Crossover/ preamp for active subwoofer help

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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.
 
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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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.
 
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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.
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?
 
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.
 
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.
 
Yes, Didi, 34 Hz is reasonable for 45 Hz tuning, you might be able to get away with lower which would not slightly roll off your bottom end.

Why do you want the crossover frequency adjustable? Making a filter adjustable involves some serious compromises or complexity penalties. If you are hoping to find your optimum crossover point and will then leave it alone, using the sockets I suggested earlier to allow changing resistor values it a good option. When done, you can remove the sockets and solder the proper resistors in place. Rod Elliot's boards are a good place to start.

If you want the crossover adjustable because you intend to experiment with different mains and subs, I suggest looking into the mini-DSP. It's easily adjustable and provides the high pass filters required for the mains to allow proper sub integration.
 
hi Bob.
I have no specific reason for making it adjustable. It just seems like thats what everyone is doing. What circuit of Rod's or yours can you suggest for a ported sub? Maybe Ill use sockets with fixed resistors as you suggest.
Please if you have schematics for this as well as a sub sonic filter and willing to share my email is riaandd@gmail.com
Regards
 
I'd use Rod's 24 dB/octave LR board. probably crossed in the neighborhood of 80 Hz. Use a 1K resistor in series with each low pass output instead of the 100 shown and tie them together to feed your sub directly or through a high pass (rumble filter)

What are your mains? If they are sealed with an F3 around 80 Hz then only populate one high pass filter section on Rod's board. Cut the trace feeding the second section (or use an axial cap for the input and you have a 12 db/octave high pass filter section available to use as a rumble filter. Be sure to install a resistor in the unused high pass filter input Notes as 2R/22K to ground to prevent it from taking off and creating noise.
 
Here is what I did.
I have decided to start my circuit with Rod's mixer/phase circuit as in project 71 on his site. Then Im going to go through a 12db/octave sallen key LPF set to about 80Hz. Then Im going to go through a 18db/octave hpf set to 20Hz.
All in all im using 4 opamps.
My question is, do I need to have any gain control somewhere? The power amp has gain.
Should I perhaps just place a volume pot between this circuit and the power amp?
The amp and this filter circuit will be placed in the same box which is ported.
 
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