I have built / assembled a Eurorack case for synth modules. You don't need to understand Eurorack to understand the simple circuitry or the problem that comes up.

In the attached picture, I have a commercially-made power supply that delivers plus and minus 15v, and plus 5v. The power supply has isolated transformer terminals which are connected to the hot and neutral AC lines. I connected the AC ground wire to the power supply chassis, and it is also the ground for all circuitry, such as the DC power sources and the audio "common" or shield line. (The AC connections have a wood shield over them for safety, preventing fingers from touching "hot" wires.)

As an aside, in the diagram, I used red for +15v and +12v lines, green for -15v and -12v lines, yellow for +5v, and black for DC common and chassis ground..

I am using an NAD preamp and power amp to take a line input and deliver it to speakers. The NAD equipment is plugged into the same outlet as the power supply.

Eurorack components need plus and minus 12v (and some also need +5v), so I built a simple bipolar regulator on a board using a 7812 and a 7912 in the standard configuration, very similar to the application notes in a data sheet, with the addition of some diodes for protection. I tested it with the power supply, and they work fine together. The schematic is below.

I used a pre-built Eurorack power distribution board which makes the three DC voltages and ground available to all modules that plug into it. There are two dist boards, for expansion, and they are bridged to provide power to the second one.

I installed a basic synth module (based on Doepfer's DIY Synth board):
1. a shielded 1/8" stereo cable runs from the module to the RCA inputs of the amplifier, and
2. the ribbon cable for the module is plugged in there and in the power distribution bus.

It worked fine, and there is no unexpected noise. Because the synth output is a mono jack, only one channel of the amplifier plays the synth; the other channel is silent, having no signal.

Next I installed three commercial synth modules, all from the same company. Two of them work, albeit with a loud buzz, when connected in the same way detailed above. (I have not tested the third.)

I made no attempt to ground the audio common connections, which are jacks on the panels of the commercially made synth modules. I assume that the manufacturer has the audio ground connected to the DC ground, but I don't know for certain.

Eurorack ribbon connectors are often unkeyed, and it is essential that the red edge of the ribbon wire is connected to the -12v pin of the connector. I ensured that this was the case; it would not even turn on if it was not

The problem is the buzz, of course. I suspect there is a problem that can be solved by rearranging connections. I would prefer not to add components to isolate the ground, such as an optocoupler.

Can anyone suggest how I should debug or fix this problem?

 

I'm still studying your problem, see if I can understand it. But in the meantime, I suggest you replace the 1uF capacitors at the regulators' outputs with 1ouF each, and also add 0.1uF ceramic capacitors in parallel to those. That will take better care of possible transients.

 

The application notes for the regulators said to use 1uF tantalum, or if using electrolytics, then it should be 10uF. The board is tiny, and it would be tough to add more, though I could probably sneak a couple of 0.1uF caps in.

 

The application notes for the regulators said to use 1uF tantalum, or if using electrolytics, then it should be 10uF. The board is tiny, and it would be tough to add more, though I could probably sneak a couple of 0.1uF caps in.View attachment 101033

Adding the 0.1uF I suggested is quite possible if you use SMT types. They won't fix your problem, but they can't hurt either.
As for the ground problem, for starts I suggest you try to isolate it to one particular component (such as when you add it and then remove it from the system), and then see if interconnecting their chassis makes a difference.

 

I'd probably use a cheater plug to float the earth ground of the power supply to see if it's a ground loop in the earth ground. Is the noise low frequency or high?

The picture looks likes there's room for many more parts.

 

The buzz is a 60-hz line current frequency.

I don't know whether the "picture" you talked about is the diagram showing the power supply and bus, or the tiny circuit board, which really doesn't have much room.

I designed this framework for flexibly adding many more components in a modular fashion - both standard Eurorack synth modules, but also my own "standard" modules with various functions for designing and building a lot of projects. I made two plywood cabinets with metal strips that are tapped to allow attaching modules with screws as the physical foundation, and am designing a power and bus foundation to make interconnection easy and reliable. The goal is eventually to be able to breadboard for experimentation, but with base functions like power and audio amplifier built in to make it easy to test, and adding more base functions over time.

 

I disconnected the ground pin from the circuit. It WAS connected to the PSU chassis, which is also the common ground for my devices; disconnecting it means that the only connection to the house current is the primary winding of the power transformer, which is not connected to anything else.

The hum is still there, unchanged.

 

The solution was most unexpected.

I was using a 1/8" stereo plug to connect from a module to the amplifier. A guy on Muff Wiggler suggested the following:

"Some output sockets (eg. Cliff as used on many doepfer modules) require mono output cables as they actually have the 0V (/ground) connection on ring instead of sleeve."

I collect plug adapters for reasons like this. I had one that has a 1/8" stereo jack on one end and a mono plug on the other. I put it onto the end of the 1/8" stereo plug to dual RCA plugs, and NO MORE HUM/BUZZ!

Thanks to all who read or even thought about it.

 

Glad you could fix that pain in the backside... you weren't the only one who learned something new in this thread.

Good luck!