Radio station mixing desk monitor cut out

Discussion in 'The Projects Forum' started by dorrissey, May 26, 2010.

  1. dorrissey

    Thread Starter New Member

    May 26, 2010
    I'm head of systems at a student radio station, and whilst my knowledge of audio based equipment isn't too bad, my knowledge of circuitry doesn't extend much past what I picked up in school, and doing some basic work on electric guitars.

    We recently inherited a new mixing desk at our radio station, its a PA desk rather than a Broadcast desk, but we were offered it at such a low price we couldn't turn it down. The main difference in the types of desk is that a broadcast desk has a monitor cut off, i.e when the microphone volumes are turned up, the monitors in the studio cut off.

    My plan is to build a small circuit which will create a monitor cut off. My initial thoughts are to use a tamper switch in contact with the microphone slider, which will turn the monitor amplifier on and off. I'd also like to use the circuit to turn on an 'on-air' light, whenever the microphone slider was up. My initial thoughts were to use a relay to switch on and off the monitor amplifier power, and similarly to switch on the on air light. I'd also like to power the entire thing by mains, rather than battery, as it will be in constant use by lots of different people, and i don't want it to be cutting out when the battery dies.

    Basically, i'm looking for any help with regards to putting together a simple schematic and list of parts I would need.

    Sorry if i'm not 100% clear in what i'm trying to achieve, i've tried to be concise without writing an essay!

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Your idea sounds feasible, although I'm not sure what a "tamper" switch is. The class of switches known as "microswitches" are available with operating levers. Some are pretty long, so they might allow you to place one where the slider would operate it. The class of switches can be had in DPDT, so operating both functions would be easy. Where to place it is the hard part.

    Do not attempt to run this directly off the mains. Use a wall transformer for safety.
    The DC voltage can operate relays to cut the monitors and to turn on the On Air indicator - easily lit by several LED's.
  3. flat5

    Active Member

    Nov 13, 2008
    Can you provide a picture of the "microphone slider" area?
    What is the model number of the mixing desk?
    Are you talking about a "talkback" system between control room and studio?
    If so, a talk switch in the control room should open (or shunt) the audio line to the control room monitors not disconnect power. Do you have service information (schematics) for the desk?
  4. dorrissey

    Thread Starter New Member

    May 26, 2010
    Tamper switch might just be what it's refered to in a few stores here, but yeah, just a microswitch with a large lever:


    The mixer is a Behringer Eurorack MX2804A.

    Here's a stock image of the mixer:


    My initial thoughts were to have the switch and cicuitry outside of the mixer casing, with the switch simply below the fader. The fader I'll be using for this is on the far left.

    I don't have schematics for the desk, i've done a quick bit of looking online and not found anything.

    A talkback function is standard on a recording desk, this is live PA desk and doesn't have that facility.

    In essence there's three outputs from the desk, the main output which is broadcast, the phones output which is fed into the headphone amplifier, and the control room output which is fed into the studio monitors.
  5. dorrissey

    Thread Starter New Member

    May 26, 2010
    Let's put this on hold, just recieve word this morning that we've been given a large funding cheque which i can go and spend on a lovely new radio specific mixing desk!
  6. rickmartin

    New Member

    Sep 26, 2009
    Darn that check! Took all the challenge away ;)