Reading the specifications of various mic preamps, I notice that the amount of gain that different models provide seems to vary quite a lot, from figures of 35 or 40 decibels quoted for many preamps on soundcard interface devices, to 60dB and above on stand-alone preamps, channels and desks. How much gain do I realistically need for using run-of-the-mill dynamic and condenser mics? Do some mics need more gain than others, and if so which ones?
Technical Editor Hugh Robjohns replies: It is certainly the case that nearly all of the mic preamps built into budget and moderately priced desks and channels generally offer a maximum of around 60dB of gain, and this is usually more than enough for recording the vast majority of close-miked sound sources with typical dynamic, electret or condenser mics.
This 60dB figure happens to be about the maximum that can be obtained easily from the current generation of simple single-chip preamp circuit designs, while still achieving respectable noise, distortion and headroom figures. The fundamental problem in trying to design amplifier circuits capable of providing more than 60dB of gain is the increasing danger of the circuit becoming unstable and oscillating. Inherent crosstalk between the various components and circuit-board tracks can result in the amplifier's input capacitively coupling to its output, resulting in an electronic howlround. This sort of design fault may be audible, but it more often occurs at ultrasonic frequencies with the only clue being a disappointing performance (if you are lucky) or some deep-fried chips and wisps of smoke (if you are not)!
It is probably self-evident that if the preamp design is optimised for a lower maximum gain, better technical specifications can be achieved. Modern electret and condenser mics generate significantly greater output levels than most moving-coil (dynamic) mics and so don't require as much preamp gain, and there are some very cost-effective models available these days. So, it is increasingly unnecessary to provide more than about 40dB of gain when close-miking most sound sources — which is what the majority of home recordists will be doing. Consequently, this figure is becoming increasingly common in many budget preamp designs.
However, there are some situations when you will need a lot more gain from the preamp. Examples include using much more distant mic placements (as you would need to when recording classical music), recording the spoken voice with relatively distant placements (as you would in radio drama, TV or film applications) and working with vintage ribbon mics. In these sorts of situations it is more usual to require 70 or 80 decibels of gain — but this requires an altogether different approach to the preamp's circuit design, which is much more costly and difficult to achieve.
Almost all preamps in this category make use of an input transformer to provide some 'free' voltage gain, and employ a multi-stage amplifier topology — both of which are expensive to implement, take up a relatively large amount of space, and are heavier than the simple single-chip solutions favoured in most budget designs.