Low Pass Filter question

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Hey everybody,
Hopefully there are some good filter designers that can help me real quick.

I just completed half of my 2.1 amplifier system, the half being the subwoofer amplifier using a parallel lm4780. I am a little dissapointed in the rolloff of my 2nd order active low pass filter since I can hear faint voices coming through the sub during certain songs.

I've decided that I want to make a 4th order low pass filter and am wondering if i can duplicate my 2nd order design and connect them together to create the 4th order. My 2nd order low pass filter is attached below. I'd rather not trash my filter since it is soldered nicely together.
 

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synthius said:
I've decided that I want to make a 4th order low pass filter and am wondering if i can duplicate my 2nd order design and connect them together to create the 4th order. My 2nd order low pass filter is attached below. I'd rather not trash my filter since it is soldered nicely together.
Which filter type is it? Basically, cascading two filter sections squares the frequency response and doubles the phase response. If your filter is a butterworth, this isn't a problem - you'll just end up with a 4th order Linwitz-Riley. If it's another filter type, then you will run into problems with dynamic range, stability and other issues - as well as having suboptimal phase performance.
 
post 1 shows a cascaded pair of unity gain low pass filters with Q=0.7 (Butterworth) in each, giving a net Q=0.5 (Linkwitz Reilly), but the frequency is only 11Hz.
Have you lost a decimal place somewhere?
4pole @ 11Hz should not have any perceptible voice in the pass band.

What have you actually built?
 
Hi andrew,

I actually build the schematic in post 1. I've attached the pspice simulation below

When you say the frequency is at 11Hz are you referring to the -3db cutoff frequency? If so I have that at 89Hz which can be seen in the Pspice simulation.
 

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1uf and 10k is defiantly what i'm using for the simulation. also, I used that schematic to build that circuit which I am utilizing in my amplifier. I agree that a corner frequency of 11Hz would undoubtedly remove any voices within the passband but that is not the case with my actual circuit built. That only leads me to believe that the circuit built from the schematic in post 1 most likely has a corner frequency of what I have observed in the simulation(89Hz) and not the 11Hz you are describing. I tested the circuit on an oscilloscope with a function generator and the signal was not attenuated until the frequency was above 70Hz. Thats when it really started to rolloff steeply. You sound like you know what you are talking about which another reader agreed too. If you're bored maybe you can simulate and see what you get??

HMMMM..... :cannotbe:
 
synthius said:
Now back to my original question, Can you add 2 more stages to my circuit to create a 8th order low pass filter?
Yes, you can - but with such steep rolloffs you would have to match the highpass and lowpass curves very very accurately to get a flat response. This would entail matching Rs and Cs very closely.

If you are using only a lowpass, then you are going to get a big lump/dip in the response near the crossover, as the rolloff of the mids is going to be much, much slower than 48dB/octave. It would take extremely careful design and construction to get such a steep filter to work properly.
 
It would take extremely careful design and construction to get such a steep filter to work properly.

having said that and thinking about the faint mumbles of words that i can barely hear through the sub, I might increase the R's to 12k which will lower the f-3dB to 74Hz and hopefully alleviate the problem.

Once I integrate the stereo amplifier into the system and listen to all channels I'll look back at this and think "why was I being so picky??"

For now I have bigger problems to tackle like constructing the stereo amp and LCD driver.

Thanks for the help all!
 
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