A very persistent and hard to identify grounding problem

[mosc]

Here at my studio for that last several months I have had some strange problems. The symtoms were varied but included very low noise levels in almost all of my synths and audio component. Yestartday I accidentally discovered the cause which is confusing enough to mention in this post.

While working on my Eurorack modular system I discovered a mysterious grounding problem. I have two medium sized Eurorack racks. The smaller one on the right in the following photo has Expert Sleepers ES-3 and ES-6 modules I use for optical audio input and output to my RME UFX interface to my windows computer. There are no audio wires connecting these Eurorack modules and my audio sysem.



Not only that, all of my equipment is connected using an isolating AC power unit with battery backup and sine oscillator. With this setup I believed (see below) I would be immune from noise problems associated with ground loops.

Yesterday, I was trying out some presets in my Disting NT module. That's in the rack on the left. Two audio patch cords go from the left rack's audio output mixer to the Expert Sleepers ES-6 DC coupled audio to optical interface on my RME UFX. I noticed a "hum".

After a lot of fussing, I discovered that if I connected another audio patch cord between the two racks the noise went away completely. This cord has no audio path being used. Now all of my synths sound clean as the day they were born.

I hope this story is helpful. The explaination isn't obvious to me. Grounding is very mysterious. I learned two lessons that I already knew.

1) If I believe it, I know it's wrong!

2) Look for the problem where you don't think it is!

[Grumble]

I had a similar hum problem. I have to add that almost all my patchcords are made from a single wire.
The idea is that output impedances are low and voltage amplitudes are high.
So what I did was connecting all cases (eurorack, mixer, amplifier) together with a wire and my hum was gone.
Remember: also an isolated powersupply will have some leakage current what will result in hum.

[SynchronousArts]

Don't know if you still have this problem but I've found the information from this site to be useful for solving ground noise problems.

http://pin1problem.com/