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love2all View Drop Down
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    Posted: 22 August 2018 at 11:49am
Greets SP crew,

Quick question - i have a double 15inch enclosure with a fane 15xb and fane 15300tc installled. Prior to upgrading the drivers the box had 9 blown piezos wired into a blown passive xo. XO has since been binned and i dont want to run the top section passively. How does one go about wiring and powering 9 piezos safely?? What ohm load will read on the amp etc?? The box already has cut outs for the piezos but i could swap them out for bullets but inclined to see what i can do as they so cheap. Any components out there the same size as piezos i could use as a better alternative?

Thoughts welcome
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Tonskulus View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2018 at 12:30pm
Piezo has no dc resistance at all, so amp sees it like "infinite" load.  However, at high frequency AC there will be some current passed.
Connect all piezos in parallel with one 22-33ohm/10W resistor.  Connect one 1...4.7µF/100V film capacitor in series with that paralleled piezo+resistor array.
Even more protection can be made by series connecting smallish 12V/15W or so light bulb.

Piezo is voltage limited speaker, if you exceed the rated voltage which is something like 30Volts.. it will burn out.  Putting small capacitor in series, prevents low frequency/high voltage peaks reaching piezo element thus making it more reliable. Sometimes there might be high freq peaks too, which need to be attenuated, so parallel connected resistor or series light bulb will help a lot there. 






Edited by Tonskulus - 22 August 2018 at 12:35pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2018 at 1:04pm
Originally posted by Tonskulus Tonskulus wrote:

Piezo has no dc resistance at all, so amp sees it like "infinite" load.  However, at high frequency AC there will be some current passed.
Connect all piezos in parallel with one 22-33ohm/10W resistor.  Connect one 1...4.7µF/100V film capacitor in series with that paralleled piezo+resistor array.
Even more protection can be made by series connecting smallish 12V/15W or so light bulb.

Piezo is voltage limited speaker, if you exceed the rated voltage which is something like 30Volts.. it will burn out.  Putting small capacitor in series, prevents low frequency/high voltage peaks reaching piezo element thus making it more reliable. Sometimes there might be high freq peaks too, which need to be attenuated, so parallel connected resistor or series light bulb will help a lot there. 





Maybe just a translation thing (and, like most non-UK based posters, your English is really good!), but technically piezo's have infinite resistance at DC, so present no load - ie no current flows.
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Tonskulus View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2018 at 1:12pm
Originally posted by DMorison DMorison wrote:

Originally posted by Tonskulus Tonskulus wrote:

Piezo has no dc resistance at all, so amp sees it like "infinite" load.  However, at high frequency AC there will be some current passed.
Connect all piezos in parallel with one 22-33ohm/10W resistor.  Connect one 1...4.7µF/100V film capacitor in series with that paralleled piezo+resistor array.
Even more protection can be made by series connecting smallish 12V/15W or so light bulb.

Piezo is voltage limited speaker, if you exceed the rated voltage which is something like 30Volts.. it will burn out.  Putting small capacitor in series, prevents low frequency/high voltage peaks reaching piezo element thus making it more reliable. Sometimes there might be high freq peaks too, which need to be attenuated, so parallel connected resistor or series light bulb will help a lot there. 





Maybe just a translation thing (and, like most non-UK based posters, your English is really good!), but technically piezo's have infinite resistance at DC, so present no load - ie no current flows.

Yes, exactly.  Only AC will pass, so piezo is more like capacitive load. 


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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2018 at 3:42pm

Piezo devices offer a very high impedance (over 1000 ohms) which is why, you can parallel multiple devices on an amplifier without overloading the amplifier during the process. 

Bear in mind Piezos are not efficient compared to what is offered today. The price of Bullet Tweeters are so inexpensive today, there is really no reason to focus on Piezo Tweeters. Out of the two, the Bullets offer a more smoother response whereas, a Piezo's response is more jagged. 

The average 1-watt/1 metre dB response for a Piezo is around 94 dB whereas Bullets are 101 - 102 dB

Best Regards, 




Edited by Elliot Thompson - 22 August 2018 at 4:14pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Conanski Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2018 at 11:39pm
Originally posted by love2all love2all wrote:

i have a double 15inch enclosure with a fane 15xb and fane 15300tc installled.
Why? Is that all you got or are you trying to something unconventional? These two drivers need to be in separate sub enclosures if the drivers are wired in parallel and will see any bass.

Originally posted by love2all love2all wrote:

Prior to upgrading the drivers the box had 9 blown piezos wired into a blown passive xo. XO has since been binned and i dont want to run the top section passively. How does one go about wiring and powering 9 piezos safely??
Piezos and bullets are super tweeters and therefore should be crossed at no less than 8-10khz, having blown my share of these back in the '80s you really need to heed this advice, nothing makes piezos sound worse and blow up easily than using them with no crossover. I bet you are planning to use that second Fane as the mid driver, there are good reasons why that isn't a great idea but if it's what you got then it can be made to work. It a waste of an amplifier to power the piezos alone so I suggest you put a hefty passive x-over between the 15300tc and the piezos/bullets at 8-10khz and biamp the box.. lows to the 15xb, mix/highs to the 15300/piezo combo. The piezos will also need a 50ohm power resistor wired in series with the group, you don't need individual resistors as shown in the link unless you only have low power value resistors.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote T-Bone Quote  Post ReplyReply Direct Link To This Post Posted: 23 August 2018 at 3:09am
I built a pair of BFM SLA Pro as per plans these are loaded 6 square piezos and mounted top to bottom on the side of the cab parallel wired but has a passive crossover at 3.5K Hz. They sounded bright without EQ but with EQ they sounded wonderful. Hope I'm not losing my hearing but they seem smoother without EQ they are harsh. Vocals sounds nice but I used Faital Pro 6" instead of Eminence. I would rate these BFM DIY a 6 from 10 being top of the line PA speakers for mobile powered DJ speakers (EV, RCF, Yamaha). I use a QSC CX 502 with a 3 way Ashly analog crossover for the 4 x 6" mid woofer and crossed at not recommended 150Hz.
BASS, how low can you go!
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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 23 August 2018 at 12:51pm

Originally posted by T-Bone T-Bone wrote:

I built a pair of BFM SLA Pro as per plans these are loaded 6 square piezos and mounted top to bottom on the side of the cab parallel wired but has a passive crossover at 3.5K Hz. They sounded bright without EQ but with EQ they sounded wonderful.


The reason many cross Tweeters 8 kHz – 10 kHz is to eliminate sibilance. Crossing Tweeters that high leads to having the tweeters sound more like an exciter (Effect Processor). This can be a good or bad thing pending the type of music you are reinforcing.    

 

When I used to use Piezos in the 1990’s, I would have them crossed at 3.15 kHz (Ashly Crossover). I prefer not having the consonants rolled off.  For the type of music I was reinforcing, the bulk of the Treble lies at 10 kHz and, required 5 kHz in order to sound correct.   

 

Best Regards,

Elliot Thompson
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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 23 August 2018 at 3:02pm
I concur with what everyone else has said, personally I would use them so: 

1) Parallel all piezo drivers, (I'd probably go up to 16 units)
2) Connect a 16 ohm, 50 watt resistor in parallel with them 
3) Connect a 4 ohm 50 watt resistor or a 12v, 20 watt lightbulb in the + line, in series with all the above.
4) Drive from an actively filtered source with HPF at 7 kHz or higher. If this is not possible, a passive filter designed to work into 20 ohm impedance. (Not difficult with assistance from online calculators and +100 DIY points).

This achieves: 
1) Simple 2-terminal network 
2) Reduction of HF resonances by electrical damping (otherwise they are unnecessarily distorted in 10-12kHz region) 
3) Amplifier and driver protection, the series resistance of the resistor or lightbulb ensures that the impedance of the system never goes to zero at VHF - and the lightbulb (if used) would act as a limiter/fuse, and blow before the piezos do. 
4) Piezos don't really respond to anything below 5kHz anyway, but they do not sound good until at least 7k.
Studio45 - Box Builder Commotion Soundsystem -Mobile PA
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Earplug View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 23 August 2018 at 4:07pm
Here is one I have on file:




Similar to the recommendations above.

Earplugs Are For Wimps!
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Chris Grimshaw View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Chris Grimshaw Quote  Post ReplyReply Direct Link To This Post Posted: 10 September 2018 at 1:25pm
Originally posted by Earplug Earplug wrote:

Here is one I have on file:




Similar to the recommendations above.



Yeah, the series resistor(s) is the important bit.

Piezos will show open circuit at DC, but they head towards a short circuit high up (well past 20kHz), which can make some amps unhappy - VHF oscillation from trying to drive a capacitive load can occur.
If you add resistors in series you make sure the impedance doesn't go to zero high up, which will keep your amp much happier.
The rest of the components are response shaping (likely for a particular piezo), and the bulb will provide a bit of thermal protection if the correct one is chosen.

Given the cost of getting piezos up and running, plus their limited power handling and then the comb filtering, I'd just take a compression driver and do it properly.

Chris
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