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Build A Hum Loop Isolator

Quote from a manufacturer's leaflet:- "If hum trouble occurs when two pieces of equipment are connected together, remove the earth from one of them." Never do this — EVER! I think I know what he meant to say, but let's take it at face value. It is your first big gig. You have a mixer-amp (which makes a funny noise occasionally) but it only has a 50 watt amp and your gig is in the town hall. In the nick of time, your brother-in-law meets this guy in a pub, who says you can use his bass stack for a bit more power. When you set up the gear you start having troubles. Your amp works O.K. His amp works O.K. When you connect "Slave Out" on yours to "Slave In" on his, all hell lets loose and starts humming like the combined Choirs of the Upper Volta. Your brother-in-law remembers that you should remove one of the earth leads from the main plug. This he does (on the plug to your mixer amp, because it doesn't seem fair to mess about with the other guy's equipment) and carefully tapes back the earth wire so it can't touch anything.

The gig is fantastic, and at 11.30 when your friend has to go, they are still yelling for more. So you give him his amp back.

At 11.50 your mixer-amp starts making noises again. While you give it a swift kick, Joe the bass player picks up the mike to apologise for the poor sound, you hear a crash and turn around and Joe is lying on the floor twitching. You remember not to touch him, grab the power cable and pull the plug out of the wall socket. Joe has stopped breathing, and neither the First-Aid officer on duty, nor a young doctor in the audience, can do anything for him. He is taken off in an ambulance but is dead on arrival at the local hospital.

What went wrong? There were several mistakes made that night. I imagine neither the mixer-amp, (which you knew was faulty) nor the borrowed amp, had been checked since they left the factory. No-one checked the wiring on stage. If you must kick troublesome amps, disconnect them first. Also none of the band knew how to treat electric shock cases (The St. John's Ambulance Brigade will teach you).

However the lethal mistake was that no-one replaced the earth lead on your mixer-amp, after the other guy took his amp home: or more specifically, THAT ANYONE TOOK THE EARTH LEAD OFF IN THE FIRST PLACE. By the time your brother-in-law got things working "properly", the only safety earth to the mixer-amp was through the slave-link to the borrowed equipment - and that was disconnected at 11.30. After that you were as much at risk as if you had been running the amp off a light socket.

If that earth lead had been on, even with all the other faults, Joe would probably still be alive. (Assuming that the stage wiring was correct — and all properly professional bands check the theatre wiring before every gig — don't they?) Before you give up the music business altogether and go into the soft toy trade, let me say that we have an electrical supply system in this country which is possibly the safest in the world, and which, if everyone follows the rules, will make most amplifiers marginally safer than the average electric toaster.

Now it's easy for you to follow the rules and I'll give you a list of them later, but you still need to have your gear safety-checked by an engineer every few months, to ensure that the man who made it, and the man who put the plug on, and the 27 other people who have ever had anything to do with it, ARE ALSO FOLLOWING THE RULES.

The rules are:
1. Don't modify equipment unless you are professionally qualified to do so.

2. Don't trust any equipment which is not checked regularly for safety. Especially don't trust old secondhand gear, converted American equipment, and anything with a built-in non-detachable mains lead.

3. Find out from the manufacturers the correct rating and type of fuses for your equipment, and keep spares. Then you never have any excuse for fitting oversized fuses; never, ever use silver paper.

4. Any electrical equipment should have an undamaged flexible cable for mains supply, with no joints, and of adequate size for the current it must carry. (This information is also available from the manufacturer). Mains connectors should all be wired correctly, and you will only rarely need a fuse larger than 5 amp in a "13 amp" plug.

5. Any piece of electrical equipment used on stage must have a substantial earth lead which is reliably attached to its chassis, and runs from there to the earth pin of the mains plug, either as part of the mains lead or taped to it.

6. Anything which consistently blows its correct fuses should be sent for repair at once.

7. "Playsafe" devices in all guitar leads are an additional safety factor, but become useless and give a false sense of security, if their special fuses are replaced by ordinary, larger ones against the manufacturer's advice.

8. Buy a Martindale mains tester (Mate Tools Ltd., Hoddesdon). This looks like a 13 amp plug with neon lights in the back, and one should live permanently in each Distribution Board. Until it shows that the wiring is correct, YOU MUST NOT PLAY, and no contract on this earth should make you. If your gear is faulty better to lose your fee than your bass player: if the stage wiring is faulty, and I know of some which are, the Theatre/Stage Manager will probably get upset — he's likely to lose his licence. Don't be railroaded. This is your right, but there is also a question of tact and goodwill: professional bands carry 50 or 100 yards of heavy duty extension mains cable, so that if there is a safe socket in the building, they can work from it. If they don't want a fire on their hands they will also remember to uncoil ALL the cable before they pass any current through it. I also knew a band who fitted their distribution board with an earth terminal, 50 yards of wire, and an enormous and vicious-looking bulldog clip for the nearest cold water pipe (hopefully not plastic).

9. Use unbreakable plugs and distribution boards; the best ones are made by Duraplug, expensive, but worth it.

10. Protect all equipment from liquids such as rain water, or beer, but do not prevent a free flow of air getting to the amps.

If you think I have missed anything please let me know.

Before you can get an electric shock on stage, or anywhere else, at least one piece of equipment must be faulty, damaged, or faultily wired, in such a way that a high voltage appears on its chassis and/or input-output connections. If you follow the rules I have given, the chassis and sockets (and therefore everything connected to them) are wired directly to earth from the amp.

Unless you are absurdly unlucky any dangerous voltage on the chassis will cause sufficient current to flow down the earth lead (and therefore up the live lead) that one or more fuses blow, cutting off all power to that piece of equipment. This applies to England: in other countries, particularly America, the only solution I would trust, is separate expensive and heavy and they are definitely NOT "Auto-Transformers." (Although they may also perform the common function of changing voltage).

Let us move to the scene of your next gig. You have followed all my advice and are feeling disgustingly good and virtuous. You also notice that there is once more a loud and unpleasant hum coming from the sound-system. Now it may strike you as unfair that you only have hum troubles when your equipment is wired safely, but there is a simple and safe way of dealing with the problem. What you have is a hum loop, and all you need to kill it is a small transformer in a metal box with some sockets and things. The hum-loop occurs basically because the outer screening braid of your slave link is also one of the single input leads to the slave. If an amp is earthed correctly there is always a very slight leakage down the earth lead, which will vary from one amp to another. For this, and other reasons, if 2 amps have their chassis connected together twice, once via their earth leads and the mains wiring, and again by the outer braid of the slave-link, a small signal may be injected into the slave at mains frequency. Some professional equipment uses a "2 wire and screen" input system to avoid such problems, but if you are not so fortunate, all you need to do is break the slave link in such a way that your vocals get through, but the leakage current doesn't.

This is one of the jobs a linebridging transformer can do for you and I have been using the same one, with complete success, for the last five years. I have rewired it neatly for the photographs, and included a switch and a volume control to make it more versatile.

It is not necessary that you understand how this works — you need only mount the components and wire them up as shown in the photograph. The only difficult part is the 8-pin socket, and it's tags should be numbered on the plastic base. You will however need to be able to solder. I have mounted the transformer on the outside of the box, which makes the wiring inside clearly visible. If the transformer is removed while travelling, and kept with your space valves, it will be perfectly happy. Otherwise use the same wiring, but mount it inside the box with epoxy or capacitor clips, make all connections to the socket and then push it onto the pins of the transformer. You will have to provide a separate screw for the solder tag.

Any wires which are not short and stiff should have sleeving over them, to prevent them touching each other or the box. Also you should scrape down to clean metal, on the inside of the box, around all mounting holes (except for jack sockets).

For those of you who are technically minded, the accompanying circuit should explain what is going on. Please note, the two capacitors are primarily to prevent any Direct Current flowing into the transformer. Because of their size, they are not safety isolating capacitors, and this box MUST NOT be considered as a safe way of connecting to a faulty amp.

There are two other problems which commonly occur when slaving equipment. The first one, is that your amp may not have a "slave out" jack, and while it will obviously not produce the highest quality, you can safely connect a slave through this box to a spare loud speaker jack on your amp. Also you may wish to slave-up a small combo amp while preserving its own characteristic distortion. You can connect to the extension speaker jack if there is one, or direct to the speaker terminals with INSULATED crocodile clips. (I know this does not include the tonal effects of the speaker, but it is pretty close). In either case it is important that you turn the original amp to its normal setting, the volume control on the box fully off and the slave volume, if there is one, fully on. Then adjust the slave output by the control on the transformer box; this prevents overloading the transformer.

The second problem which may occur is that when using mixed equipment, it is easy to end up with speaker systems out of phase, for instance, when your amp speakers are pushing air forward, the slaved speakers are trying to pull it back.

You could change round speaker leads but you may not wish to alter hired equipment, in any case it takes time, and one of the rules is that you do not modify equipment. The switch on this isolator box will reverse the input to the slave, safely. To get this right, feed in a low note (about bottom A on guitar) and adjust equipment so that each speaker produces roughly the same volume.

Then place speakers side by side and listen in the centre about 4ft. away. The switch position which gives most bass is the correct one. Under unusual conditions, you may find that reversing the switch will reduce feedback troubles.

In fairness to our better manufacturers, I must say that their reliability record is impressive. I am constantly amazed that so many musicians and "engineers" use so much equipment with a total disregard of safety precautions and common sense, and survive. Many of them have, for years, been relying on the ability of modern equipment to remain safe under totally unsuitable conditions, with no regular maintenance.

Operation and use of group amplification undeniably presents a certain hazard, but there is no reason why this hazard should not be reduced to minimal proportions by a little common sense.


Should you have difficulty obtaining components, I have listed below the appropriate catalogue references from R.S. Components Ltd. Components from this company can be ordered for you by most Radio/T.V. service engineers, but they may cost a little more than others. The two capacitors may be any value over 1 microfarad. They must be paper, plastic film or mixed dielectric types. NOT any form of Electrolytic. If you have the slightest doubt about the safety of your equipment, they should both be marked with a working voltage of at least 600v DC and/or 300v AC. (NOT 300v DC).

The 22K resistor may be rated anywhere over 1/8 watt, and for the small cost involved, it is worth using a Carbon Film or Metal Oxide type.

I have used a strong (and expensive) diecast box. This is not essential and you can use a large tobacco tin, a tea caddy, a cigar box lined with kitchen foil, or any metal-lined box which has a lid, enough space inside, and will stand a few holes in the top.

Both jack sockets should be isolated from the box. I have used the type with a metal fixing bush and a stepped plastic washer. To be safe, this requires a clean, round hole, and I would prefer you to use the type of socket with a projecting plastic thread and a plastic fixing nut. Then if your hole is a little ragged, it doesn't matter.

You can use almost any volume control, Log or Lin, between 22K and 100K. The value given is the most useful, and if you are planning to use the box for tapping off speaker leads, it should have a 2 watt rating;

Qty. Description Value Voltage (Min) Watts (Min) R.S.Cat.Description
1 Resistor 22K - ¼ Hi Stab Carbon Film
2 Capacitor 1 mike 600 DC - Mixed Dialectric
2 Jack Socket 1 mike 2pole mono - Open J Skt.
1 Linear "Pot" 50K - 2 Moulded Lin. 50K
1 Octal Socket - - - V/Holder Octal.
1 DPDT switch - - - Tag H/D Toggle DPDT
1 Knob to fit Pot - - - A1 Knob (32mm) Lined
1 Gardners Wide Band Line Bridging Transformer type MU 7530.
Assorted wire, non-melt sleeving, nuts, bolts, one solder tag to suit Octal Socket mounting bolts.

(1 Transformer costs £6.93 inc. Post and VAT from Gardners Transformers, (Contact Details)).


Under certain conditions, the capacitors used (1 microfarad) may cause some loss of signal below 60 Hz. With most P.A. and instrument speaker systems, this is unlikely to be detectable, however, increasing the capacitors to 2 microfarads each, will maintain full signal to about 30 Hz. Also the jack sockets should be described as "mono".

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International Musician & Recording World - Copyright: Cover Publications Ltd, Northern & Shell Ltd.


International Musician - May 1975

Donated & scanned by: Mike Gorman

Feature by Stephen Delft

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