Amp FX Loop Hum....

[duhvoodooman]

I don't understand this behavior, so thought I'd post it here for some of you guys who have more experience with amps with FX loops:

I built a little 6-jack switch box and placed it between my distortion pedals and my time-based effects (trem, chorus, flanger, delay & reverb). The switch is then plugged into my Egnater head's input and the FX loop send & return jack. It's wired so that it will switch between running:

dist. FX --> time-based FX --> amp input --> amp FX loop send --> amp FX loop return​

when off, and when switched on:

dist. FX --> amp input --> amp FX loop send --> time-based FX --> amp FX loop return​

This lets me turn the switch on when running with the Egnater head and get the benefit of running the time-based effects in the FX loop after the preamp, so that they don't pick up any additional preamp distortion. When I use one of my other amps with no FX loop, I turn the switch off and just jumper the FX loop send & return jacks on the switch. The switch works great. But....

When using it turned on with the Egnater head, I noticed a pretty audible hum. At first, I though maybe I had a grounding problem in the switch, but when I removed the switch and just ran things with cable runs directly into the FX loop, it still hums. Seems to be coming from the time-based effects chain, since I can get rid of most (but not all) of the hum by turning them all off. I tried a couple of different power supplies and no change. The weird thing is that if I unplug the time-based effects from the amp's FX loop and put them back between the distortion effects output and amp input, I get NO audible hum--quiet as a church mouse.

Any of you guys have an idea what would be causing this behavior?

 

[tunghaichuan]

DVM,

Sounds like a ground loop problem. Try grounding only the input jack on the box and making sure that the output jack is isolated from the box. You will have to connect the ground lug of the isolated jack to the input ground.

See if this fixes your problem and report back.

tung

I don't understand this behavior, so thought I'd post it here for some of you guys who have more experience with amps with FX loops:

I built a little 6-jack switch box and placed it between my distortion pedals and my time-based effects (trem, chorus, flanger, delay & reverb). The switch is then plugged into my Egnater head's input and the FX loop send & return jack. It's wired so that it will switch between running:

dist. FX --> time-based FX --> amp input --> amp FX loop send --> amp FX loop return​

when off, and when switched on:

dist. FX --> amp input --> amp FX loop send --> time-based FX --> amp FX loop return​

This lets me turn the switch on when running with the Egnater head and get the benefit of running the time-based effects in the FX loop after the preamp, so that they don't pick up any additional preamp distortion. When I use one of my other amps with no FX loop, I turn the switch off and just jumper the FX loop send & return jacks on the switch. The switch works great. But....

When using it turned on with the Egnater head, I noticed a pretty audible hum. At first, I though maybe I had a grounding problem in the switch, but when I removed the switch and just ran things with cable runs directly into the FX loop, it still hums. Seems to be coming from the time-based effects chain, since I can get rid of most (but not all) of the hum by turning them all off. I tried a couple of different power supplies and no change. The weird thing is that if I unplug the time-based effects from the amp's FX loop and put them back between the distortion effects output and amp input, I get NO audible hum--quiet as a church mouse.

Any of you guys have an idea what would be causing this behavior?

 

[duhvoodooman]

Thanks, Tung. But the issue isn't in the switch box I built--I'm getting the hum whenever I'm going through the amp FX loop, just running the required instrument cables for connections and not using the switch at all. Seems to be power-related in some way, because if I put just true bypass pedals in the FX loop and leave them all off, no hum. As soon as I turn one on, the hum is back. I've tried several different power adapters for the pedals, and they all act the same way.

 

[tunghaichuan]

Have you tried using batteries in your pedals?

tung

Thanks, Tung. But the issue isn't in the switch box I built--I'm getting the hum whenever I'm going through the amp FX loop, just running the required instrument cables for connections and not using the switch at all. Seems to be power-related in some way, because if I put just true bypass pedals in the FX loop and leave them all off, no hum. As soon as I turn one on, the hum is back. I've tried several different power adapters for the pedals, and they all act the same way.

 

[duhvoodooman]

Just tried putting a true bypass delay (BYOC Ping Pong) in the loop by itself, with just battery power. Hums very audibly when kicked on; slight background hiss when it's off.

Would it help to power the amp itself off a different outlet?

BTW, the hum doesn't respond to either the Gain or Master volume controls on the amp....

 

[tunghaichuan]

If you plug a guitar cable into the FX loop, do you still get the hum? It could be that there is something funky with the jacks in the FX loop. If the FX were the cause of the hum, using battery power should clear it up.

tung

Just tried putting a true bypass delay (BYOC Ping Pong) in the loop by itself, with just battery power. Hums very audibly when kicked on; slight background hiss when it's off.

Would it help to power the amp itself off a different outlet?

BTW, the hum doesn't respond to either the Gain or Master volume controls on the amp....