Last month, Peter Buick examined some of the problems of interconnection in the studio. In part two, we discover the practical applications from MIDI to the future — a single coax cable. The first instalment ended with a description of earth loops. We begin this month by setting out how to solve them.

Star point earthing is the easiest way to overcome a ground loop, although it may not totally isolate the problem.

A central point is selected as the mains earth point and all mains earths are derived from here in a star pattern (ideally of equal distance). This is much less prone to earth problems than ring mains or series linking mains blocks into each other as the earth route resistances are more closely matched.

Mains blocks can be used to create a star point by using one (or more, linked with very short cables) as a parallel feed to each of the required number of mains blocks. The mains blocks must, however, be good quality as some seem to have very poor earth contacts. You may prefer to use terminal boxes or choc blocks to star them.

Another point to consider, but beyond the scope of this article, is to have a technical (clean) mains supply with its own technical earthing stake. This is fed from the fuse box separately from the dirty, domestic supply and is as free from domestic interference as possible.

Removing the signal screen is the most common cure for ground loops in totally balanced systems.

As the screen is disconnected at one end there is no circular path for the current to create an earth loop. The signal is unaffected by this method as it is between hot and cold pins and the equipment is electrically safe as the mains earths are still connected.

It is normally the destination end that is disconnected, but refer to last month's article on matching units.

The screen is not 100 per cent effective when this method is used, but, with the common mode rejection offered by a balanced system, it is satisfactory.

In unbalanced systems life is not quite as simple, as the screen connection carries both the cold and screen connections.

There are two options. A balancing system can be inserted or one end of the screen can be disconnected (though mains interference noise may be worse). With the second alternative the signal potential is derived from the mains earth of the equipment which is linked to the audio screen by the manufacturers — unless the technical and mains earths are separated.

Separating the technical and mains earth is by far the most sensible solution to ground loops. Why manufacturers do not pay more attention to it, I do not know.

In reality, there are two types of earth. The mains earth simply functions as an earth to prevent the chassis becoming live. The signal earth is really 0V of the DC power supply and is called technical earth. If this is not connected to chassis earth, there is no earth loop problem.

Most manufacturers acknowledge this and provide a means of separating the technical earth and mains earth. Sometimes this is in the form of a handy switch, other times it is a screwdriver and soldering iron job.

To be ultimately finicky, especially in non-static situations, the technical earth (0V) should be tied to mains earth so that it cannot float above mains earth.

Normally the technical earth would only float by a few millivolts. If there was a fault in the equipment, however, you could get a shock when plugging in. This is because the screen of the plug is 0V and the chassis is mains earth. If the 0V becomes live you will be touching both mains earth and technical earth when you plug in.

Some manufacturers are already building the solution to this problem into their equipment, however. By connecting a 10 ohm/1K ohm resistor between technical earth and mains earth, 0V always has a mains earth path via the resistor.

If there was a fault, therefore, most of the current would take the easier path (ohms law) via the linking resistor (as long as you are more than 1 Kohm resistance).

Audio earth goes via the audio screens as they are the easier path (less than 10 ohms) — hence no earth loop.

If anybody has any comments on this, I would definitely like to hear them.

There are only two tools which are essential for installing equipment. The first is a resistance meter, or buzz box, which can be used to test where earths are going, or, more to the point, not going.

The second tool is an understanding of what is going on. Although it is not the engineer's job to solder leads, an engineer has to be aware of the wiring standards he is using, and whether or not he is using the right lead for the job.

The variety of equipment standards, however, can make this knowledge hard to apply. This problem is central to interconnection.

All the subjects that I have covered are underscored by this difficulty:

Balanced input to unbalanced input needs to have the balanced input linked between cold and screen at the balanced end, or instability or level errors will occur. Unbalanced outputs to balanced inputs should be connected hot to hot, screen/cold to cold and screen at one end only. Differing XLR standards will cause a phase problem. Taking the mains earth off is dangerous and there are other ways of getting rid of earth loops, but you will need to know the standards of the equipment you are using.

The only answer is a unit labelling system.

PIN POINTS

• If you find a hum, it is worth checking that it is not induction from an adjacent piece of equipment. It might also be caused by strange earthing being set up when the metal chassis of two pieces of equipment are touching together or via the rack.

• It is a good idea to build some DI (Direct Injection) points into the mic wall boxes, or to extend some line inputs into the studio as well. Although much DI is done in the control room (such as keyboard overdubs) few are really large enough to cope.

• The variety of connections (phonos, 3.5 mini jacks, ¼" jacks, GPO 316s, TTs and, of course, XLRs, balanced, unbalanced and pin hots) on different pieces of equipment is often a hindrance. One studio I know has built a box with different sockets on it wired in parallel. This can then convert all sorts into the TT for the jackfield. Kelsey Acoustics' Missing Link is similar and also has a built in cable tester. Otherwise, I am sure you will be able to find someone who is able to build you something similar.

• With the proliferation of MIDI, spare MIDI leads (DIN to DIN) and some MIDI switching or patching may save time in the long run.

With the increase in MIDI controllable effects gear, the MIDI terminals need to be brought up somewhere central. A 5 pin DIN patchbay (as available from ADO and others) is something I never thought I would see!

• Speaker cables are a great disaster. Many believe in using monster cables. Unfortunately, the manufacturers never provide a receptacle with enough area to match, so, to an extent, the cable should be limited to what fits in the terminal post. Spade connectors often provide more surface area (and hence less resistance), so they should be used where possible. The use of at least 79 strand cable is recommended, not that many of you will be using anything less.

• Another frequent mistake is burying the equipment cables. They should be laid in white conduit, or similar, after the building. This saves a lot of poking about and demolishing when you suddenly realise you need another cable run for when we get to the stage after the video suite.

What that next stage will be, I am not saying. I might not even know. No doubt the future will remove interconnection problems. There is already a visible trend for self-contained boxes.

The increase in computer and digital equipment will also, no doubt, leave us with just one mains plug, an RS 232 and a coax lead so that we can see what is happening on a television screen.

In the meantime — happy plugging.