(cut and paste, most of the below information credited to JON RISCH)

Q. The Parts Express catalog suggests putting a 20-Ohm resister inline with any Piezo tweeter to make it a more stable load for an amp.Will this not also attenuate the tweeter? If so, and if I need further attenuation, can I simply add more resistance? Is there a rule of thumb for how much attenuation I will get with further resistance, or a way to compute this number?

A. The recommended resistor is to help protect the amplifier from oscillating due to the raw capacitance that is a piezo driver. Adding resistance in series with a piezo will actually roll off the highs a bit, adding more will roll off the highs noticably. To attenuate a piezo, add a series cap, which creates a voltage divider with the capacitance that is the piezo drive element. Most piezo elements run in the 0.1 to 0.26 uF range, so a cap of the same value as the piezo element will attenuate it 6 dB.

Piezo's can be crossed over, and to great advantage. I have often thought that some of the bad rap piezo drivers have is due to the "you can use them without a crossover" fallacy. Yes, you _can_ use them without a crossover, but just because you can get away with it, does not mean it is optimal.

Since most piezo's are used in inexpensive systems, the cost of adding in "unecessary" components is often never even considered.

How to cross over a piezo:

Add a resistor in parallel, and the driver can be made to look like a current driven device to any outside components, such as a crossover cap. However, to keep costs and power dissipation down, 8 ohms is way too small of a value. The impedance of most piezo's is still quite high at 20KHz, so use a 22 ohm resistor, this makes any series crossover cap smaller and less expensive, and the resistor dissipates less energy. Use of an 8 ohm parallel resistor will also tend to lose you a little bit of output level.

For most piezos, use of a 22 ohm resistor, and a 4-4.7 uF cap will allow the response to be identical to what it was in stock form, but rolls off the lows at 6 dB/oct below 1 kHz or so. This actually increases the power handling of the piezo, as it is voltage limited. Exceed the voltage used to pole (polarize the piezo element during manufacture) the unit, and it will loose sensitivity, and eventually burn out. Most pro grade piezos will handle 35 volt transients, and 28 volts continuous, which are 150 watts and 100 watts into 8 ohms respectively.

Add in a capacitor and 22 ohm resistor, and the power handling could effectively be quadrupled, as the LF voltages are not imposed upon the unit, just the HF voltages.

Piezo's crossed over in this manner don't sound as harsh and spity, and tend to be quite a bit more reliable. Many of the piezo units have a mild peak just before they roll off in the LF, so making the series cap a little smaller can actualy flatten response, and provide even more protection and smoother sound. For the smaller piezo units that cut off at 4-5 kHz, a series cap of 1.5 uF will do the trick, larger units that go down to 3 kHz can use a 2.2 uF, and the large compression driver units meant to be mounted on a horn need about 5 uF, as they do not peak, and any higher would lose the sloping output even more.

Attenuation, HF roll-off AND the crossing over can all be done at the same time. To attenuate, place a cap in between the piezo and the 22 ohm resistor that is shunting across the unit, then if HF roll-off is desired, use a series resistor in this location too. Then the series crossover cap should be in front of the 22 ohm shunt.

Looking from the amp, first the series crossover cap, say 4 uF, then the 22 ohm shunt from hot to ground, then a series cap of about 0.15 uF for 6 dB attenuation, and then a series resistor of about 30-50 ohms to tame the very top end, then the piezo itself.

Thanks for any replies, and best of the Season to all, especially Wayne, who sent me a reply on Christmas morning!