Output impedence req'd to drive power amp?

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

  1. twangchief

    twangchief Supporting Member

    Oct 30, 2003
    Altoona, PA
    I'm searching for a backup preamp. I currently use a Yamaha PB-1 and a QSC. I don't have the manual for the Yamaha so I don't know it's output impedence. Would the small foot pedal type preamps (Sansamp, MXR, Sadowsky...) drive a power amp? I've been told "No" by a saleman at a GC. He didn't offer to hook one up though. No surprise!!

  2. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    It's not so much of an impedance matching problem as making sure that the preamping device has enough output voltage to drive the power amp to full power.

    I don't believe the Bass Driver has enough output to do the job by itself. In another thread, I just recommended an Ebtech Line Level Shifter...a passive device that converts a -10 dBV (instrument level) signal to a +4 dBU line level signal that should be sufficient to drive your poweramp to full output.

    They're about $70 at Zzounds.com

  3. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Power amps tend to need anywhere from 1 v to several to drive them to full power. Those figures are with the level control at max.

    A small battery-type pedal with 1 9 volt battery just about can't do it, not enough voltage capability. Its iffy, depends on the pedal and amp in question.

    A passive booster may do the job, if the pedal has enough drive, meaning a low wnough impedance, which many do.
  4. i connect my sansamp bddi to the effects return of my 100w combo and can get the limiter to flash showing the amp output factory setting has been reached..
    the sansamp vol is on 3/4 and so is the combo amp vol...i also find if i boost bass, treble or drive i can clip the amp output limiter as well....
    i feel the sansamp is on its limit for this type of hook up...its better to connect to the combo preamp stage via the input socket....but my preamp sounds too bright..so i use the effects return...
    for over 100w i feel the sansamp may not be the way to go...use an intermediate CLEAN gain box..
    ps i am useing the sansamp on mains adaptor even though it is only 9v can rely on constant output..
  5. Phat Ham

    Phat Ham

    Feb 13, 2000
    I believe there is a modification that can be done on a SABDDI to boost its output level sufficiently to drive a power amp. It involves resoldering a jumper or something to that effect.

    Mr. Crow,

    The effects return on your combo is probably not a line level input, so it requires less voltage swing to clip the amp.
  6. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Yup. You simply unsolder and remove one resistor, the one with the < > markings around it on the circuitboard. Then it will be perfect for driving a power amp direct. No longer good inline before a preamp, though . . .