What's up with my amp?

[hernameisrio]

I have a G-K Backline 110 that I use for practice and love dearly. Here's the mystery...when I plug in my Ipod to the "Mix" jack, I can play along through headphones without issue. But when I plug into the Mix jack through my computer's headphone jack, I get major noise through the headphones...static, what almost sounds like RF interference of some sort...it's unacceptable.

However, I tested the computer's headphone jack by itself (I have a new iMac) and had no noise and no problems. AND like I said, the "Mix" output on the amp has no noise either, at least not when I'm using it with my iPod.

So what's going on? Is there some sort of grounding issue with either my amp or my computer that is causing this? I am really starting to get annoyed...sometimes I have to practice late at night, so I can't have this! I am not opposed to opening up my amp if I have to, as I know how to solder and have a basic grasp of audio wiring, but this issue just has me scratching my head. It's not a ground hum...it's just plain old noise. My computer has AppleCare but I really would like to avoid taking it in to the Apple store if at all possible. Has anyone had a similar problem?

[hernameisrio]

Okay, I figured it out...I had to use a ground lift on the amp. Wow...I've NEVER gotten so much noise due to ground issues, I'm very surprised. Oh well, live and learn.

[BassmanPaul]

You beat me to it Bro. Glad you got it sorted. My practice rig is a Mac Mini coupled into my "studio" stereo rig. Needed to use an Art DTI isolator to get rid of the hum. Now it's completely silent. Bass goes into my practice amp my B15N.

[WingKL]

If you are lifting the ground on the power plug of your amp, you risk getting electrocuted if a fault should develop in the chassis of your amp. The safest way to handle the noise issue is to use an isolator/transformer based DI like the one mentioned in the previous post.

[hernameisrio]

So in other words, run the DI between the amp's Mix output and the computer's headphone jack? So that it goes...

Bass ----->amp---->Mix out----->DI----->computer headphone jack?

I'm kinda confused, that seems like overkill. I'm sort of embarrassed for not understanding this better given my knowledge of audio wiring, but I guess I just never ran across this before and to be fair, I'm mostly just a cable monkey.

[WingKL]

It thinks the DI goes between your computer headphone output and the your amps Mix input to isolate between your computer and amp.

[hernameisrio]

Ok...well, if I can find my DI box, I'll try it...

[WingKL]

It'll probably only work if it is a transformer based DI box.

[MuzikMan]

The noise might go away if your amp and computer are plugged into the same outlet. Then you wouldn't need a ground lift.

[Sneakypete]

"Ground Lift" NEVER means lifting the ground of your mains cable! This is there for protection from electric shock in the event of a fault and is anyway irrelevant when referring to signal noise. In signal-level circuits it is possible for hum-loops to occur and one way of solving this is to 'open' the loop. Your signal cables are still shielded from noise - but are only connected to signal-level ground at one end. Don't ever remove or otherwise mess with mains ground connections but signal grounds ARE something you can experiment with if you find yourself with some weird noise problems.