Summing Amps - what do they do?

Discussion in 'Summing Mixers' started by audiokid, Mar 18, 2008.

  1. audiokid

    audiokid Staff

    I've just added the Tube Tech SSA 2B summing amp to the Pro Shop. I would like to use this topic as a reference.

    What are some great examples of what a summing amp does?
  2. Kev

    Kev Well-Known Member

    a summing amp is used to bring multiple sources together
    it may seem simple but it has some special requirements that a simple line level unity gain amp doesn't

    a simple situation of two line outputs joined to gether to make a simple mixer.
    each amp sees the other as a load
    simple solution is to feed the two sources through a resistor such that each output/amp is not overloaded (loaded DOWN)

    as this gets larger and more and more are brought together ... that summing point seems to get near zero
    virtual earth summing junction
    the signals seem to get smaller
    the following amp .. the summing amp ... needs to be low noise and high gain ... and headroom to burn
    and a high group time delay ( don't ask )
    and a phase shift under 90deg at the crossover point ( err definately don't ask)

    they are just better amps than your average line level, EQ, buffer ... general purpose amp

    some Mic-pre amps do find themselves used in this situation.
    The Neve BA283 and API2520 and cousins are obvious old time popular units

    A modern use for these things might be a PASSIVE summing unit
    for analog mixdown from Digital Interfaces

    might also be used to bring two stereo programs to one
    stereo mix with a parallel compressed mix
    mastering engineer trick

    add the tube feel of the Tube Tech and you might have some of that MASTERING magic

    all a bit too short and incomplete as an explanation
    but I hope it gets the ball rolling
  3. audiokid

    audiokid Staff

    Kev, awesome explanation!

    Would you use a summing amp for sub mixes, bus out certain groups like bass, kick, snare... and maybe even the lead vocals?

    Example, to give all the center panned tracks better imaging?

    Or... to use it for the sweet tracks in a song... kind of like adding a hook from a sound designers POV ... ear candy...

    Or... are they more for the final mix ?
  4. Kev

    Kev Well-Known Member

    all of the above

    current trend is to do sub mixes in both stereo and mono from the DAW and then sum these together in analog for a final mix

    yes imaging is one of those qualities people are looking for

    I guess the more inputs to the passive mixer there are
    the more people tend to take out of the sub groubs

    some units don't have a pan ... as these are hard to get trimmed equally across all channels
    same goes for faders
    here is why top end mixing desks are SO MUCH money ... quality parts

    back to the point
    some units have a switch for left, right and centre
    and have a fader as a rotary switch with matched resistors so each channel tracks the same

    The DAW can make a stereo synth or guitars or drums and the imagining is ok
    then these are brought into a larger mix using the passive unit and then the centres like lead vocal
    ... kick and snare perhaps.

    As for the sweet tracks
    well people use whatever trick they can to make a musical part sit inside the mix but easily heard... without pushing it on top of the mix

    some musical parts get transistors and some get tubes
    at the time of recording
    or later at the mixdown
    or a bit of both

    it's all about colour
    and then about imaging and openess and ... glue
    all at the same time

    difficult to put into words but when you hear it you will be chasing that feel for the rest of your recording life
  5. audiokid

    audiokid Staff

    Hey Kev...

    "when you hear it you will be chasing that feel for the rest of your recording life"

    beautifully put...

    now thats a signature definitely worth using!
  6. Imago

    Imago Guest

    Summing Amp

    Check out the Dangerous Music 2-bus. It's a fantastic analog summing bus for digital recording.

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