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Old 30th March 2014, 09:45 PM   #11
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Quote:
Originally Posted by krafty View Post
I wonder why the necessity of R2 in the end of this story...
I must confess I am almost throwing my towel: building and testing this it's very time consuming.
Without a resistor in series, the amp sees the piezo as a capacitor, it naturally crosses over around 2000 Hz or so.
With a capacitor in series with a resistor and the piezo, the resistor allows the piezo to be crossed over at a higher frequency where they don't sound as bad, and also are not as sensitive, the reason you don't need anywhere near as much padding as you thought.

Getting passive crossovers to sound good even with measurement is difficult, and piezos are among the hardest drivers to design crossovers for, they are nothing like a resistive load.
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Old 31st March 2014, 02:25 AM   #12
krafty is offline krafty  Brazil
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Without a resistor in series, the amp sees the piezo as a capacitor, it naturally crosses over around 2000 Hz or so.
Ok, so what do you suggest... I can't place that 39 Ohm in there, it kills the treble completely, which value should I use. I think the treble attenuation is properly done with R1?
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Old 31st March 2014, 03:35 PM   #13
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Originally Posted by krafty View Post
Ok, so what do you suggest... I can't place that 39 Ohm in there, it kills the treble completely, which value should I use. I think the treble attenuation is properly done with R1?
You have not mentioned what type of piezo tweeters you are using, there have been a vast amount of different types produced over the many decades they have been available, all with differing parameters, each would require a different filter to achieve similar response.

Assuming an original CTS/Motorola "bullet" type, one of the types of crossover below may work OK for what you are trying to accomplish, minimal reduction of HF with more reduction of <3500 Hz. The capacitor value could be reduced for a higher crossover point, and a coil could be added in parallel with the resistor for a steeper, higher crossover.

Can't help you with any more specifics, I never liked the sound of any piezos I have heard (regardless of crossover used), so never have not done any experimentation with them. They vary so widely from unit to unit that any recommendation would be only ballpark anyway.
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Old 31st March 2014, 04:07 PM   #14
krafty is offline krafty  Brazil
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The tweeter is a LeSon TLC-1 200W. It is cheapo tweeter. I would be interested in knowing of replacements I can buy for it (real driver), the same size. Please something on budget.

I made modifications to my circuit... what do you think? Can it be adjusted better?

Click the image to open in full size.
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Old 31st March 2014, 08:41 PM   #15
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Originally Posted by krafty View Post
I made modifications to my circuit... what do you think? Can it be adjusted better?
Until tested, there is no way to tell if it can be "adjusted better". And with unit to unit frequency response variation of as much as 10 dB, what works for one might not be close for another.
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Old 31st March 2014, 09:01 PM   #16
krafty is offline krafty  Brazil
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I ask opinion if this is theoretically "OK", if there is no stupid or dumb modifications from the first circuit. Notice that I removed R2 (39 Ohms) entirely.


As for experimentation to see if it's good, like I said before, I shortwired and it sound pretty decent for bare input. In the computer I could equalize a bit the output. I have no further motivation to overtest this anymore, I realize this is a hobby for the passionate. I reached a level of frustration that left with no strengths. The more I mess with it, the more I realize I have to start all over again with different drivers and total new circuit - or get myself proper JBL studio monitors.

To me, the piezo is looking very much like the frugal article, which could or is considered alright to be crossed over this way. But since you are telling you can really never know, well, not in most cases, then what is the point. The specs you are asking are similar to the graph you posted? The manufacturer is a joke: Site under maintenance. I am going to call them right now to see if they have something.

Last edited by krafty; 31st March 2014 at 09:19 PM.
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Old 31st March 2014, 10:09 PM   #17
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I ask opinion if this is theoretically "OK", if there is no stupid or dumb modifications from the first circuit. The specs you are asking are similar to the graph you posted?
A piezo is not going to be anything like a four ohm load unless it has a built in resistor, so your circuit has at least one flaw. Theoretically, drivers are supposed to be similar, and your crossover might work OK, but look at the response of some real (cheap) piezos:

Graph of ten Goldwood 1005 piezos

They vary as much as 20 dB from each other in the 5 kHz region. 10 dB sounds twice or half as loud, 20 dB is a difference of four times. Without measuring your drivers there is no way to know how far they are off from each other, or how similar to any graph you have seen, or whether a circuit may be even remotely appropriate for them.
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Old 31st March 2014, 10:13 PM   #18
krafty is offline krafty  Brazil
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unless it has a built in resistor, so your circuit has at least one flaw.
Which flaw is that? A -series- resistor missing?
I thought I was following the frugal recomendation....
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Old 31st March 2014, 10:17 PM   #19
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Originally Posted by krafty View Post
Which flaw is that? A -series- resistor missing?
I thought I was following the frugal recomendation....
Piezos are not four ohms.
Follow whatever your ears tell you.
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Old 31st March 2014, 10:32 PM   #20
krafty is offline krafty  Brazil
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Ok, I just had luck the manufacturer replied right away.
Here is the graph:

Click the image to open in full size.

Last edited by krafty; 31st March 2014 at 10:35 PM. Reason: information coming right away after I saved the post
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