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I've been going through an amp to be my backup, and am stalled at an odd hum issue. Turn the amp on and it has a fairly loud hum. Touch the front panel, or any of the amp chassis, and it goes away. Run the amp ungrounded, and much (but not all) of the hum goes away.
I thought it might be a bad internal ground. Sure enough, the ground lead was secured at one of the inside foot screws. Of course, the chassis is enameled, so the ground lead was against enamel. Cleared the enamel, used a star washer, and re-secured the ground lead. No improvement - if anything, it is a bit worse. Except if you touch the chassis, in which case it still goes away.
Does anyone have any suggestions, or have encountered and solved something like this. I am assuming that it is something really simple...
regards -- Roger
Follow Ups:
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.
What type amplifier is it?
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Buy Chinese. Bury freedom.
It is a Counterpoint sa-12 hybrid amp with greenstreet upgrades.
The neutral to ground lug on the chassis was also enamelled, so cleaned that off. No improvement in the hum. I then ran a jumper wire between the the neutral bus of a large capacitor pair directly to the ground lug - hum gone.
I am thinking that with an enamelled coated chassis like this the conductivity is pretty bad. With the neutral to ground connection on the back of the chassis, and the ground lug at the front of the chassis, there is just not a good electrical connection. I'm going to get a ring terminal and wire, and run the neutral to ground lead directly to the ground lug. Right now it runs from the middle of the chassis to the back, and instead this will run from the middle of the chassis directly to ground. Hopefully that will put paid to the problem
regards - Roger
I'm glad you fixed it but what is a "neutral to ground lug"?
The neutral wire (white wire) connects to one end of the primary of the power transformer and does not connect to earth ground (the green wire) or anything else within the chassis.
Do you mean you ran a wire between "signal ground" and "chassis ground"?
The schematic show such a wire to already exist.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I cant resist reviving this thread years and years later as got the opportunity to fix an SA12 and I feel the urge to share my findings.The schematic posted there points to a 3.48k R7 which is the cathode R of the 2nd gain stage. It is incorrect. The value of the actual resistor is 1k which makes the stock Vp of gain stage 2 to be about 90V only. The parallel follower (tube #2 labeled gate driver) will be undercurrent and sound brittle and raspy. The original design boosts the output stage bias to the max to create a warmer sound.
I have made some changes as I repaired the unit (bulging caps everywhere)
1.) change R8 to 50k to increase current and push Vp to about 115
2.) that way the Vk at tube 2 increases.
3.) reduce NFB resisor R10 to 18k (36k x2) and parallel it with a 24pF cap to suppress ultrasonics.4.) reduce C2 coupling from 0,47 to 0.1 to avoid any motorboating
5.) reduce R4 to 91k, section off the B+ from R4 to R8 using a 560 Ohm and bypass again at the top of R4 using a 30uF high quality film cap.
6.) most important of all REDUCE the bias of the output tage from the measured 480mA to 200mA (the old biasing was just ASKING FOR TROUBLE IMO)
7) replace R52 with choke so that even with the increased draw of the tube stages ripple is minimized.I cant comment on the SA100 / SA220s cos they both use an SRPP follower vs paralleled cathode follower and I am not a fan of SRPP or whatver white cathode follower. The SA12 tube stage seem to point to a much more natural sound. Voice it warmer and there is no need to run the mosfets to the ground. Obviously they will still blow when they see too low a speaker load but thats another problem.
The Greenstreet upgrade hum can be mitigated by connecting a groundpoint on thr board or the middle link of the 2 output stage main filter caps groud to the chassis connected with a 1uF cap.
Edits: 08/23/20 08/27/20
"The neutral to ground lug on the chassis was also enamelled, so cleaned that off. No improvement in the hum. I then ran a jumper wire between the the neutral bus of a large capacitor pair directly to the ground lug - hum gone."
I strongly suggest that you use a multimeter or other tester (or call an electrician) to be sure the Neutral and Ground wire at your AC outlet are correctly wired.
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Buy Chinese. Bury freedom.
I don't know what those upgrades are. Is this a new problem in this preamp, or did you buy it like this?
"Sure enough, the ground lead was secured at one of the inside foot screws."
Do you mean the green wire from the line cord? If that's connected to the chassis, touching the chassis should have no effect. This might be a problem with your house wiring.
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Buy Chinese. Bury freedom.
Obviously it is a grounding issue related to the "upgrade". The chassis either has a bad ground to signal ground or too many grounds. Try taking a wire and touching it to the chassis and the other end to another component. What you may find is that the RCA grounds have been messed with.
The dude who designed the component was pretty bright, probably a lot brighter that Mr. Greenstreet, or whatever.
Could be the unit is picking up AC hum from house wiring. Outlet grounded properly? Polarity of house wiring (Hot, Neutral) consistent?
Are the input wiring of the amp shielded? Grounded at input side, only?
Check all grounding points within the unit. Tight and clean. With least amount of potential between grounding points. Near zero resistance between points of grounding.
Good luck!
Don't forget to check the grounds on the RCA jacks.
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