The primary function of a preamp is to take a low powered signal, such as that from a microphone or a guitar, and amplify it to line level. Once the signal is at line level, it will have the optimum signal to noise ratio to be used with a wide variety of audio gear, such as converters, eq, compressors or effects units.
Even on a mixer, when you plug in a microphone, the first step is to amplify that microphones signal in the preamp, and then when the signal is at a more useable level, it is routed through the EQ, sends, and summing buss.
The basic way which a preamp works is it will receive the incoming signal, amplify it through an input gain stage and an output gain stage. A gain stage always provides a fixed amount of gain, and the final amount of gain in the output is controlled through gain reduction. Usually, the gain reduction will be from a variable resistor.
There are various elements that determine the quality of a preamp. First and foremost, the overall clarity and fidelity. Some of the lower quality preamps will smooth over the fine details, and be a little bit muddy sounding. Kind of like having a wet towel in front of a speaker. Also, the lower quality preamps have higher noise levels, which is heard as static, or hiss. This noise is always present, and as you turn the gain up, you can hear the noise more. The better quality preamps can provide high amounts of gain without as much noise.
Another aspect about preamps is the flavour that they add to the sound. Certain audio components create unique and pleasant sounding distortion. The various combinations of these components have led to many designs that sound different, many audio engineers like certain ones better than others, or find a certain preamp to sound better for specific applications.
When preamps are built into an interface, on a per channel basis, you will have the option of using them, or bypassing them. Each input channel that has a preamp will give you the choice of using the microphone input, which goes into the preamp, or the line input, which bypasses the preamp.
Since the preamp plays such an integral part of the sound, many studio engineers will acquire high quality, stand alone preamps. To interface a stand alone preamp is simple. It will have an input and an output. You plug your microphone into the input, and the output is routed to the line input of the interface. This way, when the signal enters the interface, it bypasses the interface preamps, and goes straight to the converters.
Apart from the primary function of amplifying a low level signal, many preamps will also have additional features.
- 48 volt Phantom power is standard, this enables the preamp to be used with condenser microphones. It will be a button or switch that can be turned on or off. Although it is meant for condenser microphones, it will not damage dynamic microohones, but it can damage ribbon microphones.
- signal indicator lights or VU meters to give a visual indication of the presence and strength of an incoming signal. This is useful for setting the proper amount of gain, and trouble shooting.
- hi-z input.This is a high impedance input that is optimized for electric guitars and bass guitars to be plugged into the preamp without the need for a di Box.
- input gain and output gain. Some preamps that have an input amplification circuit and an output amplification circuit will have a gain reduction control after the first stage, and another gain reduction control after the second stage. This allows the user to have control of how much each gain stage influences the sound, and provides the ability to shape different sonic characteristics.
- pad. This is a fixed gain reduction circuit that affects the input. This is useful for very loud sources which would otherwise overpower the preamp. When the pad is engaged, it will reduce the input volume by the indicated number of decibels.