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Old 1st July 2001, 03:02 AM   #1
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I need to know how to figure piezo drivers into the impedance of the entire speaker system.
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Old 1st July 2001, 05:28 AM   #2
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Rory,
Are you intending this to be a hi-fi speaker? Piezo's are pretty harsh for home use.

Grey
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Old 1st July 2001, 02:27 PM   #3
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They're for P.A. speakers
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Old 1st July 2001, 08:47 PM   #4
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Rory,
Okay.
Are you using a crossover, or letting them roll off by themselves?
Piezos have a pretty high impedance (used to, anyway--I'm assuming that nothing's changed there). Unless you're doing something odd with the crossover, it's likely that any other drivers you've got in the cabinet are going to determine the load of the speaker as a whole. Do your piezos give an impedance?

Grey
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Old 2nd July 2001, 02:09 AM   #5
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No, they don't. Does that mean that if I have a piezo horn tweeter, an 8-ohm cone midrange, and an 8-ohm cone woofer, then I wind up with 4 ohms per channel, right?

What happens if I just let them roll off naturally? Will they get to "honking" on me? Will they be damaged? I'm probably going to use a crossover. These PA speakers are for a trumpet, two trombones (I'm one of them), an alto saxophone, and a tenor saxophone, as well as a keyboard and two vocalists. These will be running out of a Peavey 400W power amplifier. I volunteered to come up with a plan for adding speakers and mics to the instruments, because I built my own subwoofer, but I realize I don't know everything (which is why I come to you).

Also, secondary question:
On the back of my Sherwood reciever, it says "Rear Speakers (8 ohms)". Does that mean that each satellite must be 8 ohms? Or should the impedance of the satellites add up to 8 ohms (4 + 4)? How does the crossover affect driver impedance? If I have two 8-ohm drivers (tweeter & woofer) crossed over, does the amplifier see 8 ohms?
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Old 2nd July 2001, 03:01 AM   #6
dorkus is offline dorkus  United States
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rory,

if you cross over your drivers properly, than the impedance will be roughly what the drivers are individuallly at the frequencies they operate, at least in nominal value. so if you have an 8 ohm woofer and a 4 ohm midrange crossed over at 500Hz, the impedance will be 8 ohms up to around 500Hz, then 4 ohms above 500Hz, or at least roughly so... right around crossover the impedance will vary. in your case, if you have an 8 ohm woofer and 8 ohm midrange, you will end up with an 8 ohm speaker overall if you do the crossover right.

as for the "8 ohm" label on a lot of receivers, this is just to make sure that you don't hook up too low an impedance to the outputs because the receiver has limited current capability and might overheat or blow a fuse if you use too low an impedance. you may actually get away with using a 4 ohm speaker with an "8 ohm" terminal, but you will not be able to play it very loudly before the amp starts to run out of juice. if you are talking about a tube amplifier, that is a completely different story - the different terminals are different taps on the output transformer - but in this case you are talking about a transistor receiver.

with your rear speakers, as long as the satellites are about 8 ohms each you are fine. even if they are 4 ohms you might get away with it, since the rear channels don't play as loudly as the main channels... but remember also that the rear amp is probably much weaker and thus may overload more easily.
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Old 2nd July 2001, 01:04 PM   #7
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Piezo's have a fairly high impedance and some of that is capacitive.

If you want to cross it over rather than use it full range, simply put an 8 ohm resistor across it and treat it as an 8 ohm driver.
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Old 30th October 2001, 06:21 PM   #8
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Smile Using Piezos

Going back a ways, I couldn't help but comment on the subject of whether or not to use these drivers. What's funny is that if you mention certain things, they elicit tremendous response. Piezo drivers are one of these. So let me first of all state that I think that the Motorola Piezo is perhaps the most underated driver in the market. Granted it is cheap cost wise, the response curve is not constant and if not tamed, it will demand the majority of the listening stage. But with the proper crossover, they are easy to civilize. In fact, the Motorola ll88A is a wonderful horn that is second to none in the market. Here is what I do. I use a 8 ohm, the higher wattage the better, resister that is placed in parallel with the driver. Also a series capacitor, usually 4.7uH will work, but it can be changed like any other tweeter. Last, I use an 8 ohm Pot which allows me to tone town the SPL. Keep in mind that with the 8 ohm resister placed in parallel with the tweeter, it can be treated as any 8 ohm driver. If you can find the article in speaker Builder, One:95', by Mike Klasco, you will see what I mean. Thanks, JohnL
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Old 30th October 2001, 11:32 PM   #9
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I am about to build speakres using piezos. My plan was to put a 45 ohm resistor and a 10uF cap in series with it, creating a countour network that will attenuate freqs 10khz+ 3db, and a hipass 1987hz 6db/octave. tell me if i'm wrong, i took info from several different sources to arrive at this idea. One question, do the motorola piezos sound any btter than the knock-offs that sell for 10% of the price?


P.S. will piezos treat me good, or will i end up spending big bucks on a titanium compression driver after i see how much they suck?
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Old 31st October 2001, 07:08 AM   #10
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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 the cap 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.

Most of the above information is credited to Jon Risch.
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