Wiki piezo XO article question

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Odd1

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Joined 2013
#1
I'm looking for just a little clarification on the piezo XO article in the Wiki.

I'm currently designing a speaker to be used in an upcoming project and I've decided to go with a piezo tweeter / horn combination. The article on piezo XOs was very informative and helped me a great deal. I do have a couple of questions though.

I'm not sure which driver I'll be using so I have ordered one each of these for testing :


GRS PZ1188 Piezo Horn Driver Similar to KSN1188A 292-450

GRS PZ1142 Piezo Bullet Horn Driver Similar to KSN1142A 292-448

Whichever I choose, it will be coupled to this horn:

Selenium HM17-25 1" Bi-Radial Horn 60x40 1-3/8"-18 TPI 264-308

Using the article as a guide, (coming from the amp) I should have a 5uf series cap, followed by a 22ohm parallel resistor, followed by another series cap for attenuation. Am I correct?

My question is regarding the 2nd series cap for attenuation. I will be using two 4ohm woofers in parallel and the sensitivity looks to be about 91db @1w. That means I only need to attenuate the tweeter by -3db at most. According to the article, a 0.15uf cap should attenuate -6db.

What value cap should be used to attenuate by -3db?

Also, the tweeter that I'm mostly interested in is a clone of the CTS KSN1188a (part# 292-450). It has the the built in protection circuit similar to the original. Does this change any of the components or values that I should use for the XO and attenuation?

There is a lot of conflicting information out there on using XOs and attenuating piezos.

Here's another thread that I'm trying to wrap my head around. It addresses CTS/Motorola drivers.

Using piezo tweeters wisely: a "how to" - AudioKarma.org Home Audio Stereo Discussion Forums

And thanks also for taking the time to help out a XO novice like myself.
 
#2
You have to have some way to measure the resulting response. Just substracting different manufactures data on sensitivity does not work. I use Omnimic but Holm impulse and PE USB measurement microphone is cheaper option.

Things like the baffle step change output up to 6 dB. So the dB differences gives you a ball park figure. If I match a 105 dB horn tweeter with a 90 dB driver, the tweeter will be padded down a lot but if it is 12 or 18 dB???
 
#3
Thanks. Measurement mic is in the near future. For now, I'm placing an order for caps and I'd like to know what value caps to get. I don't want to later realize that I didn't order the one I need. If 0.15uf will give me -6db, should I go up or down from there to get less attenuation? And by how much?

Thanks

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#4
If 0.15 uF does attenuate by 6 dB, then 0.36 uF gives an attenuation of 3 dB.

However, the attenuation depends on the capacitance of the load, which is unknown, and as DrBoar said, if baffle step has not been considered in the calculation, you actually may need much more attenuation than 3 dB.

BTW, two 4 ohm woofers wired in parallel result in a 2 ohm load for the amp. Did you mean wired in series or do you have an amp capable to drive that load?
 
#5
You could have your 5uF cap, but make the 22R resistor out of two series-connected resistors, with the ratio of the two set to provide the desired attenuation to the Piezo tweeter. Piezo tweeters, being a capacitive load do usually require attenuation using a cap, but if you are using a conventional first-order HPF as you are, then resistive attenuation at a low impedance works well.
You could use a 22R pot set as a conventional voltage divider feeding the piezo tweeter to see what attenuation is required.
 
#6
Dissi said:
If 0.15 uF does attenuate by 6 dB, then 0.36 uF gives an attenuation of 3 dB.

However, the attenuation depends on the capacitance of the load, which is unknown, and as DrBoar said, if baffle step has not been considered in the calculation, you actually may need much more attenuation than 3 dB.

BTW, two 4 ohm woofers wired in parallel result in a 2 ohm load for the amp. Did you mean wired in series or do you have an amp capable to drive that load?
Click to expand...

Thanks. That points me in the right direction. According to the specs, the ksn1188a has a capacitance of 0.8uf and the ksn1142a is 0.3uf. That being said, these are clones and I need to figure out how to measure the capacitance. Relying on the CTS specs for now, a capacitor of the same value of the tweeter gives a -6db attenuation. Half that gives -12db and double that gives -3db. Am I understanding correctly?

If so, I'll get enough caps to cover all scenarios depending on what I measure.

I did mean parallel. I want the cabinet to be 2ohm and I'll be wiring 2 or 3 cabinets together in series on each channel of the amp. Long story. Details in this thread:

http://www.diyaudio.com/forums/showthread.php?t=242692


Sent from my Nexus 7 using Tapatalk 4
 
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