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Old 21st February 2014, 02:24 AM   #1
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Default Motor Starting Capacitor

Hi. Not sure if I'm in the correct forum, but here goes...

I found a 1000uF 250VAC non-polarized motor starting capacitor.

Can I use this for a 20Hz @ 8ohm hi-pass filter?

Thanks, Al.
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Old 21st February 2014, 02:31 AM   #2
kevinkr is offline kevinkr  United States
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Moved.. It was a toss up between here and subwoofers, but I assumed it was for a conventional speaker system.
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Old 21st February 2014, 09:38 AM   #3
AllenB is offline AllenB  Australia
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Motor start capacitors are typically electrolytic. I have a small stock of them in 20 and 30uF capacities and use them periodically usually at lower/middle frequencies.

I like them because I can keep them on hand and they have been fairly consistent after being thrown into various crossovers with little regard for how hard they'll be worked.

I've also had them connected to tweeters. They don't do much to the sound, the difference from an audio purpose polypropylene is small (YMMV).
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Old 21st February 2014, 04:12 PM   #4
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Thanks, Allen. The one I found has two 1/4" spades on one end and is about the size of a D cell battery. Good to know it should work!

Thanks, Al.
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Old 21st February 2014, 04:33 PM   #5
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Heres a picture:
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Old 21st February 2014, 04:36 PM   #6
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the last time i saw a motor start cap used as a high pass was in Martin Audio single 15 bass bins to protect the driver surrounds from tearing ask me how i learned why they where in there(it's a woeful tale of reconing several drivers)
it will work but active filters would be better what's your application?
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Old 21st February 2014, 05:47 PM   #7
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Thanks, Turk! That's exactly what I'm looking to do, prevent excessive driver excursion. Also perhaps save some wasted amp power.
My current rig is 4-way active with four stereo amps, 700, 400, 200 and 150 watts RMS. The subs are two 12" isobaric, each with two 12" drivers bolted together face to face, one of each driver pairs wired out of phase.
I'm going to convert "Frankenstereo" to 3-way active with passive mid/hi. The subs will be replaced with a single 18" bass reflex or scoop (haven't decided yet). The 12's worked fine, but a little tame for my liking. I need a heart re-starter for when my heart attack arrives LOL. You know, crazy near-field!!!

Thanks, Al.
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Old 21st February 2014, 08:15 PM   #8
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Quote:
Originally Posted by Lost in Bass View Post
Thanks, Turk! That's exactly what I'm looking to do, prevent excessive driver excursion. Also perhaps save some wasted amp power.
Al,

Problem is you may not prevent excessive driver excursion or save any amp power, and may burn up your amp due to reactance.
I have some very robust PA amplifiers that can withstand near dead shorts, which appeared to be what they were driving when I tried some really large value capacitors as "protection" for some mid drivers.
Some amplifiers would react to the reactance with a puff of smoke from the output transistors.

Anyway, a ported cabinet does not have an even impedance curve, below you can see what the impedance of a nominal 8 ohm driver in a 39 Hz Fb (box tuning) looks like, and what the 1000 uF capacitor does at the various frequencies- around 27Hz, where impedance (and excursion) goes way up, the HP drops to a useless 4.5 Hz.

Get a mini DSP and you can put in the proper filters and align all your future projects without an endless array of caps and coils.

Use the 1000 uF for storing the heart starter paddle juice .

Art
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Old 21st February 2014, 11:53 PM   #9
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Hi Art. Thanks for the great info and warning! Before your post, I decided a 20Hz cut would be rather useless with a 6db filter so I figured a cut one octave higher at 40Hz (500uF) made more sense. I assume this new value would pertain to your warning as well? I'm aware of the impedance spike at the drivers Fs. If my cut was above this spike would it solve any problems? I'm not taking into account the box Fb because I haven't chosen a box yet.
Sadly, it looks like I won't be adding any passive filter at the box because I'm taking your advice. The little bit I would gain is not worth the risk of burning up an amp!
BTW, what is a mini DSP? And I absolutely love the paddle quip!!!

Thanks, Al.
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Old 22nd February 2014, 12:10 AM   #10
sreten is offline sreten  United Kingdom
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Hi,

Series capacitors on speakers work nothing like simple 8
ohm 1st order x/o's, the simple answer is very much no.

Such things (20Hz) are much easier at line level.

The free demo version of Basta! is very good for
showing what really happens with series caps.

rgds, sreten.
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