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Old 4th January 2011, 02:47 AM   #1
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Default Subwoofer filter

Hey all,
Just looking for a basic sub filter. I wired up an LM3886 chipamp and need a preamp module that prep a signal for the subwoofer.
How does something basic like this look?
Subwoofer Filter Circuit

I would probably only build the low-pass element circuit meant to drive the sub. I would send the unfiltered full signal to my L/R satellites instead of passing it through a high pass filter as this circuit does.

This is intended for musical use, btw. If you have better suggestions lemme know.

Thanks for anybody who can help!
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Old 4th January 2011, 10:00 AM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Hi,
passing unfiltered wide band music to the satellites will force the satellite amplifier and the satellite speaker to try to reproduce the bass frequencies. This potentially is setting your satellites up to fail in quality stakes and ultimately damage the drivers trying to handle the bass signal.

The Low Pass section is OK for the bass only channel.
The left and right signals need the two 4k7 before the first summing opamp.
But, this will load the signal source with a 4k7 load in parallel with the input impedance of the satellite amplifier.
Either adopt a much higher impedance for the summing opamp and accept a tiny bit of extra noise. The bass won't reproduce much of it anyway.
Or buffer the signal source to give four high current outputs that go on to feed the summer and the two satellites.

If you look up MFB filter you will find it forms the first summing opamp's function and the next f2pole filter opamp in one section.
But note the summing opamp and the MFB are inverting topology. Where are you going to correct the phase?
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Old 7th January 2011, 05:47 AM   #3
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'Not questioning the analysis, but trying to listen to a 24 dB filtered response without its reciprocal form is going to be very...well, difficult to call decent audio. Assuming your proposed drivers cover the crossover frequencies adequately, I would second using all the adapter sections presented as a full crossover. Otherwise, use a much lower (perhaps only 12dB) subwoofer rolloff in a rough attempt to track the natural rolloff of the satellites.

Perhaps the final sub. filter stage attenuator and a downstream sub. amp level control may have been intended to correct the overall gain?
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