Odd M5 hum problem

[mhuss]

I have a medium large pedal board with a few 'dirt' pedals, and which had a Line6 M5 for the rarely used effects such as rotating speaker. All the pedals use an isolated output 9v supply, and hum and noise is at very reasonable levels, all things considered. However, when I put the M5 in line (using its own independent power supply), I get an awful(!) hum from the gain pedals when they are engaged. If I disconnect the M5 power, the hum goes away (M5 still inline with the other pedals). I tried two different supplies for the M5, with no apparent difference in noise level, and the hum is there even with the M5 in idle/bypass mode - it just has to be powered up. I'm scratching my head wondering what might cause this. Any ideas?

[Rixen]

could it be this issue ?

Line6 M5: grounding issue? | Page 2 | TalkBass.com

cheers,
Julian
Rixen Pedals

[mhuss]

As part of my investigations/experiments I had removed the paint from around one of the screws, It made no difference. Fortunately I never had the boot up problem they're describing, but unfortunately the expression pedal input just stops working sometimes, so my M5 is now retired.

[MarkusBass]

I get an awful(!) hum from the gain pedals when they are engaged. If I disconnect the M5 power, the hum goes away ....... Any ideas?

What do you mean by engaging gain pedals? Do they have power supply, or they are just passive pots? With separate power supplies the problem can be easily explained.

Mark

[mhuss]

"Gain pedals" = overdrive/distortion/clean boost. "Activate" = enable by stepping on the switch. :-)
All the pedals are on a shared but fully isolated power supply (i.e., no ground continuity between outputs) except the M5, which has it's own supply. Connecting it to the shared supply made no difference. The only shared ground connection between pedals is that of the interconnecting cables.

[MarkusBass]

Is there a chance that the gain pedals in "non-activated" state are fully bypassed? Is it possible that one of the gain pedals should be supplied with negative voltage (+V on the ground)?

[Mark Hammer]

I loaned my own M5 out to a friend, and when it came back, the unit was malfunctioning and would not boot fully. I wasn't sure whether it was the pedal or the PS. The PS lit up, and would power regular battery-operated pedals, albeit with a whine that wasn't there before, so it sort of worked. I bought myself a new PS (non Line 6) and the problem was resolved. Everything works as it should now.

All of which is to say that some switching supplies like the one that comes with the M5 can be fragile. Try a different PS for it and see if the issue is resolved.

[mhuss]

All but the clean boost are fully bypassed at the foot switch. The clean boost is the last pedal in the chain, and is also used as a buffer/driver for the wire back to the amp. These are both newer pedals (opamp/diode distortion and a Klon clone), so there are no no funky positive ground situations, although the latter does have an internal B+ booster. The overall chain looks like this:
Compressor->Tuner->M5->OD->Dist->Fuzz->Echo->Clean Boost
(The fuzz is a BigMuffRussian clone and it just buzzes anyway. Not part of the discussion. :-))

Part of the problem is certainly all the plug/jack connection grounds, and I've connected a physical wire between two pedal boxes that experimentally reduced the general buzz the most. If it were convenient, it would be theoretically better to have interconnecting wires with shield connected at output end only, and use a central ground for all the pedals. However, the digital pedals inject their own unpleasant noise on shared power, so that's not necessarily ideal either.

However, the M5 is an order of magnitude worse then the rest (much worse than the fuzz!), and buzzes whenever powered, even disabled. I tried multiple 3rd party supplies, no difference.

[MarkusBass]

In such a case test M5 with Clean Boost only. Do you have the same problem in this case?