Earth Hum

[Cliffb]

Hi all,

I wonder if anyone can help please.

Just completed the audio upgrade at a local venue. It consisted of a new mixer (Soundcraft LXii7) a couple of stereo EQ's and a dual FX engine. Turned it on and - hello- I've got a buzz. Never had a buzz before with the old kit (also Soundcraft - but no other bits). Tracked it down to an earth loop between all the above and the amps (which are a distance away in a rack room).

To get around the problem quickly I dropped the earth on the mains feed to the mixer and processing kit. I've metered an earth to the mixer etc but obviously its down the screen of the main audio out cables which I'm not overly happy about from an electrical safety point of view.

The options that seem open to me appear to be one or more of the following:-

1/ do nothing and leave the kit earthed down the mix out cables

2/ reinstate the mains earth and drop the earth on the mix out cables

3/ put 1:1 isolating transformers in the mix output line (recomendations please)

 

Anyone got views on which of these would be best ...or any other suggestions?

 

Many thanks to all

 

Cliff

[David Lee]

Put your earth back on the mains for control etc as soon as you finish reading this sentance.

 

Isolating transformers from the sends to the amps is probably the best way to go.

 

Further down the line, look at dedicated power to control and amp room.

 

Now go and put that very important bit of cable back on and remember never to drop mains earth again.

[Shez]

Now go and put that very important bit of cable back on and remember never to drop mains earth again.

Repeated in case you missed it the first time. Never ever ever ever remove the mains protective earth - it's there to save your life if a fault develops in the equipment.

 

If you're sending a balanced signal from the mixer to the amps, you should be able to lift the audio screen on one end of the cable to break the ground loop. Which end to lift is a little dependant on the kit at each end - if you make up a couple of XLR ground lift jumper cables, (often useful to have as a get-out-of-jail in any case) you can try each end and see which works better.

[KevinE]

This is a good paper to read (from the Rane technical library).

 

balanced interconnections

 

And here is an abstract if you can't be bothered to click on the link!

 

A balanced line requires three separate conductors, two of which are signal (+ and -) and one shield (see Figure 1a). The shield serves to guard the sensitive audio lines from interference. Only by using balanced line interconnects can you guarantee (yes, guarantee) hum-free results. Always use twisted pair cable. Chassis tying the shield at each end also guarantees the best possible protection from RFI (radio frequency interference) and other noises (neon signs, lighting dimmers).

[dan slv-tech]

Jsut to reiterate, never drop the mains earth, its there for a reason.

 

There are several things you can do:

 

1) check that all audio is from the same mains phase

2) drop the ground on the audio at the amp end

3) check the hum isnt coming from the inserts - if so here layeth your problem, make some proper insert cables (impedance balancing)

4) use isolation TXs in the system if the above have failed

 

dan

[KevinE]

Surely the mains phase wont affect a ground loop or hum issue?

[dan slv-tech]

not impossible....

[Cliffb]

Jsut to reiterate, never drop the mains earth, its there for a reason.

 

There are several things you can do:

 

1) check that all audio is from the same mains phase

2) drop the ground on the audio at the amp end

3) check the hum isnt coming from the inserts - if so here layeth your problem, make some proper insert cables (impedance balancing)

4) use isolation TXs in the system if the above have failed

 

dan

 

OK OK - point taken guys ... re the earth. Although I'm a little bemused. If there's a circuit to earth from the chassis to a nearby ring main socket where's the problem?

I'll drop the earth at the amp end. Getting a dedicated mains is too big an ask at this stage. As it didn't hum before with the original system I doubt that that is the problem.

Re isolation TX's - any recommendations?

 

Cliff

I've dumped the inserts one by one and no change. Maybe I'll try dumping the lot first, just to be sure.

[bruce]

I'll drop the earth at the amp end.

 

NO... don't drop ANY mains earths. At all.

 

If you must do something, remove the screen connection (ie the SIGNAL ground, not the mains ground) at one end of the links between the amps and mixer, as stated in the 3rd post in this thread. Does your amp not have a "ground lift" switch?

 

Or do the job properly, and get isolating trannies. They've been discussed here before at great length....

 

Orchid Electronics http://www.orchid-electronics.co.uk/ do a nice stereo boxed version for about 30 pounds. Or you could get one of the cpc cheapies for about a fiver (aimed at the car audio market, but sound fine), chop off the phonos, wire on a couple of XLRs and job done. Or any of a dozen other suppliers.

[mackerr]

This is a good paper to read (from the Rane technical library).

 

balanced interconnections

 

And here is an abstract if you can't be bothered to click on the link!

 

A balanced line requires three separate conductors, two of which are signal (+ and -) and one shield (see Figure 1a). The shield serves to guard the sensitive audio lines from interference. Only by using balanced line interconnects can you guarantee (yes, guarantee) hum-free results. Always use twisted pair cable. Chassis tying the shield at each end also guarantees the best possible protection from RFI (radio frequency interference) and other noises (neon signs, lighting dimmers).

 

While Rane note 110 is a great resource on how to wire cables, a better source for information on balanced and unbalanced signal lines and how and why to do things can be found here. Read all the linked application papers. Bill Whitlock is the go to guy for these questions. I have been to his grounding seminar 3 times, and I hope I get the opportunity to go 3 more times.

 

Mac

[David Lee]

Although I'm a little bemused. If there's a circuit to earth from the chassis to a nearby ring main socket where's the problem?

you said you dropped the earth. Now stop trying to drop the mains earth.

[Shez]

I've metered an earth to the mixer etc but obviously its down the screen of the main audio out cables

Although I'm a little bemused. If there's a circuit to earth from the chassis to a nearby ring main socket where's the problem?

And what happens when somebody unplugs that signal cable? No more earth... Not to mention the fact that in the event of a fault, the cable screen's comparatively high resistance will likely slow or prevent the fuse blowing and the cable will also get rather hot. During which time the chassis will be zapping anyone touching it.

The mains earth is there for a good reason; "I removed it to stop a hum" won't get you very far in court should the worst happen.

[dbuckley]

Another option is to get a mains isolating transformer.

 

The local live and neutral go to each end of the primary of the tranny, and the earth goes to the transformer case. This device should be plugged in via an RCD plug, which will save the day if the transformer faults.

 

You have an outout socket which has the neutral and earth pins both connected to one end of the secondary, and the live pin connected to other end of the secondary. Run everything audio at the mix location off this output.

 

You now have a safe, local, earthed power supply, but a supply that has no reference to any other earth.

 

End of loop.

 

Note that many 'islolating' transformers are nothing of the sort, for example, UK building site transformers.

[dan slv-tech]

you could also try a couple of other things.

 

1) switch on just the amp, nothing else - does it hum?

2) now switch on the mixer with nothing connected, just the audio out lines to the amp - does it hum?

3) now patch in lines to the mixer one by one - does it hum yet?

4) patch in any inserts EQs, compressors, effects units one by one - humming yet?

 

its possible you have a bad audio earth somewhere, or wired incorrectly - is your outboard in a rack by anychance? if so id lay penny to a pound its that thats causing the

problem - all the kit is mains earthed (by now I hope so) and theyd all be earthed together by the rack, and then earthed again b audio earth in the audio cables - multiple routes to earth cause earth loops (hum)

 

If indeed it is the outboard then you will need to adress the cabling that attaches that to the mixer, prob need to make up some insert leads of your own, using inpedance balancing, which we can go through if needs be.

 

above all make sure all the kit is earthed via the mains where appropriate please- its there for a reason afterall, and it means we can all stop nagging you and sleep easy!

 

dan

[PoppaDom]

Another thing that has not been mentioned yet is the fact that it would be, at this stage, worth checking the earth loop impedance/presence of the earth at both of the locations.

I have had earth buzzes caused by a bad earth connection at one end of an install resulting in the path to earth being (due to fault not design or intent) made through the signal cable. Re-terminating of the earth cable and retest at the amplifier end resulted in the earth induced buzz disappearing completely.

Granted the install I had problems with was older than time itself but as you state you have upgraded an existing install it may well be worth examining this.

 

Regards

Dom

 

Edit: I should point out the buzz was caused by nominal earth leakage current from the amplifiers power supplies.