ISO: Audio mixing circuit with no feedback to sources

Discussion in 'General Electronics Chat' started by Probotics, Dec 14, 2009.

  1. Probotics

    Thread Starter New Member

    Aug 26, 2006
    I couldn't find anything up already so here goes.

    I'm looking for a way to mix two audio lines prior to feeding into an amp. Normally I'd simply add appropriate resistors and wire them in. Unfortunately one of the outputs is connected to a ADC (analog to digital circuit) which must measure the voltages on only the line its attached to.

    Obviously if this were simple DC I could drop in a diode, but this is sound so no go that way. I could (I suppose) simply wire in a high value resistors on the in side and hope the output drains away power when one side is high and the other is low, but I think that would mandate postmix preamping to get appropriate signal level (= more noise) plus I don't have that much control over the impedance on the out side.

    Is there some incredibly simple circuit I'm overlooking? Am I excessively worried about the interference from one line to the other? At this point even suggestions useful of search terms would be welcome.
  2. bertus


    Apr 5, 2008

    When you use a buffer amplifier for each channel,
    the feedback will be minimized as the audio signal hardly can go back through the buffer.

  3. Audioguru

    New Member

    Dec 20, 2007
    An audio mixer is an inverting opamp. Each input has its own series input resistor that feeds the "virtual ground" inverting input pin of the opamp. Due to the virtual ground, what happens on one input does not affect any other input source.
  4. Probotics

    Thread Starter New Member

    Aug 26, 2006
    Thanks bertus and Audioguru - I appreciate the help. Based on your recommendations I bounced around the web and found a wide selection of opamp based mixers and buffer amplifiers. Many of the circuits out there don't have opamps dedicated to each channel, but I suspect datasheets associated with the inverting opamps and buffer amps commonly used with audio do (or would supply enough info so I could create one from scratch).

    Audio amplification is not my field, I know its complex and if you screw up you'll get noise rather than sound so I find it a challenge to select an appropriate IC. Do you have a preferred group of ICs for opamps or buffer amps that would work in a +12 volt environment (+/-8 volts possible through voltage regulators if necessary)? Not looking for someone to do all the circuit work, just asking about favored manufacturer/lines if you have them.
  5. Mike33

    AAC Fanatic!

    Feb 4, 2005
    I like TL071 (single) or TL072 (dual opamp) for audio, but it depends on what kind of fidelity you require. They are reasonably priced opamps with JFET input, low-noise, fast slew rate. Available everywhere. There are better ones (the "Cadillacs"), but those have always done me well for guitar preamps, effects and whatnot. They are made for high fidelity applications.

    There are probably 5 or 6 schemes for mixing audio with an opamp; I like the one using several resistors with one input (simple), but your tastes may vary depending on your application....
    This can also be done using discrete JFETS, but the opamp is simpler.
  6. Probotics

    Thread Starter New Member

    Aug 26, 2006
    Thanks Mike, that gave me enough to track reviews and suggestions about the the TL072 and I'm going to try that an an OPA2134.

    Its fascinating how you can use the internet as an information multiplier for almost any subject, but it is definitely multiplying - if you don't have anything to start with you really don't go anywhere.

    Thanks again.