The mixing console forms the heart of all signal processing in any studio. Specifications and prices vary considerably and an often difficult choice is made more complicated by being unable to make a like for like comparison. Judging whether or not a particular console is suitable for your needs is often only apparent after you have tried it out, by which time it's too late. The type of instruments that have to be recorded should also influence the decision since only the most expensive console can handle anything with ease. Figure 3 shows a typical channel in an average console. There are several important parameters common to all consoles which should be examined both at the time of buying and periodically as routine maintenance. The first we will take is the signal to noise ratio. This is normally expressed in decibels (dB) which are a convenient way of relating one level to another. Leaving the maths aside for now, here are a few rules of thumb worth memorising. If we double any signal e.g. from 1V to 2V we can say the level has gone up by 6 dB (remember mixing the mics in Part One). This happens every time we double the signal strength, so a rise from 2 to 4, or 4 to 8, is still 6dB. Conversely every time we halve the signal strength we drop 6dB i.e. -6dB. Also, to amplify a signal ten times e.g. from 1V to 10V we would need a gain of 20dB. An amplifier with a gain of 40dB multiplies the signal 100 times and so on adding 20dB for each factor of ten.

Microphone Amps

A mic amp with a gain of 60dB would amplify a 1mV signal to 1V (1000 times) and we could reasonably expect the output noise to lie between -65dB to -70dB referenced to 0dBV which is 0.775V RMS. One way of looking at this noise is as an equivalent input signal. Because it is 70dB down from our reference and because it has been raised to that point by the 60dB of mic amp gain, we can say it is equivalent to an input of 70dB plus 60dB or -130dBV. Note how this corresponds to the dynamic range in Figure 1, Part One.

At the other end of the dynamic range we have maximum input or output level before distortion (clipping). It's quite easy these days to find condenser mics that give in excess of 1V (+4dBV) when miking up drums for example. This would be a problem for all but the best mic amps. One course of action is to use in-line attenuators which drop the signal level by 20dB (how many times is that?).

Phantom Powering

Many condenser mics may be phantom powered, which simply means a way of feeding power to these mics without interfering with the audio output or having to run extra cables. If you intend using condenser mics regularly phantom powering is a must. On the other hand if you have a console which is adequate in most respects but hasn't got phantom powering, it's usually easy enough to fit. Figure 4 shows the basic scheme. Note that the mic amp input must be isolated either by a transformer or capacitors. The 6k8 resistors should be at least 2% tolerance, and 1% metal film or oxide are preferred. There is often room on either the mic amp printed circuit board or on the rear of the main connector panel to mount these components. The power supply unit should ideally have very low noise and ripple on the output, though if the transformers and mics are of high quality, a less than perfect supply may still work well.

Another parameter worthy of note is the maximum output level which for most systems running off +15V should be about +20dBV. The area between this point and the nominal +4dBV average signal level is called 'headroom'. It is very important to maintain this throughout the various stages of the console if signal integrity is to be kept.

Finally, the gain controls on some mic amps work by varying the feedback, which is generally a good idea. However, some purpose built mic amp chips exhibit instability as the overall is reduced to unity (for those very high output mics). So if you are thinking of building your own, take care choosing the feedback components. In all other respects these devices seem to perform very well.

The move in recent years away from sessions simultaneously involving numerous musicians means that a different type of studio/console/recording approach has evolved. It may be found that microphone amplifiers are redundant if the console is used mostly for electro-music since nearly all synthesisers have outputs at line level.