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re: still trying to track down a hum...

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Posted on September 15, 2012 at 01:41:53
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
It's been a few months since this problem arose with my Citation V re-build, I tried many of the suggestions from that first post, but still unable to cure the problem...

I am getting a hum from both channels, if I short the inputs it goes away... if I have one input connected it's fine, if I plug the other channel in, it hums. I have tried several sources, it does this regardless of pre-amp or source.

If I plug different sources in each input, for instance I tried two identical CD players, one in each side, no hum, but if I plug into the same source, hum..

I can hook the amp up in mono and the hum doesn't go away, but is significantly, 70% - 80% reduced..

I want to say this is a signal problem, and some sort of ground loop issue, but I'll be damned if I can track it down.. there don't appear to be any grounds in the signal path other than he RCA grounds which I already tried changing, with no effect.

All the grounds for the power supply are good. I tried removing them all and then re-connecting them one at a time, there is no change or reduction in the hum.. so I don't think it's power or filter caps, all of which are new.. I have tried several different RCA cables, and different locations for power in other rooms, checked for florescent lights, changed to a 3 prong ground power cord, tried different grounds for the RCA jacks (RCA's are new) the hum is unchanged.. I have not changed the co-ax wire, but I really doubt that's the issue..

any ideas or new investigative paths?- any suggestions welcome

thanks!

 

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re: still trying to track down a hum..., posted on September 15, 2012 at 07:07:06
Tre'
Industry Professional

Posts: 9156
Location: So. Cal.
Joined: February 9, 2002
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"

 

re: still trying to track down a hum..., posted on September 15, 2012 at 10:11:35
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
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

 

Classic ground loop, posted on September 15, 2012 at 11:14:37
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.

 

re: still trying to track down a hum..., posted on September 15, 2012 at 17:28:03
Tre'
Industry Professional

Posts: 9156
Location: So. Cal.
Joined: February 9, 2002
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"

 

re: still trying to track down a hum..., posted on September 15, 2012 at 20:19:30
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
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

 

RE: Classic ground loop, posted on September 15, 2012 at 20:25:51
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
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,

 

re: still trying to track down a hum..., posted on September 16, 2012 at 02:05:33
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
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

 

re: still trying to track down a hum..., posted on September 16, 2012 at 07:37:29
Tre'
Industry Professional

Posts: 9156
Location: So. Cal.
Joined: February 9, 2002
The coax is not the problem. You have a ground loop.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

re: still trying to track down a hum..., posted on September 17, 2012 at 06:41:24
FenderLover
Bored Member

Posts: 8302
Joined: July 31, 2007
Contributor
  Since:
May 17, 2009

I feel for you. Hum is the bane of all fixer-uppers & DIY'er. Have you checked the resistance and grounding of the grid-to-ground resistors between the input and the input tube?

Specifically, the 1meg-ohm (gtg) and 10k-ohm grid resistor. I hope that I have the correct schemo. Use the best resistors for these points. It's worth the extra $$$, in my opinion. Esp for long, quiet, reliable life.

All ground points MUST have as close to zero DC resistance between wire and chassis. You may want to make a brass or copper raised bar to solder all input points (maybe even insulating the jacks from the chassis with teflon spacers). Then have a single grounding point, for the brass/copper buss. I'd put it as close to the main PS filter ground pointing as possible.

I'm sure Jim McShane has a ton of better ideas about this issue. May want to email him.

Good luck. Sorry, no magic bullet for this one.

 

re: still trying to track down a hum..., posted on September 17, 2012 at 10:45:20
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
Hey,

thanks for the reply, yes this is the same schematic I have. the grid resistors to ground measure perfect, they are "Dales" supplied by Jim of course, from his resistor kit.

I have not checked my grounds to chassis DC resistance, I will see what I get tonight, but I do have all the grounds (12 ga. copper) on one buss soldered to a bridge that is attached to the PS bracket... that bracket is screwed into the chassis with star washers. before I put those in, I scraped away any paint and made sure the screws and washers "bite" to metal. I tried last night going right to chassis, with the buss, but no change..

The RCA's themselves are sandwiched in between two phenolic plates and then those plates are screwed to the chassis. I took them out to be sure this was the case. I don' think there can be a way for those RCA's to be touching any metal.

There is one slight improvement I noticed last night, and that is, if I remove the RCA grounds from the chassis there is like a slight extra buzz that will go away, if I touch both RCA grounds to the chassis I get a tiny extra buzz, it's not much, maybe a 5% improvement with no RCA ground. This made me wonder if there is something being grounded to the chassis that is causing this loop.... something I am not seeing....

I will check the the DC resistance tonight on all the grounds and see what I get..

thanks for your reply....

Jeff

 

re: still trying to track down a hum..., posted on September 18, 2012 at 13:46:52
Tre'
Industry Professional

Posts: 9156
Location: So. Cal.
Joined: February 9, 2002
"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"

 

re: still trying to track down a hum..., posted on September 18, 2012 at 14:08:34
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
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

 

re: still trying to track down a hum..., posted on September 19, 2012 at 11:01:54
Tre'
Industry Professional

Posts: 9156
Location: So. Cal.
Joined: February 9, 2002

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"

 

re: still trying to track down a hum..., posted on September 19, 2012 at 12:10:52
DannyR
Audiophile

Posts: 338
Joined: January 21, 2001
I have the exact same problem. Please if you figure it out let me know. I feel your frustration.

 

re: still trying to track down a hum..., posted on September 21, 2012 at 03:43:27
Ron V
Audiophile

Posts: 12
Joined: December 26, 2001

Is it possible that there is some noise on your AC line? Had a similar problem earlier this year. The problem was coming out of my TV, power supply cap was leaking. Just my 2 cents worth.

 

re: still trying to track down a hum..., posted on September 21, 2012 at 14:18:17
mr9iron
Audiophile

Posts: 211
Joined: March 13, 2002
Ron,

thanks for replying, I have considered this, I have taken some steps to see if this was possible, albeit it, they might not be extensive enough to rule it out, but anything is possible..

I tried running through a variac, was told the variac would act like an inductor and filter any AC line noise out, isolate it... It still hummed, I also tried different circuits including the newest line (1 year old electric) in my house. I killed anything florescent as well, still the hum persisted.

So just to update the thread.. I have been able to reduce the hum by about 85%-90%. I have it set up with jumpers right now, and I am testing it. It's probably acceptable, but not totally solved. The rest could be AC line noise as you suggested or my pre-amp, which I am such isn't perfect.

The set up is almost illogical, but I have another V that does the same thing so I will try it on that unit to see if I get the same result, if it does then it will be hard to argue. The other inmate who respond to the thread and has the same issue, is also going to try this method and report back so we'll see if it has merit or if it's a fluke. If the other two are reduced, then I will post what I tried.. I am still, of course, grateful for any suggestions or theories that are posted..

thanks!

Jeff

 

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