am stumped. Can anyone help??

[djmiddelkoop]

The new mains filter has a metal case that's connected internally with the mains through capacitors.
If you would isolate it from the chassis and the ground of the amp you will measure half of the mains voltage on the case of the filter, if you disconnect the mains ground wire. ( don't touch it, you will surely feel that ! )
This is causing your hum.
Keep it isolated and your hum will be gone !

Dick.

[saurus]

Thanks for the input, but the filter has no case, actually. I made it myself on a piece of veroboard with a few Y-grade caps across the mains, a couple of MOVs, and some X-grade ceramic caps from each main to earth. I have an identical one in a second amp that produces no buzz.

[Centauri]

Originally posted by saurus
The only other change I made was to route the input shield grounds straight to the star point instead of to the amp boards, but the wires are bundled with the amp grounds, so there is no loop.

The input earths are quite often isolated with resistors on the amp boards to prevent hum loops with associated equipment. I would suggest restoring the input shields to their orginal position will correct the problem.

Cheers

[saurus]

Thanks, I will definitely try this, but when I look at the amp board, the connection for the input ground is right on the big, fat ground trace that goes directly to the ground-out connection. As far as I can tell there are no resistors in series with the input ground.