Piezo Tweeters

[joel2762]

I am modifying speakers I built and I am going to use piezo tweeters. I'm not sure how to hook these up. does anyone know how?

[skeptic]

They don't need a crossover network. You can just hook them up in parallel to your main speakers. Or you can use a crossover if you want to cut them off on the low end at a higher frequency. They are not particularly recommended for high fidelity use. They sound harsh compared to dome and other tweeters used in high fidelity loudspeakers and they tend to beam their sound. They can however produce very high output without damage or severe distortion. That's why they usually find their way into disco and other sound systems which need to play very loud but not necessarily to the same degree of accuracy as high fidelity speakers.

[joel2762]

These are PA speakers actually, so parallel, how would I hook them up this way?

[Mash]

+ to + and - to -. The same as the input wire.

[joel2762]

so I just connect the tweeter directly to the back of the terminals. simple enough, thanks.

[WmAx]

They don't need a crossover network. You can just hook them up in parallel to your main speakers. Or you can use a crossover if you want to cut them off on the low end at a higher frequency. They are not particularly recommended for high fidelity use. They sound harsh compared to dome and other tweeters used in high fidelity loudspeakers and they tend to beam their sound. They can however produce very high output without damage or severe distortion. That's why they usually find their way into disco and other sound systems which need to play very loud but not necessarily to the same degree of accuracy as high fidelity speakers.
While this may be a fair summary of most piezo tweeters(since they are primarily intended for non hi-fidelity applications), this is not true of all. Two examples of extremely high quality piezoelectric tweeters are the gold-dome Audux and the Gallo CDT tweeter. A crossover may not be required for operation, as far in as durability, but for proper implementation into a loudspeaker it is crucial to use a crossover network just as with any other driver. I have not personally used a piezo electric driver, but I know they are a capacitive load, and must be treated differently then an inductive load(normal vc speaker). Here is some information provided by Jon Risch on basics of piezoelectric crossovers:



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?

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 crossover 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.

-Chris

[Mash]

It is always possible that the piezo is wired opposite polarity to the 'tweeters' in your PA speakers. This will manifest itself as a droupout or cancellation in the treble. Then simply reverse the piezo connections, + to - and - to +. You have a 50% chance of getting it right the first time- much better odds than the Lottery.

[joel2762]

So I should just put a 20 ohm resistor in line?....And what if I am using a crossover? I need a 22 ohm resistor in parallel. But i'm a little confused about the in line and parallel things. How should these resistors be placed exactly?. Sorry i can't figure this out! thanks for the help so far.

[Mash]

Simple circuit diagram

+O______/\/\/\/\/\______PT________O-

OR

+O______PT______/\/\/\/\/\________O-


O = Terminal

/\/\/\/\ = variable resistor ("pot" or potentiometer- start with Rat Shack and check electronics supply houses)

PT = Piezo Tweeter

The "pot" or potentiometer may be needed to balance the relative outpot levels
The X-over in you PA speakers may effectively reverse polarity to the PA speaker tweeter by accident or design intent. X-overs are a circuit of inductors & capacitors which will introduce phase shift, i.e. effectively 'shift speaker polarity' by shifting the signal timing.

[joel2762]

I looked at 22 ohm resistors on partsexpress but there was a variety with different wattages...what one would i need?