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Mikes (Part 2)

On speaking terms.


What they do and why. Chris Dale ponders the sound, shape and purpose of modern microphones.

If you spend more than about £40 on a modern microphone you can reasonably expect it to be "balanced". In other words, it should contain a balancing transformer or electronic equivalent. The terms "balanced line" and "unbalanced line" refer to means of connection, and in this case we'll talk about connecting a mic to a mixer input.

An unbalanced line generally uses a cable containing a single conductor with a screen wrapped or braided around it. The screen is connected permanently to ground, or earth (0V), and the signal voltage is measured between it and the conductor — the conductor can be considered to be carrying the signal. The screen, as with any cable screen, is intended to prevent airborne interference — radio, for example — reaching the conductor. However, screens are only human and interference often does get through despite their stoutest efforts, in which case it merges with the audio signal and is heard on the recording, over the monitors, or through the PA.

A balanced line uses cable containing two conductors plus a screen, and at either end there needs to be a "balancing transformer" or its electronic equivalent.

The screen acts just as a screen and has nothing to do with the signal at all. The two conductors are connected to either side of the transformer winding in such a way that all positive-phase signals travel down one side, while all negative-phase travel down the other.

The signal voltage is measured at the transformer as the difference between the two signals. If any of that nasty old interference manages to get through the screening, it will enter both conductors in the same phase (you'll have to accept this as a basic law of induction) and therefore the difference between them will be a big fat zero. This equals no interference, and that's the reason balanced lines are used. They are however more expensive than the unbalanced alternative, and the transformer (or equivalent) at either end can, very slightly and depending on its quality, degrade the signal. Therefore for higher and consequently less sensitive signal levels such as "line level", where interference isn't such a problem, unbalanced systems are often preferable.

Not so long ago, when it was still hip to wear flares, the capacitor, or condenser, microphone was to be found only within the domain of the recording studio. Reasons for its exclusion from the live stage were its delicate physical disposition plus the high cost of replacement. In those days the average PA system's performance was generally fairly limited as compared to today's equivalent, and so the subtle superiority offered by the capacitor device would have been lost anyway.

But this is 1984, and we are steeped in technologie nouvelle; people have TVs built into their shirt buttons, and some modern PA systems are becoming comparable in their sophistication to the recording studio. Microphone technology has also greatly improved to the extent of producing high quality condenser mics at reasonable prices and quite capable of withstanding the rigours of life on the road. Consequently, they are becoming more commonly used for all kinds of application.

The capacitor bit of such a microphone is identical in principle to any other capacitor to be found in everyday electronic circuits: it basically consists of two electrically conductive plates positioned closely facing each other. If a voltage is created across these plates, simply by connecting one to the positive side of a DC supply and the other to the negative, an electrical charge will be built up between them. The crux of the matter is that the size of the charge, measured in volts, will increase as the distance between the plates is decreased.

In a condenser microphone, one plate is fixed and the other is elastically suspended to form the mike's diaphragm. As sound waves buffet the diaphragm back and forth the charge is seen to fluctuate accordingly, and these changes are the desired electrical equivalent of the acoustic soundwave (as discussed last month). They are however electrically minute, and hence a built-in amplifier is required to boost them to a useable level.

One of the drawbacks of using a capacitor mike is that you have to provide it with a source of voltage to power the built-in amplifier as well as to create the charge or "polarising voltage" across the plates. This is normally supplied either by a portable power supply or one built into the mixing console.

The standard voltage level is 48V although most modern mikes will operate with anything from 9 to 50V. With the exception of certain old models which you may come across, all normal condensers require what is termed "phantom powering". The name is derived from the way that the power is connected to the microphone, which makes use of the "balanced line" principles.

The DC voltage is applied at the console end to both conductors of the balanced line. As there is absolutely no difference between them, there is nothing to measure across the transformer and thus no noise or interference on the audio circuit. At the mic end, the necessary voltage is taken between one conductor and ground (0V). The fact that it is "invisible" to the audio circuit gives rise to the name phantom.

The electret condenser is a far more convenient form of the same animal which uses a permanently polarised pair of plates, obviating the need for an external source of polarising voltage. There is still a need for a small battery to power the built-in amp but they're usually small, cheap and unobtrusive.

On top of all this hassle, condensers are generally more expensive than dynamic and the question has to be asked, is it all worth it? Can there possibly be a microphone better than a Shure SM58 anyway? The answer is an unequivocal maybe, or in some cases a very definite yes. Next month I'll compare performances and you can see for yourself. Then we have the question of valves — is there no end to it?


Series

Read the next part in this series:
Mikes (Part 3)



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When Is A Computer?

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Heroes


One Two Testing - Copyright: IPC Magazines Ltd, Northern & Shell Ltd.

 

One Two Testing - Jan 1984

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Topic:

Microphones


Series:

Mikes

Part 1 | Part 2 | Part 3


Feature by Chris Dale

Previous article in this issue:

> When Is A Computer?

Next article in this issue:

> Heroes


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