I also wondered that as there are a lot of coax choice here in Europe 8" to 15": BMS, Beyma, B&C (as Perry's choice for the BB in 15" size).
The new 10" coax from BMS seems particulary smooth as some Faitals as well. Maybe than those polymer diag are less good for cymbals and drums than treated metal diag though.
The new 10" coax from BMS seems particulary smooth as some Faitals as well. Maybe than those polymer diag are less good for cymbals and drums than treated metal diag though.
No worries Perry! thanks for the tip. I've seen others post some vehement opinions on the resonant peaks due to parallel surfaces so rather than splay the wings a little to avoid this (which would screw with your DSP tuning), I asked about the absorptive material. I will keep to the original design and skip the absorptive padding - well maybe I'll try it if I can cut it to size and just slip it in & out w/o any glue or such to mess up the finish. Thanks for the reply!!
Perry, you say "I think the Cottonwood Dipoles with an SB Acoustics 15OBO350 would be better than the Lavoce woofer I used in the design". I haven't purchased my drivers yet (the butternut slabs are acclimatizing in my listening room before I start the work this spring) so the SB Acoustics woofer seem like a worthwhile upgrade. Will these work with the cottonwood/beryllium DSP files you've already made available to DIYers like myself, or would it require new files optimized for the new woofer? That would be beyond the scope of my abilities I am sorry to admit. Thanks so much for your help and patience!
@Studley The Radian Coax in the Birch dipoles is 8" and I wanted to use 15" so the woofers and mid-coax matched in size.
Radian does have a Beryllium 15" coax but the directivity in its top octave is quite a bit narrower compared to the B&C. It's also 2X as expensive. So I chose the B&C with Titanium diaphragm, which has wider dispersion in the top octave. I find this is fairly important.
I had also tried a Beyma 15XA38ND coax, which is a pretty respectable driver, but for some reason the directivity in the 4-7K region is a hot mess. In other words in that octave it didn't have a controlled directivity characteristic, so I abandoned it for the B&C.
As for the "titanium hardness" of the B&C it's only very slight and it also comes down to which amplifier you use as to whether it's noticeable.
ONE OTHER THING: If I were to build these again I would tilt the front panel back 5 degrees instead of 10 degrees. With my couch and listening room, the 10 degree tilt has them firing about 5 degrees above my head. Really not a problem with the B&Cs but it would be a problem with say the Radian 15 or the Beyma 15 coax. Changing to 5 degrees would make it just about perfect.
@perrymarshall; was the desire for a 15” coaxial related to directivity matching, or was it just aesthetics?
@perrymarshall I’m enamored by your birch dipoles, and have my finger on the trigger to order parts. Curious as to what you use(d) for amplification For both the tweeter section and the mid/woofer section. I have an NOS Valves VRD amp that is 30W/60W IN triode/UL that should be more than enough for the mid/woofer section, but I’m curious what is suitable for the tweeter section. Your input/insight would be appreciated. By the way ... It’s a small (12’x17’) room, so I’m not needing to fill a huge space. Thanks!
If you're comfortable taking measurements and tweaking standard EQ settings on the MiniDSP 2x4HD, then you could switch in the SB woofers and they'd work fine. You wouldn't have to mess with the IIR filters, you could leave those the same. If all that gets too complex then just use the Lavoce woofers.
@Studley It was directivity matching, and low-end output (Radian 8s were crossed over at 150-200Hz, B&C 15s are crossed at 110Hz and capable of playing much louder as well), and sensitivity (95dB vs 99dB) and aesthetics.
@tlarwa I use Adcom GFA2535s which are 4x60W. I think 30-60 watts is more than plenty for the woofers. For the tweeters, 3 - 5 - 10 watts would be just fine. NOTE - in the original design files I have resistors on the tweeters which you can omit and you can just raise the gain on the amp. The tweeter section in the Birch dipoles delivers about 98dB SPL 2.83v without padding.
I do have a tube amp too (Yaqin MC-13S) and with the Birch Dipoles you can hear the Yaqin tube distortion. The Yaqin sounds gritty through the beryllium tweeters. Ditto if I use it to drive the woofers instead. The Yaqin is a low-end tube amp and it seems likely other tube amps would have less distortion. I've always wondered what a good flea-watt amp (ie single ended 3W class A tube) would be great for the tweeters in any of these designs, but have never had the opportunity to experiment.
I can see the case for making those surfaces non-parallel. But this design is so shallow I hardly think it would make a difference.
@tlarwa my room is similar size to yours. The SPL of the Birch Dipoles is 95-98dB above 60Hz so in the occasional times these do demand a lot of power, it's only when there's a lot of bass around 30-40Hz. If you roll them off at 40hz instead of 25hz that also relaxes the power requirements.
Thanks for the quick response. I’m also thinking about a flea watt SE or medium wattage PP amp for the tweeters. I’m a tube guy at heart, although maybe a solid state amp for the mid/woofers may have better base control? Something to consider.
Tubes with less damping are going to change the EQ for sure, but with the DSP in place you can re-adjust that any way you want. I once built a system with Tangband 3" full range drivers in a rear-loaded horn, crossed to a sub. Active DSP crossovers. They sounded just fabulous with the Yaqin tube amp driving the satellites and a solid state amp on the sub. I think with stereo components there is sometimes a chemistry of matching amps to speakers and that was the perfect example. The same amp driving the Birch Dipole tweeters sounds gritty. The paper cones of the TBs have just the right amount of "smoothing over" to make those distortion harmonics really pleasing. It's more art than science and while it drives engineers crazy (I'm an EE) it's a lot of fun when the ultimate priority is just making a system sound great.
@Studley there's another rationale that I forgot to mention. Indulge me in some more advanced Open Baffle acoustics.
If you look at the Birch Dipoles 0-75 degrees off axis curves... (taken from https://s3.amazonaws.com/psma-website-assets/live_edge_dipoles.pdf )
You can see some subtle "bunching" of the off axis curves at 800-1500Hz, especially at 0, 15 and 30 degrees.
The reason for this is that the Radian 5208 cone is 6.5" in diameter but the baffle is 20 inches wide. Therefore driver is omnidirectional below about 1500Hz based purely on piston diameter. The only thing that makes it directional below that frequency is the rear cancellation radiation. That starts kicking in at 500Hz and below, but isn't a factor from 800-1500. Rear cancellation would start kicking in at 1500 if the baffle was only 10 inches wide.
So that means the "figure 8" of the dipole gets fatter between 800 and 1500. Here's a crude sketch:
The wide baffle also means I can get away with using the Radian all the way down to 150Hz.
A narrower 10" baffle would not have the bunching behavior between 800 and 1500. But the woofer would also start rolling off below 400Hz and I'd need to choose a higher crossover frequency to the 18s.
This is why...
One of the reasons I chose 3 x 15" drivers in the Bitches Brew design was that a 15" woofer in a 20" wide open baffle can go down to 110Hz effectively, and I still get the consistent directivity with no bunching effects. I could not use an 8" down to 110Hz in this system, but if I tried, I would still have a bunching effect at 800-1500Hz. The Bitches Brew as designed does not have either issue.
The Bitches Brew wings do show some bunching effects between 50Hz and 100Hz, but those depend entirely on where you place the microphone. I'm pretty sure there are angles where the bunching effects are absent.
@tlarwa I am giving you the latest schematic and configuration files for the Birch Dipoles with Beryllium Diaphragms.
Config files: https://evo2.org/wp-content/uploads/2022/12/birch_dipoles_config1_17april2022.xml_.zip - these are for MiniDSP 2x4HD.
Schematic:
This is different from the original schematic in several ways:
The 22uF and 7 ohm on the upper left are a Zobel network. They just ensure that the amp sees a flat impedance curve above 200Hz.
The tweeters here are wired in series with parallel resistance across each tweeter. That gives a net impedance of around 10 ohms. It also pads down the tweeters because the tweeters are so efficient that they amplify amp and DSP noise. So I needed to reduce their efficiency. The rear tweeter circuit has an 8 ohm L-Pad.
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@perrymarshall first off, I want to thank you for your help guiding me on this … it’s much appreciated, and more than generous. But I will likely NOT use the 5208C with the Be diaphragm, due to the cost of those drivers vs. the Al version (3x the price!). I see in your original write up that you used the Al version. Do you recommend I just stick with that network diagram vs. the one you show above if I use the Al diaphragm? Or would you still recommend this updated network?
