Many computer audio interfaces include one or more microphone preamplifiers. If the audio interface is designed to be used for professional audio, then its internal preamplifier will be capable of fully professional work.
Some recording engineers however will prefer to use a separate microphone preamplifier and bypass the preamp in the audio interface, either to be sure to capture the very best possible audio quality, or to achieve a different sound texture.
Microphone preamplifier features
A properly-specified microphone preamplifier will have the following features and functions…
- 48 volt phantom power. This is necessary for capacitor microphones (other than those that have their own outboard power supply unit) to function. Phantom power may be switchable, although dynamic microphones (which don't need powering) of professional quality will not care whether phantom power is switched on or not. Some delicate ribbon microphones (a specialized kind of dynamic microphone) may however in certain circumstances be damaged by phantom power.
- Gain control, to control the degree of boost given to the signal from the microphone.
- Pad. When a microphone is placed close to a very loud sound source, an attenuating pad may be switched in to lower the signal level before it reaches the gain control. This reduces any risk of distortion caused by the signal from the microphone being too strong. Capacitor microphones commonly feature a pad too. This will be used in preference to the pad in the preamplifier, unless the cable run is particularly long, or the engineer needs to switch in a pad quickly.
- High-pass filter. This reduces the level of very low-frequency sounds and can be used, for instance, where there is low-frequency 'rumble' from a ventilation system, or where footfall noise is traveling up a microphone stand.
- Phase switch. This is often omitted. However, when present, it inverts the signal polarity. This makes hardly any difference to the perceived sound. It finds use however when a microphone or cable is incorrectly wired so that the signal is inverted before it enters the preamplifier. This is important when multiple microphones are in use.
In general, transistor preamplifiers are used when the highest degree of sonic perfection is required. A transistor preamplifier of competent modern design will be capable of very low noise, very low distortion and a wide, flat frequency response.
A vacuum-tube preamplifier may be used when a 'warm' sound is required. This is created by mild distortion that occurs in the vacuum tube, which is either the central amplifying device, or is connected as an additional circuit component. This kind of preamplifier will often feature an additional output level control. This is so that the gain can be increased to achieve more warmth, then the signal brought down again in level for output.
Microphone preamplifier manufacturers include A Designs, AEA, Alesis, Aphex, API, ART, ATI, Audient, Avalon, Avid, BBE, Behringer, Blue Microphones, Benchmark, Burl Audio, Chandler, Crane Song, Chameleon Labs, Cloud Microphones, Daking, Dave Hill Designs, dbx, Focusrite, Grace Design, Great River, Maag Audio, Manley, Martinsound, Millennia, PreSonus, Radial, Shadow Hills, Solid State Logic (SSL), Studio Projects, Summit Audio, True Systems, Universal Audio, Vintech and others.