Using a crossover with a piezo tweeter...

[J.]

Can you use a normal crossover with a piezo tweeter? I know they are often used without one since they have an inverse impedance/frequency relationship and at lower frequencies their imp rating goes way up, but are crossovers sometimes used?

 

If so, how do you select component values? For instance, if you have a regular compression horn and want to wire up a simple 1st order crossover, the value of the capacitor would change depending on the driver's nominal impedance (4, 8, 16 ohms, etc). How do you choose for a piezo?

[agedhorse]

A high-pass filter is indeed used sometimes, but a known defined "ghost" load is often used across the piezo to help define and minimize the capacitive nature of the load.

 

If I am using a nominal 8 ohm crossover HPF, I tend to place around 15 ohms across the piezo to at least minimize the problem and slide the transition frequency up, which is what the HPF is generally being used for anyway.

[J.]

Thanks a bunch. I actually did find something on partsexpress.com about piezos and using conventional crossovers (I believe it was an article written by someone for promoting CTS piezos) and they recommend wiring an 8 ohm resistor across the piezo. Am I correct to assume that the ohm rating of the resister would determine the values used in the components of the high pass filter? Also, if you wired a resistor across a piezo would you still need to use a resistor inline with the piezo to eliminate chances of amplifier damage?

 

Another thing - I was browsing through a live sound forum on usenet and someone was saying that piezos can only produce about three tones simultaneously. Is that true?

[Steve_B]

J,

 

There is a bit of information about piezos on my web site. Here is a direct link to the page. If you want to view the rest of the site hit the home button at the bottom of the page. There is also a link to the CTS application notes with more useful information on piezos.

 

Steve.

[agedhorse]

8 ohm resistor will end up with a somewhat lower "nominal" impedance, though by the time you consider the phase shift between V & I, it's just a guessing game anyway. I prefer to err on the higher side, so something between 12 & 15 ohms gets my vote as getting a nominal 8 ohms, whatever that may be

[J.]

Thanks for the info guys! I was just bit by the curiosity bug recently and wanted to find out more about these things.