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Building a LPF/crossover

Philthy

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I'm gunna post this in the car audio section, even though it isn't related to cars exactly.

The deal is, that a mate of mine has an old (~10 years) no-name pro-audio 12" sub lying around, and I have a decent PA setup that I use for parties etc.
Now, I want to build this sub a box, and use it with my PA gear for a bit of extra low-end kick, but I will need a low pass filter/crossover for it, as the amp I'll be running it off doesn't have one. Now, since this is only really for a bit of fun and I don't expect it to last that long I'd like to have a go at building a crossover from scratch, as opposed to forking out ~$80 like they all seem to cost.

A bit of googling tells me they aren't that complicated but I'm having a bit of trouble finding a suitable schematic to copy. I was thinking of either 80 or 100hz as the cutoff frequency, and it will have to be an active design, able to cope with up to 500wrms, but usually only 250wrms.

I found a table that says what capacitor and inductor I want, which I will be able to get from Jaycar
Frequency 2 ohms 4 ohms 8 ohms
(Hertz) L C L C L C
80 5.6 mH 700 µF 11 mH 330 µF 22 mH 180 µF
100 4.5 mH 550 µF 9.1 mH 270 µF 18 mH 150 µF
http://ccs.exl.info/cust_cr.html

The problem is that there are so many designs that I don't know which one is the best!
http://www.discovercircuits.com/PDF-FILES/LowPassFilt1.pdf (a few designs)

Does anyone on here know anything about this sort of thing? Just a simple schematic should be all I need to figure it all out
 
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Philthy

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Sorry, turns out I don't know what I'm talking about and it's actually a passive XO that I am after (Post amp, between the amp and the woofer)
 

daron

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why
set ya amp at the right settings youll never need to change it
 

Philthy

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why
set ya amp at the right settings youll never need to change it
The amp I'm using is a 2ch PA amp. I'll be running 2x12in boxes off one channel and the sub off the other channel. It ha no filters as such, just a graphic eq for each channel.

I have it figured out now I think...realising that is was a passive filter I needed makes it easier. I'll make a 2nd order (12dB/octave) LPF with an 18mH inductor and 135uF (probably more like 133uF) capacitor. I find slightly different values wherever I look but that combination should be relatively easy to make using bits from Jaycar and the like.
 

Slippy_Trippy

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Yeah all parts easily available at Jaycar, they even have kits I think. You could also just look into a Behringer XO unit. Not the nicest prospect in the world but they work well and are adjustable which could come in handy when you tune your PA.

-Trippy
 

bezz

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wow that first website oversimplifies filter design way too much. There's a lot more to it than that.

As for the designs on the second site, they are all variants of the same type of filter. A butterworth filter is good if you want there to be minimal fluctuation in the pass band just before roll off. Problem is Butterworth filters have quite a long roll off which may be (or might not be) detrimental to your application. Also none of those filters will be suitable for your application without modification.

Finally, if you design your filter to use op amps instead of inductors it will make for a much more robust and smaller design, not to mention less susceptible to outside interference. Inductors are annoying to use due to windings being fragile, especially if using air core type. This is the Sallen-Key method, very common in design of filters. All of the circuits on your second site are an example of this.

IMO do a lot more reading before attempting this. If I was designing a crossover for audio purposes I'd most probably look to using a Bessel filter.
 

bezz

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One more thing, you might need to consider that your amp wont be very efficient when playing back just these low frequencies. A lot of PA amps tend to be focused to deliver most of their ouput at higher frequencies (mid bass, mids to high etc). You might even find that the output <80Hz will sound closer to what you would expect from less than 100w rms from a dedicated sub amp. It may be worth considering investing in a dedicated sub amplifier instead.. jaycar do an interesting looking one but then they come at a price.
 

Philthy

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The way I understand it (and bear with me, I've never had to know any of this until now) is that active crossovers, such as in that second website, are for use pre-amp as they are more sensitive to power but will sound better.
The passive crossovers (like I plan to use) I thought were for between the amp and sub, so they have to handle large amounts of power, and there were less design options (1st, 2nd order and so on only)

I realise that my amp will be less efficient, but I would probably only use this sub when doing small-mid sized RnB parties and the like, where my current 12"s can't handle the deep bass. They are fine for techno, hardstyle etc, just struggle a bit on the really deep stuff, which is where I thought this sub could give them a bit of a boost.
I don't really do enough RnB parties to warrant buying a seperate sub amp just yet, and if I get to that stage I think I would just buy a powered sub anyway.
 

Slippy_Trippy

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Powered subs are a good way to go to expand your system and they generally have parallel outs which have inbuilt crossovers for you to plug your mid/high speakers in.

If your other 12's can't handle the deep bass then chances are this no name will yield similar results, you will just pick up a few (less than three) dB because of the 1 added speaker.

Better off to save and get a 15 or 18 powered sub designed for PA IMO. Ebay and second hand is good if you can't fully justify.

-Slippy
 

Philthy

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Powered subs are a good way to go to expand your system and they generally have parallel outs which have inbuilt crossovers for you to plug your mid/high speakers in.

If your other 12's can't handle the deep bass then chances are this no name will yield similar results, you will just pick up a few (less than three) dB because of the 1 added speaker.

Better off to save and get a 15 or 18 powered sub designed for PA IMO. Ebay and second hand is good if you can't fully justify.

-Slippy
You think?
I figure since this new sub I'm getting is an actual subwoofer, as opposed to full range 12s that I already have it will be able to give a better bottom end kick, and for the $20-30 that it will cost me to set up and get running it can't hurt to give it a shot.
Plans down the track are for 1 or 2 15" powered subs, but as I said before I just can't justify that cost just yet
 
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