Mains Hum problem

[dicky]

I've been running my new PA setup in the house to test over the last couple days

As it is probably relevant to the following problem I am using the following

Desktop PC & Monitor
Numark Mixtrack Pro II
Behringer DJX 700 Mixer


The Master output from the mixer connects to a Crown Audio Pulse 21100 Amp into twin 15" + horns full range cabs
The Booth output from the mixer connected to a Citronic CX23 active crossover and bass output from this connects to a W Audio EP 1300 Amp and 18" bass bins

Amps and CX23 mounted in 19" rack cabinet - on wheels!! It's bloody heavy!

All connections are XLR to XLR apart from outputs on the mixer are phono so I have a couple of phono-XLR leads for that.

Now when I set this up at home all worked fine - I would get a slight hiss on the speakers with the volume controls on the amps set to full and no music playing. It sounds good. No hum.

Yesterday I took it all to a venue to test it out. As soon as I plug in the mixer to the mains I get a loud mains hum. Unplug the mixer from the mains and the hum goes away.

I tried disconnecting the input from the mixer - I was only using the Numark Mixtrack Pro II as an input device It made no difference with it disconneted. With no inputs connected to the mixer and all volume controls at zero it still has the same amount of hum. Loud!

Tried powering down the PC. Tried different mains leads on the mixer. Tried a different extension lead to the power outlet. No difference.

Disconnecting the phono leads from the mixer Master or Booth outputs stop the hum on the relevant amp/speakers

No matter how I set the volume controls on the mixer made any difference to the volume of the hum

The only way to reduce it to manageable amounts was to turn down the volume controls on the two amps to about 60% which turned down the hum, then turn up the master output from the mixer to full. I could then get good loud music and drown out the hum that was still there - but it was still rather noticeable with no music playing.

Now the question - what's going on? I've brought the mixer back home today and plugged it into my PC amplified speakers. No hum. So the mixer has not gone faulty IMHO

Knowing there is such a thing as ground loop hum but not knowing exactly what it is I found this useful tutorial

http://www.youtube.com/watch?v=GS-6jBk9YPM

Having watched that through I now understand ground loops but am even more confused as my kit was all running on one extension so all the equipment earths should be the same potential as they share the same earth connection.

Part of the tutorial was explaining (at 5min 45sec) how you can get a ground loop when you have an unbalanced connection (like the phonos on my mixer outputs) connected to a balanced connection (like the inputs on my CX23 and Amps).

However as I am using the same leads at home as I was using at the venue yesterday I can't see how I have a ground loop problem with this unbalanced-to-balanced otherwise it would have been present at home too? Unless a lead went faulty in someway but the problem was on both amps so that would suggest both leads faulty - which seems very very unlikely.

My gear is all in a storage unit now (not enough room here) so can't easily test it again but I very much suspect if i plugged it all back in here the problem would go away, and if I went back to where I was yesterday the problem would return. Certainly I am happy the mixer has not developed a fault.

So can anyone suggest what was going on yesterday or what else I can do to diagnose the problem?

Cheers
Rich

[Pe7e]

The simple solution would be to buy and fit a ground loop isolator between your playout and your mixer (£5 - £10 available all over the place) before you buy one, it's worth checking if your amp has a ground lift switch fitted, if it has, try that first.

[Excalibur]

When I test anything in my kitchen, there are all sorts of horrible extraneous noises. Go to a venue, and it's ( usually ) fine.
Check if your mixer has a ground lift switch fitted, if so swap it. Same with the amps. Do not do this with everything powered up. ( It's a bit like Ghostbusters, " Don't cross the streams " )



The simple solution would be to buy and fit a ground loop isolator between your playout and your mixer (£5 - £10 available all over the place)
Apparently though, he's got hum with nothing plugged into the mixer, so I don't think this is the solution. Wouldn't break the bank though.



Different venues have more or less noise sometimes, I don't know why, sorry I can't offer more help.

[dicky]

I've been thinking some more about this. Disconnecting the audio source (Numark Mixtrack) from the mixer didn't make any difference as Excalibur mentions so I also don't an isolator there would make any difference

Having watched that tutorial video on ground loops and hum again I can only surmise that with the various extension blocks, cables and interconnects I somehow got a potential difference between the chassis ground on the mixer and the chassis ground on the amps - thus causing a 50Hz AC current via the audio cable screens (the ones from the mixer phonos (unbalanced) to the amp XLR inputs (balanced) and therefore causing the loud mains hum.

This would kinda tie in with the evidence gathered from trying various unsuccesful solutions

I have looked in the various odds and ends of cable and stuff I got with the second hand 19" cab and the Crown amp - and there are a couple of what I can only assume are isolation transformers in that lot - with 6.3mm mono jack cables on one side and XLR cables on the other. These would only do for one amp though as I only have two of them and each one is mono

However - from studying that tutorial video again, I suspect if I get some heavy guage cable and strap the ground points (thumbscrew terminals) on the two amps together, then strap that over to a ground point on my mixer with the same heavy cable (again the mixer has some thumbscrew type ground terminals) then this problem will go away

Unless anyone can suggest anything else, that is the best I can come up with

Rich
Oh PS - if you wonder why I need the mixer at all rather than just using the one built into the Numark Mixtrack, it's so I can feed separate audio into the bass bins and main PA (and control the relative volumes) and also the Mic input on the Numark has no bass/treble controls. Plus it also allows me to connect other audio sources if I wish.

[Excalibur]

Couple of things. I can't do anything about the lack of mic EQ on the mixtrack, but I can lose the Behringer for testing purposes.

First, see if there is a ground lift switch on the Behringer, it doesn't seem that way from the manual, but if there is, it's an easy option, see if it helps.
Second, see if you can scrounge a better quality mixer to use. Denon x500 is class of the field, but Numark, Pioneer and others are well regarded.The Behringer DJ mixers are not well regarded. The big live mixers are fine for the money though.

Third, the clever bit ( I hope ) Run the Mixtrack straight into the amp running the full range cabs as normal. These cables will go into the female sockets on the amp. Then connect the male sockets next to them into the input of your crossover. It's commonly referred to as " Daisy Chaining ". This will still allow you to run bass only to the subs, as you do already.

Fourth, the Genius bit ( which as per standard practice, will hammer your pocket :(:(:( ) Were you to somehow source an amp like the Studiomaster AX series ( I think the Peavey PV might as well, but I'd have to look closely. ) you could run your subs with this ( it has a built in crossover you see, two in fact ) and daisy chain a full range out of it, no problem.

Good luck, keep us posted.

[dicky]

Cheers mate

I never thought of daisy chaining the amps like that - yet it makes perfect sense as you describe it

I'll need to get a couple of short XLR-XLR cables but that is no issue.

So when I set up on Thursday for my firslt live gig if I get the hum again and can't get rid of it then I can ditch the Behringer as you suggest - thank you!

I can't complain about the Behringer DJX700 too much - it cost me practiaclly nothing as part of a job lot of secondhand gear

There is no ground lift switch that I can see - I haven't looked on the amps as the heavy stuff is not stored at home.

Rich

[deltic]

just throwing my bit into the mix...if it was ok at home but not at the venue you tried it in, maybe just maybe the fault lies in the venue's electrical supply?

[dicky]

Yep I wondered if it was something to do with the mains there - but can't quite figure what it could have been.

No earth connection came to mind but having watched that video on earth loops I would imagine in that case everything would 'float' at it's own potential and there would be no current path to cause a hum plus I would have thought touching the metal casing of the mixer would induce more hum in that scenario - which it did not.

I'll be happy to reply with more info after my next venue. If everything works there fine, as it did at home I will have to agree it was a problem at that venue then try to puzzle out what it could have been, for future reference.

dicky

[Excalibur]

just throwing my bit into the mix...if it was ok at home but not at the venue you tried it in, maybe just maybe the fault lies in the venue's electrical supply?

Indeed. It is not unknown to experience issues of this kind in particular venues.

Beware appliances with thermostats and large motors, like fridges and freezers. One venue I used to work in had something which used to come through on a regular basis. You'd hear a click, and then the motor cutting in and whining.

[dicky]

Well I don't know if it sheds any light on the problem but yesterday I set up at a different venue and all was fine - no hum

I got a booking came in yesterday for a party tonight and I set up once again at this venue this afternoon and no hum

So I can only assume it was a problem at that particualar venue or some odd combination of extension leads

Rich