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Old 26th August 2009, 01:45 PM   #1
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Default Intercomponent ground loop hum

Just finished my open baffle full range speakers with plate amp for each speaker. Also just finished my simple se tubelab amp.
I'm using a cj pv2aR pre amp. The crossover for the full range is a series cap mounted in the cj pre amp.

No hum when plugging in the cd direct to the amp. ( plate amplifiers are not in play in this situation)
No hum with cj in a different stereo
No hum with speakers when used on a cheap entertainment center

The hum I get happens only when the cj unit is connected to the simple se amp. If I turn on the amp, but havent turned on the cj the hum starts.
Even with no cd etc plugged into the cj

All components have a three prong electrical cord except cj
Tried reversing that- didn't change the hum
I tried reversing the plug on the cj, didn't help.
Do I have to get a ground loop isolator? do they work?
The hum is quite noticeable but not loud enough to hear if the music is playing
Thanks, Paul
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Old 26th August 2009, 01:47 PM   #2
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forgot to add that the simple se is connected to the full range and the plate amps are connected to a 15 in H frame woofer. The hum seems to come from the full range driver.

Thanks again
Paul
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Old 27th August 2009, 12:34 AM   #3
Sheldon is offline Sheldon  United States
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You might first want to make sure that it's not inductive coupling. Have you tried physically moving your pre around, relative to the tube amp, while everything is powered up? If you can modulate the hum that way, then it suggests that you are getting inductive coupling. It can be solved by experimenting with relative positioning.

Sheldon
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Old 27th August 2009, 01:17 PM   #4
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Movement and distance don't matter. I unplugged the plate amps, and also disconnected their interconnect didn't help.
The hum starts a couple seconds after I turn it on (well before it's warmed up).
Thanks
Paul
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Old 27th August 2009, 10:44 PM   #5
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I had the exact same problem with my valve pre amp.
In teh end it was simply the input floating when nothing else was connected, it was picking up airborne mains hum becasue of the 1 meg input impedance.

I fixed it by shorting out the jack socket when nothing is connected to it.
You can buy jack sockets with an extra pair of terminals that connect when the jack plug is removed.
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Old 28th August 2009, 04:45 AM   #6
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Quote:
Originally Posted by pforeman View Post

The hum I get happens only when the cj unit is connected to the simple se amp. If I turn on the amp, but havent turned on the cj the hum starts.
Even with no cd etc plugged into the cj

All components have a three prong electrical cord except cj
Hi Paul,

I suspect that you may not have a problem with a loop because the cj does not have a three-prong power cord.
Can you give us more information on how the grounding is implemented in the amplifier and the cj?
Is there a problem with only an interconnect cable (without the cj) connected to the amp?

Dave
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Old 29th August 2009, 01:00 PM   #7
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I made a set of speakers. Plate amps (one on each side) for the Open baffle H frame woofer(s). Connected by an interconnect to the cj pre-amp.
My diy simple se (tubelab) connected by interconnet to the cj pre by interconnect going to the full range speakers (10 inch Audio nirvana on open baffles.
When I connect a cd player directly to the tubelab amp (not driving the plate amps or the woofers) no hum, dead silent.
When I connect the tubelab amp to the preamp and turn them on, the hum starts immediately, even though the pre amp is muted for about a minute.
When the preamp comes on the hum gets louder.
I've tried connecting a ground wire from pre amp to plate amp, or to ground slot in the extension strip doesn't affect it.
I've disconnected one pre - amp interconnect:hum worse in remaining channel.
The hum is not very loud it dissapears into the background with even quiet music, and seems to be accompanied by some "pink noise ".
I don't know if it is ground loop ? hum because it certainly sounds higher in freq. than 60 Hz.
I've tried finding 60 Hz hum clips on the internet and can't find one.
Maybe the supersensitive Audio Nirvana's are amplifying the tube noise of the cj?
Maybe I need to order new tubes?
My other stereo uses ~83 dB efficient Acoustat 2+2 speakers, so maybe thats why the cj unit is quiet with those?

Were you alluding to "shorting plugs"?
Thanks,

Paul
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Old 29th August 2009, 02:35 PM   #8
Sheldon is offline Sheldon  United States
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It sounds like the problem is in the pre-amp. I can't tell from your description, if hum is coming from both the full range and woofers. If the problem is in the pre-amp, I'd expect to hear it in both. Can you confirm this?

You indicate that the hum may be higher in frequency than 60Hz. Here is an online frequency generator. You'll need headphones, unless you have computer speakers capable of lower frequencies, or you can plug the computer into your amp: http://www.rhintek.com/tutorial/Frequency/index.php If you search under online frequency generator, you will also find a bunch of downloadable freeware.

I wonder if your preamp power supply is dumping excess ripple current into the power return. You might have a bad power supply cap. I don't know if this unit has electrolytic caps, but if it does, I'd consider replacing them.

BTW, I consider any hum that you can hear more than a few inches from the speaker to be unacceptable - tube amp or no, high efficiency speakers or no. The one exception you might have to live with, would be AC heated DHT amps. Even then it should not be audible at the listening position. As you note, with the amp alone there is no hum.

Sheldon

Last edited by Sheldon; 29th August 2009 at 02:42 PM. Reason: addition
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Old 2nd November 2009, 01:06 PM   #9
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Boy, was I wrong, or has the problem changed?!
I think when I was driving the amp without the preamp I didn't have the subs going.
I've known now for several weeks that the problem is the plate amplifiers driving the H frame woofers.
If everything is off ( plate amps on standby ) there is a little bit of hum.
As soon as I turn on the tubelab amp the hum is significantly louder. Play quiet music and you can't hear it.
Disconnect or turn off the plate amps and there is no hum at all.
Everything is plugged into one power strip.
Still get hum from one plate amp with one plate amp unplugged ( I have two, one for each channel)
Due to the way I have the crossover wired into the pre amp, I can't use the high level inputs on the plate amps.
I've read many internet sites on ground loop problems. Many are descriptive of the problem, some advise against doing certain things, but I haven't read any instructions on what to actually do about it?
Should I make an interconnect out of radioshack phono plugs and cat 5 cable without a shield? Or cut the sheild connection on one end of my cables? Those are the only two solutions I've read so far, along with reasons why this isn't a real good idea.
Thanks in advance for your help.
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Old 2nd November 2009, 01:40 PM   #10
Sheldon is offline Sheldon  United States
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The ideal situation is to have the preamp as the "master" ground, if you will. In other words, all chassis grounds tied to the preamp, and all signal grounds tied to the preamp, with isolation between the two (signal and chassis). For a test, use a two prong adaptor on the plate amps. This will ground the chassis via the interconnect. This shouldn't be done for long term use, as your safety earth then depends on the interconnect. But it will give you some information to work with.

If that eliminates the hum, then I'd permanently lift the chassis ground from the signal ground in the plate amp. You can do that by using a couple of 20A diodes in anti-parallel between signal and chassis (a bridge can be used here too). In parallel with those, use a film or ceramic cap on the order of 0.1uf or so. The cap is to bypass RF. This way, low level, low frequency is lifted from the chassis, while HF has a low impedance path to earth.

Sheldon
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