A microphone pre-amplifier is a device or part of a device that amplifies the signal from a microphone or a similar low level signal from a unit such as an electric guitars pickup, to a degree where it can be used with other equipment such as recorders or mixers. Since the gain required is quite high compared to most other audio signals it is important that the amplifier is well designed and preferably some sort of a Class A type of circuit.
Note that there are microphones out there that require higher gain than most pre-amps offer as standard, even high end tube pre-amplifiers often only have a gain of 50 to 55dB, transistorised amps usually a bit more, difficult microphones like vintage ribbon mics will need at the least 65dB and most pre-amplifier designers make that 70 or even 75dB to be on the safe side. If you plan to toy around with old microphones make sure that you get a pre-amp that will actually work with those or get hold of a matching transformer.
It may seem a bit naff idea to get a separate microphone pre-amplifier since the bulk of today’s mixers and recorders have a perfectly serviceable mic preamp built in.
One of the reasons for having a separate microphone pre-amplifier in a budget setting even though your mixer has one built in is simply compatibility. There have been great strides in the manufacture of budget consoles in the last couple of decades mostly to do with manufacturing technology so that today we have even low budget mixers with perfectly acceptable preamps, the budget Behringer or Yamaha Corp. consoles for instance have a remarkably clean sounding preamp that will be quite good enough for 95% of the work they are asked to do.
The problem lies with the remaining 5%, the gain circuits in budget microphone preamps are for all intents and purposes the same discrete 2 transistor designs as we see in budget RIAA preamplifiers and the same problem applies here, these circuits are cheap, clean sounding and noise free but have limited gain and limited impedance range compatibility. Fixing either of those shortcomings will involve either compromising on the sound quality, the noise aspect or price. Since in even a small mixer any increase in price will be per channel and hence multiplied across the mixer the manufacturers are seldom willing to spend even a penny extra on the third transistor that would seriously increase its usability, so if you want a mixer with better preamps you will have to spend not a little bit extra, but considerably more so for a more upmarket mixer.
The bigger manufacturers of mixers make millions of channels with built in mic amps per year, so cost savings are a huge issue for them, Behringer went so far as to start making their own low noise transistors and while the cost saving per transistor is tiny it amounts up to hundreds of thousands of euros per year.
Some makers feel they have to compromise, the American makers of budget mixers are mostly selling to their home crowd and there they are faced the reality of having classic dynamic microphone designs from ElectroVoice and Shure Inc. as de-facto standards over there in any given kind of live and PA situations, and those microphones, while not a difficult load by any means, are borderline for those simple circuits, they work, but sound tinny. In the USA a mixers performance would thus in most cases be judged on its performance with those mics so the makers have all decided to wring out a little bit more gain of or the circuit at the cost of slightly noisier and less clearer preamp.
By investing in even a budget separate microphone pre-amp you are simply sidestepping those sort of compatibility issues altogether and in addition will in most cases get a better performance and even some sound shaping features. In fact by buying a cheap preamp and a budget 20dB transformer to go with it you will be able to use just about any microphone ever made. By moving upmarket from that you will get more flexible devices and start to see even better sound, but at the same time the sound becomes something of a personal preference so while budget preamps can be ordered over the net sight unseen as utility devices it is strongly recommended that you audition the pre-amps if you are considering something more expensive.
While all microphone preamps basically perform the same function they are packaged differently towards different markets and usage needs with 5 main variants. The most common is the Studio microphone preamp, these are usually 1 or 2 channel boxes that are primarily intended to be used in a fixed or transportable environment, they are considered as much or even more a sound shaping tool as they are utilitarian and thus often feature user controls other than gain, either to modify the behaviour of the circuit and thus the sound or controls that allow you to mimic or “model” the sound of other pre-amps, typically vintage preamp models of some sort.
There is a sub-type of the studio pre out there in the form of the “preamp module”, these are designed to slot into specific types of mixers or module rack systems, they only work inside the systems since they do not have a power supply and rely on getting power from the host.
The Portable pre-amplifier is intended for true portable usage rather than use with touring etc., they are battery driven and in general much more utilitarian designs than the studio bound ones with limited if any sound shaping capabilities but like their studio counterparts are invariably only 1 or 2 channel designs. There are 2 sub-variants, one is the “true portable” microphone pre-amp, these are units that are built as small an light as possible and intended for people that need to carry kit on their own, live show tapers and nature recordists are the 2 main markets for these devices. The other type is the “location preamp”, these are ruggedized high end professional devices that are mostly used by the movie and TV industries, although they enjoy some popularity in the classical recording world as well because of clean sound due to short signal paths and battery power supplies.
Multichannel microphone pre-amplifiers like the portable pre-amps are almost always utilitarian, they are just intended to amplify a block of signals to a usable levels and seldom contain any controls other than gain, pad and phantom power, and sometimes not even all those. That is the generic rule anyway, there do exist multichannel pre-amplifiers with sound-shaping controls but they are rare, the intended market for those are recording studios, live installations and high quality commercial sound installations.
There exists a few variants on the multichannel preamp theme mostly by combining them with simple mixers, one is an array of pre-amps with simple 2 channel mixer bus, these are used situations were a sub-mix does not need to be changed after initial setup, for instance a drum mic setup in a live concert. The other common variant are PA microphone pre-amp arrays, these can be pre-set, have remote control abilities, often have some basic mixing facilities and some models even have matrix mixing or switching capabilities and the Automixer is a development of that as it is a multiple microphone preamp that will automatically mix to a single channel depending on input signal strength.
The microphone pre-amplifier & analogue to digital converter combo is getting more popular these days but that is as the name says a mic preamp that is combined with a A/D converter and thus pumps out a digital signal rather than an analogue one although most designs allow analogue output as well and some allow the use of the A/D as an independent unit. This is more of a convenience solution and only offers limited monetary savings in comparison to having a discrete setup so this sort of a device is most commonly seen in simple all digital environments like podcast studios and internet radio stations etc.
And finally there is the Voice channel but that is a combination of a microphone preamp and further signal processing, in most cases an EQ and compressor but there can be other effects and utility processors in there as well. It is inspired by the high end mixing decks of the late 70’s and early 80’s from UK and Germany that placed basic signal processors like simple compressors, limiters, expanders and gates inline not as an option but as a standard feature on every or most input channels in the mixing desk both to allow you to record onto any track on the recorder without resorting to external equipment and as a time saving feature, you did not have to move away from the work surface of the mixer to do basic tasks. This technique was so successful that it became more or less the routine and engineers using mixers not having the feature were soon recording like this by default and it cemented the split in how European and American recording engineers worked, with the Europeans “printing” effects onto tape as a rule while American engineers recorded all music clean on to tape and added signal processing in the mix.
This gave birth to the “recording strip”, a device that contains the sound shaping electronics found in the line strips of high end mixer and later became better known with the more marketing friendly name of voice channel. It is a sort of a neat idea in many ways since it gives you the basic signal processing required for recording in a single box allowing you to record directly to a recorder without a mixer, in a live situation it allows you to get the sound people are used to from a studio recording regardless of what other equipment are to be found at the venue and similarly for someone working with cheaper mixers it allow you to have some of the flexibility a larger one out having to fork out for a second hand SSL.
The downsides are that the they are seldom as fully fledged as the discrete components and usually only have a limited subset of the controls that are available on a full signal processor of the same type, this is by design since it means a speedy setup and originally the idea was not to replace the discrete signal processors but to save time in a working environment.
There are basically 2 main variants of the voice channel out there in the form of the analogue and digital channels, in both cases the only part of the device that you can bank on being of the same quality as a discrete unit is the mic preamp. The analogue ones that invariably have EQ and compressor built in, but a de-esser, expander and gate is usually considered to be needed as well, some of them have an A/D converter at the end as well. The digital voice channel became very popular for a while, these are basically a mic preamp with a A/D converter and a DSP chip for processing, some of them copy the user interface of the analogue ones down to a T and offer the same features and controls, there are however digital voice channels that have a fully featured compliment of effect processor algorithms and in that resemble a mastering processor as much as anything else but unlike those feature a mic preamp. These are highly advantageous in cases where you are working with a fixed set of setups since you can get the flexibility of discrete processors alongside memorisation capabilities. The downside is that much of the functionality is bound to be hidden behind menus and that makes them useless in situations where real time tweaking of parameters is needed.