MTM/Push Pull Project Started!

[dc270]

I have been pretty pleased with my last custom built speakers featured in my avatar. Something about a line arrays has me capture. Now I am on to my 2.0 project build.

These will feature 6, 6" AR mid-woofers (AR VP25's), 4 Tympany 1" silk dome twtrs. and 2 250w 12" cast frame woofers configured in a push pull design, each side.. The sub cabinets are 4 cubic foot types purchased from P.E. One identical 12" in back will co-drive the 12" in front.

I made a custom subwoofer abou a year ago utilizing the push pull design and was very impressed with its low distortion! It was the type of bass I have missed but did not know it, but discovered a review explaining it good points. Now I will combine the best of both worlds with an MTM upper and a push pull lower.

Below are some early pics. The MTM will be center mounted, time aligned on top of the sub cabinets. Should be interesting.
DC

 

[gdmoore28]

I'll be watching your project with interest!

Question: Why so many tweeters when one will suffice? Won't this arrangement create IM problems?

What kind of crossover design will you use?

GeeDeeEmm

 

[dc270]

Thanks for the reply "gdmoore28..." I want to get the sensitivity to closer match the surrounding mids. I tried using only 2 twtrs but four was a much better sonic fit for the MTM arrangement. The P/P woofers should see close to 97/98 dB 1w/1m. After hitting the network woofer inductor, then drop a dB or 2.
I will be using Bessel second order network. maybe. I am going to run more simulations on my cross over builder to make sure. It should crossover well at 150 & 5khz in a second order.
The thing I am most excited about is the P/P bass. It is now sharp, well define and with good dynamic punch.Should be a lot of fun! Baco99 sold me the 12 AR mid drivers little over a year ago and I have been itching to make another line array using them. These will let me dig deeper past the 200hz low end and roll off well to match up with the tweeters.
DC

 

[sol7]

So your P/P sub is ported? I thought they were mostly sealed designs.

 

[dc270]

I did listen to them a good while when sealed and they were good, but the ported allowed me to push them lower. After I get everything in place I will probably re-visit that again. Good observation
DC

 

[gdmoore28]

I want to get the sensitivity to closer match the surrounding mids. I tried using only 2 twtrs but four was a much better sonic fit for the MTM arrangement. The P/P woofers should see close to 97/98 dB 1w/1m. After hitting the network woofer inductor, then drop a dB or 2.

DC

Yep, I see your need for the extra output on the tweeters. If you do happen to run into IM issues using a group of four tweets in a cluster, consider simply using a single horn driver with a horn or waveguide. That would give you tons of headroom to play with on the high frequencies, and would likely be even cheaper than four tweets.

Don't mistake me for an expert, or even a slightly learned novice - that should be obvious. Just throwing out ideas for consideration. I may be totally wet on this one!

Anyway, thanks for posting your build and tolerating the comments. I always learn new things by following other folks' projects, and I'm sure your's will be no exception. Keep up the good work.

GeeDeeEmm

 

[dc270]

I do appreciate input and understand your intent as being helpful. Than you for that.

From what little bit of listening, maybe a couple of hours at variou volumes and music/tone selections from a Sanyo receiver, I am not hearing any muddiness or lack of clarity as it relates to IM. However the only network I used at the time was a single 4.7 cap to protect the twtrs. The mid/woofers played wide open. I disconnect one mid array, then the twtrs. The off axis sound stage is very good. The mids give very good realism and are as truthful as you can get. Female vocalist do not sound "chesty" or lifeless, but has good attack, even when not tamed by a network as yet. The tweeters, like the mids, have a wide foot print and when using them in this quad configuration have no "hiss" and blend pretty well to the mids. I would have thought the mids would play a bit higher, at least to 7+khz, but don't quite get there.
DC

 

[baco99]

So are you using the woofers in an isobaric configuration or in a dual woofer configuration?

 

[dc270]

Not the classic Isobaric, more like the classic M&K sub woofer config, one woofer mounted facing front, the other woofer facing into the cabinet with the back frame exposed. Where I saw it they called it a push pull design....Thanks again baco99 for the AR mid cones- they sound super nice!
DC

 

[baco99]

Not the classic Isobaric, more like the classic M&K sub woofer config, one woofer mounted facing front, the other woofer facing into the cabinet with the back frame exposed. Where I saw it they called it a push pull design....Thanks again baco99 for the AR mid cones- they sound super nice!
DC

So it's really behaving more like a bandpass.

 

[dc270]

The rear mounted woofer fires into the back of the front mounted woofer, bother woofers mounted in a 4 cu ft cabinet. The distortion is remarkably low, cleanest bass I have heard in a good while! All four 12" drivers have cast frames, remind me a lot of the EV 12" bass guitar drivers.
DC

 

[baco99]

The rear mounted woofer fires into the back of the front mounted woofer, bother woofers mounted in a 4 cu ft cabinet. The distortion is remarkably low, cleanest bass I have heard in a good while! All four 12" drivers have cast frames, remind me a lot of the EV 12" bass guitar drivers.
DC

Yes, but because of the ported enclosure between the two woofers, it's not an isobaric. It's something like a bandpass/free-air hybrid. Each woofer suspension is essentially free-air, but connected through the middle chamber. The middle chamber acts in part like an air suspension, but you've ported it, so you've tuned it to a specific frequency with a 12dB drop instead of a typical 6dB drop about an octave higher. It frankly doesn't matter what directions the woofers are facing, as long as electrically they are connected in the same phase relationship.

I wonder what would happen if you changed those ports to aperiodic openings by stuffing them.

 

[dc270]

Great observation Brian, I do plan on experimenting with the tuning as you suggested- I think it will be a lot of fun to see what I can get here! Aperodic loading is a perfect idea to try out.........
DC

 

[baco99]

The other design you might want to model, which I have noticed gives a much smoother response in a simulation than a straight bandpass is a compound bandpass where the rear chamber ports to the front chamber and the front chamber ports to the outside. The combination of tuning frequencies can often smooth out some bumps in the bandpass range.

 

[dc270]

Hmmmm will certainly keep that in mind as an option. Last year when I built my first P/P sub I did it on a lark just to see if the bass product was in fact cleaner, I was delightfully impressed. then started dreaming what else I would do to finish them and an MTM seem like the natural thing to do.
DC

 

[jeffe]

Glad to see you are still happily building/creating away Doug!!

 

[dc270]

Glad to see you are still happily building/creating away Doug!!

Now if I can fiure out what the devil I am doing it would even be better! Thanks for the encouragement Give me a driver, a receiver and termite free wood and there is no telling what I might do to ruin any musical rendition!
As long as it keeps the women off my lap, alcohol from my lips and my name off the bank account, my wife lets me go.
DC

 

[dc270]

Update,
I am still working on the finish paint work, but took a break from that today. I installed the rear mounted 12" into one of the cabinets. I wire the two 12" into parallel for a 3.5 ohm load. I listened to the sub and it sounded OK- then I remembered that the two 12's , according to the push pull design paper, are supposed to be wired out of phase with one another! That did the trick! The thing hits like a sack full of anvils....

I put one of the line arrays atop the cabinet and tried it out as well, really decent sounding. all I used was a 4uF cap to protect the twtrs. I ran the mids full out with no network. It does need a network non the les. I think I will be using a -6b/oct at 200 & 5khz.

DC

 

[dc270]

Will be working on the x-over next.........subs and mid/ Twtrs pretty well complete.
DC