Return to Zero (Part 8)
More About Microphones
Dave Simpson winds up the section on microphones with a few words on impedance matching and balancing
The perennial Dave Simpson concludes his introduction to microphones with a look at matching and balancing.
As you may remember from last month's article, a microphone is basically a device which converts sound waves into electrical energy. Last time we dealt with the ways in which this might be accomplished and phenomena arising from it. This month we shall look at questions relating to the signal whilst in its electrical form - in other words, points arising from the interface between the microphone and the recorder (such as a mixing desk), beginning with the problem of...
The impedance of a microphone (or anything else if it comes to that) is in effect its resistance to AC current. Furthermore impedance can generally be divided into two groups; source impedance (a measure of the load that an output is capable of driving), and input (or load) impedance, which is the resistance of the device receiving the signal. In any signal chain, the source always comes before the load impedance. Impedance by the way, is measured in ohms in much the same way as resistance.
Despite electrical theory which states that source and load impedances should be the same for maximum power transfer, in the case of microphone matching, this is far from the ideal situation. In practice the load impedance should be much greater than the source impedance, a good rule of thumb being a factor of ten.
Microphones come in two basic impedance ranges; low impedance, which is usually considered to be between 150 and 650 ohms, and high impedance which is about 10k to 50k ohms. Nowadays, most studio mics are low impedance, but some manufacturers (Shure for instance) also produce high impedance mics.
Why then are there high and low impedance mics? Basically, it's cheaper to build equipment with a higher input impedance and there are certain benefits in the way of low background noise to be had if you take advantage of the relatively high output signal from a high impedance microphone. Fine, you might say. All you have to do is to use a desk or recorder with a high input impedance and there's no problem - except that this is not quite the case. High impedance microphones have one significant disadvantage: when a signal is transferred along a length of cable, the source impedance acts in conjunction with the cable (or rather in conjunction with a phenomenon called shunt capacitance between the screen and the cable core) to form a low pass filter and as a result, a high frequency loss is caused. The amount of high frequency loss depends upon the length of cable, its capacitance, and the source impedance. The lower the impedance, the less the effect of the cable capacitance and the longer the length of cable that may be used before the onset of significant signal degradation. Such top end cut can be pretty drastic if a high impedance microphone is used with long cheap cable.
Don't despair though, if you find that your microphone is high impedance only. For a start, matching transformers are available (although you might find it cheaper to buy another mic!). Secondly, many older P.A. amps will only accept high impedance mics, so you might find that a satisfactory solution anyway. In any case, most mixing desks will accept high impedance mics at a pinch but try to limit cable lengths to ten feet or so.
The question of impedance brings us very neatly into the problem of balanced or unbalanced lines. What is a balanced line anyway? (And why do we need them?) There are two main types of microphone cable. The first is simply single core screened cable, in which the screen is the return wire of the circuit. This set-up is called unbalanced. Although the screen will give some protection against induced signals from stray magnetic fields (in other words interference and hum), the solution is only partially successful and becomes increasingly ineffective with longer cable lengths.
The answer is to use a cable with two inner conductors driven by antiphase signals, for example from a phase splitter transformer in a microphone. Any induced current caused by an external field will flow in the same direction in both conductors, and have the same magnitude. If the cable terminates in a balancing transformer at the amplifier end, the noise currents will flow in opposite directions through the primary winding and effectively cancel each other.
Most professional studio gear will have balanced inputs and outputs. Semi-professional equipment, if it has any balanced lines at all, will have them on only the mic inputs, since the low output from microphones renders them particularly susceptible to interference. In any event, most semi-pro multitracks are unlikely to have balanced inputs, so a desk with balanced outs would be of little use.
Since balanced lines have a screen and two cores, connectors capable of accepting three wires must be used. This is one of the reasons that XLR connectors are usually to be found on professional equipment. Stereo jacks can be used, but if your mic lead terminates in a mono jack it's almost certainly unbalanced.
Don't worry if your mic lead is unbalanced. In a semi-pro or budget environment, if your leads are fairly short, it shouldn't make much difference. (It won't make any difference if your desk doesn't have balanced inputs.)
In the previous article we mentioned phantom powering (ie. the power required by capacitor microphones to power the head amplifier and apply a polarising voltage to the diaphragm assembly). The normal phantom power voltage is 48 volts, but other voltages, from 9 volts upwards are to be found. Some microphones incorporate compartments to allow battery operation. These are usually cheap electret mics, but some high quality condensers (notably the Neumann U87) offer the same facility (handy if the recording is on location).
Phantom power used to be available only on high cost pro mixing desks, but now, semi-pro desks are increasingly offering the same facility. Don't worry unduly about the voltage damaging your other microphones. What happens is that the positive DC side (the phantom power), is applied through identical series resistors or a transformer tap to both the signal wires of a balanced line, and the negative side goes to the screen. Thus the DC voltage is the same on each end of the voice coil or whatever and so no current flows through the microphone, making use of either dynamic or electrostatic mics safe. You can however run into problems if you apply phantom power to an unbalanced or wrongly wired microphone.
If you have a microphone which requires phantom power, but the desk doesn't provide it, units are available for use in line between the desk and the mic to power the microphone. AKG make several, from a small battery powered unit capable of powering a single mic, to larger mains driven box from which several mics can be powered and H&SR published a suitable design in October 84. If you decide to build this however, take care how you wire the XLR sockets as a piece of artistic licence caused pins one and two to be reversed on the diagram.
There are two types of problems associated with microphones in a studio (other than having them hit by drummers). One is to do with placement, whereby two or more mics receive more or less the same signal but from different distances and consequently with different phases, so some frequencies are cancelled out whilst others become reinforced. Secondly, the wiring within the mic or mic cable may be wrong. The most common convention for microphones is pin two 'live', pin three 'return', and pin one 'screen'. Beware of wiring the odd mic with pins two and three reversed though. This will have the effect of reversing the polarity of the microphone (the phasing) which might cause problems either when recording using a stereo pair, or if the mix is summed in mono. Up-market mixing desks usually have switches whereby the polarity of any channel can be reversed, but these tend to be of limited use in a multi-mic situation since the phase errors caused by positioning seldom fall into convenient 180° steps.
I hope this clears up a few queries regarding the finer points of microphone technique and development. If you're still unsure, as a rule of thumb: if you go to a reputable dealer and spend around eleven thousand quid with him, he'll probably see you alright (Not Slapback Audio by any chance? - Ed). And remember - the guy with the biggest birds doesn't need the best microphone!
Feature by David Simpson
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