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I've defined the behaviour of the buttons so far. Buttons will light up/down etc and control relays that will live on a separate PCB (the same form factor, so they stack nice!)

Signal Flow

Here is the signal flow that I have envisaged. Doing such a relay control board allows me to use it as a mono-differential or a stereo unbalanced. As the control signals are the same too, you could stack two relay cards and have a stereo balanced system.

Line In's can be Stereo 1/4" Jacks, or balanced XLR's.

Negative and Discrete Logic

One of the interesting things I had to wrap my brain around was the negative logic caused by having the line in's muted when both outputs from the 3 button controller were low. Sounds silly really. I was thinking of using NOR Gates and all sorts, when it suddenly dawned on me that it'd be easier to simply set the default for the mute relay to be mute, then whenever either of the SRC controls was high, simply switch to the signal path.

That OR circuit was done using diodes.

A diode drop of 0.7V happens leaving the control voltage of the transistor still greater than 0.7V... actually, BJT transistors are current controlled devices, so it'll partially be decided by the value of R3.

Selecting Which Microphone To Use

The easiest path here is to use a front panel switch to select between the 3 mics available

  • Front Panel Electret Microphone
  • Rear Panel XLR Input
  • Remote Control Electret.

This switch can be omitted if you only plan to use one microphone.

In the next entry, I'll cover how I plan to integrate the system with a remote.

Written by Dafydd Roche — September 16, 2013

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