Piezo tweeters?

Not much more sensitive. I made a crazy thing last year to take to festivals; I wanted to have the lightest weight and highest output mid-top possible using what was "to hand" in the workshop. This ended up being based around an old Toa factory announcement re-entrant horn. I used Motorola piezos for tweeter duties as I had a bunch of them on the shelf recently removed from a client's old boxes. I started off with two. This was not enough so I added two more. Half an hour later I had ten of the buggers wired up. The system was still not balanced so I then started power shaping the horn (put a midrange cut filter and l-pad on it). Eventually I achieved a good balance and with some use of bitumen damping sheet managed to make the horn sound not like a tannoy horn. But yeah, it took ten Motorolas to keep up, so plan on using at least 4 per 8" unless you want to pad down the 8 and waste some power.

The impedance of a capacitor decreases with increasing frequency. The larger the value, the faster the impedance falls. Piezos are small value capacitors. At some point up in the near-radio frequencies (way above 20kHz) the impedance will have fallen to a very small value, near zero ohms or an AC short circuit. This can cause some amps to go nuts, oscillate and blow themselves up, and some amps to produce a harsh sound because of instability caused by the impedance dip affecting reproduction in the audio band. 

Edited by studio45 - 04 May 2013 at 2:40pm

studio45 wrote: The impedance of a capacitor decreases with increasing frequency. The larger the value, the faster the impedance falls. Piezos are small value capacitors. At some point up in the near-radio frequencies (way above 20kHz) the impedance will have fallen to a very small value, near zero ohms or an AC short circuit. This can cause some amps to go nuts, oscillate and blow themselves up, and some amps to produce a harsh sound because of instability caused by the impedance dip affecting reproduction in the audio band.  However, the impedance can be thought of as a relatively constant high value - 1000's of ohms - in the audio band. Thus paralleling up many piezos, still doesn't bring the impedance down to normal coil speaker values.  The output of the piezo is proportional to the voltage across it and they pass only a very small amount of current. So if you put an 8 ohm power resistor across (in parallel with) your piezo bank, the whole bank will then effectively be an 8 ohm resistive load. The resistor will be dissipating most of the power, but the piezos will respond to the voltage across it. Depending on how many piezos are being used, it might need to be quite a big resistor, like 50 watts. Finally, to guard against possible 100kHz+ oscillation because of the impedance dip, put a 20 ohm power resistor in series with the piezos after the 8 ohm parallel resistor. This resistor will not dissipate much power, so can be a bit smaller, like 10 watts. (in+)----------(20R)------(piezo bank +)     |           (8R)    | (in-)-----------------------(piezo bank -) You can then feed this from a standard 5kHz 8 ohm crossover board, and the piezos will only see 5kHz+. If you just hook them up without the resistors, the crossover filters will not be terminated properly and will let lower frequencies through.

How would you get piezos to play above a 2 inch comp horn? For instance the horn will play up to 18 k, and the piezos 18k +?

thanks..

Using this horn- http://www.parts-express.com/pe/showdetl.cfm?partnumber=294-808
loaded with- http://www.parts-express.com/pe/showdetl.cfm?partnumber=294-615
piezos- http://www.parts-express.com/pe/showdetl.cfm?partnumber=292-442

Hmmm.........already have piezos,.......and holes already cut unfortunately. Must go ahead with this, just need to find a way to get 4 piezos to only play high freq's. Any help much appreciated.

Well, you could use the circuit I posted above, with the 8 ohm and 20 ohm resistors, but use a 10KHz highpass filter instead of a 5k.