Piezo Tweeter enhancement? - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 11 Old 05-04-07, 08:46 AM Thread Starter
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Piezo Tweeter enhancement?

The New Large Advent has a reported frequency response of 33Hz to 17,000Hz +/- 3dB. I thought I might try to extend the high frequency of the Large Advent by adding a Radio Shack Piezo Tweeter #40-1218 http://support.radioshack.com/suppor...oc63/63021.pdf . Although no crossover is needed and this driver supposedly does not significantly affect the impedence of the speaker it is wired to, I found the following ‘crossover’ circuit and modifications to the driver that is said to eliminate its "spitty and lispy" fatiguing harshness:
1. 8 ohm resistor across the piezo terminals (Radio Shack part # 271-120, $1.49 each) What affect does this resistor have? Does it make the piezo an 8 ohm driver?
2. 0.47 uf capacitor wired in series (Hovland, $1.50 each from Madisound) I assume this only allows frequencies above 15,000 Hz (based on 12dB filter parameters) go to the piezo tweeter?
3. Application of damar varnish on the piezo’s cone element is said to add refinement to the sound and almost completely reduce the fatiging sibilance:
a. Remove the three screws from the back of the piezo
b. Coat the paper cone with one coat of damar
c. Remove ‘pooling’ of damar with a tissue
d. Let dry overnight
e. Reattach horn
4. An application of rope caulk to the back and sides of the horn helps to deaden any plastic sound.



I would either put the driver and ‘crossover’ circuit in a small box to be placed on top of the Large Advent cabinet and wire in parallel to the Large Advent terminal posts, or I could put the driver and ‘crossover’ circuit in the cabinet and perhaps wire it to the Advent tweeter, but I wouldn't know the best placement for it on the baffle.

Anyone who understands piezo tweeters and crossover circuits like to comment on how to best do this? Is there a capacitor that would provide a crossover at 17,000 Hz (12dB and 8 ohms)? Would a 6dB filter be better?

Thanks for any input.
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post #2 of 11 Old 05-11-07, 09:58 AM Thread Starter
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Re: Piezo Tweeter enhancement?

Well, after some research I am ready to try my experiment. I purchased two CTS KSN-1005A piezo tweeters, two 20 ohm 20 watt resistors, pairs of a variety of caps; 1.0uF, 0.68uF, 0.47uF, 0.33uF, 0.22uF, and 0.10uF, 4 ounces of Dammar varnish, and some ACE Hardware rope caulk. I have a bunch of 3/4" MDF left over from making my FleXy Table.

I will varnish the piezo's cone element and apply rope caulk to the sides and back of the horn. The resistor will be soldered across the piezo's terminals. I have made a pair of small boxes out of 3/4" MDF just big enough to put the piezos in, the back remains open so I can get at the capacitors. I will then experiment with various attenuations provided by the capacitors wired to the positive terminal of the piezo. I bought an extra pair of 0.33uF to go with the 1.0uF for 1.33uF. I think 1.5uF will let too much of the piezo through. I can try some other combos to get intermediate values. The supertweeter system is then just wired to the speaker terminals of the Large Advent. I will also try the piezo facing forward and backwards on top of Large Advent cabinet. There is some suggestion that facing backwards might improve the sound.

We will see if this will add some pizzazz to the New Large Advent without harming the overall sound quality and report back with my findings. Total cost is under $50.
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post #3 of 11 Old 05-11-07, 06:52 PM
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Re: Piezo Tweeter enhancement?

I'll be curious to hear your results.. I haven't looked, but what's the FR of the tweeter?

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post #4 of 11 Old 05-12-07, 11:19 PM Thread Starter
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Re: Piezo Tweeter enhancement?

The frequency response of the KSN-1005A is 4 kHz - 27 kHz.
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post #5 of 11 Old 05-24-07, 02:56 PM
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Re: Piezo Tweeter enhancement?

any new updates.... very interested in idea
post #6 of 11 Old 05-26-07, 11:22 PM
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Re: Piezo Tweeter enhancement?

One of teasons piezo tweeters got a bad rep is that they were most often used without a crossover. A piezo has a capacitive impedance. This is what "crosss" it over by itself, but 1/ this crossover is too low, and 2/ it drives some amplifiers crazy.

The shunt resistor provides a load that amplifiers are happy with and provides a load for the crossover. You can then use a text-book XO to good effect. The use of a large R (20-22 ohms) allows the use of smaller, cheaper, better sounding caps.

The full text of Jon Risch's article on the subject is on my planet10-hifi site (I can't post a link yet -- haven't made enuff posts)

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post #7 of 11 Old 05-27-07, 11:24 PM Thread Starter
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Re: Piezo Tweeter enhancement?

Quote:
bigataxie wrote: View Post
any new updates.... very interested in idea
I have settled on the Motorola/CTS KSN-1005A piezo tweeter, modified with dammar varnish on the cone element and rope caulk on the sides of the horn and back of the driver. I used a 20 ohm, 20 watt resistor across the terminals and a .68 uF capacitor for a calculated first-order "crossover" at 11,700 Hz. I stuffed some fiberglass behind the driver and closed the back of the box with the MDF and screws. This sounded OK, except I thought I would try it with a 50 watt, 8 ohm L-pad which allows attenuating the tweeter. So the back of the box has the L-pad and speaker wire terminals. The exercise has added some interesting dynamics to the upper end of the Large Advents, but I found so far I like it best with the tweeter facing backwards.

I must stress that one should get the Motorola or CTS piezos. The $2 imitations are likely not to please. I found a guy on ebay who is selling new original Motorolas for less than $8 a piece delivered. Martin Sound has the CTS version for about $12.

My next project is a pair of towers based on the Quasi.Mo.D.O. with the Pioneer B20FU20-51FW ($25 each) and the KSN-1005, except with a 110.05 liter box (about 50% larger/deeper than the Quasi.Mo.D.O.) and a 4" port tuned to 33.3 Hz (F3= 28.85 Hz -3dB).

Last edited by jackfish; 05-27-07 at 11:56 PM.
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post #8 of 11 Old 05-27-07, 11:57 PM
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Re: Piezo Tweeter enhancement?

The B20 is a well respected budget driver ... did you see this thread on the FR Forum?

http://fullrangedriver.com/forum/viewtopic.php?id=1643

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post #9 of 11 Old 05-28-07, 12:35 AM
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Re: Piezo Tweeter enhancement?

jackfish, thanks for the info
post #10 of 11 Old 05-28-07, 02:34 AM
 
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Re: Piezo Tweeter enhancement?

My experiance with Piezo's in the past is that they are somewhat "shrill" sounding, they definately will cut through anything though.
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