Powering a DIY active audio mixer

Discussion in 'The Projects Forum' started by ttshaw1, Jan 24, 2016.

  1. ttshaw1

    Thread Starter New Member

    Jan 24, 2016
    Hi, everyone. I want to make a 3.5mm audio mixer so I can input from my guitar amp and my computer to my computer speakers. I found a description of one on http://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer
    with a few useful schematics.
    The above picture shows the overall mixer. I'm going to skip the tone control modules. I'm planning on breadboarding everything so I can add them later if I want. For now I only want input modules and the summing amplifier.
    This is a picture of an input module with separate left and right channels.
    And this is the summing amplifier.

    I've got a few questions, mostly related to powering the op-amps. In both modules of interest, each op-amp only has one power channel shown. Should I assume that every op-amp has both +15V and -15V power lines, each grounded through a 100nF capacitor?

    I'll need a +-15V power supply. Can anyone recommend something cheap? If not, I was thinking I could invert a +15V line from a standard 15V adapter with an op-amp and leave ground alone. Would that be noisy?

    To split 3.5mm to left and right channels, can I just make contact at the tip and the middle section? Do I need audio ground for anything?
    Last edited: Jan 24, 2016
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    U2A and U2B are in the same package so the representation is correct; each package requires +15V and -15V.

    How about powering it with one of these, it looks like a good implementation and is suitable for your application:


    You will also need a small 2x18V transformer to go with it, 10VA should be ok.

    You must use the audio ground, it is the return path for the audio signal.

    Last edited: Jan 24, 2016
  3. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    If you look at the LM833 datasheet, you will see that it is a dual part. There is no "standard" way to show a single power connection to a multi-stage component on a schematic like a dual or quad opamp. The first schematic has an error. U2B should be marked U1B, the other half of U1A on the schematic.