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A Musician's Guide To Mixers


'The higher the technology — the lower the understanding of it'.


At the start of popular music as she is known today, the 'band' was commonly 3 guitars, drums and sometimes a separate vocalist with the others chipping in with mouth noises. The equipment was rarely more than the instrument amplifiers and a separate PA mixer/amplifier with an output of sometimes as much as 100 watts (although these were valve watts and not our now more familiar solid state watts). Someone in the band or a follower of it, knew enough to be able to repair a microphone lead, fix the mains cables that had been ripped out the night before, and whether to order the low or high impedance version of the mic that the singer had spat into and then thrown at the drummer on the last gig.

Show bands, speciality groups and success all played a part in the development of band equipment. The 'show band' demanded better quality PA and flexibility of operation on stage. Speciality groups with several vocalists required high quality and good balance, while success itself allowed more bands to spend an increasing amount of money on equipment.

The range of available equipment snowballed both for stage and recording use, until today, when it is possible to be completely blinded by variety in all price ranges, powers, and colours of knobs.

Experience



Musicians learn by experience about how to use microphones and loudspeakers; after all, they are professionals (or hope to be) and, by definition, gain experience by working very hard, practising, and using their ears. The part that so often gets forgotten is the little bit of education required to turn an acceptable sound into a great one - the education being necessary to get it done quickly — time is money; or more time to practice.

A little knowledge is not necessarily a dangerous thing — only if it pretends to be total knowledge. The following is a little knowledge; but right up-to-date: A 1984 guide to mixer technology and how it affects the user.

Microphones



Articles about microphones by clever people appear all the time in magazines; but how many of them explain the basic types and how they work?

1) Moving Coil


The moving coil microphone works exactly like a loudspeaker back to front. Noise moves a little diaphragm which has a coil attached to it, suspended in a magnetic field. The movement creates an AC voltage across the coil.

They are generally rugged and fairly accurate in their response. Most are 200ohm balanced (more later), some are high impedance which means that they have an internal transformer.

2) Ribbon


These work by the movement of a tiny 'ribbon' of aluminium foil suspended in a magnetic field; similar to the moving coil but the voltage generated from end to end of the ribbon is extremely small and needs a transformer in the mic head to produce a usable signal.

Typical ribbon microphone.


They are delicate, and suffer from bass lift if you get too close to them. Not used a lot nowadays, they are always balanced, and can produce excellent sounds.

3) Capacitor


Capacitor (or condenser) microphones used to be extremely expensive and out of the question for stage use. They operate by sound pressure moving a fine membrane of gold coated plastic which is very close to a fixed plate. Movement changes the electrical capacity of the mic head and, when amplified with a special type of high impedance transistor, produces a usable electrical signal.

Typical capacitor microphone.


Nowadays capacitor mics are cheap, reliable, and very good for most purposes. The cheaper ones tend to be low impedance unbalanced, the better ones are always balanced. The only problem is that they need some form of power at the head; either a battery in the mic body, or from a separate power supply (or phantom power from the mixer).

Mixer Input Stages



Mixer microphone input stages come in various sorts: the cheapest are unbalanced, medium or high impedance, sometimes with a switch for low impedance. If this is all your cash will run to, it can be made to work reasonably well with care, but all these cheaper input stages suffer to a greater or lesser degree from a variety of practical problems:

The first is the unbalance itself: the audio signal from a microphone is very, very small. Even though the cable is shielded with a copper braid, it is still susceptible to interference from other equipment nearby, and to crackles caused by movement of the cable creating tiny signals like a capacitor microphone in the cable itself. Unbalanced mic inputs should be avoided wherever possible - where they have to be used, the cable should be kept as short as possible, and not be kicked around.

Because the manufacturer decided on an unbalanced input, he will also have economised on the type of amplifier used in the 'front end'. This means that the overload margin will be suspect - that is the horrible distortion that happens when the going gets heavy.

Other problems with unbalanced microphone systems are a tendency to attract radio interference from CB and broadcast transmissions, and the possibility of serious hum problems should the case of the microphone touch any other equipment in the neighbourhood.

Better mixers always have balanced mic inputs. There are two types: electronic and transformer.

Until very recently, all good mixers used transformers in their input stages. The transformer allows the use of very low noise circuits following it, gives exactly the correct loading (or matching) to the microphone, and is the easiest way to achieve good 'balance' or rejection of all forms of interference.

The ideal transformer input stage should have an input impedance of about 1Kohm or greater. When a 200ohm microphone is plugged into it, the 1Kohm doesn't 'shunt' it too much - very little of the precious signal is lost, so the system is efficient.

Electronic balanced mic amplifiers have now been developed to be as good as transformers for most purposes although many are still a bit suspect on interference rejection, and noise is often only optimised with specific microphone impedances (they vary a lot!). A run-of-the-mill transformer input is still as good as the best electronic system overall.

Hum loop created in a typical recording system.


Connections



The standard connector for microphones is the XLR. This connector has been around for years and has proved to be by far the most reliable and trouble free audio connector ever - even though it is not particularly easy to wire up!

The convention for wiring an XLR is easy to remember:-

X — Earth or Ground, Pin 1
L — Line or Hot, Pin 2
R — Return or Cold, Pin 3

This is now the world standard — in spite of many reversals of pins 2 and 3 in the past. The 'Earth' or 'Ground' connection is the cable shield, the 'Line' is the red or brown wire, the 'Return' is the white (or blue or black) wire. If you get it wrong, the only probable effect will be a phase reversal — the speakers sucking instead of blowing!

So now the signal is safely inside the mixer. Once it is there, it is safe from interference and other problems from outside — except for dumb operation.

The Equaliser



The main parts of any sound mixer are the input amplifiers, the equalisers, the mixing amplifiers, and the output amplifiers. There is no magic about mixers - they all contain these elements in various arrangements.

The 'Equaliser' or EQ gets its name from the early days when the signal from the microphone had to be 'equalised' to get it back to the proper sound. Nowadays, EQ is used to intensify, enhance or generally to change the character of a sound. Equalisers distort the signal; the more they are used, the more they distort, so use them as little as possible if a clean, true sound is the desired final result.

Equalisers also amplify, and by putting on 'top lift', the overload margin of the mixer is reduced at high frequencies; clipping distortion is brought closer. The answer to this one is to watch the meters and listen carefully, if audible distortion is suspected, then reduce the gain of the input amplifier and give the EQ less to do.

Equalisers give the operator apparent control over 'lift' and 'cut', 'frequency', and often 'Q'. This 'Q' value is a measure of the frequency range width. A variable Q control is a powerful and dangerous tool; high Q settings (above 2 for example) cause phase shifts in the audio band that tend to be unpleasant, but difficult to point a finger at. Beware of 'peaking up' the equaliser — you could be saying goodbye to music!

Mixing, Outputs and Hum



The next part of the mixer is the mixing amplifier itself. The signals from various sources are all present at their respective channel faders, they then mix together on a 'mixing bus' which drives the mixing amplifier.

Here, the only danger is our old enemy 'overload' again. One signal at a respectable level can be handled with no trouble, but several at the same high level are likely to add up — eventually to clipping and distortion; so keep the levels back — you get no prizes for bending the VU needles.

Signals leaving the mixer go through output 'line' amplifiers to prepare them for the outside world. The fashion today is for most mixers to have low impedance unbalanced outputs, while more pricey boards have balanced outputs at specific 75 or 600ohm impedances. An important fact to remember is that there is nothing wrong with a low impedance unbalanced signal from a mixer - but watch out for hum loops.

A hum loop is really quite easy to understand if you stop and think about it for a moment. An audio signal is an AC voltage on a bit of wire; that voltage has to be measured from somewhere - its reference, and this is the screen of the wire — the other conductor. So both the inner and the screen are really signal wires. If another 'signal' (which could be hum and noise from the power supply of other equipment) passes along a signal wire, the result is interference. When the interference is caused by misrouting of grounds via audio cables, this is the classic hum loop.

The correct cure for the famous loop is to make sure that all ground connections follow the signal in an unbalanced system ie. there should be only one main ground (earth) connection and strictly, this should be at the power amplifier — but, because of safety considerations, it is often the mixer itself.

CAUTION! When chasing hum problems, always be sure that a proper ground connection exists somewhere, and that there is always a cable screen connection between all interlinked equipment — the idea of fried musician is not appetising.

Meters and Levels



The most common type of 'professional' meter is unfortunately the least accurate in terms of true measure of audio level; the VU (volume unit) meter. It was designed to give a good indication of the subjective loudness of a sound — unfortunately, this 'loudness' has little to do with electrical levels. But all is not lost, if a few simple rules are remembered the VU meter can give a reasonable indication. The golden rule is that the meter under-reads most musical signals; from 3 or 4 dB on 'smooth' sounding instruments to about 8 dB on piano, and even up to 15 dB on percussive sounds and some voices. The trick to remember is to estimate the transient or 'edgy' quality of the sound and allow for it in the reading of the VU - if in doubt, turn it down.

Many mixers are now being fitted with light column meters. These are often designed and calibrated as VU meters which is a pity as electronically it is easy to turn them into peak reading meters which are more meaningful when setting levels, and even more important today with the overwhelming use of the synthesiser with its vicious transients.

The PPM (peak programme meter) is the other main meter type. Its expense puts it immediately out of range of most non-professional recordists, but it is mentioned because it gives a true indication of those electronic peaks that give the problems. It also gives a full range of usable levels on the scale — from 'too quiet' peaks of -22dB to 'tape saturation' peaks at + 14dB.

The Dreaded Decibel (dB)



To try to make the dB understandable rather than just boring, think of it as just a convenient way of measuring sound.

Our ears have this ability to handle an enormous range of sound pressures, to measure those pressures in 'volts' or 'pounds' or anything else on a linear scale would be very clumsy, so we use a unit that operates on a logarithmic scale. You don't need A level maths to understand that; it simply means that when a sound is twice as loud, you ADD some dBs rather than multiply by 2.

Again for convenience, 0dB is chosen as a 'standard' level. Something that is half as noisy is -6dB and the twice as loud figure is + 6dB.

Just to be awkward, the VU meter is normally calibrated so that 0VU is +4dB. Cheaper mixers often operate at a standard level of —10dB (particularly the Japanese ones) and their VU meters are adjusted to compensate for that. Better mixers operate at the higher level to achieve better noise performance and to give a standard output level that is acceptable worldwide.

Hopefully these tips and info will help a musician to get a little more out of a highly technical piece of gear; or at least to see why it doesn't sound quite right! A musician has the disadvantage of trying to get good results from an alien 'instrument', but the big advantage of a good pair of ears. So use them - if it sounds right, it is right!



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Studio Focus

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Maxim MDD-1500H Digital Delay


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Mar 1984

Topic:

Mixing


Previous article in this issue:

> Studio Focus

Next article in this issue:

> Maxim MDD-1500H Digital Dela...


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