Audio Switcher - with default state on power-off

Discussion in 'The Projects Forum' started by davidelsbury, Dec 25, 2015.

  1. davidelsbury

    Thread Starter New Member

    Dec 25, 2015
    Hi all
    Hope you can help :)

    I'm wanting to build an audio switcher, for a conference audio rack.
    It will have 3x balanced inputs, and route to 1x balanced output. (all connectors will be XLR - these are professional audio devices with +4dBu nominal signal)

    I would like to select the inputs via pushbuttons on the front panel, and to have an LED indicating which input is currently live.

    I'm quite happy to run relays to switch the audio in and out.

    In an easy world I'd just use a rotary switch - however I want the switcher to always return to having input #1 selected and routed when it loses power and powers back on, regardless of which input was selected when it lost power.

    Please could you point me in the direction of where I need to look?

    Thanks, and have a great holiday season :)

  2. sailorjoe


    Jun 4, 2013
    Have you searched for what you want in the commercial realm? Seems like a pretty common requirement for a conference stack.
  3. ScottWang


    Aug 23, 2012
    Please attach the schematic, it will help the members to help you easier and has the common target to talk to.
  4. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    Here's an idea, use one of these:

    ELM309 to do the contact debouncing.

    Use an D flip flop, that would take the pressed button and set it and reset the other channels.
    e.g. Channel #1 button sets #1 and resets 2,3 and 4. Channel 2, sets #2 and resets 1, 3 and 4.

    Messy OR gates.

    That would give you radio-button functionality.

    Your power-up circuit (always fun) would basically reset 3 of the latches and set #1.

    You could use a PICAXE processor based system.

    I don;t see any reason why an OTOMOS relay could not be used.
  5. Nykolas


    Aug 27, 2013
    Here is a one of four switcher. The cap across the off switch (which could be your input #1) assures that, after power-up, this is the active input. E
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    There are lotsa small A/V switchers on ebay. Many are controllable through a serial port. If you want to grow your own, then start with Maxim's pile of video routing chips. The larger ones are controlled through I2C or SPI, so you would need a small microcontroller to interface among the buttons, LEDs and video and audio switches. Based on the feature sets, my guess is that that is the common method in most small A/V switchers.

    OTOH, you absolutely can grow one completely from scratch, with no uC or code involved. Maxim has direct-address video and audio mux parts, and the rest is just some logic and with a power-on bias. I don't know of any balanced audio router chips, but controlling two unbalanced chips in parallel is nuttin. Or you can convert from balanced to single-ended at the input buffer, route through one audio switch device, then re-balance it in the output driver.

    If you trust the various signal sources, you can eliminate the input buffers and run the inputs directly to the mux chip inputs. Not the best practice, but common enough if you are sure you don't need any signal conditioning (gain tweak, noise rolloff, source impedance control, etc.).

    So, what are your skill set and timeline?