General Overview

This project was done for a client. The goal was to create power supplies and rackmount two each PQ1253 preamplifiers and DL1656 compressors. It was decided that the power supply would be put into a separate rack box and connected with a cable to provide isolation from the radiation of the power transformers. The schematics and the modules were all that was provided. Authentic Audio sourced all components to complete this project and provided documentation as part of the rack mounting project.

PQ1253 Microphone Preamplifier

The Calrec PQ1253 is a microphone and line preamp with switchable EQ. After researching the schematics, it was determined that the preamp uses two transformers depending on the amount of gain needed in the system. There is switch logic built into the gain switch to properly connect the transformers for the input signal. The output of the preamplifier is an unbalanced output. The EQ is located on a second card in the module chassis and is unbalanced in and out. There is no phantom power switches on the module, these are located on the console, meaning that these needed to be wired. The same is true with a switch to allow for line input (with no transformer path).

DL1656 Compressor

The Calrec DL1656 compressor is a VCA based compressor that is unbalanced in and transformer balanced out. Typical compressor controls are in place with the LED's running on a separate power supply to provide isolation from the audio signal path.

Signal Flow Wiring

Tap points are provided to allow for access to the signal at the preamplifier output, EQ in and out, and compressor in and out. The connections are all normalled allowing for complete signal flow through the chain if no connections are in place.

Filtering of the power supply is done at the DC input point to allow for the cleanest signal path possible.

Power Supply

The Power Supply for this unit is rackmounted separately. Power is provided by three of the shelf power supplies. +48V for phantom, +/-15V for the audio signals, and +8V for the compressor LEDs. +5V is also needed and this is generated in the audio rack using a standard 7805 regulator from the +8V supply. All power supplies are fused with power indicators showing the status of all voltage rails. As in all personal projects - the power supply is over designed with added filtering to provide the cleanest possible power to the audio modules. The DC power connections are sent to the audio chassis via a 5 Pin XLR connector. A secondary 5 pin XLR is provided for expansion at a later date.

Documentation

A full set of documentation was completed for this project. This was to allow any technician who would follow later to understand the design process and track any repairs needed. Read the documentation here.