Relay TVC

The Relay TVC uses a bunch of Reed Relays to choose input channels and also volume taps from a pair transformer volume controls..

Reed relays are designed to carry low level signals, and should not oxidise in the same way that a conventional relay would in the same position over time.  They also make a pleasant "plink" sound when switching rather than a harsh snap... :)

A bunch of daisychanged '595 chips are driven through an opto-isolated interface (with an inverter to turn the signals the right way), and use darlington arrays on a chip (ULN280x) in order to source current for the relays.  Power for the relays is carefully filtered to try and avoid any coupling of noise into the signal path, and the digital circuitry is completely dormant when the volume level is not changing.

Originally, the design was tested with a laptop's parallel port to provide the necessary signals, but it proved easy to port the code to a PIC chip (currently using a PIC16F627).  The real challenge will come when adding remote control capability.

There are coupling caps that can switched in or out depending on whether the source has any DC offset... the transformers used are Stephens and Billington TX102s, and they are not designed to tolerate any DC on their windings.

In truth, this is probably the best sounding preamp I've heard, which is somewhat surprising given the rats nest wiring on the back and the lack of shielding!  Just having it in the circuit with another volume control seems to clean things up a bit.

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