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How It Works - Interconnection (Part 3)

If a pair of trendy phono leads for your record player can cost as much as £100 or more, how much should it cost to cable up your studio? David Mellor has the answer...


If a pair of trendy phono leads for your record player can cost as much as £100 or more, how much should it cost to cable up your studio? The answer could be quite a lot, but if you throw away those HiFi mags you can do it properly with good old-fashioned copper wire for a more reasonable sum. David Mellor tells how....

I pride myself in having an open mind and the last thing I would do is flatly contradict those who claim that they can hear sonic differences in cables. Apart from resistance effects in speaker cables I have never detected such phenomena myself, but after all - I am only actively involved in producing music recordings, I am not engaged in the much more prestigious field of passively listening to them! (Please register note of sarcasm!)

To be a bit more serious, I used to be one of those who maintained that you could not hear differences in amplifiers, till I actually got down to doing some listening tests and found that I could. I am converted to a certain extent now, but I will say that the differences in amplifiers - and the differences between cables, if any - are orders of magnitude less than other electrical and acoustic effects produced both in the recording studio and the listening environment. I would also say that the recording you produce is created on your monitor speakers and you will automatically take into account the almost zero-order effects of cabling heard by HiFi gurus in higher states of enlightenment. You do have to reach a certain standard, however, and that is what this article is all about; how to maintain your highs, reduce your crackles and, above all, thwart the dreaded 'hum monster'!

There are many opinions on methods of interconnection, much folk-lore and scientific theory. What I am going to tell you is based on accepted electric circuit theory and hard won practical experience. In other words - it works for me!

CABLE THEORY



The purpose of an electrical cable is to carry signals from one end to the other without either gaining or losing anything. What goes in is what comes out, in other words. In this imperfect world this can never happen, but there are methods to minimise the problems.

What we might gain in the cable is interference. This could be clicks generated by refrigerators, thermostats, etc, elsewhere in the building (or even the building next door), or hum - which lurks anywhere there is AC mains nearby. Worst of all is dimmer noise from lighting control units. Theatre and PA sound engineers know all about this little nasty, which can work its way into the most professional of set-ups.

There are two ways clicks and buzzes can get into your system: through the air and through the mains wiring. Mains-borne interference can normally be dealt with using 'mains filters' which plug between your equipment and the wall socket. They vary in price and capabilities from cheap and cheerful, to medical standard - which will prevent your heart and lung machine from throwing a wobbly! You will need to have a good mains earth, however, which is more your local electrician's province than mine, so consult him if you have any doubts (see local Yellow Pages). You might be doing yourself a favour in more ways than one.

Airborne interference is the topic for today because this is how gremlins can get into your music via the cabling. We all know how radio waves can travel through the air from the BBC to your transistor portable; well interference works in exactly the same way. Electromagnetic radiation (of which radio waves are just one kind) is created anywhere there is an electric field, the problem areas as far as we are concerned being mains wiring - which causes the hum problem - and switching sparks, which cause that crackly noise we all know and hate.

Figure 1.

Copper wire, or any sort of conducting wire, is heaven for electromagnetic interference. The attraction is of the 'dog to lamp-post' order and an efficient solution in either case would be some form of fence to protect our nice clean signal (or lamp-post) from being sullied. This is done, as in Figure 1, by wrapping a wire braid around the signal-carrying conductor. Interference gets into the braid (or 'screen' as it's often called) and is conducted harmlessly away to earth, or at least most of it is. This is a simple and largely effective system and has the advantage that the screen can be used as part of the electric circuit and, indeed, is in most domestic and semi-professional audio equipment. If your school physics is a bit rusty it would be as well to remember that electricity can only travel in a circle - or circuit - and that the torch bulb must be connected to both ends of the battery to make it work.

Figure 2.

The disadvantage of using the screen as part of the circuit is that it can act as an open door to hum (cue Hammer Horror music). You may have seen one of those portable TV sets with a circular set-top aerial. This works by picking up the magnetic portion of the electromagnetic wave carrying the TV signal. Any loop of wire which has little electrical resistance will do this. Look at Figure 2.

Oh no!!! A loop of wire with little electrical resistance which forms part of the signal-carrying circuit. Do I need to say that this is bad news? This is the infamous earth loop situation. Any audio installation which uses the screen in the signal circuit and is connected to mains earth at more than one point will hum like crazy. I shall explain how to overcome this problem later, but in the meantime don't go disconnecting any earth connections on your equipment or I might find myself with a lawsuit for encouraging death by electrocution. I might write in a light-hearted manner but this point is serious. Never use equipment with the mains earth disconnected unless it is earthed in some other way. People (musicians) have done this and not lived to tell the tale, so take heed.

Losses in the cable are caused by signal 'leaking' between conductor and screen. If you use cheap cable then this is what you can expect. There are ways of working out mathematically how much you will lose, but that is beyond my scope here. Wait for 'How It Works: Electronic Circuitry', if the editor will let me write it! [Permission granted - Ed.]

BALANCED & UNBALANCED



Figure 3.

The system which I have just discussed, and with which you are probably most familiar, is the unbalanced system of interconnection. The balanced system, which is normally used in fully professional equipment, uses an extra signal conductor - thus removing the need to use the screen for this function. Figure 3 explains.

The term 'balanced' really means 'equal and opposite'. The signal on conductor A is balanced by an equal and opposite signal on conductor B. (Equal size but opposite polarity.) The balanced signal - say from the balanced output of a microphone - travels down the cable to the balanced input of the mixer, where the difference between the voltages on the two conductors is measured and passed on to the rest of the circuitry. (A mathematical explanation is given elsewhere in a separate panel.)

Mathematical description of balanced line operation.
Let the signal voltages on the conductors be +V and -V. Call the interference voltage on each conductor X.

Voltage on conductor A: + V + X
Voltage on conductor B: - V + X

The balanced input stage responds to the DIFFERENCE;

+V + X - (-V + X)
=+V+V+V-X
= 2V

The signal interference has thus been rejected.

The clever part of all this is that any interference which gets through the screen affects both conductors equally. As the balanced input stage responds only to differences in voltages in the conductors, the interference is rejected. Earth loops are also thrown out of the window because the circuitry at either end of the cable pays absolutely no attention to what is going on electrically in the screen, as long as it is connected to earth. You can have as many earth connections as you like.

There is a special technical term for the amount of interference suppression a balanced input stage gives; it is called the Common Mode Rejection Ratio which is measured in decibels. A figure of 60dB would be good for a microphone input and should cover most eventualities. Much depends, however, on the construction of the mic cable, which I shall mention later.

BALANCED MEETS UNBALANCED



So far we have discussed balanced connection systems and unbalanced systems. In the real world we are going to have to use them both together, which is where it gets a bit complicated. A few diagrams and a few words of explanation will cover most combinations. I shall start with the simple situations and work up from there...

[1] Unbalanced to Unbalanced.

This is the normal semi-pro and domestic system. As I said earlier, there must be only one connection to earth in the system or you will get mains hum. There are two ways of doing this which I am afraid are both unsatisfactory in certain respects. We have to get the job done somehow though and with a little brain exercise either method can be made to work well, and safely.

Figure 4a can represent any number of pieces of equipment. As you can see, both conductor and screen are connected at both ends but only one item of equipment is connected to mains earth. The problem here is that if you are in the habit of moving gear around, setting it up and taking it down, there is the risk that you will plug in and switch on something that has not yet been connected to earth via the screen of the signal cable. If the equipment is faulty you could be in for a shock - literally.

The moral is to set up first and switch on later. Never connect anything to the mains until it has its signal wire hooked up to the one item that has its mains earth connected.

Figure 4a, Figure 4b.

Some equipment has no mains earth, of course. This is because it is constructed in a way that gives a high degree of mains safety and passes the relevant British Standard. Although you may plug this in at any time without an earth connection, it would be better to follow procedure automatically than risk making a careless mistake.

If I were reading this article I would now be asking which one item should I earth? In a mobile installation such as a PA system, I would go for the mixer. It is the most central item and needs to be closest to a good earth. In a studio, my preference would be to earth the jackfield (patchbay) - although opinions may (and do!) differ on this point.

Figure 4b shows a different way. The screen of each connecting cable is attached at one end only - preferably the end electrically closest to mains earth. Each individual item can then have its own mains earth and the cable screen still fulfills its protective function. The safety advantages are obvious but the snag is that there are more routes for any interference on the earth cabling in the building to go straight into your system. It is a bit risky if you are wiring up a large installation in this way.

[2] Balanced to Balanced.

No problems. Plug it in and away you go. The professionals use this system because it saves them having to think about what they are doing!

[3] Unbalanced to Balanced.

Figure 5a, Figure 5b.

In other words, one conductor and a screen from the sending equipment have to be connected to two conductors and a screen at the receiving end. There are two ways of doing this. Figure 5a shows the usual method for mains powered (and earthed) equipment. Figure 5b shows how to connect an unearthed item like a microphone or synthesizer without a mains earth connection. In either case, get it the wrong way round and you will get mains hum. It's a hard life.

[4] Balanced to Unbalanced.

Oh dear! Complications... I didn't tell you how balanced inputs and outputs work did I? Well it's not really necessary to know the whys and wherefores, so I shall keep strictly to what you have to know to get things to work properly.

Balanced gear often uses transformers to convert one-conductor to two-conductor signals, and vice-versa. Transformers are simple in practice and they work well enough. Unfortunately, a line output transformer costs around twenty quid and can weigh up to a kilogram. I don't have to say why some manufacturers use other methods of balancing do I? Electronic balancing is the other method, often superior to transformer balancing and very cheap. However, there is a problem. Look at Figure 6a.

Figure 6a, Figure 6b.

This shows how to unbalance a transformer output by connecting one signal conductor to earth. Many electronically balanced outputs will not allow you to do this, so you must use Figure 6b. Some clever manufacturers have an electronically balanced output that mimics a transformer, so you must use 6a. Yes, I know it's a minefield! The only answer is to consult the equipment manual and don't forget that we are back in earth loop territory again.

If your system is mostly balanced, it is usually best to make any balanced-to-unbalanced adjustments at the connector nearest the item of equipment you want to adapt to your standard. If you are unbalanced, or rather your system is, and are using single conductor cable then you will not have this choice to make.

CABLE & CONNECTORS



Enough of the theory, the brain can only take so much - and we haven't made a note of music yet! The interconnection hardware is a much more straightforward matter but brace yourself, I am going to talk cold hard sense... and pounds and pence. You can't cable on the cheap but rest assured that HiFi territory is still verboten.

Ever had a dodgy cable? Who hasn't, but if you buy good quality connectors from a reputable source and solder them correctly, then you will have few problems. I am reluctant to name specific suppliers because there are many good ones, most of them with mail order catalogues. Except in the case of GPO jacks which are solid brass with a plastic sleeve, always buy metal bodied connectors and you will have few difficulties. If you can find a supplier who offers a consistent range, then so much the better. There is nothing worse than finding a good product only to find that the next time you order it you get a substitute brand. Cable is a similar matter. Don't buy the cheapest. Go to a reputable professional audio dealer and you will be OK. Steer well clear of cheap ready-made leads; have you ever opened one up and examined how well it is put together? You could be in for a fright!

Before I go on I must say a few words about soldering. If you can't do it, find someone who can and learn from them. It's dead easy with the right tools. A bad solder joint in your system can cause you no end of aggravation which you can well do without.

Studio connectors come in several sorts which I shall list:

1 Phono
2 ¼" jack type A mono
3 ¼" jack type A stereo *
U GPO jack (type B) *
5 XLR female *
6 XLR male *

(Connectors marked * are suitable for balanced use.)

Other types of connector are either too esoteric to mention here, not suitable for studio use, or not in common use in this country. You may not be familiar with the GPO type jack which, as its name suggests, is popular with the telephone people (in pre-Telecom days). It has a quarter-inch diameter like the normal guitar jack but has a smaller tip. The reason it is used in studios is because it is superbly built to take life's rough and tumble and is the basis of the jackfield or patchbay. Don't treat the two styles of jack as compatible or you will get bad connections or damage the sockets.

The jackfield consists of row upon row of GPO sockets which are connected to every input and output on each piece of equipment in the studio. A small 8-track set-up might need 120 or so sockets, which will cost you around £500. Five hundred what!! Don't forget the patch cords to connect the sockets together - you'll need around fifteen or twenty, at just under a tenner each.

I hope you didn't hit anything hard when you fainted. If you have a recording set-up already then you will know how convenient it is to have all your connections easily accessible. I know that I couldn't do half what I do without a jackfield and the advantage of using the GPO connectors is that they never let you down, or at least very, very rarely indeed. The cost is high but there are two ways around it. One is to use type A jacks, the ordinary guitar type which are not so robust; the other is to buy GPO type equipment secondhand and clean it up with a bit of 'Brasso' like I did. You will have to hunt around for your sources but you can expect to pay around a pound a socket for the jackfield and about 50p per jack. A bargain indeed.

I am sure you know all about the other types of connector. I am not too keen on phono plugs and sockets myself, but they are an established fact and we have to live with them. Just remember to buy good quality metal bodied ones and you should be OK.

Studio cabling is a bit different from microphone cables and guitar leads. For one thing, all your cable will be out of sight and not liable to disturbance or damage. It will won't it? You can therefore use less expensive cables which are more suited to the job.

Figure 7 shows some types of cables. I would recommend the foil-screened twin, which costs around 30p per metre, for most applications. II is easy to work with and normally has good electrical properties. It is fairly stiff also which makes it easy to form into tidy looms inside your equipment rack.

It is less good in any situation where it would be liable to bending and flexing, but in its proper place you cannot beat it.

Figure 7.


GO FOR IT!



The principles behind cabling are relatively straightforward but the practice needs a little practice. There are people who do studio wiring for a living, and a nice living they can make out of it too because it is a highly skilled job. If you can solder and you are considering setting up a small studio system with a full jackfield, then my advice to you would be to think about it, plan it, then go ahead and do it.

If your project is rather larger and you have not done any serious wiring before, then you probably need a helping hand. You will need to number every cable at both ends and keep detailed lists of where every wire is intended to go. It is not a trivial matter and can be very time consuming. It is worth doing properly though because there is nothing worse than wading through a sea of cables, having trouble with dodgy leads and the like to mess up a recording - especially if you have paying clients! Still, I hope my little article will encourage a few people to have a go.

Important Safety Note

"Fig8 at the bottom shows 'the safe way' to disconnect the mains safety earth from a
13A plug. THERE IS NO SAFE WAY!"


While disconnecting the mains safety earth in the plug may break a ground loop, it also renders the equipment potentially LETHAL and can NEVER be considered a safe or wise course of action.

A much better solution for breaking ground loops can always be found by isolating the signal earth connections between equipment (ie, disconnecting cable screens), and/or by using line isolating transformers.

The content below is included here for completeness only.
Thanks to Hugh Robjohns of SOS for the advisary.


Important Note: This info should be disregarded.

The content below is included here for completeness only - see note above.

Figure 8. The SAFE way to wire up a 3-pin plug without connecting the Earth wire.


Strip back the outer insulation so that you have about three inches of Earth wire (green/yellow) free. Wire up the Live (brown) and Neutral (blue) in the normal way, trimming them back as necessary. Lay the Earth wire under the cord grip coming back out of the plug. Fix the plug top back on then trim the Earth to around half an inch.

By following this procedure, the Earth wire is clamped in position and cannot work its way back into the plug and touch anything live. It is also visible, so that anyone plugging the equipment into the mains can see that it is unearthed. Writing 'Earth Lift' or something similar on the plug top is not sufficient, as it could rub off or the plug could be taken off and used on different equipment.

Remember: if you connect equipment that is meant to be earthed to the mains without making the earth connection, then whatever method you use, satisfy yourself that it is SAFE. It is your responsibility.


Series

Read the next part in this series:
How It Works - Noise Reduction (Part 4)



Previous Article in this issue

Korg DSS-1

Next article in this issue

Spotlight


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Apr 1987

Feature by David Mellor

Previous article in this issue:

> Korg DSS-1

Next article in this issue:

> Spotlight


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