Dreaded ground-loop BUZZZZ with refurbished Adcom and PS Audio 5.5 Pre Amp and DAC

How To
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Here’s a challenging one!

Been reading thru most of Paul’s Ground Loop Hum articles and these entries.
I have the dreaded ground loop in my system that I initially noticed with an old PS Audio 5.5 Pre Amp with the external M-500 power supply put back into service… but it was not the only culprit.
System is as follows:

Adcom GFA-555 fully serviced and tested by master tech just recently with Hoppes Brian input board
PS Audio 5.5 Preamp with M-500 external supply
Bel Canto DAC 2.7
Legacy Studio HD bookshelf speakers

I went thru Paul’s check list, religiously and methodically.

Adcom alone with speakers = no hum
Adcom with DAC ONLY (no inputs!) = hum/buzz (most noticeable from tweeters)
Adcom with DAC (only input is PS Audio) and PS Audio Preamp (no inputs) = A LOT of hum/buzz
Adcom with PS Audio Preamp ONLY (no inputs) = hum/buzz

I tried with the DAC attached to the Adcom, but unplugged the power to the DAC, and there is ‘less hum/buzz’.

Went thru and turned off every breaker in the house except for the wall outlets to the stereo: same buzz. The only thing I question is the GFCI outlet that is last in the line of the stereo room wall (nothing is plugged into it, it’s by the front door).

Last thing I tried was a ground-cheater plug on the DAC, or the Pre-Amp, and the hum/buzz is minuscule at best: there, but it’s very minimal. SSSSssSSSsSssossOOooooooo…

This tells me there is a ground loop that feeds back thru the RCA input to the amp and output RCA of the Bel Canto/PS Audio.
Meaning, there is a voltage disparity on the ground potentials of the two units and their chassis via the wall outlet ground. This gets into the ground plane of the PC boards and their corresponding unbalanced in/out, and is amplified, then ‘loops’ thru the RCAs.

I don’t think there is something ‘wrong’ with either the Bel Canto DAC nor the PS Audio pre-amp, in that they themselves are creating the ground loop internally thru improper circuit design and grounding…I think?
In my head, I think it’s an issue with a grounding disparity of the house panel itself: that there is a differential in the grounds from the get-go, which carries forward and gets worse. Am I correct in that?! Even a little bit?
Thanks for reading my TED~talk!

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Can you get an isolation transformer to test the ground loop path (open/close it)?
Can you plug the device into the preamp? Don’t remember if it has a spare socket.
If the above isn’t do-able, use all the exact same socket at the wall (temporarily use an expansion plug).

Yes, the RCA use the device chassis ground as the reference, usually! Take a VOM and test that the RCA and the “green” ground are the same electrical connection, or should be. If they aren’t the circuit board has a floating reference for the ground, this is really rare.

Take a VOM and test continuity from the GREEN GROUND on the chassis, not neutral, to the chassis of the devices, both of them. Calibrate the VOM and see if the resistance is zero. The meter should “peg” to the zero calibration point. If one tests higher than zero, that’s going to make a ground loop.

If the ADCOM by itself to the wall is OK, it seems that maybe the preamp has a lifted ground when it is inserted.

You’re on the right track using the same circuit at the wall. Also, what’s plugged into that circuit? I had an Ethernet over power-line box that feed noise into my phono head amp. Took a bit to track it down. It may not be a classic ground loop but line noise injected by something in the house. I ended up going wireless Ethernet to solve the problem in my case.

Best,
Galen

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Hi Galen, and all!

WELL, I FIGURED IT OUT!!! …well…one part of it. :smiley:
SO, the refurbished Adcom GFA555 had a new input rca’s installed, and they were touching the chassis, creating a ground loop!!!
Had an isolation board made, and everything properly chassis and signal grounded, and the amp is stone cold silent!!!..however…

There is still a ‘buzz’, and it’s from the PS Audio Preamp 5.5!!
I’ll make a separate post on it, but here is what I have discovered.
The PS Audio 5.5 has the M500 external Power Supply.
When it’s plugged into the outlet, it makes a slight ‘hum’ buzz audibly, as if from the HUGE transformer itself.
When hooked to the amp, you can hear a ‘buzz’ in the speakers…disconnect the preamp…no buzz.

I took the cover off the M500 Power Supply, and found what looks like a broken/fractured (not burnt I don’t think) resistor. The capacitors ‘look’ fine, as does the bridge rectifier. I was wondering if the capacitors had given up the ghost and they were no longer smoothing things, but now I wonder if it’s buzzing from the broken resistor and maybe also worn capacitors?

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This is an excellent time to consider hooking the DAC straight into the amp. That will allow time for you to consider, am I that guy who would buy a new used Preamp, just for giggles? Or have I earned the right to a new used preamp?

Well, which one are you? :ring_buoy:

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Yikes on the parts! Those being bad sure won’t help. The resistor is so damaged I can’t tell the value, what it is or what band convention. It looks like 4, can’t be sure out of 3, 4, 5 or 6 band. The silver at the end of the color bnads has a GAP and the next color means 5% tolerance and the VERY end is a temperature tolerance if it is a six band part.

Something seems amiss for this to happen, though. If the resistor (looks to be one watt) was the proper wattage, they are near industructible. The SIZE is the give-away on the wattage. I’d go to the source on this, P.S. Audio, and see what they suggest. One filter cap cap, one resistor and one rectifier doesn’t make much of a power supply. This is some sort of pre-filter and probably not a DC power supply output stage.

On the ADCOM, the CHASSIS seems to be a separate ground (earth/green wire) and not signal ground. Normally, the ground on an RCA is also to the chassis but as you found out, not always. This is unusual, though. Check all your stuff, look for continuity to the EARTH prong on the IEC and the RCA ground. If there is no continuity the RCA is floating different from earth. And yes, that will make a current flow and a buzz.

Galen

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6

Hey, you’re definitely on the right track.

Try using an isolation transformer to see if you can break (or confirm) the ground loop—open and close the path to test it. Also, if your preamp has a spare socket, plug the device into it. If not, you can temporarily plug everything into the same wall outlet using a power strip, just to keep everything on the same ground reference.

RCA connections usually tie to the chassis ground, so grab a VOM and check continuity between the RCA ground and the green ground on the chassis. They should be the same—if not, it might mean the board is using a floating ground, which is rare but possible.

Also check continuity from the green chassis ground (not neutral!) to the metal chassis of each device. Calibrate your meter first—it should read zero ohms. If one shows any real resistance, that could definitely point to a ground loop.

If the ADCOM is quiet when plugged into the wall on its own, but noise shows up when you add the preamp, that could suggest the preamp has a lifted or isolated ground.

Using the same wall circuit is smart. Also, think about what else is plugged into that same line—I once spent way too long chasing hum caused by a powerline Ethernet adapter feeding noise into my phono stage. Switched to wireless and the issue vanished. Sometimes it’s not a ground loop at all—just noisy power somewhere on the line.

Hope that helps!

—Galen