[Early B.]

I want to make a low pass filter for my subwoofer and need to determine the value of the capacitor. I want a 180Hz cutoff. My preamp has a 94K ohms input impedance and my subwoofer amp has a 12K ohms (not sure which one is needed for the calculation, if at all).

Any assistance is appreciated. 

[Manolo]

Depends on the cap value you use. For a 0.1 uf cap accross the input the resistor value would be 8846 ohms. The formula for a first order filter is R=1,000,000/(6.28xCxF). Hope this helps!

[Early B.]

Depends on the cap value you use. For a 0.1 uf cap accross the input the resistor value would be 8846 ohms. The formula for a first order filter is R=1,000,000/(6.28xCxF). Hope this helps!

Oops. I meant capacitor, not resistor.

[FullRangeMan]

A calc pgm:
http://www.erseaudio.com/First-Order-2-Way
For 8 ohms cap=110.417 inductor=7.077 a big one.

[HAL]

It would need to go on the subwoofer amp input, but this will put a highly capacitive load on the preamp output and may make some unstable.  A series resistor with the input cap and input impedance will probably be needed and the calculation redone.

The equation is:

1/(2*3.1416*180*12000) = 0.0737uF.  You will need to parallel a 0.047uF and 0.022uF to get close.

[richidoo]

Passive filter recipe for 1st and 2nd order filters:
http://www.t-linespeakers.org/tech/filters/passiveHLxo.html
Thanks to AC member planet10 for hosting this article.

A single cap and resistor makes a 1st order (6dB/oct) filter. 180 is pretty high for a 1st order bass LP, the bass driver will still be audible way up into the midrange, not as clean as your mid driver. 24dB/oct is more typical of bass playing that high.

But a 4th order line level filter requires a current buffer, can't be passive like the lower order filters above.  Info.

Pardon if you know all this and have a special need... 

[Early B.]

Pardon if you know all this and have a special need... 

I'm totally clueless on this stuff, so any help is welcomed.

[Doublej]

Early B.

What are you trying to accomplish at the highest level? What gear do you have and what are you looking to do with it?

[Early B.]

Early B.

What are you trying to accomplish at the highest level? What gear do you have and what are you looking to do with it?

I'm simply trying to remove the higher frequencies from my third subwoofer that sits in the back of the room and fills out the bass on my 2-channel system.  My front subs are designed to kick in at around 150 - 200Hz, so that's why I chose the 180 Hz frequency cutoff.

[Doublej]

What frequency and order crossover do you have on the front subs? I believe it is much more common to use a higher order crossover on a subwoofer than it is to use a first order crossover as a higher order filter cuts off more sharply which means the subwoofer is not being called to produce frequencies above it sweet spot (low frequencies)

Here's a pre-built second order one.

https://www.parts-express.com/prv-audio-1df180b-low-pass-subwoofer-crossover-board-180-hz--294-2863

This may work too. I didn't read through the details to find out.

https://www.parts-express.com/behringer-cx2310-super-x-pro-crossover-2-way-3-way--248-664

Here's a cheap 120Hz one. Perhaps they also make a 180 Hz version.

https://www.amazon.com/Nippon-HC-101-channel-Subwoofer-Crossover/dp/B009D9KHZ2/

[HAL]

From his description he is looking for a line level crossover, not speaker level crossover.

[FullRangeMan]

From his description he is looking for a line level crossover, not speaker level crossover.

Correct if the sub is active, I missed this point.

[richidoo]

A sub in the back of the room should crossover steeply (24dB/oct) at or below 80Hz, because above that frequency sound becomes directional and you will "hear" the sound coming from behind you which isn't ideal. Below 80Hz the sub can be placed anywhere in the room and it will just "fill in" without hearing where they are.

Most commercial subwoofers have a plate amp with a built in low pass filter. Does yours have that, or it is DIY?

[Early B.]

Most commercial subwoofers have a plate amp with a built in low pass filter. Does yours have that, or it is DIY?

I have this amp:  https://www.parts-express.com/dayton-audio-sa230-230w-subwoofer-amplifier--300-813

It's currently hooked up to the LFE. When I hooked up the RCA cable to the line in on the back of the sub amp, I got a nasty hum.

[HAL]

The amp has a built in low pass filter for the subwoofer with controls on the front.  Do they only work with the line level inputs not the high level inputs?  Did you also try the line level input instead of the LFE input?  Run both right and left channels and it should sum them like the LFE.

[Doublej]

I have this amp:  https://www.parts-express.com/dayton-audio-sa230-230w-subwoofer-amplifier--300-813

It's currently hooked up to the LFE. When I hooked up the RCA cable to the line in on the back of the sub amp, I got a nasty hum.

Possibly a defective unit?  One in which the wiring on the RCAs are reversed? I would get this amp fixed or replaced instead of mucking with more parts.

The web page says it has a five year warranty so hopefully you can get the issue addressed for minimal (<$50 to ship it back?) dollars.

[Early B.]

Do they only work with the line level inputs not the high level inputs?

I haven't tried the high level inputs. Not sure how to hook it up. Would I run speaker cable from one of my front speakers to the sub amp?

Also, would it work to use the "line out" on one of front speaker sub amps to connect to the "line in" on the third sub amp? If so, will the front speaker sub amp only pass the low level signal?

[Early B.]

Possibly a defective unit?  One in which the wiring on the RCAs are reversed? I would get this amp fixed or replaced instead of mucking with more parts.

The web page says it has a five year warranty so hopefully you can get the issue addressed for minimal (<$50 to ship it back?) dollars.

I may have some more hookup options to try before going this route.

[HAL]

I haven't tried the high level inputs. Not sure how to hook it up. Would I run speaker cable from one of my front speakers to the sub amp?

Also, would it work to use the "line out" on one of front speaker sub amps to connect to the "line in" on the third sub amp? If so, will the front speaker sub amp only pass the low level signal?

https://www.parts-express.com/pedocs/manuals/300-813--dayton-audio-sa230-subwoofer-amplifier-user-manual.pdf

According to the manual both the speaker level inputs and line level inputs work with the internal active crossover.  You can connect either of them from the preamp outputs or from the mains to the amp and then adjust the crossover.   

ADDED:  You would need to run both stereo line level outputs from the preamp to the sub amp line level inputs to get both channels summed correctly. 

The crossover is an 18dB/Oct low pass for the sub.  Already a 3 pole active filter.

Bass boost does not work on speaker level inputs.

My suggestion is to get Room EQ Wizard running on a computer with a mic and use that to setup the amp.  The max frequency for the crossover is 180Hz as you wanted.   No capacitor needed at the front end.

[Early B.]

You can connect either of them from the preamp outputs or from the mains to the amp and then adjust the crossover.   

OK, I'll try the mains to the amp and report back. Thanks.