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Discussion in 'DIY Speakers and Subwoofers' started by Matt Grant, Feb 10, 2014.
To bad about the veneer Matt. Ouch.
Did you find those resistors got hot?
I'll replace it with real wood veneer at some point which should be far more durable and hopefully look nicer too if I can get it cut nicely in the existing driver cutouts.
Yeah those resistors got really hot, I started with a 10w in there and started adding more once I noticed how hot it was getting at moderate volumes. Even with all 80w of resistors in there they could still get uncomfortably hot at high playback levels, placing them in front of the port helps but only if there is low bass in the content being played back ;D. I should add this was off just a regular 100w/channel AVR.
The switch to first order took care of that problem, now they don't even get warm. The midranges move a little more without that extra LF filtering but not enough to matter. Now I have no problem spec'ing a pair of 10w resistors in parallel for 2.5 ohm value it uses.
Sorry to hear about the veneer, Matt. I did like the looks of the cherry (partial to redheads) With that much volume, What was your port tuning? How soon can I get the plans for the MTMWW. Since I have 8 of the SD215s is it possible to do a twin isobaric ported design with a nominal 16 ohm bottom end and still have enough sensitivity?
Will be watching with anticipation.
I updated the post on the previous page with more info.
Port tuning is roughly 28 Hz, but I don't have a working WT3 right now to verify, mine broke
I don't think a twin isobaric would work well, it would need a new crossover if in fact the woofers will play smoothly up to ~400hz. Are you looking to use a smaller enclosure? Because the isobaric alignment does not give any increase in output.
The midrange HP on a 3 way is always tough. I usually end up with 1st order electrical with some kind of impedance correction like an LCR or LR. But it's always tempting to go 3rd order.
You were finding the higher order HP created an impedance drop in the midrange?
Yeah at least with the 2nd order HP and transfer function I was trying to get for a flat FR in the lower midrange. FR and phase are almost identical between the 2nd and 1st order filters the only differences being the impedance (I attached a graph showing the difference there) and a little more excursion from the drivers. The dual mids can handle the 1st order HP no problem and barely start moving when cranked. I'll have to see how well the single mid fares in the WTMW and TMW designs when I build the crossover for those.
As I posted earlier, I have 8 of the older SD215-88s sitting on the shelf, so a pair of twin isobaric boxes IS possible. Since I just recently got my Marantz 2252b rebuilt :
and it has outputs for speakers A and B (4 ch) I will be using just 2 drivers per, so isobaric has been discarded. I will be using a larger box ext. dims. of 12w x 18d x 48h should let me tune it down to 24 Hz or so.
That's a nice receiver.
That enclosure is pretty big, make sure you use a decent amount of damping material in there to cut down on standing waves. I used a bunch of the recycled cotton insulation in mine, the bottom where the port is located was empty but I used a decent amount behind and to the sides of the woofers, layered some between the woofers and also placed a layer of the r-13 between the top woofer and empty top section of the enclosure.
On a side note I have updated the WTMW center post (reply #4) with updated crossover info.
[font=verdana, arial, helvetica, sans-serif]Hi,[/font]
[font=verdana, arial, helvetica, sans-serif]I just found the Nexus Designs and joined. I'm excited about using the TMWW vented tower version in my existing 2.22 ft3 mdf cabs.[/font]
[font=verdana, arial, helvetica, sans-serif]Inner dimensions are 41.75x8.75x10.5. 5/8 walls are less than ideal, but I will brace/deaden where possible. I will have a thicker 43x10 front baffle custom cut/routered for the new drivers and also make a small inner enclosure for the M.[/font]
[font=verdana, arial, helvetica, sans-serif]I'm limited to having these right against a back wall and right aside my 42" TV. The port will have to be front firing.[/font]
[font=verdana, arial, helvetica, sans-serif]Given my cab and placement specs, I'd like some guidance/suggestions on driver placement, FS tuning, & port dimensions/placement. Also any suggestions on where to get or who can make my custom baffles would be great. I am in central MA.[/font]
Welcome to the Fun.
Your Cabinet dimensions are almost perfect for a single woofer in an SC4 alignment Ported box tuned to about 28Hz, but would be undersized for 2 woofers. However your placement up against the wall will help re-enforce the bottom end. If you will be using these in a smaller room, you will also get some boost from room gain as well.
Placing the woofer closer to the floor will also boost the bottom end, at the expense of some lower mid bass. I would put the woofer center about 18" down from the top of the cab. This puts them about 20" off the floor, enough to get some help from the floor, but also up high enough to get a good solid kick drum.
As for the port, I would use 3" Precision Port Flared at both ends that is 9" long. Assuming a 2 cu ft net. i.e. after volumes for the mid enclosure, woofers, and port are subtracted. This tunes your enclosure to between 28 and 29Hz and keeps port velocity down to 18m/s @ 35w of input. Cone excursion is exceeded below 22.5 Hz.
Hope this helps.
Thanks for the help, it's appreciated. I'm a bit confused as to why one 2 ft3 cab is undersized for 2 of the SD215A-88's. The PE recommended vented enclosure for that driver is 1 ft3 per woofer. Unless I misunderstood, MtG90's design parameters say that 2 of the SD215-88's will work in 1.5-3 ft3 vented enclosures.
Also, I have no way to accurately design crossover networks or test/model driver performance, so I need to use an established design. I think I will stick with the TMWW for my cabs even though it is less than optimum. To minimize port velocity, I could use a 4" port and have it bottom vent if I raise the cabs off the floor. What would be a good height to accomplish this? I'm not limited vertically.
For a quick and dirty ( does not include the volume taken up by the driver and port ) , I use this to get a rough idea of required box size ...
Make sure you have JAVA working and all you really need to input is Vas , Qts and Fs.
I usually go for an SB4 alignment as it is USUALLY the flattest response.. for a slightly lower tune the QB3 alignment may ( depending on Qts ) give you a slightly lower F3 at the expense of a slightly larger box.
for a smaller ported box, try the SBB4 alignment, but it will give you a midbass hump, sometimes severe depending on the Qts of the driver.
Qtc is used to adjust the size of a sealed box. flattest response is usually around 0.7 but if you want a smaller box you can go as high as .9 to 1.0
My bad. I will be using the older SD215-88, not the SD215-88a. The newer model has a lower Qts so will not require the larger box. The newer model will not reach quite as low as the older model, but is still a good value.
No worries. I appreciate your knowledge and taking the time to do the calcs. I think the revised the design for those subs is a decent trade-off. Even revised they require a substantial volume.
From the description, the MTMWW sounds like it is a "full baffle step and then some" design. If I were to build these they would be placed near the back wall a somewhat pushed into the corners. If I wanted to adjust the baffle step, would it be as simple as removing the resister on the tweeter and adjusting the one on the mids, or would that not work here? I know there isn't much to work with here as the tweeter only has a 1ohm resisiter, but a little adjustment would be better than none. Thanks!
I tried not getting into full baffle step, it's really closer to 4dB above 150hz but the close coupling of the woofers to the floor slowly transitions them to 2pi under that.
You don't gain much at all pulling the resistor off the tweeter, you only gain about 0.5dB above 5k and are left with a dip between that at 1k where the midrange response rises when reducing the resistance there. You just can't get much more from the tweeter.
Your best bet would actually be to stick 1-4ohms of resistance in between the woofers and crossover, this will pull down their response roughly 1-5dB. You'll want 2-4 20w resistors to do this (PE#017-4). It's not idea but it's the only decent solution besides external EQ. Start without any resistors just to try, it may not be too bad. If it does end up too bottom heavy in your space then you can use four 4ohm resistors in parallel, still too much try just two in parallel. Finally try them in series/parallel for the full 4 ohms.
Great Info. Also good to know that these aren't "full baffle step". Thanks!!!
I finally have something to report on my build.... I finally pulled the trigger on drivers, but not the ones you would expect for my mids...
I did a LOT of modeling with PCD, and Xsim and decided to use the Dayton DC130BS-4 for my mids as they modeled better than the DS135s.
I will post a build thread both here and over at TT if and when I ever get around to actually building them...
The problem is going to be a high parts count/cost for the XOs but it does model to +- 1db from 60HZ to 20k with a minor -1.8db +1.5db blip in the top octave.
Would side-mounting the woofers in the MTMWW config require a complete redesign? I would narrow the front baffles, which I would have to recalculate baffle step compensation for the mids/tweeter, correct? Also, because the woofers would be side-facing, I'd have to consider their low end cross-over frequency? From what I understand, side-mounting woofers can really only go as high as 100Hz due to point-source issues?
Yes, the crossover between the mid and woofers is about 450hz in the center and tower designs, that is too high for side woofer placement without major crossover alteration.
Note I have updated the MTMWW tower crossover to my latest revision, see post #5 for the newest info.
Component changes are mostly to the midrange though there are some adjustments to the LF and HF filter values. This update basically fixes issues with the midrange balance I had been having. I made these changes a while back but never got around to updating things on this thread until now.
Have you revisited the TMW "Snowman" crossover? You noted an issue you heard but have not posted the crossover schematic as yet. I am pretty interested in starting these in the spring for a full 7.2 setup, but for space reasons like the TMW better than the MTMWW towers.
Really interesting project, great value in the drivers!
Wow, I just found this thread. These speakers look like a fantastic value. Parts seem to be readily available, but it doesn't look like kits are available from DIYSG anymore.
What could one expect for a reasonable maximum output level on these? I'm strongly considering building a pair of the MTMWW towers and the WMTW center, but I didn't see driver spacing or recommended enclosure volumes.
Sorry I missed your question,
I never really finished the TMW design as I was rather disappointed in the performance of that one because it really did not do anything better then the MTM design while costing more and having a more complex crossover.
Bass output from a pair of the towers will get you above 115dB with decent power behind them (>100w per channel), but you can expect output from a single speaker to approach 110dB with 100w or so. I have driven them with a 400w/channel amp up to where it tickled the clip light every once and a while with fairly high dynamic range content (not highly compressed stuff) and they managed to do very well with that kind of power, though sustained playback at that level is not advisable.
Driver spacing is not critical but should be fairly tight, I don't remember the exact spacing off hand.
Enclosure volume for a pair of woofers should be about 1.0-1.4 cuft sealed, 1.6-3.0cuft ported tuned between 28-32hz.