A preamp may include tone-shaping controls (EQ), dynamics (compression/limiting) or both:
The range of available tone shaping is broadly comparable to the range offered with mixers, from simple bass and treble controls to 3 or 4-band fully parametric EQ. Many preamps, particularly valve units, feature EQ designed specifically for vocals.
A simple low frequency cut switch, or high-pass filter (HPF), is often also encountered. This rolls off frequencies below a given point (usually 80 - 120Hz), helping to eliminate various sources of noise such as rumble transmitted from the floor through the mic stand, some vocal pops, and mains-induced hum.
See our separate Online Guide to Equalizers for more detailed information.
Compression and limiting are regarded by many engineers as essential tools in this part of the signal path, particularly for recording vocals and drums. Though compression may be applied during mixing, any distortion or clipping resulting from an excessively loud signal can be difficult or impossible to remove, and while analogue recording features a degree of tolerance to hot signals (the associated phenomenon of tape compression is often considered sonically pleasing), digital recording has no such tolerance. Digital distortion is a pretty unpleasant noise which must be avoided.
Compression is the main tool in this battle. A compressor squashes audio by applying gain reduction when the signal reaches a certain level. Usually this level (the threshold) and the amount of compression (the ratio) may be adjusted independently, though some circuits feature one simple compression control.
Subtle compression, combined with appropriate gain settings, is often enough to prevent clipping. A more extreme solution, known as limiting, completely prevents the signal from exceeding a given level. This can be achieved with most ordinary compressors by turning the ratio fully up, but many mic preamps include a dedicated limiter in addition to the compressor. In this way, the compressor may be used for unobtrusive level shaping, while the limiter acts as a final safety circuit.
This site includes more detailed information on compression and limiting in our Online Guide to Mastering.