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Simple audio mixer using FET

Discussion in 'Electronic Design' started by ectoplasm, Mar 6, 2007.

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  1. ectoplasm

    ectoplasm Guest

    I want to build a simple audio mixer (adder) (from stereo line inputs)
    of good quality and low energy consumption. I found this simple
    design:

    http://www.aaroncake.net/circuits/mixer1.asp

    and I also found this one that also uses a transistor:

    http://www-fa.upc.es/websfa/lab/111/audio/audmixer.txt

    Q1: would the simple design work well, or is it just a theoretical
    example? Otherwise, why would the second design add a transistor? (Q2)

    The second design's webpage says:
    "If two of these circuits are made in the same enclosure for
    stereo, then there can be a single power supply to run both of them.
    There should be a resistor in series with the incoming 9V+ lead so as
    to minimize crosstalk, and reduce the PLOP when it's powered on."

    Q3: How big should this resistor be (Ohm)?
    Q4: So each of the two stereo's mixers circuits should have their own
    resistor in series with / connecting to the battery?

    Q5: The second design uses a lot more power because the transistor
    draws a current while it's not used. Battery power wouldn't be
    feasible, right?
     
  2. Eeyore

    Eeyore Guest

    They're both fairly rubbish. They remind me of the stuf you could buy about 40
    years ago.

    It'll work.
    The second one has a lower impedance emitter follower output.

    A couple of hundred ohms maybe.

    In series with the positive yes.

    Both circuits draw power with no signal.

    Graham
     
  3. ectoplasm

    ectoplasm Guest

    They're both fairly rubbish. They remind me of the stuf you could buy about 40
    You sound critical.

    So, how about a circuit using opamps? I found these:

    design 1:
    http://www.mitedu.freeserve.co.uk/Circuits/Audio/6ipmix.htm
    (ignore those mic input stages)

    design 2:
    http://europa.spaceports.com/~fishbake/mixer/mixr.jpg

    Which of these designs do you think is better? Or what else do you
    swear by?
    Remember, it's just to mix some line inputs, e.g. computer and DVD
    player, TV, etc...

    Thanks,
    E.
     
  4. Eeyore

    Eeyore Guest

    You bet.

    The 'designer' reckons those are acceptable mic amps !!! ?

    The remainder is something of an improvement though. It could easily be made to run
    from a single supply too.

    Jesus Christ ! It seems like any idiot believes he can design an audio mixer without
    even knowing the basics of how to use an op-amp !

    I design my own ! Well, actually they tend to go into volume manufacture.

    You might as well make one that's worth using though !

    Can you get the binary groups ? As you seem to be using google maybe not ? I'll
    sketch something for you.

    Graham
     
  5. ectoplasm

    ectoplasm Guest

    Can you get the binary groups ? As you seem to be using google maybe not ? I'll
    Wow, you would? That would be great.

    I'm not in binary groups, but could you please send (cc) it to my e-
    mail address?

    Thanks,
    e_c_t_o

    (my e-mail address, click to see it in my profile or use above written
    name, including the underscores, <at> hotmail.com)
     
  6. Those are not very good mixers.

    Perhaps it would be simpler if you just used an opamp inverting input,
    use symmetric supply to make life easier.
    Advantages:
    1) good linearity (opamp)
    2) good noise (opamp)
    3) few caps (freq response cost)
    4) if quad opamp more available for tone control etc.

    R1 should be much higher in value then the pots, or use buffers (balanced input).


    ------R2----
    pots | |
    []<--R1 ------------ - |
    []<--R1 ---| opamp ------------ out
    []<--R1 ---| - +
    []<--R1 ---| |
    |
    ///
     
  7. Yea, but the 1u caps before the pots and the 10u cap in the output is not needed :)
    Only the input caps are needed (in case connected to something with DC).
     
  8. Tim Williams

    Tim Williams Guest

    What's wrong with two resistors? Zero power consumption!

    Tim
     
  9. ectoplasm

    ectoplasm Guest

    What's wrong with two resistors? Zero power consumption!

    You mean a passive mixer, right?

    I am doing it now... it's a solution. Input signals get a bit weaker
    in the mixed signal, but it's ok for me.
     
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