Balanced Input/Output Pre-amplifier Circuit

This Balanced pre amp circuit is designed to amplify a little line-level audio signal (from 0 dB to 20 dB), and has balanced inputs and outputs. The scheme uses only conventional components, you’ll have no trouble finding in any electronics retailer. For a stereo version, simply to make two copies.

Input stage
The input stage is focused on a four-op amp content in the TL084. It is mounted in a simple differential amplifier. It is true that this is not the best possible configuration, but it is much less critical than for a low stage level (preamp, for example) and give results, however, quite satisfactory.

Gain Adjustment
A variable gain amplifier stage is inserted between the input stage and output stage. This stage provides a setting range from 1 (0 dB gain) to 10 (gain 20 dB) around, which should be sufficient for most situations. Note that this works in asymmetric stage.

Outputs
(Re) is obtained by symmetrizing the two remaining op amp TL084. The two resistors R11 and R12 are optional, but are recommended. They provide protection against short-circuits and sustainable output. The loss of these two levels induced resistance is negligible.

Power supply
The Balanced pre amp power supply must be a symmetrical, as described on page 001 and symmetrical power on page 002 Food symmetrical. Local decoupling capacitors is achieved by C4 to C7, to be placed closer to the integrated circuit TL084.

source : sonelec-musique.com