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Pot for passive piezo volume?

Discussion in 'Pickups & Electronics [BG]' started by Eilif, Oct 18, 2005.


  1. Eilif

    Eilif Supporting Member

    Oct 1, 2001
    Chicago
    What kind of pot should I get for a passive volume control of a piezo pickup?

    The situation is that I have two piezo pickups that I want to be able to adjust the volumes of separately before I go into my A/B box and then into a buffering preamp.

    Please respond quickly, as I am running to radio shack this evening, and I need to know if I have to pick up a special pot. Thanks.
     
  2. fookgub

    fookgub

    Jun 5, 2005
    Houston, TX
    It needs to be high impedance. I'd say 5 Megs, 3 at the minimum. IMO, this is not a great idea. You should buffer the piezo before the volume pot, then use a lower value pot (especially if you're running to an outboard AB box). The passive setup will be prone to noise and high frequency rolloff, and if the impedance seen by the piezo is too low, you'll lose bass response, too.
     
  3. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    Corrupticut
    +1

    Don't use a passive pot on a piezo pickup. Use a buffer amp with variable gain. Or use a buffer amp, and then a passive pot if you must (as suggested above). It should have a very high input impedance and usually a low output impedance. You could also use a Radial Bassbone if it needed to be a stomp device.

    As to why: Piezos have extremely high impedance, and they produce very little current. Any kind of resistive losses or stray capacitance is only going to degrade the signal. That is why the ideal setup puts a buffer amp/preamp close to the pickup. The lo-Z output is great at driving the signal over cable runs and the gain keeps the losses small compared to the overall signal level.
     
  4. Eilif

    Eilif Supporting Member

    Oct 1, 2001
    Chicago
    So there are no pots that cut just a tiny bit? Would it be better to buy a cheapo minimixer and mix the signals with that before I go to the preamp?
     
  5. fookgub

    fookgub

    Jun 5, 2005
    Houston, TX
    That's a great idea. I used to play with a guitarist that did this with his clean/dirty signal. It gave him a lot of flexibility (at the expense of convenience). There's only one problem... the mixer's input impedance won't be high enough for the piezos, so you still need the buffers.
     
  6. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    Corrupticut
    A few devices are designed specifically for mixing piezo sources. The Raven MDB was one but it sout of production. D-TAR makes a mixer with A/B capability and is currently in production.

    You could get something like the K&K beltpack preamp, or any of the piezo buffer/preamp devices. I use a K&K pre for a single piezo in my ABG. I've also used the older Furman beltpack, and a few onboard and stomp-type solutions.

    You say that you want to adjust volumes prior to an A/B switch, so you might be better off with two separate buffer amps, send those to the A.B switch. I'm assuming that you are using a passive A/B stomp setup.

    I hope this doesn't sound like nit-picking. Piezo's really do sound horrible when they see an impedance mismatch. A passive volume control is a voltage divider that sends signal to ground instead of the intended source. That is about the worst thing you can have as the first thing a piezo sees in the signal chain.
     
  7. nsagan01

    nsagan01

    Nov 10, 2007
    mpls
    what do you guys think:
    piezo film under baseplate of bridge---> kemo m040 preamp/battery ---> dtar volume and tone module ---> output jack.
    is this a combination that makes sense? i'm a novice at electronics and any help or suggestions would be great.
    thanks
     
  8. arai

    arai

    Jul 16, 2007
    bump
     
  9. nsagan01

    nsagan01

    Nov 10, 2007
    mpls
    bump?