Dual 15 + coax

https://boyzpc.home.xs4all.nl/Bouwtekeningen/18Sound/K24xov60.pdf

I would also just warn you that although a horn may be specified as having a 400Hz cut-off, it's usable range (i.e. crossover point) may be up to an octave above that.

Lot's of good suggestions so far, thanks everyone, it looks like this project will happen soon.

The B+C coax looks good, specs and price are similar to the BMS. Has anyone heard both and done a comparison? I'm not sure which to choose.

Whichever driver I use I will do some tests to choose the crossover point. The horn sizes look good to reach 400Hz (just!) but I anticipate the crossover to be around 650Hz, a 24dB/oct filter which is -6dB at 650 will be way down at -24dB around 400Hz. If the 12s or 15s aren't happy that high it might have to be a lower steeper filter.

I had a bit of spare time today so I knocked up a dry run to think about the internal layout, ports, bracing, handles pole mount and such. It's very light so far, I can easily carry it with one hand. That will change though...

doober wrote: The B+C coax looks good, specs and price are similar to the BMS. Has anyone heard both and done a comparison? I'm not sure which to choose.

The BMS sounds a little smoother from 14 kHz up, but the B&C absolutely craps all over it for midrange detail and clarity. It seems to be far more linear in that region.

Assuming you’re planning to bi-amp these and use a passive network on the coaxial HF, then the B&C is the more resilient option too. They have a passive crossover for the unit that by all accounts works well:
https://www.bcspeakers.com/en/products/crossovers/0-0/8/FB4648

They also have a bloody excellent horn for it, if you can fit it in the box!

https://www.bcspeakers.com/en/products/horn/1-4/0/ME464

Unfortunately I can’t say the same for the BMS units. I’ve seen far too many shattered VHF diaphragms from those, since the power handling disparity between that part and the main diaphragm is pretty huge.

If you get the output you need at 17 V or ~20 W peak then cool. If you need more, then you need a custom passive network with complex bulb protection circuit, or to run the driver itself on two channels with DSP.

Edited by toastyghost - 21 July 2021 at 3:24pm

Kipman- the cavities are tall enough at the front to allow the drivers to be fitted from the front, avoiding the slight complication of making access hatches in the back. Of course if I used 12" drivers they could go inside through the mid/high horn cutout, its about 20mm too narrow to fit a 15" through. I've not found a 12" which out performs the 15s I'm looking at.

I will have to put some drivers in and measure where the resonance and low-pass occur, then try some experimental stuff if it is too low.


Toastyghost- I will be bi-amping the comp drivers, so no worries about over powering the HF. I don't mind sacrificing a bit of smoothness above 14k for substantially better mids, that's quite an easy decision. I have seen that horn, it looks great but is a little too large for this project. I did think of using it in it's own box and putting 2 15s in a separate box with a pole between, but that just causes other problems...


John Boom- There won't be an array, these are to be used one box per side (plus subs.) The RCF horn is 90 degrees H by 50 degrees V, shouldn't be too far off the 15s. I have heard reports that this horn does sound good, despite the slot which is much smoother in shape than many others. The 'wide' version of the Peter Morris box uses this horn, and people are raving about it.

Thanks for sharing your experiences, that gives me some good points to consider, I know there will be a fair bit of experimenting to do with the layout. I'm aware of Hoffman's law, this box more a case of as much output as possible from a given box size which goes low enough to meet subs. I don't expect the 15s to keep up with a coax comp driver at full tilt.

I mentioned the PM90 as an example of the horn sound quality regarding the diffraction slot in the RCF horn. I don't expect the horn to go all the way down to 400Hz, 630Hz crossover will hopefully be fine if the 15s still sound decent up there and beyond.

That article is interesting. I was planning to do an impedance plot myself, but from looking at the one in your link the horn unloads around 450Hz. The SPL plot is informative, it shows a roll off below ~900Hz, might need a fairly gentle low pass on the 15s, again if they go high enough. Off axis plots look pretty good.

I've ordered some components, hopefully I'll be able to test something soon.