API 212L Microphone Preamplifier

This preamp was purchased with blown traces on the power supply lines.  The traces were repaired, the 2520 Op-Amp was replaced with an original version taken from a 575B Oscillator module, and a  Direct Input (DI) was tapped at the input of the 2520 Op-Amp.  The input was wired to a Neutrik combo connector with the 1/4" input connected as the DI input.

Allison Research Gain Brain

This Gain Brain was also purchased in a non-functional state.  It was recapped and realigned based on the original operating instructions.  To provide a balanced input the Nuetrik combo connector was connected to a Jensen JT-10KB-D input transformer.  The unbalanced output was connected to a self-designed amplifier module providing 11.6dB of gain using a John Hardy 990C op-amp driving a Neve LO2567 output transformer.

Power Supply Unit

The Power Supply for this system prescribes to the overdesign philosophy.  The transformer is torroidal and the rectifier diodes are fast recovery to minimize electromagnetic fields and noise.  The +/-16V for the API preamp and the Hardy 990C op-amp are fully regulated with 1A current capability.  The +24V for the Gain Brain is also fully regulated with 1A current capability.  The +48V Phantom supply is created using a voltage doubler followed by a 48V regulation circuit.  None of the supply regulators are fixed voltage since these are typically noisier than adjustable regulators which are used throughout the design.

General System Overview

The entire system is mounted in an aluminum rackmount case provided by Sescom and can be seen below.  Note the Jensen input transformer/990C amplifier mounted directly behind the Gain Brain and the Neve output transformer on the left of the Chassis.  These provide the +4dBu to -10dBV interface at the input and the -10dBv to +4dBu on the output of the Gain Brain.  The unbalanced connections are also provided for the Gain Brain with the input wired such that the input transformer is removed from the input circuit when the 1/4" TS connector is inserted.  The API preamp has it's input available on the front panel as well as the rear and the output is provided on 1/4" TRS as well as XLR Male.  The availability of 1/4" TRS output of the preamp and input of the Gain Brain allows for easy patching using a standard 1/4" TRS-TRS cable providing a "Channel in a Box" approach