The sources of signals serving to amplify an instrument can be divided into three categories of sound transmission according to the technology it uses to produce electrical voltage:
- the sound of the air reflected by the instrument (microphones)
- the vibration of the belly or the bridge/sound of the body (contact microphones/piezoceramic pickups)
- the vibrations of the string (magnetic or optical pickup)
Most of the dynamic microphones have a membrane firmly attached to a vibrating spool. If the membrane is put into vibrations by a sound, the spool immerses in the ringlike slot of magnets inducing an alternating tension proportional to its vibrations. Simply put it is the reversed principle of a loudspeaker.
Condenser microphones have better transmission abilities than dynamic ones. They have a membrane made of metalized plastic or gold-leaf, building a condensation together with a electrode. The vibrations of the membrane caused by the sound change the distance between these electrodes and thus the capacity, which is tranformed to voltage. Condenser microphones always need a preamp and built-in batteries or a phantom power supply.
As the name already indicates these microphones are brought into an direct contact with the instrument respectively with its body or bridge. So they do not take over the air sound but the body sound of an instrument. Contact microphones are fixed on the top or the bridge of the instrument by means of glue preventing additional air sound coming from outside into the capsule.
Certain crystals (seignette salt) and certain ceramics own the ability to produce an electric charge when being mechanically deformed at their surface. As piezoelectrical (or piezoceramic) pickups transform only body sound waves into voltage they are very insensitive in regard to feedback in comparison with microphones. Differently to a magnetic pickups you do not need steel strings.
A recent development are piezo elements made of polyvinylidene film (PVDF). This plastic has if polarized in advance piezoelectric properties and can be used as a sensor for contact pickups when provided with a metal layer.
Piezo elements can also be found in alarm system for broken glass as well as (reversed) in loudspeakers and buzzers (e.g. in birthday cards).
Similar to dynamic microphones, electrodynamic pickups consists of a spool in whose inner body you can find a permanent magnet hung in a sprung manner. If the pickup is brought into vibrations by the vibrations of the instrument’s top there will consequently be a relative movement between the magnet and the spool because of the inertness of the magnet. Thus tension is induced in the spool.
Magnetic pickups essentially consist of one (or more) permanent magnets in whose field you can find a spool out of numerous coils made of copper wire. If a steel string (or a string made of another ferromagnetic material) comes in the near of this spool, the course of the magnetic field lines will be altered. Thus induction tension is produced in the spool. By this way the vibrations of the strings are directly transformed into alternating current whereas the tension is proportional to the speed of the vibration.
A common problem when using a piezo pickup is the low input impedance of many bass(-guitar) amps. Impedance is the electric resistance of a microphone or a pickup, the measure is Ohm.
The result of using a low impedance input is sound quality loss: a thin tinny sound which cannot be corrected by means of the sound equalization of the amplifier. Active DI-boxes, buffers or preamps help by adapting the high output impedance of the pickup to a lower input impedance of the amplifier.
Especially the amplifiers designed for acoustic instruments respectively double basses (such as the Acoustic Image amps, Zadow, Hevos, Phil Jones – just to name a few) have an input impedance which makes an preamp superfluous.
Why don’t have all amplifiers a high impedance input? The higher the input impedance, the more sensitive an input is for all kind of near field electromagnetic interferences. So the amp designer has to find a balance between the demand for a high impedance on the one hand and an undisturbed sound reproduction on the other. From a viewpoint of electronic engineering it is very easy to increase the input impedance, but that would also increase the amps sensibility to disturbing signals.
In unfavorable circumstances interference or phase erasings can happen when pickups with several sensors are involved. Here the waves respectively vibrations which are received by the individual sensors are blotted out so unfavorable that there will be an unbalanced sound. (If two waves are blotted out by half a wave length, a complete blotting out will occur.)
Apart from pickup systems with one or more sensors of the same technology there are also a couple of hybrid-systems which mix different kinds of sensors. Various companies offer a combination of piezo and a microphone, but also the combination of piezo and magnetic pickups are offered. The preamps offered for hybrid systems are mostly equipped with a mix-option in order to blend both signals.