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Compressor damaging speakers?

Discussion in 'Amps and Cabs [BG]' started by complexprocess, Dec 9, 2003.

  1. I was just reading another thread and someone mentioned this as an aside. I figured I'd start a new thread rather than hijack the other one.

    So the way I read it, it can be dangerous to use a compressor to squash a signal to much, because it will create a signal that is peaked for too long, too consistently, translating to speaker damage. Is that right?

    Here's a "hypothetical" situation to think about:
    Imagine a 200 watt head, matched with a 200 watt cab, being used mostly for bedroom practice and quiet jamming (read: volume will probably never be above 3, let alone cranked :)). Would a compressor in the effects loop that was squashing the signal be likely to cause damage?

    Ok, I'll admit it. this situation isn't hypothetical. It applies to some guy I know who's a little sloppy with his 3 finger attack and that's why he needs a compressor. ;)

    Oops, I think I posted in the wrong forum by mistake. Someone please feel free to telewarp me to effects.:D
  2. Actually, I think this post belongs in "Amps."

    To answer your question: No.

    Why doesn't a compressor damage speakers in this case? A compressor controls the dynamic range of a signal, effectively making the quite parts louder and the loud parts quieter. That means your speakers will move LESS. The signal is not "peaked." Quite the opposite, as a matter of fact. The signal's average level is increased and made more consistent.

    A cheap power amp when constantly overdriven, will put out speaker damaging garbage, but it's signal peaks that overdrive an amp unless you just play full-on total nasty distortion, and even then so long as the speaker can take peaks of twice the amp's output wattage it's not going to do damage. That basically means the cab need to be rated equal to or greater than the power amp.

    Compressors aren't meant to be used in an effects loop, which blends a "dry" signal WITH an effected "wet" signal. Loops are for things like reverb and chorus where you want a mixture. The idea of a compressor is that it will change the whole signal. You won't hurt anything going through an effects loop, but you won't see any real benefit of the compression either.
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    "That means your speakers will move LESS".

    Sorry, that's a non sequitur. It's also a false statement. For a given signal at a given frequency, more average power means more speaker movement.

    What I think you're thinking about, is "excursion". That's a measure of how "far" the speaker moves, not how "much" it moves. A very powerful low frequency transient can cause your speaker cone to try to jump right out of its frame.

    But that's something different from a voice coil that overheats because it's getting too much average power. Which I think was the point of the compression thread.

    Bottom line, a crappy speaker is a crappy speaker. If it overheats or the cone jumps out of the frame at anything below the rated power, it's a crappy speaker. At least, it's not living up to its rating.

    So the answer is, no, this should never happen under any circumstances if you have good quality gear. There should never be any speaker damage from using a compressor, period. If there is, it's probably the fault of the operator and not the technology. :)
  4. It depends on how you take the original question: If the damage asked about is coil damage due to excursion, the speaker will move more air over a shorter excursion because the current content is greater, ergo the speakers move less in terms of excursion. The statement isn't false; it's qualified.

    The power rating comment on cab versus amp covers coil overheating from constant high current, which is far more likely to occur from a cheap amp than compression as a process.
  5. Finger Blister

    Finger Blister

    Jul 8, 2003
    Compressors should be used in the effects loop.
    A Serial Effects Loop and your fine.
    A Parallel Loop needs to be set fully 'wet.'

    Compressors increase apparent volume before
    distortion and many are used to protect speakers.
  6. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL
    i might have been the one who wrote that in a certain thread, and maybe you guys can clear up some things for me as well...

    i was told that certain compressors can "normalize" lower signal notes bringing them up to equal volume as the louder notes.

    in doing so, most guys run these particular compressors at near blasting levels that are juuuuuust below clipping, which could stress the speakers much faster than rigs without.

    what say you? yea? nay?
  7. VellaBass


    Aug 29, 2003
    London, UK

    Not quite right. A compressor reduces the level of signals over a set threshold. This means that by using make up gain you can then raise the overall volume because the peak sounds are not so loud, which of course makes the quiet parts louder than they were before.

    There is another important consideration, called attack time, which determines how fast the compressor works. With bass its usual to have it set so the compressor doesn't kick in for, say, 50 milliseconds, so that it doesn't take too much punchiness out of the sound. Thus it won't catch a very loud, fast transient, and because the compression has allowed you to raise your overall volume, it's gonna be that much louder. I would guess that this is the likely cause of the speaker damage.
  8. BruceWane


    Oct 31, 2002
    Houston, TX
    If you are using a speaker with a power rating that is equal to or greater than the power amps rated output, then no, you cannot damage the speaker by using compression. With a 200W amp and a 200W speaker, no worries.

    However, if you are using the formula of running a power amp that is 2X the power capacity of the speaker for clean headroom, you have to be careful with compression. This is a far more common practice in PA systems; most bass players don't do it.
  9. If they're pedals, most can't handle the line level from an amp and need to be used before the input. Rack-mount compressors need a serial loop or a parallel loop with a 100% wet capability as you alluded.
  10. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Compressors squash the dynamic range. Most work by progressively reducing the gain the further the signal level exceeds a certain threshold. Essentially, they bring the peaks down closer in level to the average signal.

    But they're commonly used in conjunction with some gain boost so that the result is a higher average level with the peaks tamed to prevent clipping or other overloading. Thus, if you follow the common rule-of-thumb of using a power amp that is rated at double the speaker's continuous power rating (or equal to its program power rating), you run the risk of burning out your loudspeakers if you use heavy compression to make yourself louder. In such a situation, you should consider speakers that are rated for higher power.
  11. natrab


    Dec 9, 2003
    Bay Area, CA
    Ok, so another "hypothetical" situation (hehe).

    Say I'm running an Ampeg SVP-PRO with a few effects out to a dbx 166A compressor, then to a QSC PLX 3402 (1100w @ 4ohms) into a Goliath III cab(700w RMS @ 4ohms - I assume that means it can handle 1400w peak).

    Now would I have to watch my compressor settings for fear of hurting the cabinet? I understand that if I lower the range of volume with the compressor than I would be inclined to turn it up a bit, but would that really hurt the speakers?
  12. Thanks for all replies people. Looks like I shouldn't have a problem.
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    It that situation, I think you're still not likely to exceed the speaker's power handling capability.