From: gt3281c@prism.gatech.EDU (TABLER,JOHN ALEXANDER)
Newsgroups: rec.audio.pro
Subject: Re: PA Tweeter Advice?
Date: 17 Dec 92 19:21:24 GMT
Organization: Georgia Institute of Technology

Here is an old post of mine which should provide you with a cost effective
solution.    I wanted to e-mail this to you directly, but had no luck.



 I would recommend getting 4 , 6, 8 or 10 motorola 10005 pizeos
and building some small arrays to take your H.F. on up to 18kHz or so.  The
10005's (I may have this number wrong, but they are small, 3.5" square and
black) are reliable, inexpensive, sound good, and are easy to use.  (MCM
electronics has them MCM: 1-800-543-4330).





Here is the information on the proper use of the (Motorola) pizeo HF drivers.

First, a few points on the use of pizeos.\

  1) A pizeo must always have a crossover, regardless of what the
     manufactures may claim.
  2) For the crossover to work properly, you must provide a 'ballast' resistor,
     which simulates a voice coil resistance.  A pizeo looks like an open
     circuit at LF, and without a crossover, the LF energy will saturate the
     pizeo element, ruining the HF sound.
  3) The capacitance of the pizeo MUST be isolated from the amplifier output.
     This pizeo capacitance can be as large as 0.2uf, and if not isolated from
     the power amp output will cause the amp to heat up, usually due to HF
     oscillation.  This is done by having a resistor is series with the pizeos,
     either as part of a pad, elsewhere.

   4) HF drivers MUST be placed on an arc when arraying, if not, the polar
      response in terribaly rough.  I have used these on a 16" arc with great
      results.

 There are two good crossover topologies.  The first uses a pad, for when the
pizeos must be attenuated to match the efficiencies of a LF or MF driver.  The
second topology has no pad, but still has Rvc and Rser, which simulate the
voice coil resistance, and isolate the pizeo capacitance, respectivly.

Crossover 1,  With a pad

                        4uF
          O-------------||-------------------.
              |                 |            |
              $                 |            R            pizoes in
              $ 0.35mH          $            R  3ohm      parallel
              $                 $            R
              $                 $ 0.35mH     |____________|<
              |                 $            |              |
              |_______LF        |            R              |
              |       driver    |            R 3ohm         |
              = 3uF             |            R              |
              |                 |            |              V
              |                 |            |
              V                 V            V


Crossover 2, with no pad

                      4uF
          O-------------||-------------------.
              |                 |            |
              $                 |            |____RRRR_____ |<   pizoes in
              $ 0.35mH          $            |     3 ohm    |    parallel
              $                 $            |              |
              $                 $ 0.35mH     |              |
              |                 $            |              |
              |_______LF        |            R              |
              |       driver    |            R 7.5 ohm      |
              = 3uF             |            R              |
              |                 |            |              V
              |                 |            |
              V                 V            V


Various LF/MF drivers will work.  A small 5" has worked well for us.  We use
these as top boxes and cross them over at around 1kHz, with a seprate 15" EV
pro line from 1K down.


  Sorry this took so long to return your mail.  If there are any omissions or
questions, e-mail me back.

Bye

John T.