Pod X3 Live: Hum In Input/recorded Signal When Both Guitar In And Aux In Have Cables Connected, No Hum When Only One Is Disconnected

[db0451]

This is baffling. I was connecting up a stereo keyboard using Aux and Guitar as left and right inputs, panned accordingly and recording via the tuner, Bypass, and USB. After having to turn up the volume, I realised an intrusive hum, somewhere around –65 dB, had gotten into my signal, both for monitoring and recording (read: in the inputs, not just outputs). Further experimentation showed that unplugging either of the inputs would get rid of the hum altogether in the one that remained, but as soon as they were both connected, the hum would reappear. It does seem slightly louder in one of them, however.

 

What could be happening here? I found some old threads suggesting a ground loop, but surely the way to cure that would be to disconnect the computer or to do something else that would remove an errant AC component from the network of devices. Why would disconnecting either of the cables have any effect upon a hypothetical ground loop?

 

All suggestions would be much appreciated. I need to go and test this a bit more with different combinations of stuff plugged in. And I guess I will check it in a spectrogram to see whether the hum is concentrated at 50 Hz or not. If I can post more info later, I will. Still, if anyone already recognises these symptoms from their own experience or can theorise about a cause, please let me know your thoughts.

[TheRealZap]

you might try just putting the stereo keyboard into the fx loop return. that way you take out some internal pathways in the device. 

just something to try... i've used it for recording the vdrums stereo out this way.

[db0451]

I’ll try that in its own right since it might prove useful later if it works. But at the moment, I want to use the proper inputs because, although I do typically record the keyboard dry, I want to retain the possibility of using effects from the POD, so I really need my main inputs to not be broken due to being plagued with hum.

 

[edit] Anyway, I’m not sure what your idea would actually be bypassing. The Loop is bypassed when the Live detects nothing plugged into Send/Return. So, currently, my inputs wouldn’t be going around any diversions in the internal signal path, especially when I have the tuner set to Bypass, meaning that I doubt I have to worry about that. Also, though I haven’t tried it, I’d assume by the same logic I’d have to run a dummy cable out of Send to enable the Loop at all. Anyway, as I said, I need my main inputs working properly, so although the Loop is all well and good as a separate option for another day, it’s not a solution to this problem. [/edit]

 

Two things I found out since my first post that I need to test: (1) Ground loops can arise when several devices connected together are not all plugged into the same outlet/strip; the POD and computer are currently on one, whereas the keyboard is on another. (2) Contrary to what I thought, TS cables are grounded, so the fact there are two between the keyboard and the POD might be causing or exacerbating the problem, if I’m correctly interpreting descriptions of ground loops, which seem to suggest they can arise when there are two alternative possible paths between two grounded devices.

 

When I go back to my setup, I’ll try plugging everything into the same outlet. If that fails, I’ll try unplugging the USB. Then the PC. And so on. Maybe I can find the cause of the problem. I really hope so because I really need to have all these devices connected together and performing properly, and hum around –60 dB does not fit that definition.

 

However, I’d still appreciate any other advice anyone might have, because this is a new problem to me, and I’m not sure I’m interpreting the available reading material properly—or whether a ground loop is even related here at all.

[silverhead]

Is it possible to set your keyboard output level to Amp/Instrument rather than Line Level? You may just be getting signal overload into your X3 inputs. The Guitar and Aux inputs are expecting instrument level inputs. You might experiment with this idea a little bit by using the PAD switch with the Guitar input.

 

This is probably a long shot because it doesn't explain why the noise disappears when one input is removed, but might be relevant.

[db0451]

The keyboard only offers a slider for volume, which I presume is analogue. Overloading, by which I mean clipping, only occurs on patches that are individually loud and playing multiple notes, and I can turn down the overall volume to remediate that. Anyway, I tried the Pad before, and it simply reduces the whole signal, including the hum, with the result that the hum was equally loud proportionally as before. Even if it had somehow worked, it would only be usable for one of the mono inputs. 

 

Anyhow, I switched to another set of cables, and the hum was gone. Or so it seemed! Closer inspection seems to reveal it is still there—and still seems to disappear when either of the two inputs is disconnected at either end—but seemingly exceeded by the white noise from the keyboard itself, with the total noise floor hovering at a much more acceptable –80 dB, a figure that actually beats the quoted noise floor in the service manual of the keyboard. So, I can live with that. The hum itself is presumably even lower in amplitude than that figure.

 

The interesting thing is: the same phenomenon of hum induced by both cables being connected also occurred with another device, which was completely unplugged from the mains at the time! What does this indicate/how is it physically possible? Should I assume this means some very low level of hum is likely to occur in many (most?) situations? I hope that this is something normal and hence that I don’t have to add a problem with grounding to the list of things that might be wrong with my X3L.

 

Anyway, that confusion aside, the solution to remove the hum at an intrusive level seems simple. It must have just been those cheap cables by Stagg. Switching to a pair of proper ones by Fender seems to have done the trick. Why, I have no idea. What could be the problem with cheap cables that would cause them to transmit more hum than better-built ones? The actual signal itself, in contrast, was no louder, so it’s not like the Staggs were somehow just louder altogether.

 

Finally, just to correct myself from above:

I was connecting up a stereo keyboard using Aux and Guitar as left and right inputs, panned accordingly and recording via the tuner, Bypass, and USB.

Using the tuner and Bypass as I described is the best way to monitor this setup. However, the proper/optimal way to record it is simply to take a stereo track from ASIO using Tone 1 Dry and Tone 2 dry (inputs 7 and 8) as the respective channels; that way, the raw inputs are sent directly to the recording software, rather than risking (because I have no idea whether they really would) going through any unnecessary parts of the processing chain.

[Charlie_Watt]

^{Good Cables are worth the $$.}