Doepfer A-143-3 Quad LFO Signal Bleed

I can hear the four LFOs cycling through my amp when the module isn't patched. Unfortunately, it's pretty loud. This seems to be a common problem with LFOs in general when the rack is plugged into older, ungrounded electrical systems like mine. The signal noise disappears when I unplug the LFO from the PSU.

I assume the noise would go away if the outlet were grounded; and, eventually, I may ground that specific outlet. Until then, are there any practical ways I could at least dampen that unwanted signal? Thanks.

https://cdn.modulargrid.net/img/racks/modulargrid_853360.jpg

This is sort of...odd. How could you be hearing the LFOs unless they're running at audio frequency rates? If the "LFO sound" is actually more like a constant hum, then the problem probably isn't the LFOs at all and you're dealing with a ground loop. The big clue here is the phrase "older, ungrounded electrical systems"; grounding is ESSENTIAL for electronic instruments, processors, and the like, and if it's absent, the equipment will seek a "ground" by whatever means it finds. In this case, it sounds like it's found your A-143-3...and that's not good for that module if that's really what's going on here, because it's passing leaking AC line voltage back thru the module, the cab's distro and PS, and the like.

The practical way to disengage a ground loop is to add isolation to your modular's outputs...which is something EVERYONE should be doing for exactly these reasons, particularly if you play live and you're plugging into power systems you know nothing about. The Lifeforms Outs you have doesn't appear to have this; I would strongly suggest looking into an output module that has proper balanced TRS mono outs, not TS. Furthermore, if you make sure and find an output that's specifically transformer-isolated, this will not only give you the isolation you need, it also puts some "iron" into your output signal path that you can saturate a bit with a touch of overloading that'll give you a warmer signal provided you don't go overboard with the levels.

It isn't a constant hum unless you mean that it's constantly making noise. The sound changes accordingly when I select low, mid, high or when I turn the frequency range knob. All while the module isn't patched. Would a power conditioner like the Ebtech Hum X relieve the problem at all?

A Hum-X could help if both the modular and the amp have 3-wire AC connections. However, this sounds suspiciously like leakage across the busboard, TBH. First thing I'd do is to check the bus connection to the module on both ends and check for pin problems, debris in the ribbon connectors, that sort of thing. Then carefully reconnect the ribbon to the bus and module, feeling carefully to see if there's any resistance that doesn't feel quite right. If that doesn't work, then try replacing the ribbon cable itself, as there might be a broken wire in there that could be forcing leakage in some way.

Also, this is a good reason for investing in filtered busboards. Issues like this usually get sorted out by the filtering on the DC rails, which can be very useful if a certain module or two have a tendency to leak, as some older designs might. But yeah, this is a bit weirder and definitely more Eurorack-centric in terms of problems than a simple ground loop.

Thanks for the feedback. I'll look into some of these suggestions.