Troubleshooting ground loop/hum in dual loop switcher - help!

[mdcmdcmdc]

Hi folks,

I put together an a/b + on/off dual loop switcher box a few days ago that's giving me a real headache when it comes to ground hum.
I don't seem to have much/any issue using it at my apartment, but when it's plugged into the rest of my pedalboard and my amp at my practice space, there's a fairly loud hum coming out of it.

Here's the way I wired it:



The red leads go to the LEDs.

Should I only be grounding one side of the loop?
Should I swap the 4PDT for a 3PDT and not tie the loop RTNs to ground on the switch?
Would adding a buffer somewhere help?

Any advice here would be greatly appreciated.

[ElectricDruid]

You've got two grounds, one "signal ground" and one "power ground". That's fair enough, since the power is only used to turn the indicator LEDs on and off.

But the first switch grounds the input to the *power ground* not the signal ground. The second switch does the same thing - uses power ground in place of signal ground. If you're going to keep the two grounds separate, you need to be consistent!

[antonis]

Your diagram doesn't indicate any grounded sleeve..

[mdcmdcmdc]

Ostensibly, the sleeve is grounded to the enclosure (which also connects it to the switch/LED ground I suppose?).

Better? Worse? No different?

[ElectricDruid]

Now it looks to me like it should work, since you've tied both grounds together.

The reason for *not* doing this is that the LED currents will now flow in the same wires your audio uses, which makes ticks when you switch it much more likely. In many ways, I preferred it before, but you need to ground the signal inputs to the signal ground.

[mdcmdcmdc]

ty - I'll give it a go.

I wonder if this may just be a consequence of the mains supply coming into the building /florescent lights etc.

If that doesn't clear it up, would the next step be to use isolated jacks for the loop returns?  Something like this?




(EDIT: oops, swapped the snd and rtn jacks - fixed the image)

[ElectricDruid]

No. It might fix a loop to/from whatever the external gear is, but it does nothing to deal with separating the power and signal grounds or keeping LED currents out of the signal ground.

If the problem is the mains supply as you suspect, that would be a *massive incentive* to keep the audio switching part of this entirely separate from the powered part that does the indicator LEDs. The switching is passive and doesn't *need* power, so there's no need to mix it up in that.

[mdcmdcmdc]

It seems to be working now, thanks for the help!

In addition to adjusting the ground wiring I also went through it and shortened every connection as much as (reasonably) possible which might have helped as well.
It's a needlessly large enclosure at the moment as it's what I had laying around, so when I get a moment to grab a 125B I'll try and pack it in there.