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Piezo Cap Installation

Discussion in 'Amps and Cabs [BG]' started by Growly Lytes, Jul 23, 2012.


  1. Growly Lytes

    Growly Lytes

    Dec 4, 2009
    Downunder Oz
    Bass player
    Hi guys,
    Last week i added a tweeter (Motorola Piezo with stamp '502' on the back of it & i added a 8.2ohm 5watt resistor to protect the poweramp from the ultra highs, cab is 8ohm) to 1 cab.
    Now we can hear a buzz when playing.Maybe the Piezo isnt strong enough or its damged but when tested the day before via a hi-fi connection it was perfect.Now it buzzes installed on low notes mostly, even when i bypass the Sanamps going direct to the cabs we can hear the buzz & is disturbing.
    How can we remedy this ? I can add more Piezo`s but then il get more buzz.Its not the Piezo level , sensitivity or sound its not bad at all only the buzzing & what is causing it ?
    The cab is 8ohms & has the resistor connected to the + terminal & - on -.The local tech said to try adding a cap to block out the lows but first disconnect the Piezo & check the sound without it so thats first but what would you guys do in this case to block the lows. How do i go about hooking up a Piezo,cap & resistor all together ?In what order does it go & what terminal.Im sure its not the same way i have it wired now ?
    I read all over the net that Piezo tweeters are fine without caps & resistors but i dont think so now !

    G.L
     
  2. will33

    will33

    May 22, 2006
    austin,tx
    Peizo doesn't have a voice coil, so nothing for the cap to "see". You put a resistor across the + and - terminals, and then put a cap inline with the + wire to block lows.

    When Bill and them do it, they use a 30ohm resistor, something about insertion loss I don't understand. I guess you could put an 8ohm resistor across the terminals, then a 4uf cap inline with the + wire to make a filter. If there is insertion loss, I guess the peizo would just play a little quieter.

    I think you're supposed to still keep a resistor inline with the + wire too for amp protection.


    So I guess a resistor, then a capacitor inline with the + wire, then a resistor across the + and - terminals?
     
  3. lelandcrooks

    lelandcrooks Commercial User

    Jan 6, 2011
    Kansas
    Owner SpeakerHardware.com
    The resistor should be in the + line for a couple of reasons. The main one is to protect your amplifier from hf oscillation.

    Generally a cap is not necessary, as piezos are basically capacitors in the circuit and naturally roll off about 2.5k. Bill's are used for smoothing circuits at the crossover point. Test the piezo by itself with a different amplifier. If it's fine, it's your signal chain. If it's not, open up the back and inspect the element and internals, see if something is loose. If it's a genuine Motorola it's pretty old. If all that fails try a cap, 5 or 6 uf, poly.
     
  4. Growly Lytes

    Growly Lytes

    Dec 4, 2009
    Downunder Oz
    Bass player
    Im pretty amazed now.Last nite this was eating me because i knew the Piezo was fine before putting it in the cab,I never over powered it just tested it to hear some sound & all was fine.
    Now the "Signal Chain" is where i think i may have found the buzz.I disconnected it all & put it back together but i used a " 1/4' speaker cable " to interconnect 2 preamps.This is where i think the problem lay.I was using an " instrument cable " to interconnect 2 preamps(braided shield type) where the braid is the drain or negative.I changed the cable to a speaker cable & you can bearly hear any buzz now.You could even hear it before i changed the cable when on low volume levels & bypassing the Sansamps so it couldnt be what i first thought.
    Ive never seen any info on what kind of cable people should use to interconnect 2 Preamps, no matter what brand they are but these are both Sansamp.Sansamp doesnt say anything about this in thier owners manaul just that you can connect them.Also Sansamp doesnt have xlr inputs if i wanted to go that path only outputs.
    Il test it again tonite & make sure this info is correct.:)
    G.L
     
  5. seamonkey

    seamonkey

    Aug 6, 2004
    Should be a higher value resistor
    http://www.partsexpress.com/pdf/piezo.pdf

    Interconnects at line levels should be shielded. It could be you had a bad cable originally. Maybe it had a bad ground, or you had a ground loop.
     
  6. 1958Bassman

    1958Bassman

    Oct 20, 2007
    One resistor in series and another across the load is called an "L-pad". It's fixed, not variable, but it works the same way- lets the amp see a load it likes and reduces the level to the driver.

    BTW- piezoelectric transducers naturally pass highs, not lows. Putting the cap in series keeps this kind of tweeter from being as harsh as they usually are.
     
  7. Growly Lytes

    Growly Lytes

    Dec 4, 2009
    Downunder Oz
    Bass player

    I think your right there ! Im pretty sure the cable i was using (the interconnect) i or somebody in the band had soldered quickly at jam one day,short & perfect for patching in 2 preamps.I had tried it once or twice at band practice & it was o.k on a cab with Eminence BP102 speakers which sorta covered up this problem,they are so low-end directed its unbelievable.Then addding a tweeter & another cab with Eminence Alpha speakers it came to surface.It may not be a problem with the tweeter as it sounded much better when cable was changed.
    Ive got my fingers crossed that im on the right path here.
    G.L
     

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