MTM or TMM

xschop

I'm going to embark on another musical mod journey and will build either of the two and would like to hear opinions and experiences with both platforms from you knowlegeable folks....
Good, bad, ugly....
Let's hear it....

Schurkey

MTM is supposed to mimic a point-source better than TMM.

Makes for good ad copy, anyway.

xschop

+1 for TMM
Not into gimmicks

JPete

I think MTM looks better. I don't have any though.

Have you decided on driver size and types yet?

heiney9

MTM is hardly a gimmick, but just because there is a tweeter between two mids doesn't mean you will get the benefits of something like a D'Appolito array.

Does anyone actually do any sort of research before they start making assumptions?

A loudspeaker configuration developed by and named for Joe D’Appolito, in which a high frequency driver, or tweeter, is positioned between two midrange or low frequency drivers that each cover the same frequency range. Depending on the exact implementation the speakers can be positioned with a vertical and/or horizontal orientation. In either case the two midrange drivers serve a couple of purposes: they combine to create a larger effective woofer or midrange driver size, and they also serve to control the dispersion of the tweeter. The tweeter’s output is somewhat corralled or contained by the sound coming from the midrange drivers in a similar way to how two parallel surfaces control dispersion. There are some variations on the design where two same sized woofer/midrange drivers may cover slightly different frequency ranges, however those aren’t considered true D’Appolito designs.

The D’Appolito design specifies a third order crossover network. The tweeter is coordinated with the woofer so that at the selected crossover frequency, the drivers all have similar horizontal dispersion. (This is not easily accomplished because many drivers behave badly at the extremes of their range.)

The advantage of doing it all correctly is one of the most seamless blending of drivers possible. The result is an absence of any sudden change in directivity with frequency. This may not mean much for monitors where there is a limited listening area, but in a typical room where a large percentage of the sound is reflected by the room, the effect is dramatic.


He's a very quick overview. The D'Appolito array is highly regarded. Some of you amatuers just want to do this and that and mix this with that and then you are some kind of expert.

Speaker design is a science that takes lots of understanding and critical measurements to replicate and get right. Adding billet or swapping unknown tweeters, or adding foam strips into a cabinet does not make one a speaker expert.

Heck, I'm no an expert, but I know enough to research something so I can reasonably understand why and what is going on.

heiney9

I tend to shy away for Wiki pages, but this one has some good info.

https://en.wikipedia.org/wiki/Midwoofer-tweeter-midwoofer

heiney9

Bottom line is there are excellent examples of both types. There is no "one type" fits all. Depends on what your goal is.

xschop

I've read that...
+1 for TMM as I'm no expert on crossover design. I'm taking advice from experts here.
Even Mr. Polk was an experimenter at one time.
To answer JPete, I'll be using the same SL-5000 tweeter. MW is up in the air at this point until I get a response from the 5jr crossover thread.

heiney9

xschop wrote: »

Even Mr. Polk was an experimenter at one time.

With a degree from The Johns Hopkins University and looking to start a speaker company .

xschop

Mine is not from JHU, so does that exclude me from speaker building lol?

Can you answer my question in the 5jr crossover thread. I'll repost it here if need be.

xschop

My donor boxes showed up from fellow member. Looks like 1.1 cu. ft total volume to work with.
TMM looks fairly straightforward, from my reading, I gather that a TMM configuration can use a simple Two-way crossover. So I figure I'll mod a 5jr X-over to fit the enclosures, but I'll have to hunt down 8-ohm MW's and run them parallel as I confirmed from my more experienced speaker building friend aka nerd.

xschop

Well, I didn't want to mill off the edges of 4ea perfect working MW6502's, so I got new truncated Peerless drivers. I had to enlarge hole diameters by only 4mm and they drop right in. The old SL1000 tweets fit too so the RDO-194/8 is plausible....
I'm waiting on my 5.5mH inductors to show up to mod the X-overs and wire these puppies up. I'll also plug the small front port hole as the Bass-port in rear will suffice.

Dennis Gardner

Nice solution to the odd fit!

pkquat

I think the SL1000 plate is the same size as the polk Peerless tweeters, and the SL2000 and up / RDO plates are slightly larger.

Which Peerless drivers did you choose? How do the specs and price compare to replacement polk MW6502s?

xschop

Peerless 830657. I chose them because they are treated paper but 8 ohm so I can run them in parallel and the lower woofer will be crossed over at 150-200 hz.
I am listening to one right now and the bass hits you deep in the chest standing 5 ft away...amazing
I don't hear the cross-talk in midrange that various sites mention although I'm only using about half my 80 wpc and these are amazingly loud, crisp. The SL1000 is still twangy lol.

xschop

Still waiting on my Inductors to show. In the meantime I'm spinning out a couple of these in billet....

Polkaguy58

I like being an "amatuer" myself and experimenting with speakers is one of my pure pleasures.
More often than not, they don't yield the desired result, but I still have fun reguardless.
Sorry, but I don't subscribe to the "Speaker building is an exact science" way of thinking.
I've created my share of great sounding speakers, made out of whatever happened to be in my closet and borrowed ideas from earlier designs that I'd observed.
It's always been the thrill of the hunt for me.

xschop

In my short audio journey, I've discovered that speaker building is just like Science in which I've spent most my life. Most subscribe to a populous theory and are on that bandwagon in fear of rippling the waters and alienating the bretheren. In reality, to their detriment have only stunted their own growth and enrichment, accomplishing nothing.

As to this set of transducers with a 5jr crossover, sound like 5jr's on testosterone.
Having them cranked up a bit more, I can hear the mids with a slightest of echo, is the only way I can describe it. From my reading the individual inductors on the lower woofers will take care of this effect.

Polkaguy58

I'm still digging the 5 Jr crossovers in my Monitor 5's !
I just had an audiophile buddy over my place the other night and he was blown away with how good they sounded in tandem with the 5 Jr rear set.
This guy's got equipment (tube and solid state) that costs many times more than my little system, so I appreciated the compliments on how well the sound room has come since his last visit.

xschop

That's good to hear. I've seen reference that the Monitor 5 crossovers cut out the lowest frequencies to the active driver.

In that respect, It seems like the audio hobby is also just like the auto hobby. I've built a few custom Porsches sub $10k that walk all over their $80k peers.

xschop

Erse Inductors showed up finally.
A question for the more versed in this area...I would like to make a phenolic spacer and mount this inductor on the X-over housing, but it looks like it will violate rule #5... (Cred goes to WMG post I found)



Will this be OK as in rule #6?

westmassguy

Yes it will, but why go with the laminated steel, and which Inductor is it replacing?

xschop

Great, thank you...again.
Lam. I-core is lower resistance in a higher inductance (> 1mH) is the consensus from various readings I've encountered. It was also good enough for the JHU graduate in the LSi9's that bear his name.... as well as the RT55's from the pics I've seen of their cramped X-overs...This site's schematic drawing link for them has never worked however.

It is not replacing, but adding a low-pass filter to the 5jr 2-way, effectively making it 2.5 way.
I also deciphered the value using a formula I found from a gentleman that has accomplished a few 2.5 way designs. It takes in account "baffle step compensation" which is new knowledge to me as well.

Again thank you.

westmassguy

Understood. Just be sure to observe polarity. The 5jr/jr+ crossover uses a 2nd order filter for the high pass, and a 1st order for the low pass. The Tweeter polarity is reversed.

xschop

Luckily the tweeter and woofer leads are long enough to reach from the 5jr crossovers. I swapped in my spare set with the 1.2 ohm resistors and it tames the sl-1000 down bearably at high volume. The Peerless MWs have extra 1/8" spades that I wired with fastons to the soldered ends of the new inductors. I can definitely hear the mids with more clarity and the lows sound more extended... More tinkering to do....Doing A-B test.....5 Jrs still win the smoothness category and the TMMs take the bass trophy....

xschop

Trying to get the last bit of crankiness out of the factory tweeters.... Prussian Blue Molecular weight 650g/mol at 4 coats..... Sounding alot better...

xschop

Ok, more Polk crossover shenanigans....
Before I order more Mills resistors to mod another set of 5jr crossovers, I wanted to run this by the forum here.....

I removed my already modded set with 1.2 ohm Mills resistors (verified by ohm meter) and threw in my un-touched set with 1.3 ohm factory resistors and the sl-1000's sound MUCH better yet.

So I was curious what 30 yrs will do to the resistors.....

The factory 1.3 ohm both read 1.6 ohm now.
I checked my later factory 1.0 ohm resistors at 1.3 ohm now.

Main question is, will running a Mills 1.6 ohm resistor be OK in the long term?

RandyCroissant

Did you add in the lead resistance?

xschop

No, I just want to try a Mills 1.6 ohm resistor with new Caps since these factory ceramic resistors are actually 1.6 ohm and make these SL1000's sound pretty good.

xschop

I went ahead and ordered the Mills 1.6 ohm resistors. Parts Exp ships thru USPS, so snail mail is fitting.

I procured a caulking tube of the Power Grab this time. It works great around the house besides speakers...
I'll install these and remove the temp. rags I had blocking the ports. I found an article that said Polk put the extra 1"x 8" front tubes in to tune out some unwanted resonances caused by the foamed polyurethane drivers. These treated paper cone Peerless are pretty close in timber to the 6502's it seems....

7075 billet is what I had on hand.....

xschop

Does anyone know where to get insulated binding posts that can go thru 1/2" aluminum plate?