Two Channel Audio Mixer

Discussion in 'The Projects Forum' started by Seddie777, Jan 30, 2014.

  1. Seddie777

    Thread Starter New Member

    Jan 28, 2014
    Hey guys, just looking for a couple fresh pairs of eyes to look over a small project I am currently working on. Feel free to throw in any errors you see, or even any suggestions would be great. Here is a run down of what the circuit (attached) should be. First off this mixer is actually for stage use, it will be mixing a post-amp "house" audio signal of about 10Vp nom (30Vp max) with a pre-amp electronic drum signal of about 2Vp. So I'll discuss the circuit left to right. First the three signals (labeled in the diagram) are fed into a summing amplifier. The house signal being dropped down to a much more manageable 1/10th of its input via the voltage divider. Also included is a DC offset of +5V to compensate for the lack of a negative voltage on the negative rail of the op amp (An LM318 since I know it is difficult to read in the schematic). From there the +5V has a 3.3K resistor to keep the gain at 1 (I know I missed the feedback resistor in the diagram, but it will be a 3.3K. Just keep that in mind as I discuss the rest). Then both the house and drum inputs have 5K potentiometers to allow for both <1, fractional gain as well as >1 gain to increase volume when needed. After the op amp, a high pass filter to decouple the earlier DC offset and another 5K pot for total volume control. After this I was debating on throwing another op amp of -1 gain to re-invert the signal, but I'm not sure if this is necessary as this is simply for audio purposes. This circuit will be powering a simple pair of headphones of impedance 16Ω or 32Ω, so very little power will be drawn. Thanks for taking the time to read everything. I would like to say I have a decent knowledge of electronics and circuitry, unfortunately all of my knowledge is from my university courses. Which as I have so quickly learned, is very little when venturing into the real world. I say this so that if I have overlooked some major flaw in this design you understand why :D.
  2. t06afre

    AAC Fanatic!

    May 11, 2009


    Jan 20, 2014
    No feedback resistor?

    Look at page 15 in the document linked to by t06afre
    Start there and make simple summingresistors for each stage.
  4. Seddie777

    Thread Starter New Member

    Jan 28, 2014
    Yeah, there is a feedback resistor, as I mentioned in the post I just forgot it in the diagram, it will be a 3.3K.

    I'll look into the circuit on page 15, thanks.