Can somebody help with a simple 12dB filter? - diyAudio
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Old 27th December 2007, 03:51 AM   #1
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Default Can somebody help with a simple 12dB filter?

I'm going to incorporate a Dayton SD215-88 ( http://www.partsexpress.com/pe/pshow...number=295-480 ) into a portable boombox. I'm going to power the DVC sub with a 2 channel amp. I dont have any impedance plot except for the Dayton provided one. I'm wanting to cross it over at around 200hz @ 12dB. Can anybody help me out? How close could I get by using the Dayton provided impedance curve?

Thanks!
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Old 27th December 2007, 06:00 AM   #2
GM is offline GM  United States
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Greets!

Pretty close once you add a zobel to flatten it, but the 3.75 ohm non-inductive power resistor and 85 - 88 uF cap's going to be kind of large to fit in a boombox, not to mention make it run hot, so a line level XO seems a better choice: http://www.t-linespeakers.org/tech/f...ssiveHLxo.html

GM
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Old 27th December 2007, 08:11 PM   #3
Svante is offline Svante  Sweden
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Well, passive filters at 200 Hz is often problematic due to the impedance curve of the driver. Below is an example of a cookbook filter (green curve) and what happens when it is applied to the complex load of the driver. When the values are "fiddled" a bit, it looks a bit better (blue curve).

Click the image to open in full size.

I think this highlights the need for simulations when designing passive filters . The impedance of the blue system is probably too low to be useful, though.
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Old 28th December 2007, 12:07 AM   #4
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I'll try the PLLXO route then. It seems easy enough (even for me). I have no idea how to choose R1 though. But if I were to choose 10kohms would this be correct for a 2nd order lowpass @ 200hz?

R1 = 10kohms
C1 = 80nF
R2 = 100kohms
C2 = 8nF

Is this right?
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Old 31st December 2007, 09:56 PM   #5
owdi is offline owdi  United States
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Quote:
Originally posted by GuyPanico
I'll try the PLLXO route then. It seems easy enough (even for me). I have no idea how to choose R1 though. But if I were to choose 10kohms would this be correct for a 2nd order lowpass @ 200hz?

R1 = 10kohms
C1 = 80nF
R2 = 100kohms
C2 = 8nF

Is this right?
You pick R1 and R2 as a compromise between insertion loss and the impedance your preamp sees. You need to know your preamp's output impedance, and your amplifiers input impedance, too get an idea of how much wiggle room you have.

For example, if your amplifier's input impedance is 100kohms, then your above filter would result in about 3db insertion loss. If the input impedance is 10kohms, then insertion loss is closer to 11db. As you reduce R2, insertion loss gets smaller, but so does R1.

Since R1 is the lowest impedance the preamp will see, you can't reduce it forever. I think 10x output impedance is a minimum, but hopefully someone with more knowledge than I will chime in with a better answer.

Dan
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Old 3rd January 2008, 05:07 PM   #6
gfiandy is offline gfiandy  United Kingdom
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Hi, is there no way you can use an active filter before the amps. You will get a much more predictable crossover and better control of the drive units as there will be less resistance between the amp and the driver coil.

Regards,
Andrew
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Old 9th January 2008, 05:16 PM   #7
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Quote:
Originally posted by gfiandy
Hi, is there no way you can use an active filter before the amps. You will get a much more predictable crossover and better control of the drive units as there will be less resistance between the amp and the driver coil.

Regards,
Andrew

+1 active is always better than passive... cheaper and better sound quality.
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Old 9th January 2008, 05:39 PM   #8
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I'm in planning stage of similar project. Planning to use a DVC sub woofer. Was going to use active filter but concerned about Power supply. Going to run car radio with ac power supply. An active filter would require another amp and would be considerable challenge to power supply.

See if this helps. http://www.apicsllc.com/apics/Misc/filter2.html
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Old 19th January 2008, 07:44 PM   #9
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Quote:
Originally posted by davidlzimmer
I'm in planning stage of similar project. Planning to use a DVC sub woofer. Was going to use active filter but concerned about Power supply. Going to run car radio with ac power supply. An active filter would require another amp and would be considerable challenge to power supply.

See if this helps. http://www.apicsllc.com/apics/Misc/filter2.html

AC PSU? Look into getting +-5v from your cars electrical system, it shouldn't be too hard. Try not to use AC in your car unless you are making your own SPMS.
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Old 19th January 2008, 08:02 PM   #10
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Quote:
ry not to use AC in your car unless you are making your own SPMS.
Maybe I wasn't clear. The project is a portable boom box. Based on car stereo which will give me FM, CD, and external inputs such as MP3 player.

Plan to use AC power instead of battery. Tho, a batery could be used if no power available.

Am now considering a matched component system and be done with it.

Like this
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