Power amp gain all the way up ?

Discussion in 'Amps and Cabs [BG]' started by eyoyo, Dec 17, 2014.

  1. eyoyo


    Jan 18, 2014
    Dear Power amp experts,
    I am using a preamp (Amepg SVP-CL) + power amp (QSC RMX1450) trough an Ampeg SVT-212AV cab.
    Power amp spec is 2x450W (I use one channel) under 4ohms load and cab is 600W 4ohms.
    Some people told me that it was more safe to set the power amp gain all the way up and adjust volume with the preamp master.
    I understood that it was better for speakers if I do this because the preamp will never saturate.
    I'm scared for the power I could send to speakers coils if I do so.
    On the other hand my preamp master is always below 9PM when I play. Isn't it weird?
    What do you "preamp + power amp" users do and why ?

  2. The power amp can put out its full rated power even if your attenuators are below "full on". Just takes more input signal.

    Just use your ears and you will be fine.
    seamonkey likes this.
  3. eyoyo


    Jan 18, 2014
    I didn't mean I wanted all the rated power.
    I only want to know what is the best way to adjust masters on both preamp and power amp
  4. Depends on the pre, how noisy it is, what kind of color you want out of it, and where the combination of settings leaves you.
  5. TrevorOfDoom


    Jun 17, 2007
    Austin, TX
    Power amp at full volume will yield the cleanest tone.
    My suggestion: turn up your preamp gain to where you like it, then adjust the power amp volume to where it needs to be to put that tone into the room. Done.
  6. Roland GR 88

    Roland GR 88 Commercial User

    Sep 16, 2013
    Ontario Canada
    Retail store manager
    You won't harm anything. Run the amp at full, adjust the pre for taste. If you like a little dirt then back off the pwr amp as you raise the pre amp. TrevorDoom has it right, find your tone and adjust volume as needed.
  7. eyoyo


    Jan 18, 2014
    I'm quite reassured but I still don't understand why it's better to adjust power amp to a high level!
  8. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    I do it the way the Applications Engineer at QSC recommends, which is pretty much SOP in the pro sound world:
    Setting Gain Structure with a Pre & Power Amp Setup

    Last edited: Dec 17, 2014
    ddnidd1 likes this.
  9. I have an RMX1450 - I just set the gain on it like a master volume control. There's a signal clip light on the front too, in case you turn up your preamp too high.

    Only time I've cranked the master volume to full on an amp was a guitar tube amp. Running the power tubes at full gave it better tone, though might have been shortening their lives.

    Setting the master at full just seems like you're setting yourself up for potential disaster. A little crackle in a dirty pot when turned, and suddenly you have blown speakers. Well, not really, but I just don't see any reason to crank the power amp.
  10. eyoyo


    Jan 18, 2014
    I crank it all the way up because I read it's the way to use a power amp. I just don't know why it should be used like that. Nobody seems to agree!
  11. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Chester, Connecticut
    Former Technical Communications Developer, QSC Audio
    No, it won't.
    seamonkey, Jim Carr and B-string like this.
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Chester, Connecticut
    Former Technical Communications Developer, QSC Audio
    If that were the way to use a power amp, why would the manufacturer even bother putting knobs on it?

    The gain controls on a power amp are there so you can make it work with a lot of different things. So stuff can work with other stuff. If your preamp can only put out a weak trickle of signal, you'll probably have to crank the power amp gain way up. If your preamp can put out a decent signal, let it do so and turn the amp gain down.
    ddnidd1, jastacey and B-string like this.
  13. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    This is true. Feeding a power amp, with focus on the RMX1450, is a bit of a crapshoot as there's no consistency in preamp outputs brand-to-brand, model-to-model. The SVP-CL (sans modification) is notorious for its relatively low output, as one example. The trick is to set your gain structure (via power amp attenuators) to establish a linearity of response when boosting the master on the preamp. This will provide the best response with minimal clipping and optimal signal / noise ratio.

    When paired with a known line level device like a mixer, the suggested attenuator setting is ~37% or the 11:00 position. Sounds a bit absurd but it's true as I verified by running pink noise through the system. With the 1450 attenuators fully CW (no attenuation), I can easily clip the amp with mixer slider at -15 dB.

  14. The other thing to consider is the noise of the amplifier itself (aside from fans). With no input when you turn the amp up full do you hear a hiss? If you can turn the attenuators down to a level where that hiss is not heard and still have enough volume for the situation that you are in then do so.
  15. groovatron


    Mar 11, 2010
    It's pretty simple really. If you are clipping the power amp output, turn it down. If you have headroom, play with the input level of your preamp for tonal reasons. If you find the sweet spot, use the power amp gain to adjust overall volume. Max output is max output. Whether the gain comes from the input signal or the amp itself doesn't really matter. Just don't overload the power amp and you will be fine.

    As far as your speakers are concerned, there are many more factors involved. You can blow a woofer running it under it's RMS, especially when using effects in your signal path. Too much of a certain frequency or unaccounted gain from a pedal can cause damage in certain circumstances.

    In the end, you must use your ears. Balancing your tone with your equipment is key. Know what your gear is capable of and know how hard you can push.

    Not sure if any of this helps at all, but everyone has their opinions and I'm not one to tell you in absolutes:) Good luck.
  16. JACink


    Mar 9, 2011
    I'm not saying it is THE correct way of doing it, but the way I have always set up power amps is as follows:

    I set whatever I am running into the amp at the maximum gain level that I will be running at when I am maxed out. I then adjust the gain on the power amp to the maximum level before the speaker/cab etc. starts to distort, in other words, I find the max of the speakers and then back it off a little.

    If this amp is for use by another person, I usually install the great little metal plates that QSC provide to cover the gain controls.

    Obviously, it all depends on what changes you make to the signal going into the amp, as groovatron said, certain effects can cause clipping, so you have to use your ears and some common sense.

    One thing I have learned over the years is that if it is a DJ or a Keyboard player that is going into the amp, make sure any knobs they may have within reach are set to max before setting the power amp levels (or their channel levels on the mixer), as they will slowly turn everything up during the gig! :)
    Last edited: Dec 18, 2014
  17. baxter_x


    Nov 27, 2013
    I usually set the power section of my SVT 4 Pro at 75%. Then adjust my volume with the preamp gain.
  18. I think it's kind of wrong to call the knob on the poweramp a gain knob. It does not add gain, it just reduces the input signal. One can sometimes achieve a lower level to noise ratio when letting the preamp work on a level it's comfortable with and then use the poweramp's sensitivity knob to adjust the output volume.
  19. JACink


    Mar 9, 2011
    What is the common name for it in English? I remember referring to it as a volume once and that caused a huge uproar in the thread.
  20. baxter_x


    Nov 27, 2013
    Absolutely! I should have raised this before ;)