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In Reply to: re: still trying to track down a hum... posted by mr9iron on September 15, 2012 at 01:41:53
Are the input RCAs grounded to the same point?
Are the internal coax cables only grounded at one end?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Tre,
yes, the RCA's are grounded to a single point on the chassis, I have tried to ground them to separate points, but no change. I have also disassembled the plugs to see if they where touching anything, but they are secure in a phenolic plate.
The co-ax appears only grounded off the RCA's at that end, the other end hooks up to the 12by's via some resistors (sorry I am not a supper tech expert to explain exactly that circuit path but that's the simple basic description of where that coax goes) I have double checked those resistors to make sure they are right(all of which are new in the amp) and they come out perfect...
are you thinking it needs an additional ground?
thanks for your response Tre!
J
Chris, Tre,
bummer, I tried the set up exactly as you guys described with new high quality coax cable, still same exact result, no change.. I ran a solid 12 gauge wire connecting the grounds of the RCA plugs and then from the center of that wire to the signal ground, no change, still hums. I also tried grounding them separately, and not at all, still all the same result..
could the loop be coming from somewhere on the board? It seems like I have exhausted everything on the RCA end...
thanks for your suggestions, much appreciated..
Jeff
"I also tried grounding them separately, and not at all, still all the same result."
The "not at all" indicates that the RCA connectors are grounded somehow. They must be touching the chassis.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre,
yes, I was expecting the same result... but with both grounds off the RCA's, the amp is the same.. the real bad hum which you mentioned, will only occur if I disconnect the other end of the coax... and I have to disconnect it on both coax cables for it to do the big hum.. I know that sounds outrageous, but I kid you not, this is what is happening. I tried grounding the other end of the coax to other points on both sides, but no difference..
It sounds insane, I know, but that end of the coax is suspicious that it behaves like this. The path to the 12by is pretty simple and I can't see any other place it can ground.. I do think there is something up with the other end and not the end of the RCA's
It was suggested that I check DC voltage from all the grounds to chassis and the RCA grounds and the filter supply grounds all drop to "0" DC volts.. when I checked last night.
again I seriously thank you for your replies on my thread, I am losing all hope on this lately..
J
Take a pair of RCA interconnect cables and cut one end off of each.Solder the wire end to the circuit at the 12by7. Positive to the grid stop resistor (10K), top of the 1meg resistor, and ground (shield) to the same ground point of the grid resistor (1meg).
Do both channels. Disconnect the in-chassis input cable.
Connect the other ends to your preamp. Make sure nothing is grounded. If any of your equipment has 3 prong plugs, use a cheater adapter so nothing is earth grounded.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/19/12 09/19/12 09/25/12
That resistor on the 12by7 is called a grid stop resistor. It's fine and not your problem.
I have had this type of hum problem before. I used solid 12ga wire from one RCA jack to the other and then grounded the center of the 12ga wire with another length of 12ga wire.
BTW It's good that the coax shield is only grounded at one end.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre,
I have some really good quality coax that is shielded, I can try it out, and I do have some 12 gauge wire I can make new ground wires with..I didn't think the existing coax was a problem since when I hooked up one side it didn't hum, but it's not too bad of a job to change it...
I will follow up and let you know if it works...
thanks...
Jeff
The coax is not the problem. You have a ground loop.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Your left and right RCA input jack grounds should be tied together right at the jack. Then then that single ground should be tied to the signal ground of the circuit like it was prior to the modifications. When you use your meter you should measure less than 1 ohm resistance to ground. The hum is limited to the input section, not the power supply or anything a cheater plug will help you with because it stops when you short the inputs.
I will bet that if you disconnect the ground from the RCA jacks, one might still be grounded through the chassis. You might also have a bad solder joint on the RCA jack ground. You can't create a loop if there is only one path, somehow you have more than one. Keep looking, you will find it.
Chris,
thanks for the info, the stock set up had each ground of the RCA meeting simultaneously at a solder tab on the chassis. I am guessing this is the intended "signal ground" Both you and Tre suggest tying the RCA ground together first and then go to the signal ground. I thought this is what occurred on the old set up, but I guess it's slightly different. I'll give it a shot.
thanks so much for your reply, I'll keep digging, appreciated..
Jeff,
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