Active Filter Network

[Matttcattt]

I currently have my sub crossed at 100Hz, and I have a linkwitz transform. I feel that it is time to create proper preamp with high pass filters. Basically, is my current circuit ok, or should I add some unity gain buffers anywhere? I calculated the values for the filters using the SLOA093 document from Texas Instroments. I have built each filter seperately on breadboards, and they work ok. I just dont know if I can stick them together like I have.

[joensd]

You can have a look here and compare that with the TI papers.
The sheets includes everything like subsonic filter, input buffer, linkwitz transfrom and variable gain.

[Matttcattt]

Thanks for the link, but ive already had a play with the Linkwitz Tranzform Program, and other spreadsheets.

[theChris]

all filters show are 2nd order, likely making them 12dB/oct unless they are signifigantly overdamped which i'm betting against given the closeness of values.

note that the source impedance is not shown. you may acheive better results adding a buffer on the inputs.

have not checked the values but it is likely the rumble filter and linkwitz transform filter will be near the same frequencies.

[Evan Shultz]

Yes, all your filters are indeed 2nd order, 12dB/octave.

You need a buffer at the very end to prevent the pot from getting loaded down.

[Matttcattt]

So adding buffers at each end is the only change I need to make?

Are the values of the components ok?

[edit]: Sorry if ive confused anyone with the orders of my filters. I thought 2nd order was 6db/oct.

[richie00boy]

Yeah, a buffer at the front end would be a good idea because there are a few stages to drive and you can never be sure what the source impedance is. A buffer on the low out mono is also needed as mentioned for the pot.

I would also add a resistor of 22-100 ohms at the output of each op-amp that will drive a cable to swamp against capable capacitance possibly sending the op-amp into instability.

You might also like to simply parallel two 0.1uF for your sub low-pass as well instead of using a 0.22uF, but this is being a little picky and I guess you are short on space. I'd also look into cascading two of those filters to cut off more sharply.

[carlosfm]

Originally posted by Evan Shultz
You need a buffer at the very end to prevent the pot from getting loaded down.

Why not place the pot before the rumble filter instead?
No additional buffer needed.

[richie00boy]

I may disagree. Depending on the value of the pot, the filter will be affected.

[Matttcattt]

Originally posted by richie00boy
I would also add a resistor of 22-100 ohms at the output of each op-amp that will drive a cable to swamp against capable capacitance possibly sending the op-amp into instability.

The buffers, or all the opamps?

You might also like to simply parallel two 0.1uF for your sub low-pass as well instead of using a 0.22uF, but this is being a little picky and I guess you are short on space. I'd also look into cascading two of those filters to cut off more sharply.

Which filters? The lowpass?