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Old 15th December 2010, 10:25 PM   #21
vlucian is offline vlucian  Romania
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Once again, it is the music coming from CD what I still hear in that channel with the volume all the way down ...
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Old 16th December 2010, 11:05 PM   #22
vlucian is offline vlucian  Romania
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Any idea, please? Thank you.
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Old 17th December 2010, 12:16 PM   #23
DF96 is offline DF96  England
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It has to be the grounding of the pot, or the grounding of the next stage. Somehow they are not being referred to the same signal potential.
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Old 17th December 2010, 01:12 PM   #24
vlucian is offline vlucian  Romania
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Ok, if it's related to the pot then why only one channel? (Grounding of the pot, you mean connecting the metal housing of it to the star ground? I already did that.)
Grounding is same for both channels. For better understanding I attached a simple sketch. I didn't draw details about pot and selector wiring.
thanks
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File Type: jpg star groundig1.jpg (47.4 KB, 51 views)
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Old 17th December 2010, 02:01 PM   #25
DF96 is offline DF96  England
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No, not grounding the pot housing (although that can help) but grounding the ground terminal of the pot. I don't understand your diagram. You seem to ground the volume pot via the output?

The critical thing is that the input cable screen, the pot ground terminal, and the ground of the next stage (presumably the cathode circuit) are all grounded to the same point. This can be a star ground - each of these three has its own lead to the star.

Alternatively, the input screen is connected to the pot ground, then the pot ground is connected (perhaps via screen of wiper output cable) to the next stage ground - a form of busbar ground. In effect, the pot is electrically part of the input cable.

In both cases the input socket is isolated from the chassis, as otherwise you get a loop.
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Old 20th December 2010, 11:02 AM   #26
vlucian is offline vlucian  Romania
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I agree my diagram is not very friendly.
RCA connectors are on a plastic panel but aren't they connected through cables shields wich are both connected to the star-ground of the source (in this case a cd-player) as in attachement #4? I'm asking this because I have noise only when I connect both RCA cables to the amp.
I tried both your variants none with succes. I tried to make some diagrams for those 2 cases.

I tried to understand post #33 in Help! ground hum and that's diagram #3. I'm not sure it's applicable.
Attached Images
File Type: jpg grounding1.JPG (32.5 KB, 45 views)
File Type: jpg grounding2.JPG (32.5 KB, 44 views)
File Type: jpg grounding3.JPG (22.5 KB, 38 views)
File Type: jpg grounding4.JPG (36.8 KB, 38 views)
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Old 20th December 2010, 11:15 AM   #27
roline is offline roline  United States
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Put 1u poly caps in series of the ground of the RCA jacks to have a AC ground but DC open to see if music goes through without the hum. That would confirm a ground loop in that portion of the circuit. I've used both shielded twisted pair and just twisted pair for the input hookup. For twisted pair, I use the ground from the PCB at the amplifier input , following it through the pot, switched input to the MP3 or RCA jacks. When the twisted pair has a ground shield, I tie the shield to the star ground of the entire amp.
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Old 20th December 2010, 11:48 AM   #28
DF96 is offline DF96  England
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The general rule is that from any grounded point in the entire system there should be one and only one path back to the reference ground point. In addition, none of the signal ground connections should have noise injected in - so signal and power supply ground currents must be kept apart.

Your first diagram shows loops within each channel, as the cable screen is grounded to the star point both at the input socket and at the valve stage. I would disconnect the lead from the input socket to the star point. (Diagram #2 has two loops per channel!)

Then try disconnecting the screen of the cable to the volume pot from the input socket on one channel only. The pot and internal wiring will be grounded at the valve stage. The external cable screen will be grounded at the CD player. These two grounds are connected via the screen of the other channel.
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Old 22nd December 2010, 08:04 AM   #29
vlucian is offline vlucian  Romania
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Thanks df96, I think I solved the problem ... It was similar to your last solution. I disconnected one screen from the coresponding RCA ground. I think the diagram will explain it better than me. Now I only have to connect everything via imput selector (I have 2 pairs of RCAs). Thanks again to all.
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