Filters...which one to use?

[theChris]

for some simple reference, read a paper i wrote:
http://www.ee.siue.edu/~cstahl/e592b.pdf

this paper detials the common non-inverting single amplifier biquad in the general case and then expands to two of the three common cases.

case 1 -- equal resistance, fixed gain of 1, Q adjusted by capactiors.
case 2 -- equal capacitance, fixed gain of 2, Q adjusted by resistors.
case 3 -- equal capacitance, equal resistance, Q adjusted by gain.

i didn't cover case 3. for this circuit a high gain makes component tolerance requirements unreasonable. for a general purpose filters i prefer case2, which allows the Q to be adjusted with resistors which are found in 1% tolerance.

now you just need to find a look-up table for the desired Q and F values for the filter you want.

also look into "filter free" by nuhertz. or filter pro ect... filter free will give you up to 3rd order filters.

[sparky3489]

Ok, here they are:

--Subs

Blaupunkt TSw1200

Qes 0.38
Qms 4.06
Qts 0.347
Fs 25 Hz
Vas 4.178 cu. ft.
Ze 4 ohms
Re 3.69 ohms
Xmax 0.22 in.
SPL 90 dB
Pe 200 W
BL 12.5
Sd 0.549 sq.ft.
air volume displacement 0.12 cu.ft.

--Amp

Sony Xplod XM2200GTX

200 WRMS X 2 @ 4 OHM
250 WRMS X 2 @ 2 OHM
500 WRMS X 1 @ 4 OHM

50 - 300 Hz , 12 db/oct Low-pass filter
0 - 10 dB (40 Hz) Low boost

--------------------------------------------

Space I need to work within:

Width - no more than 29"
Heigth - no more than 14.75"
Depth - my entire trunk if needed

Would be best if front of box was at a 79.5° to fit nice and snug against the back of my back seat. My seats fold forward.

[30bandgeek]

Filters...which one to use? Post #1
I need a HP filter that cuts off everything below 45 Hz with a 24 dB roll-off.

If you designed PCB's, which would you choose?

1. Two 2nd order Sallen-Key Butterworths cascaded

2. A 4th order Sallen-Key Butterworth

3. Passive RC HP filter

4. Some other filter I didn't mention - please explain

Why may i ask? i hope you know passive subsonic filter make the speaker sound like poo... I mean you could use 600w a/c rated Poyproplene caps and big air ribbon coils and it would still be yukkie sounding. Just to spout out a question on a rolloff without giving the senerio of the rest of the circut does no good if you expect a honest real life agreeable awnser

to awnser your question, use a active filter linkwitz-riley to do a fair job at rolling off 24db.

[sparky3489]

30bandgeek,

Did you happen to see my second post (post #4)?

Oh and two 2nd order Sallen-Key Butterworths in series IS a 4th order Linkwits-Riley (I just found that out myself).

By winISDs calculations, I can pump 70 WRMS to my subs before excursion begins to occur with my current design.

[theChris]

for the purposes listed, a linkwitz-riley filter is not really appropriate. the only advantage is is large band-wagon.

i'd say a typical butterworth filter might be more appropriate, if your going active.

i am not entirely sure, but you should check to see if basic RC filters will work. maybe a C-R highpass on the amp input, and a C-R filter on the amp output.

[sparky3489]

I don't think I need anything on the output (tell me if I'm wrong) since I'm using a 4th order bandpass bass box. The upper frequencies are already taken care of. It's the lower frequencies (below 45 Hz) at higher wattages (above 70 WRMS out of 200 WRMS possible) the I'm trying to cut off to prevent excursion.

Now if I have the gross dimensions of my box

Vf = .76 cu.ft.
Vr = .87 cu.ft.
tuned at 60 HZ
port dia. 3.94"
port length = 9.54"

Excursion is just below the line @ 200 WRMS with a 4th order Linkwitz-Riley set at 33 Hz my excursion graph looks like this @ 200 WRMS:

[30bandgeek]

i guess i didnt understand the question but i do now.

Personally i like to be able to feel the bass below 45hz, also subsonics make a huge diffreance in how spacial the system sounds, but if it is just a bass machine olny for spl well than, sounds like you know what your doing and good luck