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Mod for rack-mount compressor for dual-band use?

Discussion in 'Effects [BG]' started by AaronBass4, Jun 3, 2014.


  1. For years I've run dual-band compression by running the biamp output of whatever pre I was using into a rack-mounted 2-channel compressor. Then, summing the outputs with a Y connector before going into the power amp.

    It occurred to me that I could save some wire clutter, reduce possible points of failure, and save the expense of the y-connector if I simply go into the compressor and hard-wire the outputs together. Electrically, this is the same as what I have been doing for years with the Y-connector. The grounds are already connected so I would just need one wire to connect the conductors going to the tips of the output jacks. I could get fancy and put a SPST switch in the added wire to make the mod switchable.

    I think there is a Rane model that has this feature (ability to sum the outputs), maybe the DC24. Is it possible that this is all they are doing or is there something fancier that they are doing and I should be doing?
     
  2. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    Shorting the outputs of pre and power amps together is not recommended and can potentially cause catastrophic failure.

    Manufacturers usually insert some resistance in the path of the output jacks to protect the circuits for the occasional cable short or miss-wiring. And even though passive mixers use series resistances, they are usually designed to have a light load (large resistance value) for the outputs of the source amplifying circuit.

    I would suggest that you contact the compressor manufacturer to ask if shorting the outputs together for mixing is safe with their product.

    -Frank
     
  3. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    The Rane DC24 uses active mixing to combine the channels before the output amplifiers.

    -Frank
     
  4. Thanks for your replies, Frank. I guess I've just been lucky. I've been running this way for more than 20 years (as an occasionally gigging hobbyist). I've used Behringer Autocoms - the 1200 and the 1600 - both of which are still in service and I've not blown up any amps. I suppose what I am doing is putting the 2 output impedances in parallel and changing the effective impedance of the summed output. Maybe the new impedance is still OK for the compressor and the downstream power amp in my case. Does that seem reasonable?

    BTW, I am totally sold on dual-band compression and hear only great things about your dual-band compressor. However, I'm not a pedal guy and I like to have visual indications of input, output, and gain reduction. Any plans to do a rack version of your compressor with lots of blinking lights? I probably couldn't afford it but I sure would consider developing a plan to hide the expense from the finance committee if one were available.
     
  5. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    Yes, you are effectively paralleling the two output impedances together, but it is a bit more complex than that because you have two low impedance source amplifiers before those safety resistors. The circuit may be perfectly fine with shorting the two outputs for a composite signal, but there are possibilities that there can be parasitic oscillations from each amp fighting against the other trying to correct for their proper signals. A lot of amplifier circuits will have decreased bandwidth with the excessive loading, which may happen when two low impedance outputs are shorted together.

    For this type of voltage signal coupling, the standard method is to have low output impedances feeding high input impedances. Shorting the outputs basically makes each output see the low impedance of the other channel along with the high impedance of the power amplifier input. The power amp input is at a high enough impedance to not be affected by the two channel short before it… so it won’t be harmed. But if you short the outputs of a two channel power amplifier that isn’t designed to do so, then the amplifier will usually fault and shut down its outputs to prevent damage.

    It is impossible to say if there will be a problem or not with shorting the compressor outputs without knowing the circuits involved. I just have seen enough problems over the 30+ years working with electronics to confidently say that I do not recommend doing it without the manufacturers’ approval. Those Behringer units may be totally OK with it, but there is loads of info for very simple passive mixers on the net that will be a much better method of combining these signals. You can always take a low impedance output, split it and feed several high impedance inputs without issues.

    Another harmless issue from mixing signals from crossovers is that there can be nasty phase summing that causes cancellations and/or peaks in the signal. Again, without knowledge of the circuits involved no one can say how it will work out.

    As for us building rack gear, we would definitely like to be in that market someday. But with the recent changes we have made with staff and production around here, we are basically a new business that has a lot of work to catch-up to where we should be before any new ventures are started.

    -Frank
     
  6. Thanks Frank. This is great information. I really appreciate the time you took to share it with me. I will dig around and see if I can find some information on a simple passive mixer circuit. Maybe I'll try bread-boarding something and see if I can package that inside the compressor. I had the 1200 apart last night and there is a ton of space available for something like this.

    ... and I'll be watching for your rack version of the 2-band optical compressor ;-).
     
  7. Sure enough, there is lots of information about this available . . . now that I know what to look for. Looks like its as simple as adding a 1K resistor between each output and the summing node. The output impedance of one of my compressors is 40 ohms. The input impedance of one of my amps is 20K. If I'm thinking about this correctly, the 1K added output impedance will significantly increase the isolation of the connected outputs since 1000 is large relative to the output impedance, and, since 1K is small relative to the input impedance of the amp, the attenuation should be minimal. When I get some time, I will try this.
     
  8. Well, after all I've read and learned about this now, I'm not sure what to make of this. . . . I finally got around to asking Behringer about this and they said I should be able to just connected both outputs of the compressor to one input of the power amp. I've already ordered all the parts to make a simple passive mixing box based on a circuit recommended in an article Rane published (Why Not Wye?). I will probably go ahead and make the box, connect it, and see if I notice any difference.
     
  9. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    Even if you do not hear a difference, the method that Rane describes is the proper method of combining output signals. The Rane engineers are top notch.

    -Frank
     
  10. I built the summing box shown in Figure 1 of the attached Rane document. It worked but the attenuation was much higher than I expected and I was not able to drive the power amp to get anywhere close to full volume. Not sure where to go from here. I am looking into some op-amp circuits for recovery. I might just give in and buy a combiner pedal like a Boss LS2. Been trying to avoid that because I don't generally do pedals and prefer to keep everying in the rack.
     

    Attached Files:

  11. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    Since your amp has a 20Kohm input impedance you can remove the 10K resistor to ground after the 470 ohm summing resistors.

    -Frank
     
  12. Thanks Frank. I will try that - snip, snip, and done. Family is asleep so no more window-shaking tonight . . . .
     
  13. OK. 10K resistor to ground is gone leaving just the 470ohm resistors connected to the summing node. Output is improved quite a but still just barely touching the power amp even with the compressor output peaking at +12 or so. Next, I connected the output to my trusty old Trace Elliot GP7SM 250 at the instrument input with active pad engaged. That did the trick. All kinds of headroom now. I'm thinking that I should go more aggressive on the summing resistors - maybe 1K or even 10K. In the circuits I found online, most had 1K or 10K resistors and no resistor to ground. Since I'm going back into an instrument level input, I can get away with a lot more attenuation now. It's hard to say if there is a difference - I'll need to crank it up with the band before I can tell if anything good happened (that's audible).
     
  14. I changed the summing resistors to 10K and now the output from the summing box 'looks like' insturument level from an active bass at the input of the Trace head. When playing with the band it's hard to say if there is a difference. Maybe high range (crossed at about 300HZ) is a little more present. Maybe the higher energy output of the low channel shorted to the high was causing a ducking effect on the high channel??? I certainly could be imagining this. IN any case, it sounds good and I'm comforted to know that I'm not causing any consternation in the output stages of the compressor - so, I'm going to leave it like this for now.

    If anyone is watching (besides Frank) and cares to know anymore detail, please let me know. Frank - thanks again for your insight.
     
  15. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    I've certainly been following this with great interest--just didn't have anything near Frank's input to add. :)
     
  16. Ha! OK. So it's the three of us - me, the master comressor builder, and the master compressor archivist ;-). This method of dual-band compression and the benefits are obvious to me . . . but I'm an old guy still using equipment from the 80's and 90's when, perhaps, cross-overs in bass equipment were more of a thing.
     

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