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Input impedance mod

Discussion in 'Amps and Cabs [BG]' started by geoffzilla, Sep 15, 2004.


  1. geoffzilla

    geoffzilla

    Oct 30, 2001
    Nashville, TN
    Endorsing Artist: Aguilar Amplification
    I'm pretty sure this is the wrong forum, but I can't think of where else to ask. Feel free to move.
    I'm interested in finding out how hard it is to alter an amp's input impedance. I've seen that some companies use 10m ohm inputs to accomodate piezo pickups. Is it reasonably easy to modify a standard input jack to do this? Could it be within reason for a local amp tech to swing this?
    Thanks.
     
  2. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    Not really, lowering the input impedance is easy, but (significantly) raising it, that usually requires a complete redesign of the input.
     
  3. Yes there's someplace better to ask this question - AMPS :rolleyes:
     
  4. notanaggie

    notanaggie Guest

    Sep 30, 2003
    A clever tech could do that, by adding an "FET follower" stage to an SS amp input...

    If it has a tube preamp, it could be very easy............. change the resistor.

    Clever techs sem to be in short supply. What kind of amp is it?
     
  5. geoffzilla

    geoffzilla

    Oct 30, 2001
    Nashville, TN
    Endorsing Artist: Aguilar Amplification
    Sorry Hambone,
    I wanted to find the advice of some of the builders. Thanks for moving.
    Tim,
    It's a Hartke 3500.
    I used to do a more like 90/10 split of electric vs. upright gigs, but am now more like 50/50. I'd like to be able to use the upright pickup straight into the amp and get a decent sound.
    I realize that I could just buy a new amp, but feel odd about it, since there's really nothing awefull about my current rig.
     
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If you're having trouble with using a piezo pickup into your amp just add a pre-amp, like a Fishman, that's designed to work with a piezo pickup.
     
  7. geoffzilla

    geoffzilla

    Oct 30, 2001
    Nashville, TN
    Endorsing Artist: Aguilar Amplification
    billfitzmaurice,
    I know you're right, I was just interested in the "all in one" approach. After looking at Bob G's website though, I realized how small and inexpensive some of these are. It looks like it may be more expensive to try to mod the amp, than to just put a preamp in my gig bag.
    I guess that's why more people haven't mentioned doing this kind of mod.
    Thanks,
    Geoff S
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Modding just about any modern amp is not easy what with everything on printed circuit boards. If it was me and I needed a pre for the piezos I'd put it in the ax and not have anything extra to plug in or carry. BTW, chances are the input impedance on the Hartke is not the problem but the low output voltage of the piezo is, so modding the Hartke would involve more than just impedance and could end up being a large can of worms that you don't want to open.
     
  9. geoffzilla

    geoffzilla

    Oct 30, 2001
    Nashville, TN
    Endorsing Artist: Aguilar Amplification
    This isn't really an option with a double bass! :p
    I think you're right though, about opening a can of worms that would best be left alone.
    It was an idea I thought I'd ask about. Turns out maybe it's a bad one.
    Thanks for the input.