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Pre/Power amp levels

Discussion in 'Amps and Cabs [BG]' started by ivanthetrble, Dec 8, 2003.


  1. ivanthetrble

    ivanthetrble

    Sep 9, 2002
    Orygun
    For those who use a pre amp/power amp set up, what do you do with the levels? Do you turn the power amp up all the way and control the volume with the pre amp master or do you turn the pre amp up all the way and use the power amp level to control volume? Other options?
     
  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Hi ivan, what I do depends on which preamp I'm using. If I have a preamp that goes into crunch mode when I turn up the volume control, I set up a little differently. I usually do three things in sequence. First, I turn the power amp up just loud enough so I can hear it, and I dial up the sound I want on the preamp. Second, I crank up the power amp all the way. Third (and this is where it gets dicey), I fine tune the preamp sound so it works best at the volume I'm using. For this last step, if I'm using a tube preamp (or something that tends to distort at higher volumes), and if the preamp doesn't have a master volume control, I'll have no recourse but to use the power amp level control as the master volume. But for any other preamp (if the preamp either has a master volume control, or is "clean" in the sense that the sound doesn't change much as you rotate the volume control through its range), then I'll use the preamp volume control to set my level (and leave the power amp cranked all the way up). Examples: I used to have an SVP-Pro that I absolutely hated, 'cause it distorts "quickly" as I raise the preamp volume. Same for an Alembic F2B, which sounds great at low volumes but doesn't have a master volume control. On the other hand, I have a solid state pre that I've been using lately, that I adore 'cause the sound doesn't change when you go up or down in volume. On that one, I leave the power amp cranked and just dial in the level I need on the preamp.
     
  3. I get blasted often but here's what I do:

    I turn up the amplifier all the way and use the pre-amp for gain. I have the "master" (sometimes called "post") gain/volume of the pre-amp set so that it's "almost ready" to clip the input level of the amplifier (around 3 o'clock, 7-8, etc... really depends on the amp and pre). This is a personal preference but it ensures I will have the cleanest possible signal travelling from the pre-amp to the amplifier. I then use my "pre" (otherwise known as input volume or level) gain/volume to control how loud I want to get. I run a compressor in the effects loop set as a limiter.

    Many people might say I'm crazy but IME as long as I use restraint with my volume I'm in no danger (ie; I've never damaged anything). What a lot of people seem to not know is that a power amplifier can produce it's full rated power (or more) even if the gain controls are set to less then full. IOW, if you send a hot enough signal to an amplifier it can (and will) produce it's full rated power regardless of how far you have the knob turned up (unless of course it's in the "off" position;) ).

    The way I set things up on my rig makes it handle transients (spikes in the input signal) really well. I have never had my clip lights come on even when playing with a heavy hand in a loud band situation. The only time things distort is if I purposely make them (which I do with the tube sometimes). Once again, this is all just stuff I've found works for me, YMMV. I'm really happy running my rig this way because it lets "how I'm playing" dictate more of what sound comes through the rig vs. the rig "interfering" in that process.

    Sorry for the book guys! :bassist:
     
  4. Chasarms

    Chasarms Casual Observer

    May 24, 2001
    Bettendorf, IA USA
    You are correct in that you risk no damage as long as you properly control levels.

    You are also correct that a power amp can easily make max power without being cranked. But there is a flaw in your gain structure.

    Setting the post gain output on the preamp to its max, non-clipping level does not guarantee that the power amp is getting the cleanest possible signal. In fact, it ensures that any noise induced into the signal in the preamp stage is amplified to its maximum potential.

    In addition, the lower the preamp gain, the lower the s/n ratio. So it isn't helping you there much either.

    If your compressor/limiter is set up correctly, you won't clip anything anyway.

    These aren't insurmountable. If I had a really bad sounding bass plugged into a really nice preamp, I might experiment with a less radical version of what you have going.

    The biggest issue is the frustration in dialing in your tone. The preamp gain has much more of an impact on your tone than do the post output or the power amp output. As you are doing, any change in volume level means a change in tone.

    Or course, if you have a really hot onboard pre in the bass and run the rack pre pretty much flat, it won't matter much either way.

    To answer the original question, the most common way is to control overall output using the preamp master (post gain) if it has one.
     
  5. I do this with my Eden/PLX:

    Set gain for best signal/noise, set amp at a moderate level say around 10:00 and adjust volume with master on pre. If I need more volume then available (rarely) when the master reaches around 12:00, I'll add more volume at the amp.
     
  6. 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
    Most power amps have a lot more gain than most situations require. It's generally safer and quieter (less noise and hum) to reduce the power amp gain somewhat and hit it with a hotter signal from the preamp.

    If the preamp can't cleanly produce a hot enough signal, then turn up the amp gain to make up whatever you can.
     
  7. ivanthetrble

    ivanthetrble

    Sep 9, 2002
    Orygun
    Thanks Bob, I was hoping you would chime in on this thread. Since I have a tube pre-amp (Aguilar DB680) it makes sense to adjust the pre-amp gain until it starts to distort, then back it off a bit since I like a clean tone. I usually have the pre-amp gain about 12:00 and the master at about 12:00 and adjust the power amp to the sound level I want.
     
  8. Chasarms-

    To clarify; What I meant as to the "master" gain dail being "near clipping the amplifier in-put" is that I have it set to give the amplifier the hottest signal I can WITHOUT clipping the amplifier in-put (by setting the "pre" at 12 o'clock and distorting the out-put signal purposely I found the level it happened at and backed off a few "notches"). On my pre-amp this is around 3/4 travel of the "master" dial (3 o'clock, #7-#8 in relation to #10 being the loudest, etc...), not all the way up.

    As far as S/N ratios go I'll gladly put up with a little "hum" to have the dynamic control from my fingers instead of having to fiddle with knobs. FWIW I have EMG's in a Fender Jazz, run the pre almost dead flat (slight eqing for different rooms), and achieve dynamics through various playing techniques. I believe I clearly stated these settings as a personal preference. I suppose my main point was to offer some of my knowledge about "gain structure". I hope it helped!
     
  9. ivanthetrble

    ivanthetrble

    Sep 9, 2002
    Orygun
    Hey, another Portlander! Thanks guyplaysbass!
     
  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
    No problem with that, Guy. But optimizing your gain structure can usually help reduce the hum portion quite a bit, and you can still play with dynamics. In fact, IMHO, good gain structure is important for dynamics.

    Some bass/cable/rig combinations are inherently susceptible to hum, though, and there's only so much you can do to improve it.
     
  11. Mister Ex

    Mister Ex

    Feb 23, 2003
    I've used many different combinations of preamps and power amps over the years. Bar none, the absolute biggest problem I've had in optimizing my tone is gain staging. It's something I still struggle with to this day. I understand the concepts, but the practical application of gain staging in a gig environment can play hob with what I perceive as my 'best' sound. For example - my current rig is a Status Empathy 5 into an Alembic F-1X or an Aguilar DB659, then into a Stewart World 2.1, then onward to Bag End S-15D and S-15XD speakers. It's a nice rig, but it's taken a long time to find my sound, and sometimes I still struggle with it. First off, there's the Status bass. It has 18v electronics, and the HyperActive pickups put out a ton of signal. So - I should use the active input on the F-1X, right? Well, maybe. However, I've never liked the way that active inputs on any preamp change my tone...

    Well, I just realized that I could get really long-winded with this, so I'll cut to the chase.

    1) Gain-staging, if done correctly, can make your tone shine, make your bass easier to play (dynamics), and remove that ugly wart on your finger.
    2) Bold Statement Alert: I think that when many players criticize certain preamps about being too 'wooly', or 'brittle', or 'too whatever', they may in fact be suffering the effects of gain-stage-itis. I know that I sure have!

    Now, if I can only find where I left my talent...
     
  12. That's priceless!!!:D

    I think that's what was at the heart of the original question in this thread. So, what is it that makes "good" gain structure? I stated what works for me. Obviously (and I would be pretty ignorant to think differently) it's not the only way to do things. So, any other ideas/input???
     
  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
    Good gain structure involves making optimal (or close to optimal) use of the dynamic range of each stage of your system. This involves having a strong but unclipped signal all the way through.

    The reverse of good gain structure would be to have weak signals partially buried in the noise floor, and then a lot of gain in the last stage (the power amp) to bring the signal up to the level that drives the loudspeaker(s) to the SPL you desire; the problem is that the high gain at the end would boost the noise by an equal amount, too.

    The problem is that there is no universal way to set up your gain structure. Some players play harder than others; some instruments are hotter than others; some preamps can put out 14 volts RMS and others can't put out more than a volt. And different power amps will usually have different maximum gain.

    But thankfully, that's why we have knobs.

    The idea in optimizing gain structure is to make your signal strong early in the signal chain (even at the bass, if possible) so you don't require much gain in subsequent stages. But at the same time, you don't want the signal so strong that it clips. So there's some balancing required. It's worth experimenting with.
     
  14. ivanthetrble

    ivanthetrble

    Sep 9, 2002
    Orygun
    "The idea in optimizing gain structure is to make your signal strong early in the signal chain (even at the bass, if possible) so you don't require much gain in subsequent stages."

    Now that makes sense! It seems like a weak initial signal could carry noise along with it only to be further amplified later by cranking the power amp.
     
  15. 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
    Yup, you got it!