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Using 2 pre-amps with one amp

Discussion in 'Amps and Cabs [BG]' started by JOME77, Apr 3, 2003.


  1. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Georgia
    I have been using a Demeter HBP-1 and Alembic F1-X with a Stewart 1.6 for a few months now. I've been wiring the Demeter output into one side of the Stewart and the Alembic output into the other side of the Stewart. For speakers I've been using two Goliath Jr III's (one connected to the Stewart Left side and one to the right side). I plug my bass into an A/B switch and choose either or both preamps. Many times I'd like to just use one of the Jr's or just my Bergantino. Would there be problem a using a Y-cable from both preamp outputs into one side of the Stewart or do these signals need to be isolated from each other? I considered another A/B box to switch the preamp outputs but I don't believe that you're suppose to make/break the input signal to a power amp (when it's powered up). Any suggestions?
     
  2. You DON'T want to be doing that (using a Y-cable)! My knowledge is rudimentary and I defer to the experts here, but you don't want to be driving the output of one preamp with the output of another.

    The only thing that I can think of that makes sense is for you to find a line-level mixer with inputs that can handle the voltages your preamps put out.
     
  3. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    You can use a Y-cable, but you may loose some signal level and may experience impedance problems.

    Any decent A/B box will be able to switch without any noises.
     
  4. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Georgia
    JMX,
    So switching the input signal to the amp via an A/B box will not harm the amp? In some cases I might want to go from the Demeter to the Alembic (or just the opposite) in the middle of a song. This would change the input to the amp while the amp is under a load. I guess that this is my major concern. I didn't see anything in the amp manual addressing this subject.
    Thanks for any help you can provide!:)
     
  5. Not to question JMX's knowledge on this subject, but this seems just plain wrong. Of course I don't have an engineering degree (in electronics anyway), so perhaps you should hang off until you get a few more opinions on this. Good luck.
     
  6. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    You don't "drive" one output with the other.
     
  7. I would think that an a/b before the pre's and a mixer of some sort (if the pre's use 1/4" outs) like this:
    [​IMG]

    if its XLR:
    [​IMG]
     
  8. JMX, If you are using a Y cable, the outputs of your preamps are connected together. Whichever preamp you're using via A/B is running into (eg. driving - may have been poor word choice) the output of the other which is not a good thing. Anyhow, not78pbass has the correct solution shown.
     
  9. 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
    Don't use a Y-cable, because you'll end up with the preamp outputs trying to drive into each other's low impedance.

    Use a mixer or an A/B switch.
     
  10. Bob - the voice of reason! Thank you.
     
  11. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Georgia
    Bob,
    Would switching preamps (via an A/B sw) in the middle of a song cause any potential damage to the power amp?
     
  12. 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.
     
  13. Steven Green

    Steven Green

    Jul 25, 2001
    Pacific NW
    Oh man, I shouldn't have read this thread...I've got so many potential rig combinations running through my head right now! GasX!:eek:
     
  14. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Yup, adding a mixer can create all sorts of possibilities...

    I originally added the mixer so that I could play guitar through my bass rig (with the Vamp 2) and use the drum machine for practice. Then it dawned on me that I could use all the preamp sources at the same time by plugging my bass into one channel and sending the effects send and aux outs to various preamps (takes a 4 bus mixer).

    Preamps that can be used simultaneously (although I haven't done it yet...)

    Pearce BC-1
    AMP SL-1
    Behringer Vamp2 (great distortion for bass):bassist:

    Takes a lot of cables though...and scores high for BLF (blinking light factor) :D
     
  15. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Georgia
    Thanks for all of the quality input guys! I guess now I have to decide if I'd rather have 2 A/B boxes or go with a mixer and 1 A/B box. I guess that I'm leaning toward 2 A/B boxes so I could change preamps on the fly while playing.
     
  16. Tecx

    Tecx Radio Rock Leads To Sterility

    Jun 9, 2002
    Halifax, NS, Canada
    but won't you have to switch both A/B's at the exact same time so you don;t inbterupt the signal.... You know what I mean?..

    Alex

     
  17. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Georgia
    Tecx,
    You are correct if I had the A/B switch for my bass on A or B. If I have my A/B switch for my bass on A & B (both) I could switch the other A/B switch (preamp out signals going to the amp) to A or B (Alembic or Demeter). I think that would work.:meh:
     
  18. Tecx

    Tecx Radio Rock Leads To Sterility

    Jun 9, 2002
    Halifax, NS, Canada
    or you could build a A/B box yourself that would do both at the same time.... or build a A/B - C/D box with both switches right next to one another so one foot could switch both... I could draw a plan up foor you...

    Alex

     
  19. ttyR2

    ttyR2

    Feb 23, 2003
    Athena, OR
    When you say A/B switch, are you talking a simple mechanical switch, or an electronic switching device? If just mechanical, seems like there would be a huge opportunity for loud pops and whatnot. Just wanted to make sure I understand the situation.
     
  20. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Georgia
    Alex,
    Sketch it up and send it to me. I'm open for the best solution. Thanks for your help!:)