Chasing hum in the recording studio

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Ilya

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Hi chaps. This is my first post here, so I beg your pardon if anything is wrong.
I'm a recording engineer at the studio. The main problem is the studio has (presumably) bad ground and poor power. There's a hum (not very loud) in the monitor path, but it doesn't appear as loud in recording though. Moreover, if I deadpatch the input of the desk (that is if I insert the connector in the console input at the patchbay and it terminates hot and cold from this input to my converter output) I still hear the signal from the converter. Very softly though, but it's there anyway.
This led me to the thought that we have problems with the ground.
The most awful thing is that local electricians have no clue about ground loops and stuff like that, I doubt they know exactly how and where is the ground itself.
I want to troubleshoot this to get a better understanding of what to do to eliminate this. Balanced power distribution and separate ground are quite expensive, so I don't want to throw the money out for this if those issues could be fixed without them.
So I'm asking for help. If anybody here could suggest a procedure or some steps to get a better grasp on this problem, I would be really grateful.
 
I've had decent luck narrowing down the offending section by using one of these, piece by piece, on the any equipment that has a three prong power cord. It's not your cure, but a decent troubleshooting tool:

pRS1C-2266744w345.jpg
 
I'll bet that you might have a ground wire out of the panel weither it touchs ground might be a different story.

To ID the location of you hum put a paper towel roll to your ear, notice I said Paper Towel Roll, no a hard pipe or anything else for your safety.

The paper towel will help isolate and even ampliy it to your ear, once you have found it.

The panel is Grounded by a ground rod, everything else is bonded at all electrical devices.(if even present)

I don't know European or much of other electrical codes in other country, but it sounds (pun intented) that in fact you might just have poor quality electrical service!

Do ask for references from the electricain that you might try to hire!
What jobs they have completed to date, Good Luck!
 
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This forum is probably the wrong place to ask that question, this forum is about North American AC power and more importantly the rules and safety associated with US power.

But some good places to search are:
Bill Whitlock at Jensen Transformer
Jim Brown at Audio Systems Group
RANE Corp. application notes

I have a long list of papers if you want to search further.
 
@ mdshunk:
Actually, the main piece of gear is a large Euphonix desk with 30A twist-lock connector. I'm a bit hesitant to cut the ground off of this baby... I'll check the other stuff though.

@ cadpoint:
Where should I listen hum with this paper towel?

@speedskater:
Excuse me if it's the wrong place to ask. I was googling and found this forum. Read quite a few threads about studio problems here. Didn't realise it's for US only.
Thanks for suggestions. I've read some RANE and Jensen papers, but all of them were dedicated to the grounding inside the gear, not the power system. I'll take a closer look at them shortly.
 
I'm sorry, I didn't mean that you were wrong posting here. I meant that some other forums spend a lot more time on the subject. The John Sayers Forum is one:
http://www.johnlsayers.com/phpBB2/index.php
The newsgroup (remember newsgroups?) rec.audio.pro is another.

Bill Whitlock and Jim Brown write lots of papers about system problems.
Don't cut the power cord ground.
I'll PM you about my list.

Kevin
 
Not entirely sure it's a "ground" problem. Many times a "hum" is present when the trims are set too high, whether on the input channel strip, or the mixdown busses. I've also found "aux", "control" and "headphone" trims too high causing a low-grade hum. "Hum" is not always from "ground loops". Good luck to you. By the way, Euphonix is a great desk! :smile:
 
Number one issue I have encountered is neutrals grounded downstream from the main neutral to ground bond.

Do a zero sequence reading on the feeders for the recording equipment panel.

With all equipment on measure neutral to ground voltage the further from the neutral to ground bond the higher the voltage depending on load.

Measure current on any conductive components in the studio.

There are ways of masking the issue but IMO it is better to repair/fix.
 
Thanks everybody for your input.
I've measured the voltage between Neutral and Ground and there's about 1V there. I can't tell more precisely ATM 'cause my DMM has only 200V AC scale.
I've talked to the local electrician and he showed me the main grounding connection of the building and suggested taking ground from there. Although he couldn't tell me where's the bond between N and G.
Is there any advantage if I ask him to bond the new ground with neutral at our studio panel?

wbalsam1, yeah, I know about that. Unfortunately this hum doesn't depend on any volume control. It's constant.

Brian, I've measured ground current (inserted my dmm between the jacks sleeves at the patchbay) and it didn't show anything. However between hot and ground or cold and ground there is about 700uA (I measured with no signal of course).

Kevin, have you recieved my PM?
 
Kevin, have you received my PM?
Darn, I forgot to hit send. The list is on it's way.

Some of the common problems are:
a) The infamous XLR connector pin 1 problem.
b) Using legacy vacuum tube equipment with a small capacitor on the primary connected to the chassis.
c) Switch Mode Power Supplies with a small capacitor to the chassis or asymmetrical leakage capacitance to the chassis.

Kevin
 
I'm now thinking about the step-down transformer and it occured to me that it doesn't have the secondary connected to ground. I remember that in some circuits I saw one leg of the transformer tied to ground. So I'm wondering if I should do the same with my step-down transformer?
Just wanted to make sure before I start getting the new ground wire in.
 
Ilya said:
I'm now thinking about the step-down transformer and it occured to me that it doesn't have the secondary connected to ground. I remember that in some circuits I saw one leg of the transformer tied to ground. So I'm wondering if I should do the same with my step-down transformer?
Just wanted to make sure before I start getting the new ground wire in.
Click to expand...
Not sure a step -down transformer is the right answer or at least in the right context. Did you mean an isolation transformer?

An isolation transformer is the most effective means of removing common mode noise. In fact that is exactly what a balanced power system is, except it eliminates the grounded circuit conductor (neutral). However that does not mean a ground is not used, quite the contrary as it would violate US electrical codes and be dangerous to do so even in UK system.
 
I mentioned earlier that the desk is 120V and is fed via a step-down transformer. I wanted to find out if it's correct to groung one side of the secondary or not.
I'm researching the balanced power now and it seems like a good solution. Although very expensive. Maybe I'll be able to order the balancing power trafo at the local manufacturer.
 
Ilya said:
I mentioned earlier that the desk is 120V and is fed via a step-down transformer. I wanted to find out if it's correct to groung one side of the secondary or not.
I'm researching the balanced power now and it seems like a good solution. Although very expensive. Maybe I'll be able to order the balancing power trafo at the local manufacturer.
Click to expand...
OK help me out here. In UK you use 240 correct? IS the 240 center tapped to ground for fault clearing, or is one side of the 240 grounded. It is grounded somewhere I know, it has to be for safety.

Ok here is the problem using a step-down transformer used in the way I think you will use it. If the input is 240 and the output is 120 and you ground either end, you are creating a unbalanced system where one side is hot, and the other is grounded. I assume your equipment has a ground conductor ran with the circuit conductors for OCPD operation and safety? Where this becomes a problem is since the EGC is ran with phase conductors in an unbalanced circuit, the ungrounded hot conductor will induce voltage current in the EGC from capacitance coupling. This voltage/current is seen as a noise by the amplifiers and like any amplifier amplifies the signal and is heard as HUM.

This is where balanced power comes into play. There is nothing magic going on here. IT is nothing more than a precision wound transformer, and works exactly like any other single phase transformer with a center tap on the output. The difference is the output across the transformer is 120 volts, with the center tap being 60 volts to L1/L2. The center tap is grounded, but no neutral or grounded cricuit conductor is used. The equipment is powered from L1/L2 at 120 volts, and the ground is ran with L1/L2 for OCPD operation and safety. What makes it different is the ground is at zero sequence with the phase conductors L1/L2, therefore no induced voltage or current can be induced leaving you qith a clean ground reference.

Now with that said there is danger of equipment malfunction, and special requirements. Here in the USA both L1/L2 have to have OCPD installed, that means twice the breakers. Also GFCI are requirend on every circuit for added safety. Also voltage drop requirements are enforced so for longer runs you may have to oversize circuit gonductors and EGC, as a result of th elower voltage to ground of 60 volts of a 120/60 system.

The danger for the equipment may not be appearant but can be real. Much of the equipment may have RFI, surge protection devices installed between neutral and ground. Typically the voltage rating of these devices are low. So when you use a Balanced Power System what was once the neutral circuit that only has a small potential difference of say up to 5 volts, now has 60 volts. So things like MOV's, capactors, etc rated at say 10 volts now have 60 and let out the magic smoke. This is very easy to deal with by checking each piece of equipment and removing the filters (violates FCC rules and warranty), or changing them out to the appropiate voltage via manufactures assistance and keeping the FCC and manufacture happy.

There is also one more safety problem. th epower switch on the equipment. It only switches off the hot side and not the neutral. So when you turn it off and think it is safe to stck your hands inside, you still have what was once neutral is now hot with 60 volts. Again not a problem with qualified personnel who are, should be aware of the situation.

Now with all that said, I have worked in recording studios early in my career. I also have a lot of experience with Balanced power in data center. IMO you can do just as good by using an isolation trnasformer to establish a new ground refference point free of common mode current (noise), and then run dedicated ciscuits to each piece of equipment. It would be less expensive than balanced. If the area the equipment is in is small and confined, you can buy a plug-n-play transformer. in either isolation or balanced isolation, All you need is one circuit from the main, preferable a dedicated 240 for the power, plug it in, then plug all the equipment into the recepts. No major electrical work needed, just one dedicated circuit from the main. The new ground is established from the EGC from the main bonded to the Xo of the transformer output.
 
In our country we have a 380V 3-phase distribution system, which delivers 220V on each phase. The PE is bonded to the Neutral of 220V, thus creating an unbalanced system. It is supposed that N is tied to PE at the service entrance.
We have only 1 feed of 220V and that forces us to put all equipment on this phase (including air conditioning, lights and heaters).
The desk is currently connected like this: 220v Hot and Neutral ->transformer-> 120V floating -> mixer. The ground of the mixer is connected to the ground at the studio panel.
Now, what concerns me regarding the step-down transformer is that it's floating. I'm curious about this and think that if I connect it in unbalaced manner (that is tie the secondary to ground) it may actually be more "compatible" (for the lack of the better word) with the main 220V system.
Thanks for commenting on the balanced power question. Actually, we'll have 120V here, not 60V, which is potentially more dangerous. Moreover, I was repairing a unit which had only single-throw switch. In the case of balanced power it will be always "semi-on" with 120V in there.
Anyway, I think I'm going to try grounding the secondary and see what happens :)
 
Sounds very like it, doesn't it.

If your stepdown transformer is an isolating transformer of decent manufacture then you can bond one end to earth and then you have a 120V standard American supply.

The biggest pain in recording studios is 19 inch rack mount equipment and multiple earth paths. All your 19 inch gear should be mounted with insulated bushings so that there is no common earth paths between the ears of the equipment.

Ideally, you want to run the whole studio off one plug, and then you have a single ground point for all the electronics. If you use multiple plugs and especially multiple plugs on seperate circuits that go back to a distribution panel then you are almost guaranteed to have circulating currents on signal earths, with the attentive rabid hum.
 
Yeah, the desk is purchased from a canadian studio and is rated 120V, so I ordered a step-down transformer for it.
Unfortunately, powering the whole studio from one receptacle is impossible, because the desk alone is rated 30A (at 120V).
Now, regarding rack gear. I've made a test when I disconnected everything including patchbays and left only the desk and master-patchbay section connected. All other gear was physically disconnected from both mains and signal wiring. And the noise was still there (step-down trafo wasn't bonded to ground though). This leads me to the conclusion that the mains is bad. Particulary I've measured the voltage between neutral and ground and there was about 1V floating.
I've tried bonding the secondary to ground and it didn't make any difference. However I'll do a "disconnect everything" test once again and see if anything changes.
 
dbuckley said:
The biggest pain in recording studios is 19 inch rack mount equipment and multiple earth paths. All your 19 inch gear should be mounted with insulated bushings so that there is no common earth paths between the ears of the equipment.
Click to expand...

In decent equipment, it doesn't matter how many earthing paths exist since the signals won't be referenced to the chassis. Classic large audio system design* dictates solidly-grounded chassis & racks (same as telecom). And, even the if individual rack mounts were isolated, the chassis would still be connected by the EGCs.

BTW, aside from balanced signal connections, another classic wiring technique is to lift the shield at one end of an unbalanced connection to break any loops. The signal reference must, of course, come from somewhere, but you only need one per device, not 2/4/8/etc.

*see Audio Cyclopedia (H. Tremaine) or similar works
 
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