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Old 26th September 2005, 09:48 PM   #1
DC Dave is offline DC Dave  United States
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Default Sub xover for chip amp

I'd like to incorporate a simple low pass filter in my chip amp to run a small subwoofer. One requirement is that it must have a variable cutoff frequency. I was surprisingly unable to find anything in my searches online that I wanted to use so I came up with this idea.
See the attached pic.

I think what I sketched is a 2nd order Sallen-Key Butterworth lowpass filter with unity gain.

Can someone tell me if I am on the right track with this design?

From my calculations with the 5k resistor and pot set to zero resistance the low pass will be at around 225hz -3dB.

With the 5k resistor and the pot set to 20k ohms for a total of 25k ohms the low pass will be around 45hz -3dB.

I used this formula: R = .707/(2*3.14*F*C)

Will this work? Is there a better way? I'm trying to keep it simple. I've never designed a crossover before.

Thanks,
Dave
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Old 26th September 2005, 09:51 PM   #2
DC Dave is offline DC Dave  United States
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Another idea I came up with is this one, combining 2 1st order filters to create a second order crossover.
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Old 23rd November 2005, 10:17 AM   #3
Wynand is offline Wynand  South Africa
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I would also like to use a circuit like this. Could this be used with a LM3876 or 1 Channel of a LM4780?
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Old 23rd November 2005, 10:25 AM   #4
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Your first circuit it correct and the one to use. The second circuit has a Q of 0.5 which is too droopy IMO.
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Old 23rd November 2005, 11:14 AM   #5
Wynand is offline Wynand  South Africa
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thanks for the reply
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Old 14th February 2006, 11:40 PM   #6
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Dave,

Been some time now. How did it work? Thinking of doing same. What op-amp did you use and what voltages were applied.

Thanks,

David
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Old 15th February 2006, 12:29 AM   #7
DC Dave is offline DC Dave  United States
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Yes, it has been a long time and I have gotten bogged down with work and other projects. So I have not made much progress. I plan to incorporate this design into a six channel preamp that is on my list of things to do. I will be using OPA132 or OPA134 opamps for the filter, powered by +-15 volts.
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Old 15th February 2006, 12:43 AM   #8
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Thanks,

Heck, I might beat you to it!

Dave
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Old 16th February 2006, 09:04 AM   #9
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I've used a circuit similar to the first one a few times and have had good results.

Basically this circuit:
Sub X-over

I think in the circuit I'm currently using I tweaked the caps a bit,for a lower range,and used a TL082 op-amp,with the second op-amp in the package used as an output buffer,with a bit of gain.
It's been working well for me.
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Old 21st February 2006, 01:11 PM   #10
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Default Variable X-over frequencies

Hey all:

I actually built a variation of the original X-over posted by DC Dave about 12 yrs ago, and it worked very well! My ckt was an adaptation of the two-taw x-over that first appeared in the July 1988 issue of Audio magazine. (At least, I think it was July 1988).

This was about the time that old-school car audio was the rage, and few brave souls were venturing into the DIY realm. This x-over was fixed at 120Hz, but I made it variable by replacing the two 100k resistors with a 2-ganged 100k linear pot, each section in series with a 11.3K 1% metal-film resistor. This yielded nearly the same 45-225Hz x-over frequency range.

Since it was a fouth-order Sallen-Key, it was in F and I could select between 12dB and 24dB per octave, simply by selecting where the output buffer was connected.

I also did its hi-pass counterpart for a complete two-way stereo x-over, slope-selectable, continuously variable independant left- and right-channel crossover.

Having the magazine article provide the formula for the cut-off frequency helped me choose the range I wanted, and the values of the tuning resistors to use.

If I find any information on this, I will post it here.

Cheers,

Steve
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