Two Channel Power Amps and Multiple PreAmps

Discussion in 'Amps and Cabs [BG]' started by Big String, Nov 7, 2002.

  1. Question: I own a QSC PLX 2402 and a Demeter Pre amp. Can I add another pre amp, say an Aguialar 659 or Alembic FX1 or Kern to my rack and have both my Demeter and one of the others hooked up to my QSC in different channels with the amp set in the Stereo mode?

    If I can, I would be able to switch to a different pre by just plugging into that pre or even maybe a A/B box.

    Am I nuts or what? :eek:
  2. Thanks San
    That gave me some ideas, but I don't think I read anything specifically to what I was asking although very close. Most of the discussion was on looping through another pre. I'm thinking one amp one cab and two pre amps. Did I miss something?
  3. you can definitely do it. An A/B box will work fine. Or you can just plug into the different pre's.

    the only other consideration is what speakers will be use. If you run the poweramp in stereo mode, each output goes to a seperate cab - so you'd need matching cabs, unless you want a different sound from each channel.

    Or I guess you could use a "Y" cable into one cab.
  4. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    Sure, you could run stereo using 2 preamps, one preamp per channel. One common usage is the John Entwistle method of running a clean signal (for clear bass) on one side and an effected signal (for distorted top end) on another. Or you could put delay/chorus/flange/etc on one channel, leaving the other clean.

    Keep in mind that if you kept the amp in stereo mode, one preamp pre channel, you'd have to use two separate speaker cabs. If you wanted to A/B between two different preamps, that would mean using just one speaker cab at a time (unless you had a switch that did A/B/A+B). If you wanted to A/B between 2 preamps and use all your speakers, you'd have to recombine the signals and effectively run your amp in mono. Hope that makes sense, and if I've forgotten something I'm sure others will help out.
  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
    No, you'd better not do that. It won't work, and you could end up with a hefty repair bill.
  6. thank god there are experts around here.

  7. Fuzz, I didn't think of the fact I would have to use two speakers, and if I went mono/parallel I would not be able two have both preamps on at the same time. Drat! Maybe the looping is the way to go. The Demeter has blending on the effects/loop so I could bring it in. I was trying to keep the preamps seperate.

    Quoted from the mentioned older string:
    This way I could plug into the A/B, run one side to the Kern (powered by one side of the 2.1) and run the other side to the SWR IOD (powered by the other side of the 2.1). You could then add speaker cabs to taste, and simply switch back and forth between solid state clean, tube distortion, or a combo of both. This is the rig I would have right now if it weren't so expensive! Hope that responds to your original question and that it give you ideas of what you might be able to do.

    :rolleyes: :rolleyes:
  8. 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
    You would have to switch both the input and the output of the preamps. You could switch the inputs just by plugging into one or the other, and use the A/B switch to select the pre that feeds the power amp.
  9. Hi Bob,
    So. If I'm understanding correctly I would have to hang an A/B box somewhere near the back I guess and plug both preamp outputs to the inputs on the a/b box and the one output of the a/b box to the amp. .
    Now if I would like to switch preamps without unplugging and replugging could I put another a/b from the bass? I know that was not my original desire, but everybody got me thinking, can you smell the smoke?
  10. jerry

    jerry Doesn't know BDO Gold Supporting Member

    Dec 13, 1999
    If the amp is run in the stereo mode, it would be like having two different poweramps.......i.e. you could have a Sansamp for distortion running from one side to a.......say a cabinet without a tweeter,
    and the other side with your Demeter into another cabinet like your Bergie 2/12 for a clean sound. You would just need a A/B box between you bass and the preamps. Sounds like your worst than me Big String when it comes to messing with my stuff:D
  11. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    Let's see if I have this right: you'd like to be able to switch back and forth between two preamps or use them both at once or whatever. I think you could do that using just one A/B box, but yeah, you'd have to split the signal coming from your bass and then recombine it before running into your power amp. I think this would work:

    Run the cord from your bass into an A/B switch

    Run Channel A out into Preamp A

    Run Channel B out into Preamp B

    Combine the outputs of the preamps (run them into a simple "Y" connector)

    Run the single output from the "Y" connector into your amp... that way you could run it mono or stereo or whatever.

    You would simply use the A/B switch to go back and forth... and I do believe that Whirlwind (and other companies?) make an A/B/AB switch where you can run channel A, channel B, or *both*.

    I think that's do-able; the only weirdness might be balancing signal level when both preamps are on versus using either one or the other.
  12. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    P.S. you're not crazy for wanting to try stuff like this... there's no significant danger if you experiment *before* the power amp (as Bob said, don't go playing with Y connectors AFTER the power amp!!) That's how you learn about gear. I did all kinds of wacky stuff back in the day; I once blended a dirty guitar Rockman with a clean bass preamp (sounded pretty cool), and I've messed around with various stereo and biamp configs. Lately I've been using KISS methodology because I'm lazy. :p
  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
    C'mon. Don't do that, either. Y connectors are not mixers, either at speaker or line level.

    Doing that to preamps might not burn anything up, but they won't be happy, either.
  14. coyoteboy

    coyoteboy easy there, Ned Supporting Member

    Mar 29, 2000
    Sactomato, CA
    If one channel of the power amp has parallel inputs, then just run one output from each pre- into each parallel input at the poweramp. No Y cords there. I'm starting to play around with multiple preamp routings myself.
  15. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    Thanks for the correction, Bob. Can you elaborate on "not happy"? Did I endanger my gear back when I played around with Y connectors or was it simply a "sounds like crap" issue?
  16. I can't stop laughing at Ad Naueseam's Avatar...

    Bob, Jerry, Fuzz, Geshel, Coyote, Sanctum:

    Thanks to all of you for the "input" (heehaw). I at least hope we got some more creative juices flowing. I do have an headache from trying to piece it all together. I must confess that I complicate the heck out of something, but yet I want to keep it simple :D But that's just me. I'm think Bob's suggestion about one a/b and me just switching preamps is probably the easiest although I couldn't go back and fourth say in the middle of a tune. This is o.k. cause I don't now if I'd use it much anyway. My main line of thinking was just a different sound completely for a different tune, it's not like I'm playin' geetar again. I thought maybe the "Ampeg" sound to the ultra clean sound the Dem makes. The little mixer that Gesh linked looks pretty cool too, now tell me how to hook it up.

    Jerry, I'm probably the worst gear nut on the whole blue planet....does OCD come to mind? I have to expel my energy some how.
  17. I dont see how you can do this. On my QSC, if its run in parallel mode, only the input into channel one is used. So you cant connect two pre's into this input, without a "y" cable (already labled a bad idea) or another kind of a/b box. The parallel mode combines one input into both poweramps for output ...... it doesnt mix two signals for output.
  18. 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
    Paralleling the inputs is the same as using a Y cable. It is for feeding two or more inputs from one source, not combining sources. Don't do it that way.