Humming Amp Head

[Johnny Wishbone]

Hi all, newbie with a problem for you to hopefully solve:

The problem concerns my bass head (Crate BX220H) which I've been running for about 8 years with no worries. Lately I've been having problems at gigs with the XLR Line Out feeding a lot of noise to the PA (depending on the PA it's either a constant kind of low hissing/humming sound or occasionally an intermittent sort of random clicking, both of which can be quite loud). I suspect it's an earthing problem.

Up until recently it happened once in a blue moon, but now the humming's started happening at rehearsal (I line out into an old PA to spread the bass around the room so our guitarists can hear it better) so I'm pretty keen to sort it as I've got a shedload of gigs through the summer.

Is there a simple solution, or should I get the head looked at by a tech?

Hope you can help

Edited June 28, 2007 by Johnny Wishbone

[Johnny Wishbone]

Bugger - this should probably be in the "Repairs & Technical Issues" section

[Muppet]

Do you have a ground lift on the amp? If not or you do but this doesn't help then you could be there for ages trying to trace the problem, so if it were me I'd admit defeat and get it looked at properly.

[Johnny Wishbone]

Sadly no, plus I've tried all the suggestions on the common faults page (changing power leads, unplugging FX, changing basses etc) and it still hums even with nothing plugged into it at all. Looks like I'll need to get it looked at

[BOD2]

It's possible you could have a "ground loop" problem. But note that this would most likely show as a humming noise and not a hiss or any random popping noise. If you're getting hissing/popping it's likely to be something different.

To cure a ground loop you can disconnect the earth wire from ONE of the XLR connectors used to connect the DI out to the PA. Only disconnect one end. This breaks the loop without losing the earth connection completely. Note that it's perfectly safe to do this (the earth on an XLR cable is NOT a safetry feature) but never remove the earth from a mains plug.

If you try this and it works then make sure to mark the XLR cable so that you know it has the earth disconnected at one end.

[Johnny Wishbone]

It's definitely more of a hum than a hiss, and I've fixed the clicking, which was an unrelated fault (dodgy XLR cable). I suspected it might be a ground loop so I'll give this a try at the weekend. Many thanks for your help.

Incidentally I guess this doesn't really solve the problem outright unless I take my own XLR cable to gigs, right? Or am I correct in thinking that most DI boxes have a ground lift anway, so I should be OK?

[BOD2]

Most DI boxes have a ground lift but it would be safer not to rely on this.

What you could do is make a short XLR male-female lead (half a metre or less) with the earth removed from the end that fits into your amp's DI socket. You can daisy-chain XLR leads so if you always plug that short lead into your amp then you 'll be able to plug any other XLR lead into the free end of that lead.

[Johnny Wishbone]

Top stuff - I'll be off to Maplin's shortly then. Again, many thanks.

[Steve_K]

BOD2's suggestion does sound like the most likely "cure", though if it's got a ground loop there will be a reason why; so when you get a break from gigs it would probably be better to have it looked at to fix the problem, rather than a workaround.

[Johnny Wishbone]

Yep, tried the quick fix but no joy, so I've got a tech to have a look at it whilst I've got a couple of weeks until the next gig.

Had to DI direct into the PA at last night's gig. Slightly odd experience not having my rig behind me.

Thanks though guys. I appreciate your advice.