Pre-Amp / Power Amp Woes

Discussion in 'Amps and Cabs [BG]' started by m.pisa, Jan 9, 2015.

  1. m.pisa


    Jan 9, 2015
    Looking for some advice on properly tuning my new set up. Using a SVT-BSP tube pre-amp into a Crown XTi 2002 driving Ampeg an PNHLF 1x 15 and Ampeg PNHLF 2x10. That's 2000w driving a 4 ohm speaker set up with 575 & 550 watts power handling respectively. The problem I'm having is that my pre-amp gain & level are turned up to the point where my peak LED is on more than off. The output level on the channel out put is cranked. My power amp is not peaking and I'm driving my speakers with the power amp on 8 and I still feel like I need more volume in my tiny practice space. I feel like I'm missing some critical piece of the puzzle. Any ideas?
  2. Your amp requires a large amount of signal to drive it fully. Your pre may not have that much output on its own. As long as your speakers are not sounding like they are going to die, you are fine. I would recommend turning your amp up all the way and controlling the volume from the output on your pre.
    taylor16 and DWBass like this.
  3. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Syracuse NY
    Endorsing artist: Dingwall Guitars
    I'm guessing this is your problem:
    XTi 2002 Sensitivity (for full rated power at 4 ohms): 1.4 Vrms

    A quick look at the Ampeg manual makes it look like the omitted the output spec for the unit, but I'm guessing that you're probably getting about half that.
  4. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Whenever I see a post like yours I immediately suspect use of an improperly wired bridging cable. Are you 100% positive that you have a cable wired 1+ and 2+ on the amp end and 1+ and 1- on the cab side?
    agedhorse likes this.
  5. m.pisa


    Jan 9, 2015
    Thanks! Is there a way to boost the output from the pre?
  6. You would either need something like the ART Tube MP, or similar.
  7. m.pisa


    Jan 9, 2015

    Hmmm. I must plead ignorance. I have no idea what that means. Oh I got it. I was wondering why the speakons were labeled like that. I'll make sure I have the correct arrangement. Thanks!
  8. m.pisa


    Jan 9, 2015

    I did look for the output rating and was unable to find it as well. Thanks!
  9. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    You only need the specially wired cable if running in bridge mode -- I assumed you were doing so because you mentioned the expectation of 2000 watts.
  10. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Syracuse NY
    Endorsing artist: Dingwall Guitars
    Its hard to truly know that that's the problem without much in the way of specs, but if I were you I'd take that preamp and test it out with a power amp that has a .775 input sensitivity and see if that does it for you. I don't think the 'clipping' light on the Ampegs are designed as a 'warning light' and are intended to show that you're saturating, so getting it lit isn't a danger, but still.

    Take the preamp to a local store (or a GC, whatever) and 'shop' a couple power amps with .775 input sensitivity (Crest, Peavey, some QSC) and see if that fixes your problem. If it does, you know what to do.
  11. will33


    May 22, 2006
    One of those Ampeg pres didn't have a lot of juice at the output but I don't remember which model.

    Yes, an additional gain gadget like the TubeMP or other will fix it if that is indeed the problem.

    The amp may also be able to be modded to accept lower input voltage as well.
  12. DWBass

    DWBass The Funkfather

    Question? Why do you need to run 2000w?? I run my system in parallel mode (into 2 cabs) and I have plenty of wattage. If I need more volume, I'll add my 2x12 4ohm cab on one side and the 2 single 12's on the other pushing 500 watts to each side. I run the amp wide open and control volume from the preamp. Also see if that amp has an input sensitivity switch (some do).
  13. sonic 7

    sonic 7

    Aug 10, 2011
    Queens, N.Y.C.
    put an ART tube MP between the pre and power amp. This worked wonders for my rig, totally transformed it. AS was mentioned earlier , a lot of bass pre amps just don't have a strong enough signal to properly drive the power amp. The tube mp is less than $100.00 and worth every penny, it also has a high pass filter that may be useful. I don't really see it as adding any "tube" sound its really just about hitting your power amp with a good strong signal.
    wmhill and rigtec like this.
  14. Try the XLR balanced outs of the pre into the amp instead of the 'preamp out' jacks. You'll probably get a much hotter signal(and be able to adjust it) that way. Plus you can run the cabs stereo with one dirty and one clean.
  15. rigtec


    Jan 30, 2012
    Hi m.pisa,

    I did a search as well to see if your pre-amp is properly matched w/the "Crown xti 2002" and as 'CrazySkies' has stated, the amp's 'full output' requires a line input of 1.4V (+5dbu); and Ampeg does not give gain output specs on their "SVT-BSP" pre-amp. But I also noticed on the Crown spec sheet that this amp will only deliver it's full rated output at 2ohms stereo or @4ohms bridged. So at 8ohms/ch (stereo) you'll only see 475W/ch. You'll only see a full 2000W if you 'Bridge' the amp at 4ohms; not sure if you did that! Here's the spec sheet:

    Worst case scenario, get a "Line amplifier" (line out booster) $30 bucks from amazon!

    Best to you!
    Last edited: Jan 9, 2015
  16. m.pisa


    Jan 9, 2015
    Thanks for all the great info. This fancy new rack set up is a lot more involved than just plugging in. I am running in bridged mode so driving 4 ohms. I'm going to try the ART line amplifier and see how it goes. I also might try running stereo with a high pass to the 2 x10 and low pass to the 1 x 15. We'll see. Id imagine adding the line amp will improve the whole setup regardless of how I go out from the power amp.
  17. Not without a specially wired bridging cable, you're not, as Passinwind pointed out.
    agedhorse likes this.
  18. friendlybass


    Jul 19, 2012
    I'm no wizard at this kind of stuff, but maybe you pres output impedance and your power amps input impedance. Happens with pedals sometimes,a buffer is the solution!
  19. m.pisa


    Jan 9, 2015
    Where would that bridging cable go. I thought it happened internally by just selecting bridge mode in the menu. In between the cabinets?
  20. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    Okay, here's an easy way to do it:

    *You need a bridge speaker cable which can be accomplished via either banana plugs spanning the two center "red" terminal posts or Speakon plug wired +1, +2 inserted in output #1. As I understand it, both enclosures are 8 ohm and daisy-chained with a resulting ~4 ohm load.

    *When connecting the preamp, use 1/4" output marked "Clean/Mix" just to keep things simple. Avoid the DI / XLR outputs as these are more appropriate for sending signal to the mixer / FOH.

    *I hate this particular amp based solely on the input configuration: XLR only, no 1/4". Even my Crown XLS Drivecore 1500 / 2500's offer a 1/4" input. Since you're stuck, you'll need to snag a 1/4" T/R/S (balanced) >< 3 pin XLR cable to make the necessary connection. Here's a handful I found @ Sweetwater:

    A balanced cable should work even when the preamp's output is not.

    *Double check your Crown's set-up for bridged mono's easy to goof up if not careful. I don't know if the the XTi is anything like my Drivecore but my units require little if any attenuation for best performance / optimal gain structure.

    *There's a really good chance you will not need to add any peripherals (boosters, etc) if you follow the above first. You'll be out the cost of the TRS / XLR cable and you can return that if the results are less than satisfactory.

    BurningSkies likes this.