Audio Pre Amps

PreAmps are an essential tool for audio mixing. PreAmps are used as both boosters and buffers. They isolate individual components within the mixer chain and allow for precise level control. While many inputs can vary in gain - the preamp allows one to adjust the gain to level out the signal variations. Then when an Audio Signal is applied to a mixer matrix (Sometimes called an analog "Combiner" or "adder") there is an inherent loss. A low-gain preamp is used to bring the gain back up and counter the loss of the mixing matrix.

Naturally, for quality - you want to introduce as little noise as possible in an audio chain. This is why I use these hi-gain low-noise transistors. Every powered device has some inherent noise - this is because as electrons (electricity) flows through the circuitry it produces inherent background (or thermal noise). For practical reasons - most professional preamps are designed with a target noise floor of -90dbm or less.

Here we will present two versions of the same Transistor Preamp. For our purposes we will be using the 2N5088 NPN "Low Noise" Transistor. We are going to present the same preamp with two distinct gain factors.

The firs preamp will produce a gain factor of 4X (or + 12 db). It is a basic common emitter circuit that offers plenty of gain while drawing little current. It is also "ideal" for use in a mixer-matrix (where you need to overcome the gain loss in the mixer/combiner Matrix).

The beta of the transistor (2N5088) actually has very little impact on the amount of gain for this circuit. The tonal influence is essentially flat.

The pre-amp can be powered with a single 9V Battery. It will operate efficiently with any voltage from 9V through 16V D.C.

This second pre-amp will produce a gain factor of 17X (or +25db).

In this configuration the Transistor has been flipped . At first glance you would think it is an emitter follower - but in fact - It is an inverted transistor amplifier.

While the choice of transistor in the first circuit has little to do with influencing the tonal quality of the sound passing through the PreAmp. In this circuit the transistor will have more influence on the sound of the circuit so it might be possible to try different devices in this setup and get a variety of tonal flavors. It may be necessary to adjust the value of R1 when other transistors are used in place of the 2N5088. This preamp is used when you absolutely have to have that extra gain. Most of these high gain preamps are used in the line input or "channel" PreAmps of a mixer.

This preamp can also be powered with a single 9V Battery. It will operate efficiently with any voltage from 9V through 16V D.C.