4. Basic Features
By definition, any mic preamp has at least one mic input and one line output - a multi-channel preamp (see below) will have several of each. Digital outputs (often available as an option rather than as standard) go one stage further than converting to line level, enabling direct connection to other digital equipment. Digital output may take the form of S/PDIF or AES/EBU (2 channels), or multi-channel formats such as TDIF and ADAT (light pipe).
The addition of an insert point allows the connection of an external processor such as a compressor. This is neater than chaining the compressor between the preamp and mixer/recorder, and also allows it to be connected before other onboard sound-shaping controls such as EQ.
Valve or Transistor?
Though modern valve-based mixers are something of a niche product, inevitably costing several times as much as their transistor equivalents and requiring more maintenance, valve preamps and channel strips continue to flourish. Valve preamps, like valve guitar amps, are often said to have a warmer and more musical sound than transistor based products, and are particularly popular for recording vocals. Like all other audio equipment, preamps will be transistor-based unless otherwise stated. Many preamps marketed for their valve sound are in fact hybrid designs employing a combination of transistor and valve circuitry usually just a single valve, in fact.
Almost all preamps feature some form of volume adjustment. This may be just a single control, or separate controls for preamp gain/drive, which may be employed to add subjectively pleasing valve (or valve type) distortion if desired, and output volume.
Many manufacturers offer the convenience of several mic preamps within a single box. For reasons of space and cost, there is usually an inverse relationship between the number of channels and the number of controls per channel. The commonest channel formats are 1 (mono), 2 (stereo), 4 and 8.
Anyone who cares enough about their sound to use an external mic preamp will almost certainly be using condenser mics most of the time. These require a connection which carries power to charge the condenser, standardised at 48V. Phantom power is therefore a standard mic preamp feature, though it is usually switchable, per channel on multi-channel units important as some older ribbon mics can be damaged by phantom power.
This switch has various applications, including correcting a phase problem elsewhere in the signal path, but is most commonly used in situations where two mics are facing in opposite directions, for example when miking the top and bottom of a snare drum, or the front and back of a guitar amp. In these instances, the two signals will be out of phase - one of them must be reversed before they can be mixed together in order to avoid phase cancellation effects.