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Compressor and EQ order

Discussion in 'Sound, Music, and Intercom' started by renegadeblack, Dec 10, 2008.

  1. renegadeblack

    renegadeblack Active Member

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    First things first, I'm the light guy at my school but my sound guy refuses to join this forum because he's realized that he's a KOB.

    OK, so we've had a compressor sitting around for a while and we want to set it up. We need to get some cables for it as well as a new power adapter. We currently have things set up as Sound Board > EQ > Amps. Where in the chain should the compressor go? We aren't quite sure. I was thinking before the EQ, but I'm not sure. Figured someone here would know.
     
  2. Eboy87

    Eboy87 Well-Known Member

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    It depends on what you want to use it for. Generally compressors are inserted in any of three places: on the channel, on the subgroups, right before the amps. Don't take that as gospel.

    The channel: you use the channel's insert jack to run out to the compressor and back. Generally for theatre, I avoid compressing individual singer's channels. However, I do put a bit of compression on things like bass, drums, and acoustic guitar. You'll need an insert cable for this, which is a 1/4" TRS jack going to two 1/4" TS jakes. I believe most console have the tip as send on the TRS plug and ring as the return. You can make your own, or Banjo Hut has them for around $10 last time I looked.

    The subgroup: I'm not sure the cheaper boards have inserts on the subs, but if they do, you can use this technique. On one run I was on, the male leads were sent to subgroup one, female leads to sub 2, male chorus to sub 3, female chorus to sub 4, etc. We had Focusrite Red 3 compressors inserted on the lead channels, and dbx 160's inserted on the chorus, just to tame them a little bit for the system. Keep in mind that if you do it this way, the loudest singer/instrument/whatever will in essence determine the overall level. It's the one that'll trigger the compressor, which will bring everything down in addition to the loudest thing. This problem doesn't exist if you compress on an individual channel basis.

    Before the amps: If I have an analog driverack, I like compressors right before the crossover. In this case, they act more as limiters than compressors, protecting the system from clipping. Most DSP units have limiters built-in to the individual outputs that you can control, so the highs go into limiting before the lows, to take an example. If you go this method, you simply hook it up in-line between your EQ and crossover (or amps if you're running full range). You'll need one channel of compression for each driveline if you want to compress all of your outputs, or you can just use two and limit the L-R sends. A note of caution with system limiters: if you keep running the system hard and into heavy limiting for an extended period of time, the overall power going to the speakers increases, and in most cases will burn up the voice coil.

    One other trick that I've learned since working, but haven't tried yet. If you have really long drive lines to your amps, you can put a compressor inline with the board's output. Set it so that it's not compressing anything, but you can dial up makeup gain, creating a sort of line driver to drive your amps. Again, I've never tried it; it's just hear-say.

    Finally, with compression, a general rule of thumb is that if you can hear the compressor doing its job, you have too much compression going on. I suggest that if you (or your sound guy) have never used compressors before, hook it up on your CD channel and experiment with it for a little while to see how it affects the sound.
     
  3. museav

    museav CBMod CB Mods Departed Member

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    It depends on what you are trying to do. If it is a comp/limiter being used for system protection, then it should be the last thing before the amplifiers. If it is a compressor being used for 'artistic' purposes, then it depends on the effect you want, but if the EQ is a house EQ and not normally adjusted then I would put the compressor before the EQ so that it does not affect the equalization being applied.

    Personally, I usually avoid compressors on the main outputs except as part of a limiting scheme for amplifier and speaker protection and especially if not a multi-band compressor. Compressors are typically much more effective and less obtrusive when applied to specific channels or groups, when used on the main outputs you can have a peak on any single input cause everything to be compressed.
     
  4. museav

    museav CBMod CB Mods Departed Member

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    Wow, at least our responses were pretty consistent and for once mine was the shorter response!

    I'm not clear what the advantage would be of the above unless it was simply a matter of the maximum output level or output impedance of the compressor versus the console. If both devices have the same maximum output level and the same impedance I have difficulty seeing what would be gained. Maybe if it is a really long line and even with the amp input wide open you couldn't get sufficient signal level then placing the compressor, or any active device, at the amp end might let you use it to compensate for line losses, although with a corresponding increase in noise levels. Or am I overlooking something (wouldn't be the first, nor probably the last, time)?
     
  5. Eboy87

    Eboy87 Well-Known Member

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    Apologies for the hijack Renegade.

    I wish I had a compressor to try out this theroy myself the other day. The setup on the show I'm on now is a Mackie 1402VLZ (an old one) running through a 5' XLR to an ART amplifier in the rack. I moved the mixer out of the booth to the tech table during tech week, because mixing in the booth is like mixing from a pillbox. I had to add a 30' XLR cable to the left and right outs to make it to the 5 footer in the booth, so each output was driving 35' of line to the amplifier.

    Now, I never changed the level of the sounds effects going to the mixer from Qlab. When I had the 35' line, a sound played with the mains faders at unity, and the channel fader at, say, -5dB was the perfect volume. Now, changing nothing, when I moved the mixer back to the booth and got rid of the 30 footer, playing everything at the same level sent the amp into clip, and had people diving for cover covering up their ears because it was far too loud.

    Nothing changed except the 30 foot drive line. My hypothesis is that the mixer didn't have enough oomph, for a lack of a better term, to drive the cable run to the amp. I would have liked to see if the compressor as a line driver would work.

    BTW, I've also heard of this technique being used in some of the shows in residence here in Chicago to drive their runs.

    Long story short, I'm assuming that if there is an impedance mismatch in the console output and the amp input, and you add a couple hundred feet worth of cable in between, there might be some advantages in using some type of line driver to take the strain off the console's output. As I understand it, a low impedance dropping it's load into a high impedance means the largest amount of voltage is transferred, correct? Or do I have that backwards?

    I have no idea if that's the OP's situation or not, I'm guessing no.

    Please correct me if that logic seems flawed.
     
  6. museav

    museav CBMod CB Mods Departed Member

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    Yeah, sorry for the hijack. :oops:

    30' of cable should not have much loss at all, certainly not the significant difference you noted. The 1402VLZ XLR outputs are +28dBu, 120 Ohm, balanced outputs. When they first came out one of the complaints with some of the Mackie mixers of that period was that the outputs were too hot, many of the downstream devices of the day couldn't accept much more than maybe a +18dBu input, and you sometimes had to actually pad down the mixer output. Anyways, unless the compressor is capable of an output much greater than +28dBu, which is unlikely, and/or has an output impedance much lower than 120 Ohms, then I don't see any obvious advantage.

    If you were using the XLR outputs, is there any chance you accidentally bumped the output pad switch on the back panel of the mixer when you moved it? There is a 30dB pad on the back for the XLR outputs that is intended to let you connect the 1402 into mic inputs. This was commonly used for submixer applications or I used it several times in existing gym systems so they could mix an announcer's mic, wireless mic, some sound sources and whatever else was needed and then feed that into an existing mic input. Just guessing here, but changing the pad setting would certainly create a significant difference in the level.

    You have it correct, impedance bridging, where the input impedance is greater than 10 times the source impedance, is used in modern audio electronics to maximize voltage transfer. Impedance matching, where the source and input impedance are equal, is used to maximize power transfer. Maybe that is more of what they are actually trying to do for a long run, get a device with a higher output impedance to maximize power transfer, sort of like a telephone line.
     
  7. TimmyP1955

    TimmyP1955 Active Member

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    Master: EQ or comp first? Depends on what the EQ is doing. If the EQ is compensating for system and room problems, put the comp first. If the EQ is compensating for program material problems (mic proximity effect, etc.), put the comp after the EQ.
     
  8. museav

    museav CBMod CB Mods Departed Member

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    If the comp/limiting is for system protection then you want it last even if the equalization is system EQ, it really is a matter of how both units are being used. It is also easy for personal preference to come in here, for example I rarely use a compressor on the main outputs in live sound other than a comp/limiter for system protection so I tend to assume that purpose.
     

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