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Stereo audio mixer build

Discussion in 'Audio' started by elkbird, Jan 18, 2018.

  1. elkbird

    elkbird

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    0
    Jan 18, 2018
    I'd like to put together a mixer for 3 mono inputs and 3 stereo inputs plus a stereo effect send/return.

    I've found the following schematic that I'd like to adapt for this project:

    http://www.all-electric.com/schematic/simp_mix.htm

    which comes with a nice write up, so I think I have a pretty decent grasp on everything that's going on here.

    Adding another mono and another stereo input is straightforward. For the effects loop though, I'm not sure what's the best/proper way to implement it.

    I'd need another unity gain amp for the send, right? Here's how I'd think to implement it:

    use SPDT (mono) or DPDT (stereo) switches for each input to divert the signal path after any panning/balancing to the effect send. The signal path would then go to a unity gain amp (just like the ones for the main channels) then to an output jack. The return would then go have an input jack that receives the return signal, and this would sum at the node leading into the main channel unity gain amp.

    Questions:

    Should I have a diode at the effects return to protect the pedals/make it work properly?

    The effects return could re-enter the circuit post-unity gain amp. Should it go there? Why or why not?

    How mono-friendly will my stereo effects loop be? If I only have a mono effect in the chain, will the use of summing cables be sufficient to make it work as expected? Is there an elegant way to add a "MONO" button that will turn the loop into mono?

    Thanks for reading.
     
  2. AnalogKid

    AnalogKid

    2,299
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    Jun 10, 2015
    All three active mixer schematics have the same problem with the output. Placing a 10K pot between the opamp output and the connectors means that the output impedance varies between 100 and 5100 ohms as the pot is rotated. Not good. Better to put the pot between the first and 2nd stages. This gives a fixed output impedance of 100 ohms.

    Also, the feedback capacitors are too small. I don't think you need a 340 kHz or 590 kHz lowpass corner frequency in an audio system.

    Separate from that, post a block diagram showing the overall system you want, with each input and output clearly labeled.

    ak
     
    davenn likes this.
  3. elkbird

    elkbird

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    Jan 18, 2018
    Good to know!

    I've edited the diagram to show how I intend to implement the effects loop. I did it for only the right channel of a single mono input, but hopefully it's easy to extrapolate how I intend to go about the rest of the inputs.
     

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  4. AnalogKid

    AnalogKid

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    Jun 10, 2015
    You cannot run audio through a diode (and have it remain audio). Why do you think you can?

    ak
     
    davenn likes this.
  5. elkbird

    elkbird

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    Jan 18, 2018
    If I remove the diode is that a reasonable way to design the effects send?
     
  6. davenn

    davenn Moderator

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    1,758
    Sep 5, 2009
    agreed

    a diode in the audio path will cause horrific distortion of the audio signal ... why did you put a diode there ?
     
  7. elkbird

    elkbird

    4
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    Jan 18, 2018
    My thinking was that it would prevent loading current into the output of the effects devices, but in retrospect that doesn't make much sense.
     
  8. Wayne Phillips

    Wayne Phillips

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    Jan 7, 2018
    I would say don't use a diode on the effects send..if you look on most effects sends you will see a cap of small value say 2.2uf to even 10uf..
     
  9. tedstruk

    tedstruk

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    7
    Jan 7, 2012
    In my experience(of which is very very small) the balance system must be balanced to something... can't be balance to ground. ie. you can balance a midrange to a bass and a treble, or any combination there of, but you can't balance a high end to a low end without a midrange. on my latest guitar, I split the coils, I drove each coil with the switching and balance potted the center split so I have a kind of leveler between the front middle and rear transducers. It makes pickup switching alot more effective.
     
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