IntroductionThe Dayton Reference series of drivers may be the best value available at this time due to their low cost relative to their excellent performance. A popular combination of drivers is the use of the RS180 7" aluminum midbass driver along with the RS28a 1" dome tweeter in an MTM configuration. There are already several different crossover designs for this driver configuration and I've added a couple more based on a request from someone a while back. All of these existing designs are 2 way MTM designs including the Dr. K MTM, Natalie P., Modula MTM and DB. I have created a 2 way MTM crossover for these drivers as well as a 2.5 way MTM version which has some advantages and is the only 2.5 way version for this driver configuration that I know about. The main advantage of the 2.5 way version is there is no comb filtering between the two woofers in the top end of its passband since the lower woofer is filtered below 1kHz. This means that when used vertically you will have a more uniform vertical lobing pattern with fewer dips in the power response. Because of this 2.5 ways can be used horizontally as a center channel speaker and you will only have to deal with the crossover phase interactions as you move off axis horizontally. This still isn't an ideal center channel but it works much better than a 2 way MTM with all of its comb filtering problems.
All of these designs will be represented using Speaker Workshop simulations using measured response files that I found on the net. All of the designs use the same measurements to derive the response so even if the final response doesn't perfectly match the actual response of the design, the relative differences between the designs will still hold true. So if you compare the different crossovers by considering the relative differences in the response it will give you a good idea of how they sound relative to each other.
All of these designs assume that the cabinet design matches that for the Dr. K MTM project presented in the PE Project Showcase including the driver locations. Some minor modifications are acceptable to still get good results using these crossover designs. The tweeter can be centered if desired and the baffle can be slightly narrower or wider with minimal affect on the performance of the crossover as long as you don't go too far (1" max). The baffle can be extended to the floor to make a tower speaker but may result in a slightly warmer midrange due to the extended baffle for the bottom woofer. The design can be made ported or sealed, it will only change the bass response. A sealed enclosure usually requires less volume but has a higher -3dB frequency.
Cabinet DesignThe cabinet that was used to model all of the crossover designs was the one used for the Dr. K MTM project located in the Parts Express Project Showcase. This cabinet is a 1.0 cu. ft. finished cabinet available from PE. The cabinet is 9" wide by 22" tall by 13.5" deep. The tweeter is offset horizontally by about 0.75" and is centered vertically on the baffle. It is best to keep the RS180 drivers as close to the tweeter as possible. These cabinets give roughly 16 liters per driver in volume and are ported using a 2.5" diameter by 7" long port located on the back, opposite the tweeter which gives an f3 around 40 Hz. The measurements shown indicate the response for a sealed enclosure of unknown volume so the bass extension for a vented version will be much better and closer to flat down to below 50 Hz. If you want to alter the cabinet I would recommend that you leave the baffle width the same or if you want to change it, widen or narrow it by less than an inch total. Larger volume cabinets will yield a lower f3 but will reduce the power handling in the bass region due to driver excursion issues. There are some recommended enclosure sizes in the PE writeup for the Dr. K project and I'll include the section below...
Since I figure this will be popular as a tower, I went ahead and modeled a couple of floor-standing variations just to give people an idea of the alternatives. One is a “standard” alignment that has good bass extension but still a relatively compact size. This is 1.2 cu. ft., tuned to about 38 Hz via a 3" x 8.5" port. Using 3/4" MDF, this would translate into a cabinet of roughly 9" width, 40" height, and 9" depth, externally. A larger “extended bass” version of the tower would be approximately 1.7 cu. ft., tuned to 35 Hz via a 3" x 6-1/2" port. Again using 3/4" MDF, this cabinet would measure roughly 9" wide, 40" tall, and 12" deep. Note for these larger cabinets, a 3" diameter port is preferred, and a 4" port is an option if you are going to be running the speakers at high power.
All of these crossovers will work with any of the cabinet designs described above, the main difference will be in the bass extension.
Crossover DesignsIn this section I will present the crossover schematic, frequency response and impedance response for several different crossover versions that can be used with the RS180/RS28 MTM cabinet. If you wanted to do a TMM design then I strongly recommend that you stick with the 2.5 way crossover version because there will be less phase problems between the bottom woofer and the tweeter. The 2.5 way version has the advantage since the bottom woofer is filtered below 1kHz the distance to the tweeter doesn't matter unlike the regular 2 way designs which have to get good phase alignment with the tweeter and can't be done if the midwoofers are at different distances from the tweeter (and you can't follow the rule of thumb of keeping the bottom woofer within 1 wavelength of the crossover frequency from the tweeter).
The designs that will be presented include my "RJBAudio" version, a 2.5 way RJBAudio version, the original Dr. K MTM, Jon Marsh's Modula MTM, Dave Brown's dB version and the Natalie. Individual frequency response and impedance response plots will be provided for each design as well as a crossover overview chart that lets you compare the responses of the different design using your mouse and an interactive overlaid plot.
RJB Audio RS180 MTM two way designThe first crossover design I will present is the first one that I came up with when somebody wondered if the original Dr. K MTM crossover could be improved upon. I tried to go for a slightly warmer midrange because every aluminum cone driver I've worked with so far sounded a little bit better to me if the top end of the midwoofer response was attenuated slightly and the original Dr. K MTM actually has a slight peak there. I also tried to flatten and extend the tweeter response by using a notch filter and it works quite well but causes the impedance to dip slightly below 4 ohms above 10kHz. The crossover schematic and response plots are shown below.
This design has been built by at least one person and that person said that he preferred the sound of my version over the other versions he tried including the Natalie and original Dr. K but I think he preferred the Natalie over the Dr. K.
RJB Audio RS180 MTM 2.5 way designI came up with the 2.5 way version on request as well. The 2.5 way version has some advantages over any of the 2 way versions. The 2.5 way version can be used with a TMM driver arrangement and can even be used as a center channel because you don't have to worry about comb filtering between the two woofers. Use as a center channel still isn't as ideal as having a vertically arranged tweeter/midrange combination because you will still get some horizontal lobing due to the phase interactions between the tweeter and the midwoofer (but since the 0.5 woofer is filtered below 1kHz it won't interfere with the response). Even as a vertical speaker the 2.5 way version has advantages because the vertical power response will have fewer off axis dips that would normally arise due to comb filtering. Instead the 2.5 way version has a much more uniform (but not symmetrical) vertical power response. The 2.5 way version is shown below.
I don't have many reviews of the sound of this version but somebody was able to listen to a single channel and liked it based on initial impressions but said that it was a bit warmer sounding than my 2 way version.
Dr. K MTM designI think that the original Dr. K MTM design was what got this RS180 MTM craze started. This design is probably the simplest of the bunch but doesn't provide the flattest response. From the responses I've received my versions and the Natalie versions are generally preferred over the Dr. K crossover sound. The details are shown below.
Jon Marsh's Modula MTM designJon Marsh is a big fan of steep slope cauer-elliptic type filters when working with metal drivers and came up with this design for the RS180/RS28a combo. The main complaint of this design is the huge number of parts required which can put the price of crossover components well beyond the price of drivers. The response has a bit of extra tweeter attenuation compared to some of the flatter designs but I haven't heard any specific reviews relative to some of the other versions. The crossover and response details are below.
Dave Brown's dB MTM designI don't know much about dB's design but I did "steal" his idea of using a notch filter on the RS28 to get a flatter, more extended top end response. The main issue with this crossover is that the tweeter level is a bit low and it uses quite a few parts. The details are below.
Natalie MTM designPlease forgive me for not remembering who designed the Natalie version for these drivers but I would have to say that it is one of the most interesting crossover designs I've seen and it has a very nice looking response. The crossover is a sort of mutated series crossover that is like nothing I've ever seen before but the response looks great and the parts count isn't too bad. The details are shown below.
Crossover Response ComparisonI will present the frequency response including the response of the RS180 drivers and the RS28a driver as well for all of the crossover designs presented above on a single plot and allow you to select which one you want to view for a quick comparison.
So there is a summary of the performance of these different crossover designs. Based on the responses and my limited experience with these drivers my favorite crossover would probably be my 2.5 way version because it has some advantages that the others don't and has a nice looking response. Although not an ideal center channel the 2.5 way version will work better than the 2 way version when the speaker is placed horizontally. Also the 2.5 way version is the only one that can be used in a TMM configuration all of the others must be MTM designs. These responses might be a little bit off relative to the response that you might get with your actual drivers but the relative differences between the crossovers are accurate so this comparison will tell you where one crossover might be a bit stronger than another in a certain frequency range. I hope this analysis makes your choice of crossover for these excellent drivers a bit easier and I would appreciate some feedback if you get a chance to try one of my versions relative to some of the others (my email is under the "About Me" section).