Dismiss Notice

Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

Level settings for pre-amp v. power amp

Discussion in 'Amps and Cabs [BG]' started by Inseam, Jun 18, 2004.


  1. Inseam

    Inseam

    Jun 17, 2004
    Pittsburgh, PA
    I'm a newbie here, my first post. What a great site!

    I just ordered my first power amp and cabinet. Always used combo amps before. My question is this: how should the relative levels of the pre-amp and power amp be set? That is, should the pre-amp output level be relatively high and the use the power amp level to set the volume, or the other way around? I'm told that the Stewart power amp had great "headroom", which I understand in a general sense, just wondering how best to take advantage of it.

    FYI, I'll be using a SansAmp PSA rack pre-amp, a Stewart World 1.2 power amp, and an Acme B-2 cab. I've heard the Acme is power hungry, so I plan to use the Stewart in bridged 4-ohm mode. But I would be nervous turning up the Stewart all the way for fear of damaging the cab with all that power.
     
  2. Passinwind

    Passinwind I Know Nothing Supporting Member

    The ideal is to leave some headroom at each gain stage. You set input gain just below clipping, at the loudest output from your bass. You set your tone controls such that they won't overdrive the preamp output stage, unless you want that. You set your master gain so that it matches your power amp's input sensitivity, and can drive the amp fully. Then you turn the power amp input to where you have the desired volume.

    But I would be nervous turning up the Stewart all the way for fear of damaging the cab with all that power.

    You can still overdrive your speakers with the power amp input turned down. The amp's input sensitivity is what changes, not the power output. IOW, if you send a really hot signal from your preamp, and it's very distorted, your amp can maybe still develop full power with the input attenuator at, say, 12:00, and would actually be more likely to hurt your speakers than a totally clean gain staging as detailed above. Make sense?
     
  3. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    My non-technical way of looking at it is this: I set up the preamp for optimum tone (too much gain and it might distort), and signal-to-noise ratio (too little gain and you'll notice background noise). Then I let the power amp do the work.

    Have fun, that's a sweet rig!
     
  4. Inseam

    Inseam

    Jun 17, 2004
    Pittsburgh, PA
    Fuzzbass phrased it the way I would have done it with no other guidance. So that makes me more comfortable.

    Any advice on how you "set your master gain so that it matches your power amp's input sensitivity"? Right now, that statement goes over my head. I never had to worry about this before. Maybe it will make more sense once I actually get the gear.

    One other related question. Someone told me that you take advantage of "headroom" by turning the power amp all the way up (or nearly so) and then control volume with the pre-amp output. That didn't make sense to me, and I think you are saying that I was right to think so. I thought "headroom" meant you didn't have to drive the amp to its limits to get the power output needed, hence cleaner power. But this guy had some experience with power amps, so that's why I came here for some second opinions.
     
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis

    If you keep the power amp turned down, which many people feel is less scary to them, you can cause problems.

    1) the preamp has to put out more signal. Many QSC amps (for example) are -10 dB at half on the level control. That takes 3 times the input voltage to get to the same volume as when the control is full. Some preamps just don't have that much output.

    2) More preamp output means you are closer to preamp clipping. Then if you also feel uncomfortable with the master at full, and keep IT at half, you could toss away ANOTHER 10 dB. Now you need NINE times the voltage from the preamp to get full power.

    3) If you get preamp clipping as a result of trying to compensate for all these losses by more gain, it sounds bad.

    4) Clipping is also harder on speakers, since there is more power in a clipped signal than an unclipped one at same general output amplitude (voltage). Any preamp clipping will be faithfully reproduced by the power amp, since you have probably kept it at a low, unclipped level.

    Now for "headroom".

    If you have turned everything (amp and preamp master) down some, you have LESS "headroom".

    Headroom is generally accepted to be the the difference between the usual signal level and the clipping level (max power).

    Since as per above, you have pushed closer to clipping in the preamp, you almost certainly have LESS headroom. The difference between your "average" volume and the most you can get without clipping is smaller.

    So, it makes a lot of sense to turn up the power amp to max or near max, and use the preamp master and gain to set your volume etc.

    Only if you get too much background hiss with amp up and preamp master down, does it make sense to turn down the power amp somewhat. (assuming you could actually use full power of the amp)

    Otherwise you are just throwing away dynamic headroom. That can strip off and clip string pops etc, squashing your sound.


    Now, if you play a small bar, might be different. Your amp may be so big you can't use the full power of it without being ridiculous.
    Then turning down the amp may make sense, since that way even if you book the pre you won't get louder than the level you set, without getting into clipping. Effectively you have reduced the max power of your power amp.

    But in that case, you DON'T WANT headroom, you are trying to reduce it so you don't blow everyone's ears off. So in reality, you are using the reduced headroom that occurs when you turn the amp input down.
     
  6. Passinwind

    Passinwind I Know Nothing Supporting Member

    Fuzzbass phrased it the way I would have done it with no other guidance. So that makes me more comfortable.

    That's the beauty of Talkbass, we get to hear different takes on the same subjects. Bob Lee's Ellington quote, "If it sounds good, it is good", comes to mind here, no?

    Any advice on how you "set your master gain so that it matches your power amp's input sensitivity"? Right now, that statement goes over my head. I never had to worry about this before. Maybe it will make more sense once I actually get the gear.

    Jerrold did a nice job explaining this, and I didn't.:cool: My point is to avoid needlessly clipping your preamp output. Say you only want to run 500 watts max into the B2. I would set my bass, gain, and pre-master all as hot as they'll go without clipping. I'd turn the power amp down to the desired max volume. Then I'd use all those other controls to drop the level to where I actually want it, depending on what sounds best. You have headroom to burn. In other situations, keeping the power amp all the way up can make sense. In practice, nearly every one I've heard sounds better attenuated at least a little bit, assuming the preamp ahead of it is up to the task. But then, I think any properly designed system always allows for more amplifier power than you think you need, or can use.
     
  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
    Good way to put it.
     
  8. Inseam

    Inseam

    Jun 17, 2004
    Pittsburgh, PA
    Thanks to all for the input. Can't wait to get started.