For a while now, I've wondered if gains could be had by using a midrange driver that is more naturally suited to, well... being a midrange. The Lambda TD15M Apollo is a remarkable driver in many ways, but despite its curvilinear cone, is probably best crossed below 3kHz due to the off-axis response pattern - which is fundamentally limited by the size of the radiating cone.
I was after more flexibility and also the chance of using a pair of midranges in MTM around the Raal unit. In my own experience, I've generally enjoyed listening to MTM equivalents (for example, JMLab Utopia Mini vs. Micro) somewhat more... a part of this is probably due to the radiating pattern being a little closer to the ideal point source when the crossover is suitably done, in addition to the usual benefits of increased power handling.
At first, I was looking at conventional woofers. The SBAcoustics Satori range have had extremely good press... a very advanced motor system combined with a free-breathing basket and a paper cone with well-controlled breakup modes... in fact, on initial inspection it looks like it could be usable up to 10kHz! Very impressive indeed.
A couple of small flies in the ointment. The efficiency is poor, as it is not designed as a pure midrange... it's in the region of 88dB/W, and there's what looks like a very big dip at 1.2kHz. And as it's probably designed to go in a vented cabinet, that powerful neodymium magnet system results in a Qts of 0.33. I've no interest in putting my midrange in a cabinet, so this is going to be underdamped and with somewhat rolled off upper bass... a quick sim suggests a -1dB of around 200Hz free air, plus there's the baffle step on top of that... and I don't like having any significant baffle to speak of. Tricky.
Ok... so maybe these aren't the best choice for me. What else? I thought about some Jordans... they are nice sounding units in my experience, but once again, low Q and very low efficiency - a poor match on paper to the Raal ribbons.
So... something that is designed to be used free air is a good starting point. What about a planar? The B&G Neo 10s might prove to be a good fit... they had a nice write up on Zaph Audio... and over time he hasn't liked many planars - mainly due to their poor measured performance... these managed to shirk the trend.
I do not have a great past experience with planars. I did have a pair of Stryke Audio RTW2s (also known as the Silver Flute YAG20), a long time ago. They were cheap, and had a sound to match. It was smooth, sure, but incredibly muddled... when Zaph measured the Silver Flute version, it became apparent exactly why... the graphs show strong harmonic and intermodulation distortion. I had no interest in purchasing any planars after that until I'd heard some good things.
Bohlender and Graebener have had a good reputation for planars as long as I can remember. Before the new Neo family, they were probably best known for long planars such as the RD75. And I mean long... the 75 indicates 75 inches long! While the RD range are reasonably expensive, they are far from unreasonable to have a driver that covers such a wide frequency range. Their disadvantages... ferrite magnets and a steel frame makes for a fairly inefficient and heavy unit.
The Neo10s, as the name suggests, use neodymium magnets instead of ferrite. While this hasn't had much effect on the weight, due to the heavy steel frame to give the unit rigidty, it has had a considerable effect on the efficiency, In fact, the Neo10s are more efficient that the Raals. It's also had another, less desirable effect... cost. The Neo10s are around 270 euros each before tax here in europe at the time of writing.
But still... they looked promising... so I bought a quartet to try. As with the RDs, the number gives an indication towards the diaphragm size...
Based on a few implementations I'd seen, I thought I'd try a vertical arrangement... probably staggered and mirrored to reduce edge diffraction and gain a more even tonal response. To do this in a low cost manner, I decided to use the thinnest plywood feasible and mount it up using PVC waste piping. This meant a self-supported frame for a total cost of probably less than 30GBP. The piping was hot-glued to the back of the plywood, and then angled pieces were used to create a basic frame...
It isn't the last word in rigidity, but it wasn't handling the bass frequencies... the crossover is around 500Hz so the air shifting is mostly being handled by the bass unit. I've misplaced the measurements, but it was quite smooth across the critical area in the listening position, but the baffle was providing too much colouration, despite the bloom working very well in some recordings. Dipole systems do not respond well to thick baffles - you gain in rigidity, but the sheer bulk creates other problems... the ideal (for me, anyway) is to not create any early reflections.
So I reconsidered my options. Perhaps a metal frame would work better. So I went back to B&Q and got some perforated frame... this is the kind of stuff that shelving is sometimes made from... angled steel with elongated holes. Steel angle is very rigid, even in long lengths, so once the drivers are mounted, it should not need any real bracing if I keep the form narrow.
I would be able to mount the Neo10s directly to the frame, but the Raals are a little narrower. I bought some aluminium sheet and made some helper pieces to allow it to be mounted in place. Pairs of L brackets allowed the unit to stand up with reasonable stability.
I thought they looked promising, and in some ways the sound was dramatically better... the "woody" sound had gone, but they were extremely position sensitive. I'd forgotten one of the important aspects of any MTM... combing. Depending on listening position, sound from the top and bottom midrange drivers will cancel out... the wavelength dependent on the distance between the two units.
As everything here is mounted vertically, the null point is in quite an audible spot... right in the midrange. Not good. In addition to this, the extremely narrow baffle in combination with the free air mounting meant the Neo10s were starting to roll off around 1kHz... agh. That's too high, the Dipole15s are not happy going above 500Hz without some serious EQ, and they should really be left to lower frequencies to get a nice, even response with no breakup.
Ok... so the vertical arrangement was not working well. What if things are turned 90 degrees? Well... the sweet spot vertically will be much smaller, but the drive units can be brought into a much tighter cluster... this moves the combing much higher up where it is generally less audible, and having the Neo10s horizontal will extend the bottom end much further as the baffle width increases.
By some remarkable coincidence, the holes for the Neo10s and the holes for the Raals match up perfectly. Chances of that? Pretty small, but I'll gladly take it. This allowed me to bring the acoustic centres as close together as feasible with no need for a custom faceplate.
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So this is what I ended up with. A much wider spacing also meant the bass driver could sit in between the legs, although things were going to get much more wobbly near the bottom - to ameliorate this I added a single supporting strut from angled aluminium to stop the frame from twisting excessively.
The result? Good integration between the Neo10s, much better than before. Plus, the Neo10s now go down to 500Hz meaning they also fit much better with the bass drivers. The crossover still needs some work, but already there are things I'm hearing that I very much like.
Although about this time, the potential arose to try out a slightly different solution to the midrange upwards...
