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What are the chances I will do any damage? (Bob Lee perhaps)

Discussion in 'Amps and Cabs [BG]' started by chaoslord, Jan 20, 2003.


  1. Hey guys and gals. Here is my dilema. I am using a QSC RMX2450 bridged mono into my Ampeg 810. So I am bridging 2600 watts into a cab that can handle a peak of 1600 watts. Now the main reason for such a large power amp was to get a nice clear sound with plenty of volume and not have to worry about clipping the amp.

    I have always been told that overpowering your speakers isn't a problem unless you are pushing the amp to clipping or really thumping the hell out of things.

    I have also always been told that the proper way to run a poweramp is to crack it and adjust volume with the source that is running into the poweramp (in my case an Ampeg SVP-BSP preamp).

    My question is should I be cranking my poweramp and adjusting on my preamp or should I be cranking my preamp and adjusting on my poweramp insted so that I don't give my speakers too much juice?

    Any help is apreciated.
     
  2. You should be fine with that setup. You'll get better signal to noise and probably better preamp tone if you crank the preamp to a point just before the input clips. Crank it 'til you see the clip light while you're playing hard, then back it off a tad. Crank the outputs of the pre in similar fashion, then use the power amp's attenuators to get to the volume you need. That's what they're there for. Turning down and up using the amp's attenuators back will have less effect on oyur tone than turning your preamp up and down as well.
     
  3. I have the input on my preamp cranked to just before clipping, so it is getting the full amount of juice there. I just wasn't sure wether it would be safer to use my preamp's "Master volume" or my poweramps master volume.

    Thanks for the advice, anyone else agree or disagree?
     
  4. I use the power-amp as the master volume. My pre-amp doesn't have a master or a clip light (F1-X) I just crank it up to distortion and then back it off until the distortion only breaks the peaks. Then I use the power-amp to set the volume. My power-amp is 1000 watts into a 400 watt cab. I've not had any problems and I've been playing this way for over a year 2 to 3 nights a week.
     
  5. 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
    Cool! You're getting good advice before I even get here. ;)
     
  6. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    But don't you get more headroom if you set the power amp levels to max and use your preamp to control volume?
     
  7. 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
    No.

    The knobs on a power amp are gain controls, not "headroom" controls.
     
  8. amper

    amper

    Dec 4, 2002
    US

    Umm...no. The knobs on an amp are not gain controls, they're *attenuators*. Depending on the S/N ratio of the components involved, you may get a better result by opening the power amp up all the way and controlling your volume from the preamp, rather than the other way around.

    It all boils down to this--which is cleaner, your preamp or your power amp? The only way to be sure is to hook up an oscilloscope, unless the differences are obvious (which they may be with your gear...).

    The bottom line is, an amp has a fixed amount of headroom/gain/amplification factor/whatever you want to call it. The only level control in an amp is the level of signal entering it or exiting it, which is changed with an attenuation stage.
     
  9. amper

    amper

    Dec 4, 2002
    US
    Actually, I should modify that...there may in fact be some amps out there that change the amplification factor. I believe London Power's amps follow this type of philosophy? Maybe?
     
  10. Phat Ham

    Phat Ham

    Feb 13, 2000
    DC
    Although the knobs on the amp don't control gain by boosting but only by attenuating, they are still gain controls aren't they?
     
  11. 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
    No, they're properly called gain controls. They may be attenuators in a particular part of the amp circuitry, but they do control the overall gain of the amp. Thus, they are gain controls.

    The way to maximize overall signal to noise ratio is to maintain a strong signal-to-noise throughout the system. That means you get your signal boosted early in the chain to keep it well above the noise floor of each successive stage. Then you don't need as much gain at the power amp stage, and you don't boost the noise as much. If you've got a noisy stage in the system, having a strong signal level going into it is the best solution, not boosting a weak signal and lots of noise by maxing out the power amp.
     
  12. amper

    amper

    Dec 4, 2002
    US
    Well, Bob, you're more familiar with industry terminology than I am, but I think the term "gain controls" implies that they actually change the amplification factor of the circuit. As I understand most amplifier circuits, the amplifying component amplifies at a fixed ratio (assuming you're not *really* changing the gain ratio, ala London Power's Power Scaling/Super Scaling), so I find "gain control" a less-than-optimal description of what's really going on in the circuit. They're not changing the gain ratio, but the starting point (in the case of an input attenuator), or reducing the level of the output (in the case of an output attenuator). Of course, no matter how you look at it, the result is the same...

    But the S/N thing...true, you do want your output to be as high above the noise floor as possible, so running your preamp as hot as possible will generally provide the cleanest signal, assuming you're comparing apple to apples. I still think that adding another attenuation stage (i.e., the input controls of a power amp) can degrade a signal. I think it's probably better to use a lower power amp than to turn down the input to a more powerful amp, assuming you're still using enough power to reproduce your peaks cleanly.

    Thankfully, in most equipment, these concerns are for the most part irrelevant. With well designed circuits, most people won't notice a significant noise difference between using a high level setting on the preamp/low level on the power amp and a low level on the preamp/high level on the power amp. Of course, it's still better to follow best practices, as you suggest, but if you find you get a better sound by opening up you power amp and turning down the preamp than the other way around, who am I to argue? All sensory perception is personal, no?
     
  13. amper

    amper

    Dec 4, 2002
    US
    Anyway, the important thing to understand is that power ratings of amplifiers assume maximum input level. If you're attenuating the input of your power amp below its max level, whether at the preamp exit or power amp input, you're not getting the maximum wattage out of the amp. So as long as you don't send the speakers more power than they can handle, you're not going to blow anything (in the speakers that is, I can't vouch for the building you're in or your listeners' eardrums!).
     
  14. 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
    There are no "output attenuators" on power amps. That would have to be like a power soak and would be a really poor way of reducing an amp's gain.

    Let's look at a typical power amp: it has an input stage, which is essentially a buffer and, in pro use, a differential amplifier so that you have balanced inputs. Usually they have a gain of 1 or 2. Next are the gain controls, which are passive attenuators and thus have a gain of 0 to 1. Next is the output section, which provides the oomph, with voltage gain and current capacity; it might typically have a gain of anywhere from 10 to 40, depending on the model.

    From input to output, these gains multiply:

    Code:
    [Gain] = [Input gain] × [Gain control] × [Output stage gain]
    Thus, if you have an input stage gain of 2 and an output stage gain of 20, varying the gain control knob between 0 (minimum gain) and 1 (maximum gain) will vary the overall gain--the amplification factor--between 0 and 40×.

    Turning an amp down does not reduce its power capability; it simply reduces its gain. The result would be that it takes a higher input voltage to make the amp reach full power. Using a lower-power amp will cut down on your headroom, which may or may not be a good idea.

    You might get your best sound and performance with the preamp turned down and the power amp turned all the way up. You might also happen to go faster in your car by not stepping on the accelerator than by flooring it. But in both cases, the smart money would be bet on the opposite scenarios.
     
  15. amper

    amper

    Dec 4, 2002
    US
    Power soaks do actually work well in some instances, like for guitar...but this is a bass forum, so I guess that doesn't really apply. I do however, also play guitar, and I find that an SWR Interstellar Overdrive (which has a power soak built-in) is just the thing I need to get the sound I'm after, which is power stage saturation, but at 5 watts, that would blow any normal power amp to smithereens. The power soak allows me to bring that output back down to line level to drive effects, and then I amplify the resulting signal with a non-saturated power amp (in my guitar rig, a Mesa 20/20). For bass, I run the Stella clean.

    But, and you're the guy who works for an amp company, not me--I thought that the way most amps worked was that they amplified at a fixed ratio, say hypothetically 120dB. Now if your input max is 1V, you're going to get X watts of power to your outputs, but if you attenuate that input to .1V, you're still amplifying it the same 120dB, but your output will now be only .1X watts. Right? You're not really changing the amount of gain in each stage, you're just changing the input level.

    BTW, of course using a lower-powered amp will cut down your available headroom, but if the amount of headroom left is sufficient to reproduce your peaks cleanly, why would you need a more powerful amp? All things being equal, I owuld bet that taking that higher powered amp and attenuating the input so that your gain is reduced--in the question that was posed, the poster was concerned about overpowering his speakers, so attenuation is a requirement here--would be likely to result in a lower S/N ratio than simply using a lower gain factor amp and not attenuating the input. Yes, the difference would most likely be small, if not imperceptible, but I think eliminating unnecessary attenuators and/or gain stages in a signal chain is a good thing.

    But I'll stop rambling on now...it sounds like we're talking about two sides of the same tree and not realizing it's the same tree...thanks for the conversation!
     
  16. In my opinion, the way to make a higher powered amp safer for a given speaker cab would be to minimize available pre-amp headroom by cranking it up and turning down the gain/attenuators on the power amp. The pre-amp can only output so much and then goes into clipping. This is sorta like using a compressor. (sort of) The real danger with using big power is not playing the guitar but accidentally dropping something or having a cord short out or some sort of runaway feed back problem that drives the amp into clipping and frying you cab.