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Old 18th December 2008, 07:59 PM   #1
Ilianh is offline Ilianh  Canada
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Default Piezo crossovers

Hi there,

First of all I havn't posted on this forum since 5 years. Lot's of things have changed around here ( for the best).
I have more free time ( Even with newborn 2 kids ) and I dug all my vintage full rangers off the closet.

Now......

I mounted myself some bookshelf OB's with a nice full range.
Since the fullranger goes to 15khz I created a "PHY HP" tw-37 "clone" with a motorola piezo.

Now... the question part:

I dont seem to get any serious frequency response changes when using different crossovers.

First: Here's the frequency reponse of the driver with no crossover.

NO XO: http://tinyurl.com/3zhs9k

Then the conventional 20 ohm resistor across piezo terminals and a small cap (5.3uF) , to roll off at 15khz. That should be a first order 6 db per octave roll off.

Here's the Frequency response.
6DB XO: http://tinyurl.com/4kh8kq

Then, seeing it did not do the job (I want 15KHZ!!!!) I did a 12db/octave at 15khz.

It was done using a 8ohm resistor across the piezo terminals because I only had those coil valus off hand. caps is 6.6uF and coil is 169 uH

Here's the Frequency response.
12DB XO: http://tinyurl.com/5x9h6q

Now... does anyone see anything wrong?

I'm about to get angry and start creating gainclones and active crossovers.

Thanks for any input on this issue.
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Old 18th December 2008, 09:34 PM   #2
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A piezo-elecric driver's reactance is capacitive in nature, so it will not simply act as a uniform load to the 1st order filter - as might a moving coil do, more or less.

20 Ohm + 5.3 uF gives a -3dB point at 1500Hz. (ignoring the reactance of the piezo element at 1.5kHz whatever that might be, but its not going to be realy small)
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Old 18th December 2008, 09:53 PM   #3
Ilianh is offline Ilianh  Canada
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Thank you for the reply Steerpike,

I based my calculations on these piezo application notes:
http://www.pulsardevelopments.com/pr...l/piezoan.html

These are the same as provided by CTS I think.

Any references to calculation formulas taking the capacitive nature of the piezo in account?


[EDIT] I'm sorry on the original post I wrote 5.3 uF when I meant 0.53uF [/EDIT]
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Old 18th December 2008, 09:53 PM   #4
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http://www.zillaspeak.com/Crossover.asp

I think you need to be using smaller capacitor values. Hope the chart above helps.

Godzilla
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Old 18th December 2008, 09:57 PM   #5
Ilianh is offline Ilianh  Canada
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Once again. My error, I meant 0.53uF and not 5.3uF...
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Old 19th December 2008, 12:48 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

Your numbers seem to be out by a factor of ten.
e.g. 6.6uF should be 0.66uF and 170uH should be 0.17mH.

/sreten.
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