Noise generated by alien earth loops plagues many a studio — but earth loops themselves can be surprisingly easy to avoid, as Simon Bateson explains.

If I told you there was a guaranteed way to avoid suffering earth loops when wiring up equipment to your desk inputs, which required no more than a quick modification to your cables (or the building of some adapters) and which allowed all the equipment to keep its earth wires attached, would you be interested? Well, there is, and this method also applies to PA systems — possibly even more so than to studios, since there are much longer cable runs, less control over the equipment to be plugged in and no time to spare!

The advice following is not secret and has probably been demonstrated before, but is well worth repeating and explaining. The first principle to remember, and which gives the clue to how to approach earth problems, is that a signal source such as a guitar amplifier or pedal produces a signal which is a voltage difference between the output 'earthy' side (a jack socket body, for example) and the output line (the jack socket contact). The desk input — or whatever we are feeding — is required only to measure that difference.

In a multi-device installation such as a studio we usually connect all earthy sides together, happily assume that they are connected by infinitely small resistances, call this theoretical common point 'earth' and measure all signals relative to it. In reality earth wires do have some resistance, and that is the start of our problem.

Figure 1 shows the most common occurrence of an earth loop, when the circuit completed by the mains protective earths and the signal interconnection screen forms a single-turn transformer, picking up any magnetically-coupled AC fields from nearby mains cables. A significant current can flow through the screen, which has a finite resistance (RS) and therefore develops a voltage (VI) along its length. This voltage is in series with the signal path and is therefore added to it — it is 'series mode interference'.

The usual way to reduce the effect of the earth loop is to break it at some stage, or to insert a resistor which reduces the magnitude of the current flowing — but this will only work reliably if it is the mains earth side that is broken. If we break the signal screen or insert a resistor there will still be a voltage induced in the loop and the device output voltage difference will not be accurately transmitted to the receiver.

On the other hand, if we remove the mains earth and keep the signal earths connected, the receiver will accurately measure the source output voltage difference. But then there is no protective earth, since signal leads cannot withstand earth fault currents — and no earth at all if the signal lead is unplugged!

The safe and correct answer is to use the balanced inputs on the desk even for unbalanced sources. Figure 2 shows the idea. A balanced input only measures the difference between the + and - input lines and does not amplify 'common mode' signals — that is, if the + and - inputs suddenly both went exactly 1V positive relative to earth, the desk would detect no signal because there would still be no difference between the + and - inputs.

This idea works as long as we don't overload the balanced input by exceeding its 'common mode range'; for most electronically balanced inputs this is up to about +/-10V. Transformers, of course, can have common mode voltages right up to the point of insulation breakdown, which is why a transformer is the ultimate earth-loop breaker, but we're trying to be affordable here.

We take our balanced differential inputs right to the signal source socket, thereby measuring the output voltage difference which, after all, is what we're after. We do need some sort of earth interconnection to make sure that if, for some reason, there is no mains earth on the signal source, the balanced inputs are kept within their common-mode range. We don't need a very good earth connection though, so to prevent significant earth currents flowing we insert a series resistor of, say, 47 ohms in the screen connection.

And that, more or less, is it. Most desks with balanced microphone inputs also provide balanced line inputs, but just in case you need to deal with big signals, for instance from a guitar amplifier output, use the attenuator circuit shown in Figure 3.