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

How To
1

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!

2

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

1 Like