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Old 31st December 2013, 08:38 AM   #1
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Default Crossover/ preamp for active subwoofer help

Hi All.
Busy building a Hifi sub. Enclosure unfortunately is ported, but it will do. Using 12" woofer. Just finished the power supply, amplifier and DC protect circuit.
I need a design/ schematic for a crossover/ preamp for it. Must have variable control for crossover freq and a gain control also a phase switch.
If anyone has a schematic or design they are willing to share, that would be great.
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Old 31st December 2013, 12:17 PM   #2
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Made this for my brother, isn't pretty but works fine. He needed the same basic capabilities. It is second order Sallen-Key, variable frequency with about (IIRC) +/- 12dB boost / cut and inverting / non-inverting outputs. Any 'audio' opamps are fine (jfet input for the filter is best). Output caps are not shown on my diagram but best to include them.
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Old 31st December 2013, 12:44 PM   #3
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Jason's circuit is fine for just a sub, but I prefer also high pass filtering the mains. If the mains are sealed Jason's circuit will work crossing at the mains F6 to get a Linkwitz Reilly crossover. Ported you need two filter sections at F6. You also don't remove the low frequencies from the mains, leading to increased distortion. I try to go an octave above the mains F3 for my sub crossover. It all depends on your priorities.

For a simple solution with boards available, try Linkwitz-Riley Electronic Crossover You could tie the two channels together after the input buffers, which would require cutting traces to add a buffer (Jason's circuit to the left of R3 and R4) to feed a single low pass filter.

Using Jason's circuit you can make the frequency adjustable using sockets to allow swapping out the resistors at R10 and R11. Like these: 929870-01-09-RA 3M Electronic Solutions Division | Mouser

You can also use a 2 pole multi position switch to switch resistors. Get a make before break type to avoid pops while changing. Potentiometers could be used if you go to 24 db/octave, but tracking between sections must be very tight. Also, if you go with a 12 dB/octave filter as shown one resistor must be twice the value of the other.

With a ported sub, consider adding a high pass filter to protect the sub from below tuning frequency signals. Not an issue for most music, unless you use vinyl. Beware of some movies with extremely loud low frequency effects. See Active Filters for topology and how to calculate the component values.

Last edited by BobEllis; 31st December 2013 at 12:55 PM.
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Old 31st December 2013, 01:52 PM   #4
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Old 1st January 2014, 07:18 AM   #5
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Quote:
Originally Posted by BobEllis View Post
Jason's circuit is fine for just a sub, but I prefer also high pass filtering the mains. If the mains are sealed Jason's circuit will work crossing at the mains F6 to get a Linkwitz Reilly crossover. Ported you need two filter sections at F6. You also don't remove the low frequencies from the mains, leading to increased distortion. I try to go an octave above the mains F3 for my sub crossover. It all depends on your priorities.

For a simple solution with boards available, try Linkwitz-Riley Electronic Crossover You could tie the two channels together after the input buffers, which would require cutting traces to add a buffer (Jason's circuit to the left of R3 and R4) to feed a single low pass filter.

Using Jason's circuit you can make the frequency adjustable using sockets to allow swapping out the resistors at R10 and R11. Like these: 929870-01-09-RA 3M Electronic Solutions Division | Mouser

You can also use a 2 pole multi position switch to switch resistors. Get a make before break type to avoid pops while changing. Potentiometers could be used if you go to 24 db/octave, but tracking between sections must be very tight. Also, if you go with a 12 dB/octave filter as shown one resistor must be twice the value of the other.

With a ported sub, consider adding a high pass filter to protect the sub from below tuning frequency signals. Not an issue for most music, unless you use vinyl. Beware of some movies with extremely loud low frequency effects. See Active Filters for topology and how to calculate the component values.
Mmm, very interesting!!! Would you mind elaborating on this a bit more.
So, if I understand correctly, with a ported sub you need a high pass filter as well. Unported?
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Old 1st January 2014, 02:16 PM   #6
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Active High Pass Filter
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Old 1st January 2014, 11:55 PM   #7
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Default active subwoofer with protection

Hello
greetings hope this helps
warm regards
andrew lebon
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Old 2nd January 2014, 03:06 AM   #8
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didi - with a sealed sub, compression of the air inside makes a natural excursion limiter. So there is no need for a high pass unless you want to do something like Bag End does. With a ported speaker, as the signal drops below the port tuning the cone is unloaded and only the driver's suspension limits excursion. With a low enough signal and enough power this could lead to woofer damage from overexcursion. Most music will stay above 30 Hz or so, but tympani, synthesizers, LFE and warped vinyl can have low enough signals to potentially damage a ported sub.
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Old 2nd January 2014, 11:05 AM   #9
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Originally Posted by BobEllis View Post
didi - with a sealed sub, compression of the air inside makes a natural excursion limiter. So there is no need for a high pass unless you want to do something like Bag End does. With a ported speaker, as the signal drops below the port tuning the cone is unloaded and only the driver's suspension limits excursion. With a low enough signal and enough power this could lead to woofer damage from overexcursion. Most music will stay above 30 Hz or so, but tympani, synthesizers, LFE and warped vinyl can have low enough signals to potentially damage a ported sub.
Hi Bob. Thank you soooo much for that explanation.
I completely understand now.
So I have found my ported sub tuned freq to be around 45 hz. Found Rod's sub sonic filter schematic which i will then set at half an octave lower at 34 Hz. Do you agree with this?
Now all i need is an ajustable low pass filter.
My question however is, at what freq range will the Adjustable lowpass filter need to be able to set at?
Also do I place the sub sonic filter before the lpf filter or after?
Thanks again.
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Old 2nd January 2014, 11:11 AM   #10
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Originally Posted by andrewlebon View Post
Hello
greetings hope this helps
warm regards
andrew lebon
Thank you Andrew.
Nice design. Was this for a vented sub?
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