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31st January 2011, 10:21 AM  #1 
diyAudio Member
Join Date: Jul 2008

Super tweeter addition
I will add a super tweeter with my fullrange and wanted to know how to calculate the capacitor value.
I dont know wich impedance value i have to use for the calculation of the capacitor. Do i take the impedance of the super tweeter wich is 8 ohms or i have to use the combination of both drivers, both are 8 ohms wired in parallel wich give 4 ohms? Thanks 
31st January 2011, 08:34 PM  #2 
diyAudio Member
Join Date: Nov 2010
Location: Glasgow, UK

You would calculate it based on only the impedance of the tweeter  after all the capacitor would only be in series with the tweeter.
Keep in mind that 8 ohms is most likely only the nominal impedance of the tweeter in the 24Khz region. By calling it a super tweeter I'll leap to the asumption that you're planning to cross it over fairly high  at least 8Khz. Most tweeters with the exception of some ribbons have impedance curves that rise fairly high towards the top of the treble range, so at your planned crossover frequency the impedance could be 1520 ohms or more. You have two choices at that point  (a) you could add a zobel network to the tweeter (something not normally done when using much lower crossover frequencies of ~3Khz) and then calculate your capacitor based on the new relatively flat impedance curve  this will give you a predictable 6dB/octave roll off, which might or might not match the natural high frequency roll off of the full range driver. (I presume you'd be running the full range driver "open") (b) you could allow for the rising impedance curve and make the capacitor value smaller than what you would calculate for 8 ohms  you will also end up with a first octave or two rolloff which is steeper than 6dB/octave but less than 12dB/octave, due to the lossy equivalent inductance and series resistance of the voice coil. This might or might not be a more suitable match for the full range driver. Are the driver sensitivities fairly well matched or do you plan to pad the tweeter as well ? 
31st January 2011, 09:37 PM  #3 
Speakerholic
diyAudio Moderator

Mr DBM has given it to you in detail and he is correct but what I might do in your case is simplify it and:
Buy 6 caps, two of each: 2.0, 1.5 and 1.0 mfd. Try them each to see which suits you the most. Buy a few 2 and 4 ohm resistors too. then you can add series resistance or Lpad if you find it a little hot. 
1st February 2011, 05:07 AM  #4  
diyAudio Member
Join Date: Jul 2008

Quote:
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Thanks 

1st February 2011, 05:08 AM  #5 
diyAudio Member
Join Date: Jul 2008


1st February 2011, 05:19 AM  #6 
diyAudio Member
Join Date: Sep 2006
Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.

What he means by the supertweeter being hot is the higher sensitivity ..which is the case at 108 DB.....as for padding, this refers to the "LPad" a variable "Volume control" for your supertweeter.
You will need to "turn down" the power going to this tweeter! _________________________________________________R ick......... 
1st February 2011, 05:35 AM  #7  
diyAudio Member
Join Date: Jul 2008

Quote:
Thanks 

1st February 2011, 05:53 AM  #8  
Account disabled at member's request

Crossover calculator.
Quote:
Passive Crossovers, Capacitor and Coil Calculator With super tweeters the crossover frequency can be 8KHz, 10KHz or even 12KHz. It's subjective. 4 and 8 ohms are only ballpark numbers, the actual impedance is likely lower. The only way to know for sure is to measure, run an impedance curve. If you were to use an ohmmeter/multimeter and measure the DC resistance of the tweeter's voice coil you'd have somewhat of an idea of the tweeters impedance. I kind of, sort of guesstimate a tweeters actual impedance to be about ten percent more than it's DC resistance. If you add two tweeters together in series, then of course the impedance doubles. Last edited by AEIOU; 1st February 2011 at 05:56 AM. 

23rd February 2011, 01:49 AM  #9 
diyAudio Member
Join Date: Jul 2008

After several hours of listening and tryouts I finally got the real thing! Huge improvement due to LPAD addition.
For now 2.2uf for 9k Hz cut and Rs 5.6ohms + Rp 3.4ohms for 10.5 dB, all this for Coral Beta8A matched with Coral H105 super tweeter. I also have a 470uf cap to cut low frequencies to the Beta8A, this clean and open the midrange (im using an high efficiency subwoofer). Thanks a lot without the LPAD i would not had the wonderfull result i have now. 
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