In Reply to: Re: Hum when power is off (ground problem?) posted by Pär on September 26, 2001 at 05:15:46:
Well - Both are grounded to mains (ie with the green wire). I did try lifting the ground on one at a time to see what would happen and it did not help.
Yes, I am using unsheilded ICs, so I will try to move them around to see what happens.
A suggestion. If you pull the IC's and no more hum from the amp and speakers... check all the ground points (especially the input and filter cap star grounds) of your DIY unit.
Kinda sounds like a solder point has gone bad or a tube in the DIY or both.
I pulled the IC from the TT and the hum remained. I measured .08VAC on the output of the phono stage (with it turned OFF). I then removed it from the preamp, thus leaving it standing all alone (still off) and measured the same .08VAC.
I then unplugged the phono stage from the mains and the AC died...
Removing the earth ground caused alot of popping noises through the speakers...so I put that back.
I tried putting a small cap between the grounds of the in/out jacks and the rest of the ground system, but that did not help either.
Off hand, it sounds like you have a ground loop between the AC cord's ground and the chassis's ground.
Where did you place the green ground wire from the AC line? Is it grounded to another ground source, from the components? If so, try removing it from the its present location and ground it to the chassis in an isolated spot (either by bolt or solder), far removed from any input and star ground of the power supply and filters.
Any help? Are the ground points of each input jack and any control pots grounded to one point? Sometimes running a strap from each input ground to each pot, then to one single ground point can help eliminate loops. Keep this ground point as far from the AC green ground wire point and the output ground wires as possible.
Check the ground points of the filter caps and the transformer. They should be grounded to the same place on the chassis or very close to each other. The AC line's green wire should be grounded away from this point.
Did you use any shielded wires in your system? If so, how are they grounded to the chassis? If you ground both ends, believe it or not, you can induce a potential and noise. In addition shielding can act as an antenna and pick up EMF.
Try disconnecting one end of the sheilded wire. Try grounding the other end to a star ground with all the other shielded wires (one end, only) to the power supply ground. Or remove the shielded wire all together, in one jumper and see if the noise decreases.
Are the wires to the heater filaments (from transformer to each tube's heater) wound tightly in a helix around each other? This helps to cancel out AC hum from these leads.
Or is the heater filament power supply rectified VDC? This can be a source of noise---if the filtering within its rectified circuit is not good or has a poor grounding point.
Are the transformers aligned 90-degrees from each other? And any choke attached to the chassis?
Finally, how are the filter caps? Did you form them, when the unit was brand new? Do they appear to be okay. No visual signs of distress?
Check any coupling transformers to match the impedances. Check are wiring to-&-from any transformer and choke for good continuity.
Good luck. Hum, sometimes a stubborn companion of tube equipment.
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