PIEZOS |
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love2all
Registered User Joined: 30 October 2015 Location: london Status: Offline Points: 131 |
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Posted: 22 August 2018 at 11:49am |
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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
Registered User Joined: 15 September 2017 Location: Finland Status: Offline Points: 425 |
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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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DMorison
Old Croc Joined: 14 March 2007 Location: Aberdeen Status: Offline Points: 1647 |
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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
Registered User Joined: 15 September 2017 Location: Finland Status: Offline Points: 425 |
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Yes, exactly. Only AC will pass, so piezo is more like capacitive load. |
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Elliot Thompson
Old Croc Joined: 02 April 2004 Location: United States Status: Offline Points: 5172 |
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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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Elliot Thompson
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Conanski
Old Croc Joined: 26 January 2006 Location: Ottawa, Canada Status: Offline Points: 2515 |
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T-Bone
Registered User Joined: 27 December 2005 Location: Tampa, FL USA Status: Offline Points: 292 |
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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.
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BASS, how low can you go!
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Elliot Thompson
Old Croc Joined: 02 April 2004 Location: United States Status: Offline Points: 5172 |
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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, |
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Elliot Thompson
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studio45
Old Croc Joined: 16 October 2007 Location: United Kingdom Status: Offline Points: 3863 |
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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.
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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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Earplug
Old Croc Joined: 03 January 2012 Location: Europe Status: Offline Points: 7199 |
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Here is one I have on file: Similar to the recommendations above. |
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Earplugs Are For Wimps!
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Chris Grimshaw
Registered User Joined: 10 September 2018 Location: Sheffield Status: Offline Points: 281 |
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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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