Just checking: google suggests JC-40 is a Roland Jazz Chorus 40W guitar amp. That's correct?sprnva wrote:I'm running outputs 3+4 from my Scarlett 6i6 into the front of my JC-40
One check I did myself: outputs 3+4 of the Scarlett 6i6 are technically identical to outputs 1+2. That is: these are balanced outputs on 1/4" TRS.
What you have just confirmed is that this phenomena is only present when the JC-40 amp is added to the equation. And if there is indeed a ground loop (quite plausible) then you need to look at the cables connected to that thing. There's just two. Or three, depending how you count:sprnva wrote:I can hear hard drive activity etc as static noise in the amp. I don't hear it in my studio monitors connected to outputs 1+2 or in my headphones.
1: mains power. Gotta be the 3-prong type, so with ground. This ground is half the cause. Break it here, and the loop is broken. What surely helps is literally cutting the ground connection of the amp, but that's not advisable for an amp that is directly connected with bare metal strings you are firmly holding in your sweaty hands. So that's no option. You did try a variation on this:
That could have helped if mains power in the other room was wired to a different ground pin (that literally goes into the ground) but in most houses there's just one. Sometimes for the kitchen or bathroom there's a separate one.sprnva wrote:Running an extension cable in from another room and connecting either the amp [...]
So for the mains connection we're out of safe options. That means we have to look at the other wire:
2. Audio wires, of which you have two (left & right) so that makes three in total. The balanced output of the audio interface go into presumably unbalanced inputs of the amp. I wrote presumably because electric guitars rarely ever have balanced outputs.
What works is hum elimination boxes. These provide galvanic isolation. That is, the signal has to pass a transformer. These have nice properties: alternating currents do pass (by induction of nearby coils) but decent currents cannot pass. And since it's the direct connection of the ground wire giving the loop problem, hum destroyers will work.
But there's a cheaper option! Make use of the fact that the interface provides balanced outputs (plus, minus and ground) while the amp only needs two out of these three. What can work is a special cable:
* at the interface side leave the ground disconnected, but Tip & Ring are connected.
* at the amp side Tip goes to Tip, but Ring goes to both Ring & Sleeve (or just Sleeve of a mono plug)
Or stated in another way: take a regular guitar cable. Take off the mono plug from one side. Put back a stereo plug. On the stereo side connect the cable shield not to sleeve but the Ring of the plug. This way the ground at interface side is not connected, but the other two conductors are connected.
Or stated in another way: take a regular balanced TRS cable. Take off the stereo plug from one side. Put back a mono plug. On this mono side leave the cable shielding disconnected (also don't let it touch the casing)
