LithMTM

Background:

The Lithium design is a really small monitor that sounds a lot bigger than it has any right to, and even though it is not the lowest cost design around, it is still pretty reasonable. Once it was out for a while, interest developed for an MTM version. I have never done an MTM before - generally speaking, whenever I have contemplated doing an MTM, I have always wound up going a different direction (either bigger, more expensive single woofer or dedicated midrange and tweeter). In this case, I can see a clear case for the MTM. The Dayton ND-105 is impressive to be sure, but it is a 4" woofer trying to operate down into the 40's. Having a pair of these gives the displacement of a high excursion 6" woofer while retaining the attractive dispersion characteristics of a 4" midwoofer.

Cabinet Design:

The original Lithium shared cabinet dimensions with Paul Carmody's Overnight Sensations design, so the MTM has the same dimensions as the OS MTM's (which is the same as the Nano Neo cabinet). The cabinet is 15" tall by 6" wide by 10.5" tall and made using 3/4" MDF. DIYSoundGroup.com sells the OS MTM cabinet for less than $25/each, so that is a good option if you don't want to cut your own or want that CNC precision fit. I discarded the front baffle and made by own with different cutouts to accommodate the drivers (Dayton ND105-4 and Vifa BC25SC06-04). The 1.5" by 4" flared port sold at Parts Express tunes this cabinet to 50 Hz (a few Hz higher than the Lithium, but in the same ballpark). I located the port behind one of the woofers (the cabinet's brace interfered with the preferred location behind the tweeter and I would rather keep the rigidity that the brace provides.) The interior is lined with R-8 Ultratouch Denim fiber insulation affixed to the cabinet by 3M 77 spray adhesive.

Crossover Design:

The Lithium crossover did a good job beating the ND-105 high frequency output into submission, so I wanted to retain similar performance for the MTM version. I also wanted to keep a similar crossover point and slopes for both drivers. Like the Lithium, I used a 4th order electrical low pass on the ND-105 with a resistor on the first shunt leg. Likewise, I have retained a second order electrical high pass on the Vifa Tweeter with a shaping resistor on the shunt leg and a two resistor L-pad.

Initial Impressions:

Like the Lithium, the LithMTM is clean and resolving. The MTM version does sound more dynamic and, predictably, can deliver more SPL without strain. With the woofers aligned vertically, tonal balance is optimal with the tweeter at or near ear level. Part of the point of an MTM is to minimize floor/ceiling reflections - so none of this is a real surprise. Unfortunately, this also means that if you use the LithMTM horizontally as a center channel, there are limits to how far off axis you can be and still get good sound quality. We're not cheating any laws of Physics here. The original design thread, with some comments by others was posted to Parts Express Tech Talk. Another builder used the design in a soundbar project.