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DIY Center channel crossover help!


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#1 of 22 OFFLINE   Patrick Sun

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Posted February 11 2001 - 02:28 PM

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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#2 of 22 OFFLINE   Mark Wilcock

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Posted February 11 2001 - 08:40 PM

Hi Patrick,

I'm no expert, but I think your heading up the wrong path.
If you add another midrange you should recalculate the values for the relevant lowpass cross depending on whether you connect them in series or parallel. You are going to get either a 4 or 16 ohm load for the midranges, I don't think there's any way around it.

If you connect in parallel you might start by halving the 0.68mh inductor to 0.34mh. Your midrange level will be different, you might want to use an l-pad. (You would need a different level setting resistor network, and you will change the dc resistance of the inductor by changing it's value.) Passive crossovers are a nightmare, unless you really know what you are doing (in my limited experience anyway).

If you add a large (for a crossover) value resistor is series the the load you'll screw up all sorts of things - levels, damping factor, somebody help me out here.

Will your midrange driver work well without the woofer? Can it handle lower frequencies? (if it's sealed then probably not.) This is probably the biggest problem you face as the midrange driver will probably only handle 300-400hz and up, not 80hz or so, like it would have to if you were going to run the speaker as 'small' on your reciever.

Keep up the Sonosubs, you are the king of the digital camera! I'm gotta build a pair soon.

#3 of 22 OFFLINE   Hank Frankenberg

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Posted February 11 2001 - 11:40 PM

Patrick, I would not use a conventional MTM configuration and lie it sideways. Manufacturers sell their MTM's as center channels so they won't have to design and market a separate speaker, IMO. The narrow vertical dispersion characteristic (good thing - minimizes ceiling and floor reflections) of an MTM would be destroyed by laying it on its side. Actually, a quality two-way would work better, depending on how much horizontal dispersion you need (small OR wide sweet spot?).

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#4 of 22 OFFLINE   Greg Monfort

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Posted February 12 2001 - 02:38 AM

I'm not familiar with the components, but a 4" mid, and 10" LF implies an XO point well above 100Hz. And of course adding a second mid won't lower it. Then there's the problem of blending to the sub, or to the L/R speakers, if none is used.

If it were mine, I would use the 10" too, and mount the HF/mid oriented/spaced the same as the mains, with the 10" to one side, and copy the XO's.

GM




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#5 of 22 OFFLINE   Brian Bunge

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Posted February 12 2001 - 03:01 AM

Patrick,

I agree 100% with Hank! Why not just build an MT with the same mid and tweet with the same crossover (dumping the mid's highpass, of course)? The improvement in soundstage was immediate when I took my ACI Emerald (5.25" midrange and 1" tweeter) and set it upright! I no longer hear a discrete center channel plus L/R information. Now all three speakers blend together seamlessly across the front stage!

If nothing else, I'd try this first and then if you aren't satisfied you can toy with the MTM design. You should be able to throw this together rather quickly since you already know all the component values. All you need is a simple test enclosure to see if it's worth going all out with cabinet construction.

I also agree with Greg. ACI is currently building a dedicated center that uses the same 5.25" midrange and 1" tweeter vertically oriented and then will have two 6.5" woofers flanking the MT. It should be quite nice!

Let us know what you decide!

Brian
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#6 of 22 OFFLINE   Patrick Sun

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Posted February 12 2001 - 03:12 AM

The only reason I wanted to go MTM sideways was because I only have about 6"-7" of vertical space for a center channel speaker (that's about how "tall" my current center channel speaker is, while being about 15" wide). If I went the MT route, and put the speaker on its side, where's the "mid-point" of that speaker for the sweet spot? I've given thoughts of just doing the MT, but my current HT setup won't let me since my gear in in the rack on top of the RPTV. Any other ideas on how to go the MT route within my constraints?

Thanks for the input, guys.

Oh yeah, I have plenty of spare MDF from my subs to build a small enclosure for this little project. Posted Image

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#7 of 22 OFFLINE   Hank Frankenberg

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Posted February 12 2001 - 04:28 AM

The way Brian describes ACI's new design IS the way to design a center channel. I just completed mine and will post photos as soon as I learn how. Here is the source, and they are the absolute lowest-cost D-I-Y resource I've seen: http://speakerpage.com/k05c22.htm

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#8 of 22 OFFLINE   Greg Monfort

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Posted February 12 2001 - 04:36 AM

You can't lay an MT down and expect good horizontal sound dispersion. If there's not enough room to mount it vertically, then there's no good options as I see it.

Probably the lesser of evils would be to mount two of the mids butted up against each other hoizontally, and trim as much of the HF's faceplate off as practical so that it can be nestled in between the 'cleavage' of the mids. Orient it so that the HF is above the mids, to reduce reflections off the TV screen.

There will still be some off axis comb filtering, but the bigger issue is still going to be the acoustically large gap between the CC and whatever it XOs to.

Frankly, it's probaby best to optimize imaging, and delete the CC. It's what I did since there's no practical way for me to mate a reasonably small conventional CC to ~full size theater horns, especially in my modest sized room.

GM

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#9 of 22 OFFLINE   Patrick Sun

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Posted February 12 2001 - 06:12 AM

Hank/Brian, interest CC idea. Maybe I could find smaller woofers and implement a modified design.

Hank, how are the woofer connected to the crossover, do they present a 16ohm series load or a 4 ohm parallel load (if each woofer is a 8 ohm driver) or do they present a 8 ohm series load or a 2 ohm parallel load (if they are 4 ohm woofers)?

Greg, now, that would have been interesting to see, a CC with full sized horns...


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#10 of 22 OFFLINE   Dustin Haug

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Posted February 12 2001 - 07:55 AM

Pat if you figure out the speaker orientation problem there are products out there that you could use to make your 2 8ohm speakers appear to be an 8 ohm load. Veritas used to make a product called an "Accumatch" (I think that was what it was called). Richard Clark (Tom Nousaine's AutoSound2000 buddy) was the designer if I'm not mistaken. They were mainly used to make high impedance speakers appear lower. But they worked both ways, so you could wire your 8 ohmers in parallel then use the accumatch to bring the load back up to 8 ohms. Just an idea. Posted Image

[edited because I can't spell impedance]
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#11 of 22 OFFLINE   Greg Monfort

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Posted February 12 2001 - 08:01 AM

Actually, I did have a three chnl horn loaded stereo early on in my first house for a short period of time, but I also had a 26ft x 45ft great room with a cathedral ceiling. The rig was straight out of a cinerama movie house, but didn't sound as good as two chnl IMO, so sold the extra LF horns and all three multicell horns, and subbed the smaller V-O-T 511/500Hz horns. I kept the extra LF drivers for spares, as they are so expensive to replace, when they can be found since they're long out of production (and highly desired).

My photo albums burned up in a fire, but you can see what each chnl speaker looked like here (A4): http://www.audioheri....pro/page_5.jpg
Total height/width, less HF horn, is ~84" x 80.5".

GM

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#12 of 22 OFFLINE   Patrick Sun

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Posted February 12 2001 - 09:07 AM

GM, Whoa! Muy impressive!

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#13 of 22 OFFLINE   Bob Sorel

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Posted February 12 2001 - 10:06 AM

Hehe, Greg! Boy, do those old VOT cabs look familiar! I used to use 4 of them on each side of the stage, with 4 multi-cell metal horns (again on each side) for use as a PA system at the beginning of my band days Posted Image

I never even saw those A2 and A4 style cabs. Those were really big monsters!

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#14 of 22 OFFLINE   Greg Monfort

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Posted February 12 2001 - 10:25 AM

Yeah, those were the good old days when Altec ruled! Posted Image

The A2,A4, and there was an A6, A8, with two multicells, (for really large theaters), were what was behind the movie screen.

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#15 of 22 OFFLINE   Patrick Sun

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Posted February 12 2001 - 12:35 PM

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:

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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#16 of 22 OFFLINE   Ken Cline

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Posted February 13 2001 - 12:22 AM

Damn Greg,

That brings back some great memories. I used to work at Altec Lansing(1977-1981). Like you said; "Back when Altec ruled" Posted Image

Just before Altec closed the doors(mid '80's I believe) at their Anaheim facility, I got a chance to go to 20th Century Fox for a private screening in one of the Executive screening rooms. The reason we were going there was Fox had one of the original VOT system still in use. From what I remember it still sounded great.

Ahhh...the memories....

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#17 of 22 OFFLINE   Brian Bunge

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Posted February 13 2001 - 04:36 AM

Just as a FYI, here's some more information about ACI's new center which was posted in the official ACI forum over at SMR:

"Progress is being made on the new speakers, however not as quickly as Mike
would like. The hard part is the power amplifier and the pre-amplifier for the dual
6.5" woofers. Powering the woofers will result in several advantages over passive
systems.

The system will have incredible dynamic range. Since your existing center channel
amp will essentially be working only above about 200Hz, power demands are
reduced.

The internal pre-amplifier contains the usual with a twist. Two twists to be
exact! A two-band parametric equalizer will allow the user to precisely tune the
woofer section to the acoustic environment it is placed in. Center channel in a
cabinet above or below the screen? No problem, tune the system for flat
response. There will be a control panel on the front of the speaker to make for
easy adjustments.

Mike and company are doing everything they can to make this the very best
speaker it can be. Their goal is to set a new standard. The matching left and
right speakers will use the same bass-mid and tweeter, but the cabinet will be a
tall, slender tower. The dual 6 1/2" bass units will be replaced by a single
long-throw 10" sub.

Stay tuned."

Brian
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#18 of 22 OFFLINE   Patrick Sun

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Posted February 13 2001 - 02:37 PM

I'm thinking about using the Vifa P17WJ-00-04 (4 ohm) driver. I would use 2 of them in the D'appolito design (in my case, the W-(T/M)-W configuration).

The drawback is that they are polyconed, and this might color the sound due to my midrange made out of a paper cone. Plus I would probably have to increase the size of enclosure a little more (vs. using the paper cone version). I am wondering if anyone has some suggestions in creating a 2nd order filter to get the bass cutover frequency around 400-500Hz using 2 of these P17WJ-00-04's connected in series (for a nominal 8 ohm load in keep with my tweeter and midrange drivers). I'd be happy with the low end F3 of around 60hz, if that's achievable using 2 of the Vifa woofers in this center channel speaker design.

FYI, my main speakers have paper cone woofer (10" Accordian Edge Eminence), so that's the concern about using the polycone version of the Vifa's. You can't buy the paper cone version M17WG-09's anymore from websites.

Anyone want to take a stab at my design problem? Anyone know of other paper cone woofers in the 6.5" diameter form factor that would work for my application?

So...where should I begin to get the bass crossover 2nd order values? I see that have a crossover calculator on the Audio Labs main page (but I was wondering if I needed to do some compensation for the woofers as I target the F3 at 500Hz) which gives me some preliminary values for difference filters (this is not taking into account the voice coil inductance of the 2 woofer drivers):

Linkwitz-Riley: L=5.09mH, C=19.90uF
Bessel: L=4.41mH, C=22.80uF
Butterworth: L=3.6mH, C=28.13uF
Chebychev: L=2.55mH, C=39.80uF


But I'm not sure how to model the rest of the woofer load in the network to keep it smoothly integrated with the midrange network. Do the specs for this woofer driver help?

I have a link to many Vifa drivers here .

The specs for the P17WJ-00-04 are:

SPL=88dB
P=70W
Z=4 ohms
Re=3 ohms
Le=.33 mH
Ce=.33 uF
Fs=37Hz
Vas=42 liters
Qms=?
Qes=?
Qts=.28
Xmax=+/- 4mm

The original bass crossover from my main speakers looks to be a Chebychev 2nd order with F3 at around 400Hz.

Using the F3=400Hz, the other LC values are:

Linkwitz-Riley: L=6.37mH, C=24.88uF
Bessel: L=5.51mH, C=28.50uF
Butterworth: L=4.5mH, C=35.16uF
Chebychev: L=3.18mH, C=49.75uF

---

The only enclosure requirement is to keep the total height of the box around 10" (I've resigned myself that that's the minimum height I will need to mount both the tweeter and midrange vertically and still have 3/4" on the top and bottom layers of the enclosure. If the enclosure is 2 feet wide, that's okay, and a foot deep, I could live with the form factor, and I plan on stuffing it with polyfill if it keep the internal volume down.


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#19 of 22 OFFLINE   Patrick Sun

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Posted February 15 2001 - 01:44 AM

Here's an update:

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).

I've decided to use the Vifa P17WJ-00-08's (but I may change my mind and use either the P17SJ-00-08 or go down to the 4 ohm version of the P17WJ's and wired the woofers in series, the 8 ohm drivers would be wired up in parallel).

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 think I may have to re-do the calculations for the other driver if I decide to go with the other driver.)


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#20 of 22 OFFLINE   Brian Bunge

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Posted February 15 2001 - 02:58 AM

Pat,

That sounds pretty good! I have one question for you, though. What ever happened with the retuning of Sunosub II? Did you ever finish it? If so, what did you end up going with and how does it sound?

Thanks,

Brian
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