Bubba po

I've had to borrow a mixing console for a show tomorrow night, but whilst in the process of checking compatibility and connections with my own gear I discovered it is putting out a quite loud hum.

The hum volume is independent of channel gain.

It is not being generated by the amplifiers.

My own (small) Studiomaster desk is not causing the same hum.

There are no fluorescent or low energy bulbs on in the house.

I've tried several different mains power leads.

The hum isn't present on the headphone PFL output.


The desk is a 12 channel Soundcraft Spirit Live 42 with an external PSU.

Any more suggestions? I'm at me wits' end.

Sonic'

Grounding problem?

balanced / non balanced cables

other than that not really sure

Freak

If you arent already, plug your gear into the same wall socket using a splitter/multiway...... could be caused by being on different parts of the home circuit or different rings. Common cause of crap like this.

If that fails, turn everything off, then turn the desk on by itself- see if its still there. if its not, then turn each piece on one at a time and see if it reappears.
Might need a lot of plugging and replugging to find this.

Could just be a ground issue- you could lift it from the desk, but its not exactly safe (or legal nowadays more than likely), or failing that, its a fubared desk..


I have about 35 pieces of gear in my studio- didnt get one hum when i first powered it up.... was NOT expecting that!

Sonic'

I thought lifting grounds was still commonplace

Bubba po

How might I cure this, if that's the problem?

I've just tried connecting the amps to the desk directly without the multicore in the chain and the hum is far less prominent. Do you think it's possible that the multicore shielding is not properly connected to ground or is just foofed? ?

Bubba po

Thanks for that. I've already done most of the tracking down procedures, but I'm willing to go through it all again. I remember one club I used to play at that had about ten rotary dimmer switches for the house lights and an external neon sign. The hum was louder than the bloody band at times.

Sonic'

It is possible that the multicore is the problem, but you say you dont get this on your desk, is that using the same multicore too?

Is it across all the channels, or does that not matter, my mates studiomaster has a couple of fubarred channels that hum like buggery, so they have been muted and taped over so they dont accidently get used

It was a while back now but I did read some interesting articles on fitting ground lifters on multicores/splitters and it was pretty much a 50/50 argument on their use or not

Can you try and isolate the hum any further, and have you tried what Freak suggested re dhome circuits

Bubba po

Erm... I've not used the multicore with the other desk, LOL. I'm bringing it out of semi-retirement. All the channels w*rk. . Guess I'm going to have to dismantle the stage box and check that all the shielding is connected properly to ground.

Power Junkie

iTrader: (1)

Lift the ground on the power supply and see if that helps,

Sonic'

The desk is a 12 channel Soundcraft Spirit Live 42 with an external PSU

Ahh, wasn't sure if you had tried the multicore or not on your desk, with you saying you dont get the hum on yours

I changed all mine last year for digital as it doesnt go out on the road much anymore, was going to build a stagebox but all those wires etc put me off as it looked like a boring job

Power Junkie

iTrader: (1)

does it still hum when the desk is not connected to anything?

Bubba po

How do you mean? It only hums when it's connected to the amps. If I lift (presumably you mean disconnect) the ground on the desk, isn't that a bit dangerous? Or are you just advising that as a diagnostic tool?

Bubba po

There is absolutely no point building a multicore/stagebox. Buy one off the peg. Trust me, it isn't worth the w*rk!

Power Junkie

iTrader: (1)

Yes remove the earth just to see if it stops the hum, then you will know if it is the problem.
and not connected to any multi core/inputs just desk into amps, also try mute all of the channels then if that helps unmute or solo 1 by 1

Power Junkie

iTrader: (1)

Adam Hall do some good deals if you go through Thomann

Bubba po

Right, I've taken the earth wire off the power lead to the mixer and the hum's disappeared. What now, Batman?

Power Junkie

iTrader: (1)

Right, it is an earth loop, Have you tried it on a different mains circuit?
are you using XLR's to the amps? if so you will be ok to run without the Earth on the desk (it will earth through the gnd to the amps)
other than that if you can find another psu that will work,

Bubba po

Thanks very much PJ, but you've hit the nail on the head in that last post. It's now sorted, and it's all down to me being a feejit. The input connectors on my amp are a type I hadn't come across before, that are a black moulded xlr/jack combination. I hadn't realised that the XLR-to-jack connections I was using were unbalanced. So I swapped them for standard XLR balanced connections and .... Hey Presto! It's now hum-free. (My little mixer has jackplug outputs but the cable run was only about three feet. )

Power Junkie

iTrader: (1)

thats ok, glad you sorted it

ScoobyDoo555

Sorry for the late delay of reply - earth loops!!!

Dan

Sonic'

Glad you got it sorted Bubba

I did mention balanced / unbalanced in my first post

I dont think I will ever build a stage box, not these days, probably never buy one either, unless I would be earning money from doing it

ScoobyDoo555

Stage boxes are a piece of p1ss to make - try doing 24-way DL patch leads

Dan

Power Junkie

iTrader: (1)

Nooo not the DL's i have had many a bad day doing them

ScoobyDoo555

lol!!! I've gone cross-eyed doing them How ya doin PJ?

Just been having a some fun with Roland's new Digital Snake system and an SSL AWS

You busy?

Dan

Bubba po

I assumed my leads were balanced via a stereo jack at the amp end, when in fact they were foofing mono.

Bubba po

My Peavey one deffo would be - the inputs are in banks of four on circuit boards, I found out tonight.

ScoobyDoo555

Power Junkie

iTrader: (1)

I need to try the Roland D S, been busy playing with a big Funktion One rig sounding great now

ScoobyDoo555

PM me about the DS - it's a GREAT bit of kit. Cheap (ish) too

Gotta go to bed now - been a long day!!!

Dan