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Discussion Starter #1 (Edited)
I've been toying with the idea of trying to build my own high pass filter. According to what I've read something similar to an FMOD (line level filter) can be built with simply a capacitor. Placing a 1uF cap in line looks to be a 19Hz filter (although only 3 db per octave). Has anyone tried something like this? Is it really that simple? Any input would be greatly appreciated.
 

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Most HPF's are second order or higher to be effective. Assuming a 4 ohm load, a 2nd order HPF at 19 hz would require a capacitor in series followed by a parallel coil. The cap would need a value of 1480.6 Microfarads and the coil would need a value of 47.3 Millihenries. There's a calculator here:

http://www.the12volt.com/caraudio/crosscalc.asp#ccc

I'd be curious to find out what both pieces would cost.
 

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Discussion Starter #4
Thanks for the links. What I am seeing appears to be speaker level values. Wouldn't they be different if they were line level (between the pre amp and amp).
 

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You have to assume much higher terminal impedance on a line level system.
 

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Discussion Starter #6
Rated input impedance of the amp is 10k ohms. Is that the impedance value I need to use?
 

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You would have to design it for a lower impedanse so the input impedance of the amp doesn't affect the circuit. Typical output impedances for line level signals are 100 - 600 ohms.
I have seen a circuit for a line level baffle step correction filter.
Do a search on that.
 

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I've been toying with the idea of trying to build my own high pass filter. According to what I've read something similar to an FMOD (line level filter) can be built with simply a capacitor. Placing a 1uF cap in line looks to be a 19Hz filter (although only 3 db per octave). Has anyone tried something like this? Is it really that simple?
Well yes, it is that simple, but it would quite ineffective since it wouldn't have a slope large enough to make a dent.

The inline capacitor would result in a first order high pass at -6dB/octave. This is insufficient to protect your driver. A proper filter would have -24dB/octave slope. That would require an active filter stage.

Rated input impedance of the amp is 10k ohms. Is that the impedance value I need to use?
Yep, the output impedance of the previous stage has no effect.

So, with an input impedance of 10K and the 1uF capacitor you mention, the filter would have a knee of 16Hz.

brucek
 

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Discussion Starter #11
If I build a first order filter at 24 Hz it appears to keep excursion within limits. I know I'll lose about 3 db in the 20 Hz range but it seems like a good trade-off to keep from damaging my new driver until I can spring for a better HPF/EQ. The driver is a CSS SD12 in a 4.92 cubic foot box tuned to 21.5 Hz @ 500 watts. Am I miisng something?
 

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If I build a first order filter at 24 Hz it appears to keep excursion within limits. I know I'll lose about 3 db in the 20 Hz range but it seems like a good trade-off to keep from damaging my new driver until I can spring for a better HPF/EQ. The driver is a CSS SD12 in a 4.92 cubic foot box tuned to 21.5 Hz @ 500 watts. Am I missng something?
A 24Hz high pass 1st order filter will already be down -3dB at 24Hz. This is the cutoff frequency, below which attenuation will increase at 6dB/octave. Above the cutoff, attenuation increases rapidly to 0dB.

The 24Hz filter would require a 0.68uf capacitor (standard size).

brucek
 

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Discussion Starter #14
What are you powering the sub with?

Right now I'm still powering it with a Behringer A-500. Just trying to stay on the safe side.
Most of the time the sub is fine without a filter. Movies like War of the Worlds are another story.
The excursion scares me to death even at low volumes. I am afraid something will sneak in on me in a movie I haven't seen at decent volume and bottom the sub. It just seems nuts not to spend a couple of bucks to protect against it until I can get something better.

Thanks for the help folks
 

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Discussion Starter #15
Closest I could get with what they had at RS was 22.5 Hz. (4.7uf + 4.7uf + 1uf in series for about .70uf). It definitely helped. WoW at higher volume (-26 db as opposed to -32) had lower excursion. I'm not going to push it higher than that with this setup.
I'll get something better in a couple of months but I'm not as worried about something sneaking in on me.
Thanks to all for the help.
 

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It definitely helped
For fun, I graphed the first order filter function in Excel and converted it to decibels and transferred it to an REW file to plot it. I was interested to see how the attenuation looked at higher frequencies.

Anyway, if it's of any interest, that's your filter below. You can see the cutoff frequency of 22.75Hz is down 3dB, so my formula was correct.

You're down about 8dB at 10Hz, so I suppose that's a little bit of protection - but not a lot.

You can also see what you're doing between the cutoff and 100Hz........

first order 22.75Hz.jpg

You might consider getting a single capacitor.. :)

brucek
 

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Discussion Starter #17
Thanks for the help and the graph Bruce. I'm now looking at a 2nd order with a resistor going to ground between two capacitors. What are your thoughts on that? I would also be lowering the cutoff frequency.
 

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I'm now looking at a 2nd order with a resistor going to ground between two capacitors. What are your thoughts on that? I would also be lowering the cutoff frequency.
No, the free ride you get with the first order filter comes to an end when you try and cascade another stage because of insertion loss over the passband. You get away with it in a first order because the insertion loss is quite reasonable, yet the tradeoff is a weak slope of only 6dB.

The second stage of the filter would have a set of components in approximately a ten fold of the first, so the capacitor would be ~0.068uf and the resistor would be ~100K. This means that at the cutoff frequency, the capacitive reactance of the second stage would be about 100K. Since the input impedance of the amplifier is only 10K, you can see that over the small passband we're using, the loss would be overwhelming. Just not usable.

This is where you see the beauty of an active stage. No impedance concerns, no loss concerns, and slopes of any order you desire. But then, an active filter requires a power supply, PC boards, some knowledge of electronics, etc, etc. .....

brucek
 
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