Idea: low-pro MMTMM center channel, RS100/RS28A

[fusseli]

The idea is a low profile horizontal MMTMM center, that would match to bigger L/R mains based on the RS line.

I've never tried a 2.5-way before, so this is my first run at it. Frankly I'm not sure if this is the best way to model it in PCD. Using the baffle below and planning for a horizontal MMTMM (using Zaphs measurements), driver CTC spacing suggests an Xover point to the tweet at 1500Hz or lower. The outter pair of RS100s must not play above 800Hz or so.

23"x5.5"x12" (~.6cuft with 1/2" mdf) sealed and stuffed cab is looking good for an HPF at 80Hz, perfect for a center channel. A little bigger & vented and it can go lower but that's not really necessary.

[fusseli]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

Initial run at an Xover, few components, both horizontal and vertical off-axis look awesome as the power response might suggest. Currently doing a parallel 3-way config in PCD, with just an LPF on the midrange. Is this the correct way to do a 2.5-way such as this?

[joeybutts]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

Very cool. I have nothing to contribute other than I love liitle speakers in multiples.

[kmibb]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

driver CTC spacing suggests an Xover point to the tweet at 1500Hz or lower.

That doesn't seem accurate, I would think higher. You're using the distance between the center of the tweeter and center of the inner-most midwoof, right?

[fusseli]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

No, CTC for the inner pair of woofers since this will be horizontal (8.5" CTC --> 1500Hz). The upper extension of the pair of woofers is the limiting factor, not the CTC with respect to the tweet. Low extension of the tweet is definitely mandatory. The 4.25" CTC to the tweet would be in the 3kHz range... not real sure how to consider that.

Oh ya, factoring 3dB of BSC right now.

Also... impedance is looking real low, maybe 3 ohms nominal. May try series/parallel combo of the RS100-4 instead of all parallel RS100-8. Although crazy sensitivity in the 93dB range would be cool haha

Using RS100-4 the box size requirement is smaller, could drop down to a smaller vented config maybe just 8" deep (.4cuft)

[DanP]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

Initial run at an Xover, few components, both horizontal and vertical off-axis look awesome as the power response might suggest. Currently doing a parallel 3-way config in PCD, with just an LPF on the midrange. Is this the correct way to do a 2.5-way such as this?

There is also an option in PCD for a cascaded 2.5 where the woofers would utilize the midrange lowpass with an additional (cascaded) circuit to drop the passband even lower.

Using multiple small drivers in a 2.5 and a low reaching tweeter is a good way to get a capable, very low profile center channel. You should definitely build this.

Dan

[fusseli]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

Got it, figured out the 2.5-way in PCD. Here's my official first crack at the Xover RS28A is acoustic LR4 at 1200Hz and just 7 total Xover components. Overlay shown is 30deg off-axis horizontal. In RM I did .1cuft per woofer tuned to 65Hz.

[GranteedEV]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

A 2.5 way horizontal center speaker doesn't make sense to me. That's 6db of BSC (already a lot more than usual), plus augmentation off the TV / TV stand.

I would cross the RS28 lower as well, to keep the the horizontal dispersion wider (remember your two ears will recognize horizontal phase/lobing etc instantly from a center directly in front of you..). I hate to say this, but you're going to have some definite issues voicing this. Vertically it could work very well though.

[Pete Schumacher]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

A 2.5 way horizontal center speaker doesn't make sense to me. That's 6db of BSC (already a lot more than usual), plus augmentation off the TV / TV stand.

I would cross the RS28 lower as well, to keep the the horizontal dispersion wider (remember your two ears will recognize horizontal phase/lobing etc instantly from a center directly in front of you..). I hate to say this, but you're going to have some definite issues voicing this. Vertically it could work very well though.

2.5-way does not mean 6dB BSC. You can still dial in any amount you want.

2.5-way means the outer woofers roll off before the inner woofers. The amount of overall BSC is still up to the designer.

[fusseli]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

Horizontal orientation still works as long as the roll-offs are appropriate for the driver spacing. That's the whole point of the 2.5-way, the outter pair of woofers are too far apart to play high, so they get extra roll-off. There is no lobing issue in this initial Xover and horizontal off-axis looks pretty good.

[rickyj]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

For closer spacing use the RS100T with truncated frame. By the way I like the concept it will be similiar to the Cynosure in the project showcase.

[fusseli]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

For closer spacing use the RS100T with truncated frame. By the way I like the concept it will be similiar to the Cynosure in the project showcase.

The cynosure (2.5.5-way) was kind of an inspiration, yes! I will see if using the truncated frames and RS28 faceplate will make much of a difference.

Argh. so only the RS100-8 comes with a truncated frame, not the RS100-4

[fusseli]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

Well here's another whack using the truncated frame drivers. A bit of an improvement and looking good off-axis. Overlay is 30deg off the horizontal.

[Wolf]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

Do you want a 4 ohm center channel?

Normally we recommend 8 ohm centers, and this is perferrable for most center channels, as amps are rated for 8 ohm impedances for center channels most commonly. If you used the T-8's instead, also note their specs are a lot different from the normal 8 ohm RS100's. I think a line of trunkies will be beneficial in this instance.

As to xover points, the CTC from closest mid to tweeter is the minimum or highest point you can xover. Not CTC of midranges. In the case of a center channel, the lower the better to make the lobe and horizontal dispersion larger, most typically approaching 1kHz for the larger format varieties. Since you are using a smaller center format, your xover point is probably fine, and 2kHz may even be doable if you wish.

Now- your response is very peaky. Try a 1-3 ohm resistor in series with C2, and/or maybe decrease C2's value, as that is a bit large. This will help droop the knee and flatten out the response. 'Tanking' it would also be a probable idea to nuke any breakup that would otherwise come through.

The RS28A is not as deep a reacher as the F, and may have issues with a 1.2kHz LR4. I recommend moving to 1.5kHz, personally, and also increasing one electrical order on the tweeter to make the phase line up easier.

As to the 0.5 sector, Part of the reason for the peaking response there is the large value coil, and the highish Fs of the RS100 being a smaller driver. If you want to not use full-comp, and minimize this issue, then a 20-30 ohm resistor across the 0.5 midbasses can minimize the effect of the Fs, smooth out the response a bit, and take a bit of sensitivity off of them to not compensate for full 6dB of BS. You can also likely remove the cap on the 0.5 section.

Otherwise, you're off to a good start!
Later,
Wolf

[diy speaker guy]

Re: Idea: low-pro MMTMM center channel, RS100/RS28A

n this instance.

As to xover points, the CTC from closest mid to tweeter is the minimum or highest point you can xover. Not CTC of midranges.

Why?

In PCD, the further you move the mids apart the worse the power response gets. Since they are both playing the same frequencies they interfere with each other.

Am I missing something?