Twelve years since first writing about the Syncron AU7a, I can finally offer a neat phantom power conversion. I converted a few of these in the past by hacking the original board or even building a tube circuit, but this is a better, tidier approach. A big thank you to Robert at Russell Technologies for designing boards and a better circuit from my sketches.
|My Syncron AU7a converted for phantom power
The Syncron AU7a was one of the earliest transistorised condenser microphones, and was also sold as the Fairchild F-22. It ran on four mercury batteries, which had a tendency to leak after a few decades and are now obsolete. Conversion to run on phantom power seems sensible, but the original circuit used a P-channel JFET and positive ground, which are not compatible with modern phantom power supplies. A new circuit and a new PCB will make things a lot simpler.
|Syncron AU7a - circuit for P48 operation
|New circuit board for Syncron AU7a
|Transformer side of the circuit board.
|New AU7a circuit in place awaiting clean-up.
|Inside the AU7a with the new circuit board.
|Syncron (green) and Sony C48 (blue) responses compared.