Hi there,

If you can help quick please do! I'm back there in morning to try to solve this. Thanks

I just installed my company's standard medium system in the main room at a venue, all works fine. The promoter had requested the audio to be piped into two other rooms, which we brought small systems for, and which requires a 75m cable run taken from a second LR out (using L only) on desk. When I connect the cable run into the small pa in room two, there is a low freq hum generated. This hum affects the previously trouble free main room system also.

The set up is as follows: Control, FOH, mons and b'line are split between two 16 amp outs of a big distro unit that powers most of the main room (its a convention thing with stalls etc). There is a 75m cable run to another room. And another 50m run that feeds a single box in the third room. (In the heat of battle I ignored room three and just tried to elimate the hum between main room and room two.) All cable runs are just 'loop thru' the boxes.

Also important: disconnecting the earth in the plug of active box in room two solves the prob. And the cable run is along a ceiling channel which has all sorts running on it, including mains cables. No boxes have g lift on them.

Um .... help! Thanks.

Hi, Is the main power unit powering all the rooms or are they powered within each room? Dave

--------------------
My head hurts!

Thanks Dave. Its a different distro in the other room. Third room is socket on wall.

Im gonna try patching a short XLR with shield pin disconnected. Would dropping the run from line to mic level via a DI and using ground lift help? (ie does mic level travel cleaner along long runs?)

Quote aekoi:

---

Im gonna try patching a short XLR with shield pin disconnected. Would dropping the run from line to mic level via a DI and using ground lift help? (ie does mic level travel cleaner along long runs?)

---

If you were going to do that in the first place - why did you need the di's grnlift in the second place?
Did i miss something?

--------------------
17ft here! Too deep for non divers.

Quote aekoi:

---

Im gonna try patching a short XLR with shield pin disconnected. Would dropping the run from line to mic level via a DI and using ground lift help? (ie does mic level travel cleaner along long runs?)

---

Dropping the level to mic level will not help, however isolating the ground by either disconnecting the shield at one end or using a DI box with a ground lift will help. Ideally you would use a 1:1 transformer for this purpose. You will need to isolate the audio ground of all equipment connected between the two rooms/distros. NEVER lift the AC/Mains ground. It is a safety hazard.

Seablade

If you are in UK, one of these works...

http://www.maplin.co.uk/ground-loop-isolator-33172 Probably not great fidelity and you will have to watch levels but better than hum! You will of course have to get creative with the connectors but twisted wires and gaffer tape is fine at line levels.

As Seablade said DO NOT LIFT mains earths. ESPECIALLY in a "public place". If anything were to happen you could go to prison. Certainly any 3rd party insurer would walk away!

Dave.

Bases have been covered really... Ideally you'd fix this in the cabling, but time is money and I've used a 1:1 some times in installs to sort these sort of issues.

Dave (ef37a) is right about the Maplin gizmo - it'll work, but not really what I'd use on an install for the reasons Dave has said.

The ART DTI is a useful one-size-fits-all solution because of it's variety of connectors - recently reviewed by Hugh in SoS.

My favourite audio-gadget company of the moment is Orchid Electronics in Exeter. They have some 1:1s - not tried one of these, but the company's reputation is good with other products.

Right Mike, but I think the OP needs to fix it today!

Maps open at 09.00.

Dave.

Ah! Good point Dave.

But if he's in the vicinity of a GAK, Andertons, PMT, DV247 etc (other retailers are available) shop he might be able to get one of the ARTs or an ART CleanBOXII.

But if he's not then the Maplin kit would be a get-out-of jail card for today....

Sorry I didnt get back, nodded off! the above replies will solve the issue probably. If not try alternative power socket/distro configurations. good luck! Dave

--------------------
My head hurts!

A ground loop occurs when there is more than one ground
connection path between two pieces of equipment. The duplicate
ground paths form the equivalent of a loop antenna which very
efficiently picks up interference currents. Lead resistance transforms
these currents into voltage fluctuations. As a consequence of
ground-loop induced voltages, the ground reference in the system is
no longer a stable potential, so signals ride on the noise. The noise
becomes part of the program signal.

A ground loop in the power or video signal occurs when some
components in the same system are receiving its power from a
different ground than other components, or the ground potential
between two pieces of equipment is not identical.

Usually a potential difference in the grounds causes a current to
flow in the interconnection line. This in turn modulates the input of
the circuitry and is treated like any other signal fed through the
normal inputs. Here is an example situation where two grounded
sets of equipment are interconnected though signal wire ground and
the mains grounding wire. In this situation there is 1A current
flowing in the wire which causes 0.1V voltage difference between
the grounding points.

Because there is voltage difference between the equipment, the
signal in the interconnection wire sees that difference added to
signal. This can be heard as humming noise on the wire because the
AC current cause the voltage difference of those ground potentials
to be also AC voltage. This is one reason for this 50 Hz or 60 Hz
noise you hear in the audio signal (or see in video signal as
annoying horizontal bars).

Another problem is the current flowing in the signal cable grounding
wire. This current passes though the cable and through the
equipment. Of the way the current passes is not well designed this
can cause lots noise to the equipment or other kind of problems
(like computer lockups). Lots of designers count on ground
being ground and do not optimize their design to eliminate
their sensitivity to ground noise.

Quote aekoi:

---

Also important: disconnecting the earth in the plug of active box in room two solves the prob.

---

Ah-ha!

The building does not have a unified earth and there is a difference (that you can measure with your trusty multimeter as a few volts AC) between earth in room one and earth in room two.

Once you have identified this difference, tell the venue manager that he needs to get an electrician in to solve what is a serious problem.

You solve your problem by getting all your power - and therefore earth - from the one source. So now you need a nice long length of power cable.

I'm assuming we're talking about a UK installation here... in which case I'd be VERY surprised if the building didn't have a 'unified earth' -- meaning all earthed sockets in the building are connected back to the same central earth point eventually. It would be contravening the UK IEEE wiring regulations for a start... And running power from the first room to a second PA system via a 75m long mains lead is likely to cause more problems than it solves unless you know what you're doing and use appropriate cabling.

In fact this is a very simple and trivial ground loop problem, which is entirely normal and entirely too be expected if you are running audio between two seperate systems powered from local mains sockets separated by a reasonable distance within the same building.

It often happens when powering equipment from sockets on opposite walls of a small domestic bedroom, and it's absolutely guaranteed to happen when powering from rooms that are many metres apart in the same building.

As has been said, the loop is formed through the audio ground connections, via the mains safety earth connections of one room's equipment, through the building's mains supply earth cabling, and back via the safety ground wiring of the other room's equipment. It has nothing whatever to do with acting as an aerial!

The currents that flow in the building's ground wiring are perfectly normal, safe and expected and these currents will inherently cause a voltage to develop across that wiring because the wiring itself has a finite resistance. The equipment in different rooms will therefore 'see' slightly different ground reference voltages, and that will cause a 'ground loop current' to flow along the audio cable screens between them.

If the equipment is badly designed, that ground loop current will find its way into the audio circuitry's ground reference and inevitably be added to the wanted signal, whereupon you'll hear it as a hum, buzz or other unwanted but continuous noise.

The correct, safest and only guaranteed solution is to isolate the audio circuits between the two systems using proper audio line isolation transformers. Mike has already mentioned two extremely cost-effective and decent quality options: the ART DTI or the ART CleanBox II units -- both around £40. However, there are many more including offerings from Orchid which are just as good but even less expensive, up to the extremely expensive and stunningly good Lundahl-based offerings from Canford.

If none of these area available, then using a DI box to take the line output of one system, pad it down to mic level through a transformer with the ground lift activated and pass that signal on to the other system will brake the ground loop perfectly well. it's a shame to have to knock the signal down and reamplifiy it -- and there will be a small noise penalty -- but that's considerably better than the hums and buzzes.

As has been suggested, the other quick and dirty 'help what do I do now?' instant solution is to break the screen connections at one end of the audio cable(s) running between the two rooms... BUT this will only work if the audio connection is balanced and going to a proper balanced input. If you have to resort to this in desperation you should break the screen at the destination end only, and mark the cable in some clear way so that you can fix it again after the de-rig. Also, be aware that breaking the screen on a long cable run does introduce a significant risk of RF interference... so it may well cause more trouble than it solves.

Invest in some line level transformer isolation boxes -- they are extremely useful problem solvers!

Hugh

--------------------
Technical Editor, Sound On Sound

As Hugh says, earth currents, like death, taxes and noise, are always with us.

They can be quite large in some situations, in the data industry where lots of shielded cables run from one comm room to another, use is made of Parallel Earth Cables, "PECs" heavy, low resistance wires that divert much of the current from the data shields.

Apart from residual earth voltages in different rooms there is no guarantee in a large building that you are on the same phase! This should not occur on the same floor I understand but buildings get "added to" and lekkies are not always scrupulous as to where they pick up a new feed from!

To pararphrase Mr T. "Get some TRAFFS!"

Dave.

On the phase thing, you get different phases on sockets in the same room, never mind the same floor all the time.
Perfectly standard in venues of any significant size and not any kind of problem (The old 2M phase separation rule went out the window years ago).

Running gear from different phases DOES NOT cause hums and buzzes, and it takes an awful lot of simultaneous faults before it becomes a safety issue.

What we have hear is a straight forward earth loop, nothing a line level transformer won't sort out, or even simply telescoping a shield if the circuit is balanced.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

Quote Hugh Robjohns:

---

The currents that flow in the building's ground wiring are perfectly normal, safe and expected and these currents will inherently cause a voltage to develop across that wiring because the wiring itself has a finite resistance. The equipment in different rooms will therefore 'see' slightly different ground reference voltages, and that will cause a 'ground loop current' to flow along the audio cable screens between them.

---

Slightly different ground reference voltages = no unity!

OK, I learnt this stuff in a different country and the way I understand things, a unified earth is unified when and only when the earth does not rely on the earth wire alone for its unity. If there is a difference between two parts of the building's earth, then it cannot, by definition, be unified. The very structure of the building and the ground it sits upon have to provide the unity.

The problem with giving answers to questions on a forum, is that one assumes too much or too little. In this case, I am assuming that the OP has done everything as he should, in particular, bearing in mind input impedance and the effects of long cable runs.

Dan. My point about different phases is that it would probably mean a ring from a different cabinet and hence even more earth wire resistance? The place where I worked but one there were cabs stretched out over some 10mtrs of a wall!

Red. Can't see how you can ever get a "unified" earth with zero potentials between distant earth pins?
Maybe from an electrical engineering perspective a "couple of millivolts" is as close to perfection as is required? But for audio work, two mVs is only about 50dB down on +4 OP level!

Dave.

Quote The Red Bladder:

---

Slightly different ground reference voltages = no unity!

---

I hear what you're saying, but you will only find what you are calling a 'unified earth' -- which would involve multiple grid-like earth paths -- in buildings with very specialised earthing arrangements... such as a recording studio complex.

You will definitely not find that kind of arrangement in any normal domestic or general commercial building because it is not a requirement and is very expensive to achieve. The important point is that this situation is not caused by a mains wiring fault and it does not require the urgent summoning of an electrician. Your recommendation was inappropriate, hence my response to save any possible embarassment and further confusion.

Quote:

---

In this case, I am assuming that the OP has done everything as he should, in particular, bearing in mind input impedance and the effects of long cable runs.

---

As you say, assumptions are the mothers of all ****-ups!

hugh

--------------------
Technical Editor, Sound On Sound

Just back and out again first thing so I don't have a lot of time to absorb the suggestions you guys have taken time to contribute, but many many thanks for these .

I cobbled together a solution this morning, using short XLR patch cables with the shield disconnected. The on-site sparky told me that hums in the system or very common, even in the main hall only (our job was the first time that one audio feed was used in all three rooms).

I didn't manage to solve it completely, and I agree with whats being considered above: that the long cable run may be contributing significantly. I did do some searching on 75m ish line level runs and no great problems seemed to be flagged. The additional run (probs another 75m) was a 'day before' request which I didnt have much time to consider.

I'll grap a cuppa thru the week and check out all the above advice and suggestions. And I'll post details of my fix.

I'm both grateful, and even a little touched that so many of you have tried to help. Gold stars to every-one.

Its all over now, phew!

I took several XLR patch leads with shield disconnected. I used one of these at the destination end (room 2) which didnt make much difference. (It was quite difficult to judge as there were now lots of people setting up and making noise, very different therefore from the day before).

I tried a cable at control at the start of the 75m run. This seemed to lessen the main room FOH hum, but didnt cure it completely. I then noticed that the FOH hum was only coming out of one PA stack (both stacks were fed from same 16amp socket). I tried plugging stack into the AV crews gangs - still hum. Then I tried connecting the offending stack via loop out of the quiet stack - rather than xover - and humming disappeared.

Back to room 2: still hum, cant tell if better of worse cos too much ambient noise now, HPFs on boxes masked it adequately. Same in room three. Once FOH was solved in main room, room 2 and 3 were less crucial anyway.

So thats that. Im still not sure of exactly what was causing the prob. The non earthed IEC didnt seem to make any difference also on the second day. There was one test on first day where it didnt make difference also but i put this down to loose earth wire touching pin, i opened and taped it which, I'm sure, stopped the buzzing immediately. Too much going on too fast to be positive now tho, since it defo made no odds the next day.

I will always carry the right kit to diagnose and solve similiar probs in future. This was quite a big job, with lots of aspects that you seldom run into in the normal course of 'rock gig' saturday nights that I mostly do.

And thanks again too everyone.

Glad you got though the job!

If you are preparing a problem solving kit I strongly suggest a couple of diecast boxes each containing an OEP Z3003E 10k:10k input transformer.

Not only will these break earths but have an interwinding screen that is very good proof against RFI. Use as close to the "recieving" kit as possible and connect via a hefty braid and foil screened cable.

Canford Audio stock them.

Dave.