DIY center channel crossover help!

[Patrick Sun]

Okay, here's what I want to do:


Use the tweeter and midrange components from my DIY speakers and create a center channel speaker with them. I want timbre-matched speakers for my 3 main fronts.


I want to do a MTM center channel speaker (with it lying sideways like most center channel speakers that lie near the top or bottom of the TV).


I was mucking over the 3-way crossover and thought I could re-use basically the crossover for the tweeter, and do a little work on the crossover for the midranges.


I wouldn't really need the highpass portion of the midrange crossover, thus I could jettison the 3 mH inductor and the 40 uF capacitor at the beginning of the midrange crossover. So I would keep the inductor as the lowpass component for the midrange. Now the tricky part is how to wire up both midranges and still make it an 8 ohm center channel speaker. Each of the midranges is a nominal 8 ohm load (I'm using a totally sealed midrange, the Peerless 1385 (KO40MRF) 4" Midrange driver). In the original crossover, there's a L-pad to attenuate the sensitivity of the midrange and get it in line with the tweeter which also has an L-pad attenuator built into its crossover.


Would I be nuts to wire both midrange drivers in series for a nominal 16 Ohm load, and adjust the resistor with the value of 20 Ohm to 10 Ohms.


Here's a link to the original crossover. Look at the midrange crossover section, and see where the 20 Ohm resistor in the vertical position of the crossover, that's the one I want to change to 10 Ohms (the L-pad gives me an impedance just a tad over 8 Ohms with the 2 midranged in series and the resistor changed to a 10 Ohm resistor.)


I fear that I have screwed up the current divider from the original midrange crossover if I put 2 of them in the network of midranges.


Any ideas if I'm going down the right path?




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[Patrick Sun]

Okay, dug up my old crossover notes to find the F3's for the 1st and 2nd order networks that are used for the overall crossover network.

What I came up with for my main speakers crossover F3's (click on the link that's in my initial post for the crossover I am using):

Tweeter hi-pass F3=4800Hz

Lower Midrange hi-pass F3=460Hz
Upper Midrange lo-pass F3=1872Hz (this seems low)

Woofer lo-pass F3=410Hz

I'm still thinking about going for the W-(T/M)-W design as follows (I'm basically copying this design):

Now, if I found a couple of 4 ohm woofer drivers in the 6"-8" diameter range that would handle from 80-500Hz, I think I could still utilize my existing crossover from my main speakers. Any recommendations for such a woofer driver?

How about box size? Would 0.75ft^3 stuffed do the trick?

I was also thinking that if I change the 0.68mH inductor on my midrange to something a little lower, I could extend the upper F3 to around 2400Hz, I'd need to replace that inductor with a .53mH inductor. Would it be worth the risk for a slight hump in that region?

Just typing outloud.




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[ThomasW]

Patrick

I've used both vertical and horizontal MTM configs for a center channel. Personally I don't feel that one is seriously flawed over the other.

As for you idea, this involves completely redesigning the XO. In theory your original XO should have been designed by measuring the drivers while mounted in the baffle, then taking those measurements and combining them with the T/S data to create the optimal XO. So any change in the baffle, the drivers the number of drivers etc, means that the XO design process must be begun from scratch.




theAudioWorx
Klone-Audio

[Patrick Sun]

Thomas, I have no idea where to look for the T/S parameters of my tweeter and midrange. Well, I can try checking at the local shop that carries them, but that might be a while before I can stop by the store. Plus, it's a rather inexpensive project using some (flying by the seat on your pants) choices. And I don't have any good modelling software at the moment. If it sounds horrible, I'll start from scratch and chuck out the crossover.




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[ThomasW]

Patrick

IMO a properly designed/engineered speaker is much more important than a timbre matched speaker that's not optimized.

If you're not set up for doing an optimal XO design then I suggest going with a kit. Or atleast using one of the speaker supply houses that has a LEAP design service.




theAudioWorx
Klone-Audio

[Patrick Sun]

Thomas, I will make one attempt to muck about with the woofer crossover section (Ed's helping me a bit on it as well for my 1st try), and if it doesn't sound too good, I'll start from scratch if I'm not too depressed from the initial effort.




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[Patrick Sun]

I think I've decided to use the Vifa P17WJ-00-08's for the 2 woofer drivers in my center channel project. They'll be handling the low end from around 60Hz to 500Hz, maybe 60Hz-400Hz.




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[Patrick Sun]

Ed P suggested that I walled off the woofers into separate compartments, and I thought it was a nifty idea! I think I'll incorporate the woofer compartment woofers idea and play around with the woofer crossover towards the end (figuring out how to do some tweaking with LC pairs for the best sound).

So how does one do the calculations for determining how large you should make a box for a given driver?

Well, I had to dig out my audio engineering course notes (through quite a few boxes of college notes), and played around with some enclosure calculations. I decided to do vented woofer compartments within the big enclosure. So I shot for a Qt of 20 for a really small box, used it to get my "a" (alpha = 3.5), which allowed me to get the Vb from Vas/a and that was about 12 liters. Using the same Qt chart for "a" I was able to get "h" and F3/Fs, which I used to get my Fb (63Hz) and F3 (59Hz). This was the ball park I was aiming for to get my low end response.
Then I needed to get the port tuned, so I decided on a 2" wide port, and plugging through the formula gave me a port length of 3.63". If I used a 3" port, the length was getting too long for the box (around 9.25"). My Infinity RS3's have a 2" wide port about 4" long, so I think my math is roughly in the right neighborhood.

So for each woofer compartment, I included 1.5"x1.5" bracing on the back side (I plan on using wood screws to screw the back panel into the bracing with weather stripping in between the bracing and panel - I did this with my main speakers). The bracing and the porting volumes have been included in the final volume calculations (I hope). I came up needing 790 in^3 of volume for each woofer compartment. I used this to get the form factor for the depth of the compartment as outlined below.

My internal height will be 8.5" due to the tweeter and midrange heights. For the woofer compartment width, I needed at a minimum of 6.75", so I arbitrarily went for 9" in width to keep the depth down. This meant I needed a depth of 10.33".

The middle compartment will house the tweeter and midrange drivers, and will be 5" wide, they'll have enough internal volume in their compartment.

This means that using 3/4" thick wood, I'll wind up with a box whose outer dimensions (WxHxD) will be 26"x10"x12". This will easily fit on the RPTV.

Ta-da! Q.E.D!

(I may have to re-do the calculations if I decide to go with one of the other 2 Vifa woofers under consideration)




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[ThomasW]

Well Pat as Pink Floyd says:
"What's the frequency Kenneth?"

That's generally the determining factor for box size woofers. One can of course "force" a woofer to work in any size enclosure up to certain limits. Just use WinISD, it's the same for woofers or subwoofers. Or you can grab the "demo" copy of LspCAD and punch in the data. I think the XO tool is included in the demo. But you probably can save the data files

http://www.libinst.com/LspCAD.htm

Also it's usually much easier to wall off the midrange with a PVC subenclosure, and give the woofers as much space as possible




theAudioWorx
Klone-Audio

[Patrick Sun]

Thomas, thanks for the links. I neglected to mention that the midrange I'm using is a sealed unit, so I could recover lots of volume if I engineer it correctly and wall off certain areas, and punch some holes in other areas.

I'll be running through more ideas/calculations tonight when I have access to my "tools".




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