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Send/return balanced mic signal to compressor

Discussion in 'Recording' started by quesne, Dec 10, 2008.

  1. quesne

    quesne Active Member

    Nov 10, 2008
    Hi Guys:

    I have an Alesis 3630 compressor which -- based on how I am reading the documentation -- has two unbalanced channels which can be used separately or as the R-L sides of a stereo signal. I also have a Presonus Firestudio Project.

    I'd like to patch the compressor in to the back of the FSP using the 2 sends and returns, but I do not want to compromise the quality of the signal I am recording and/or monitoring (i.e. I do not want the return signal to be cut in half since it would be unbalanced).

    I have a balanced patchbay, and it would be nice the have the compressor attached so I can use it in the first one or two mics whenever I want (i.e. patch them in when I want to). Could anyone give me an idea as to the best way to do this?

  2. Link555

    Link555 Distinguished Member

    Mar 31, 2007
    North Vancouver
    Unbalanced is fine if your cable runs are short. and you are careful about keeping your audio cable separate from your power cables, or at least they cross at perpendicular.

    Use the +4VdBu Switch on the 3630, this will pad input and adjust the output for +4VdBu.

    Balanced connections are differential, so the positive and return paths are floating. Usually when these connections enter a piece of audio gear the balanced connection is converted to a single sided (or unbalanced circuit) via a transformer or differential line receiver such as a SSM2141. This just ties your audio signal down to common reference point, i.e. the circuit common.

    If you want to use your balanced patch bay the best way is shown in the following PDF


    I hope this helps,
  3. Boswell

    Boswell Distinguished Moderator Resource Member

    Apr 19, 2006
    Home Page:
    The 3630 is not designed for handling mic-level signals. The place to put it is between your mic pre-amps and your ADC, or else patched in as an insert device if your pre-amp has insert jacks.

    The 3630 is unbalanced, so it needs some care when using it with balanced equipment. For the 3630 output, treat it as balanced but with the -ve signal lead grounded. Most balanced equipment will accept this as a full signal (no 6dB loss).

    For the 3630 input, you have several choices:
    (1) use a balun transformer
    (2) drive always from a pre-amp with floating transformer output
    (3) just connect the +ve signal of the balanced pair and take a 6dB signal hit (the 3630 has up to 20dB of make-up gain, so it's not a huge problem)
    (4) use as an unbalanced insert device

    Alesis designed it expecting it to be used as (4), so that would be the best bet, if available.

    Looking now at your FireStudio Project, channels 1 and 2 have line outs and returns which can be used in an insert-like fashion. But these are balanced , so you can't get away without some compromise.

    One way of wiring the 3630 through your patchbay is to normal the FSP send/return through the patchbay and patch in the 3630 in method (3) above. You would have to use threshold settings of 6dB less than expected and an extra 6dB of make-up gain.

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