what type of capacitor to protect tweeter in active filter setup - diyAudio
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Old 9th December 2004, 07:47 PM   #1
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Default what type of capacitor to protect tweeter in active filter setup

I've been researching what type of capacitor to use to protect my tweeter when converting to an active filter. The only place I found online that made specific recommendations said polystyrene was most preferable and that electrolytic caps can be used as long as they’re bypassed properly.

Polystyrene caps seem to be being phased out and I didn't find any at Digikey, who seems to have it all, and if they don't have polystyrene, I doubt I'd have much luck locally.

Concerning bypassing the electrylytics, I don't know what is required to do that. If someone could clue me in, I'd appreciate it.

I'm going to be using a DBX Driverack PA as x-over and am looking to use an 18uf bi-polar cap in series with the tweeter. I've heard the arguments that this protection is not needed, but the arguments that it is needed are more persuasive to me.

thanks
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Old 9th December 2004, 08:00 PM   #2
tiroth is offline tiroth  United States
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How about this?
http://www.allelectronics.com/cgi-bi...204&type=store
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Old 9th December 2004, 08:10 PM   #3
Grahamt is offline Grahamt  Canada
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I think the caps you are looking for are polypropylenes. Keep in mind that I know little about active crossovers but wouldn't a fuse be better than a reactive component like a cap?
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Old 9th December 2004, 08:17 PM   #4
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Polypropolyne could work, but then so should many of the others. The recommendation was for polystyrene. I don't know enough about the types of caps to know their different characteristics for audio apps. Concerning the fuse, it would not filter low frequencies, but would only protect against excessive voltage/current. Protection against the low frequencies is what is needed.
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Old 9th December 2004, 08:23 PM   #5
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Quote:
Originally posted by Grahamt
I think the caps you are looking for are polypropylenes. Keep in mind that I know little about active crossovers but wouldn't a fuse be better than a reactive component like a cap?
No because the problem is excursion not power.

People use a capacitor to prevent excess voice coil motion from an amplifier's DC offset at turn-on.

I looked at what my amp did on power up with my scope, found that the DC was less than the 1.5V I got with a D cell checking for driver polarity, found that good polypropylene caps providing a Fc couple octaves below the cross-over point cost at least as much as replacement voice coils, didn't want to screw with the cross-over's time alignment, and stopped worrying about it. No problems in > 500 turn on/off cycles.

YMMV.
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Old 9th December 2004, 08:26 PM   #6
tiroth is offline tiroth  United States
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Polystyrene are uncommon above 1uF. Unless you are willing to pay through the nose for an extremely debatable difference, polypropylene will be just as good.

Either one is worlds above a bipolar electrolytic.
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Old 9th December 2004, 08:46 PM   #7
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Polypropylene is just about the only choice in high value, high quality caps. The series capacitor is really to protect the voice coil in case of amplifier failure, where you could possibly find the full rail voltage on your driver. At HEL, we use a a 75uF and a 5.1uF in parallel to provide 80uF of series protection capacitance on our 2 way passive crossover for use with a bi-amped system. This crossover provides the passive crossover from the midrange to the tweeter in a 3 way system. 80uF gives a -3db point of 250Hz, which is an octave below (or more) below the active crossover point of 500Hz. If you are using a tube amplifier, then this capacitor can be eliminated, since you cannot pass DC through the output transformer. Amplifier failure like this is rare, but I have seen it happen. The replacement voice coil assembly for a high quality midrange compression driver can cost over $100, so $20 for the series capacitors is good insurance.

https://secure11.websitecomplete.com....asp?prod=2675
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Old 9th December 2004, 09:35 PM   #8
Grahamt is offline Grahamt  Canada
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Quote:
People use a capacitor to prevent excess voice coil motion from an amplifier's DC offset at turn-on.
I see now. Sorry if I caused any confusion.
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Old 9th December 2004, 11:53 PM   #9
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Thanks for the advice. Polypropylene it is. I'm running my two way speakers with a bi-amp arrangement using a quad-gainclone, so the protection will be worth the price of the caps, not fully knowing what to expect from an amp that I made myself.
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