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Old 13th October 2008, 07:00 AM   #1
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Question Third order crossover query

OK this may be a newbie question but I have been unable to find the answer, perhaps some-one can help steer me to the information on line or provide it here.
I have been playing with a third order crossover on a second rebuild of an experimental OB 3 way.
I built the cross-over according to normal usage placing the smaller capacitor first and the larger value after the parallel inductor, then I started thinking along mechanical filter line and wondered what would happen if I used the physical analogy and placed the larger capacitor first in line, so I did so and listened again.
By my ear the reverse of normal practice sounds much better smoother and with less ringing on certain cymbal sounds and bells ( I was listening to Mike Oldfeild and "Tubular Bells ) can some one try and explain to me in as symple as possible a manner??
Thanx in advance
Ted
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Old 13th October 2008, 11:59 AM   #2
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This sounds strange. Normally, what you are doing would create a serious electrical resonance in the filter.

What component values are you using? Any series (or parallel) resistors? Do you have an impedance curve for the tweeter?

Standard passive 3rd order high pass filters often need some trimming to avoid ringing, as component values are quite critical. The inductor should not be smaller than calculated - it is better to use a SLIGHTLY larger value. The first capacitor (see from the amplifier) should normally be a little less than 1/3 of the second capacitor.
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Old 13th October 2008, 08:10 PM   #3
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Thanx for reply, in this instance not a tweeter but an older fullrange Alnico 6inch crossed at 500, as yet I have no way of measuring
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Old 13th October 2008, 08:26 PM   #4
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Quote:
Originally posted by Moondog55
Thanx for reply, in this instance not a tweeter but an older fullrange Alnico 6inch crossed at 500, as yet I have no way of measuring

This does sound strange. I would see if you could find another enthusiest in your area with measurment capability... as previously stated you might find that you have lot of peaking in the crossover at the crossover frequency...


I've found calculated crossovers to be of zero value, the important acoustic slope is the combination of the electrical response of the crossover and the acoustic response of the driver...and calculated crossovers don't take into account the driver response....

That said...

If you like the way it sounds...and you don't think that you're causing a problem with impedance then it could be just what the Dr. ordered.

Scott
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Old 14th October 2008, 06:11 AM   #5
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Quote:
Originally posted by SpeakerScott

I've found calculated crossovers to be of zero value, the important acoustic slope is the combination of the electrical response of the crossover and the acoustic response of the driver...and calculated crossovers don't take into account the driver response....

I agree 100%! Calculated values are merely a starting point.
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Old 14th October 2008, 06:19 AM   #6
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Don't know if you are getting actual 3rd order acoustic slopes or not, but from what I've read you do NOT want to use them with open baffles since the dipole null will not form correctly. Think I read this on John K's site.
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Old 14th October 2008, 09:35 AM   #7
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The bigger cap might have a better power-handlig capasity. When placed after the big one the smaller will not see a full signal anylonger and given this easyer job it can handle more power without screaming


The dynamic or max power capasity in passive filters are underrated by many builders
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Old 14th October 2008, 01:04 PM   #8
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Quote:
Originally posted by PaleRider
The bigger cap might have a better power-handlig capasity. When placed after the big one the smaller will not see a full signal anylonger and given this easyer job it can handle more power without screaming

Doubtful. The current carrying capacity of the capacitor (say that 3 times fast!) is most assuredly sufficient for the job.

Quote:
The dynamic or max power capasity in passive filters are underrated by many builders

I agree. Typically when using ferrite core inductors which saturate quite easily, and quite harsh sounding when they do. I have some 15mH ferrite cores that might be waiting a long time for use again....they sounded like hard clipping when saturating. Iron cores are better, and of course...air cores are best..but with copper pricess $$$$

Scott
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Old 14th October 2008, 08:07 PM   #9
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In this case caps are NP 100 Volts, I chose 3rd order to stop driver bottoming out as its rated power handlind is 3watts when used full range.
And I experimented becauase driver was a dollar as part of a grab box.
I have to say that usually I only hear ringing in drivers when using bypass caps of different types; I have put this down to different grades of capacitor having different rise times thus giving resonance,
Still don't understand the why common/ standard method works, I find electronics counter intuitive at times
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Old 16th October 2008, 11:27 AM   #10
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Hi,

for a 3rd order Butterworth this should cause a ~10dB peak
and that peak has a serious impedance dip to low values.

/sreten.
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