BMF 15s - Two-way Active DSP Build

HiFiRuss71

Distinguished Member
Now, this bit seemed to cause some debate (i.e there was literally no in between on the opposing opinions) on a popular DIY Speaker Facebook group and one that is waaayyyy less combative than almost any forum. What have I done? I have damped the rear surface of the horn. Shock!

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One stance is that by the time you have bolted 2kg of tweeter to one end and screwed the mouth to the baffle, the horn will be well damped and what I have done is unnecessary. The other stance is that people can't understand why you wouldn't do it.

I'm in the latter camp and feel I do understand why you should do it. I did it on the mid horn of my Klipsch Forte IIIs and it definitely made than sound a bit cleaner and less 'busy' when playing out high volumes and I do mean high - 100dB+ peaks on music sounds a lot louder than it does in home cinema.

What it cannot do is make anything worse and as I was staring at more than enough spare Silent Coat to make it work, why not? You decide - opens link to Facebook.
 

HiFiRuss71

Distinguished Member
With basically all of the dust and fluff making out of the way, we're into the home stretch of making the thing pretty. You've seen the leatherette vinyl back in post #13, but now it is time to actually apply it...

By the way, I had intended to gloss wood effect vinyl wrap (car style) the side cheeks and that may yet still happen, but now as a Plan B. My head has been turned by Shou Sugi Ban and now I have seen it, I cannot un-see it. I will do it, but it may yet fail if I cannot get the requisite flat, gloss finish over the top. Still, I now have a Plan B...

Back to the Vinyl.

The cans of contact adhesive said "not suitable for plasticised PVC". I know what unplasticised PVC is, because you see it in gutters, windows doors, etc. I assumed my leatherette is the former, so I did some off-cuts to find out the issue with the by now, multiple MDF off-cuts available. You can use contact adhesive two ways. Spray it on one surface and apply the other after a few seconds. Spray it on both surfaces and push together after a couple of minutes.

I learned that the one coat method (you do it a bit thicker) gives a bit of latitude to move/re-apply the vinyl. The two coat method sticks and that's the end of it. I accidentally found out the 'issue' I might find by accidentally over-spraying the leather side of another off-cut. The adhesive dissolves the surface. Wipe it off quick and it is destroyed. Wait for it to set and the surface is destroyed. Clearly, it dissolves PVC. I left the successfully applied samples for over a week and nothing had leached through and both were stuck like limpets, so I went with the more cock-up proof single coat method.

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Spray a clean face of the box (more on the bare MDF edges as it soaks up glue) and progressively roll out the roll chasing it out with a rubber veneer roller toward the edges of the cabinet to avoid bubbles. I masking taped the free edges (to be trimmed and stuck later) to maintain some tension whilst the glue dried. Copious amounts of newspaper everywhere, changed at every turn of the box and held against anything being sprayed to avoid overspray. Turn a finished face over on to a spot of glue and you'll never, ever get rid of the blemish.

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It's actually one of the quicker steps in the build; about an hour per cabinet. So quick, I forgot to take photos of how to do a really tight join of the two ends on the bottom of the cabinet.

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The baggy looking bits are where I have rollered over the edges of driver and amp openings.
 

HiFiRuss71

Distinguished Member
It's all gone swimmingly up to now, but next comes the first time if I can find out if the extra millimetre (+/-) of clearance I left for the vinyl is enough or not. Will it hide the bits that are less than perfectly straight as they were hand routed by eye? It's a bit late if the answers are no...

I did a rough craft knife trim up of the leatherette covering the openings. Once the various bits have been clamped a while, I'll lift them out, apply a thin smear of PVA to the rear and use the drivers/amp to seal the vinyl in and trim up accurately.

Well, the mid/bass driver is a very snug 'interference' fit. That's as iin 'I'm glad there's two 7" ports to sneak a rubber mallet through to 'persuade' it out again, but otherwise the rank amateur guesses have worked out better than I dared hope.

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Looks like I've got enough clearance for the proposed chrome trim ring for the mid/bass driver.

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There's only two out of eight bolts holding the horn in, so I haven't tightened them too far, but it looks like the 2mm vertical upstand on the horn will submerge pretty much bang on into the baffle and the vinyl against the seal on the rear of the horn (see picture in post #26) will give a seriously air tight seal plus room for adjustment.

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The recess for the plate amp was the one that actually worried me most, but it looks great.

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The most current picture. The grey square bottom left is the smoked Perspex cover to the IR remote control eye and status lights with its protective layer still applied. The hole looks almost butchered compare to even the amp recess, but the vinyl has hidden everything, not to mention provided an air tight seal - the Perspex has simply been pressed into place and it stays put without assistance. Golden.

In the next few days I will staple and trim back the free edges of vinyl. I've decided to cut precise (as I can manage!) rectangles of vinyl to infill the 1mm void the vinyl wrap causes, prior to sticking the wood end cheeks on. I guess this will give some sort of constrained layer damping (CLD if I mention it again) effect too, but really it's a matter of convenience with material available.
 

mattkhan

Distinguished Member
this bit seemed to cause some debate (i.e there was literally no in between on the opposing opinions) on a popular DIY Speaker Facebook group and one that is waaayyyy less combative than almost any forum
Odd, never noticed that as being a contentious topic before. I have some vague memory of it being a HOM related topic though which, if memory serves, is a bit more contentious so perhaps it is driven by that?

I'm glad I am not the one who has to move those speakers :) they look really nicely made/finished
 

HiFiRuss71

Distinguished Member
Odd, never noticed that as being a contentious topic before. I have some vague memory of it being a HOM related topic though which, if memory serves, is a bit more contentious so perhaps it is driven by that?

I'm glad I am not the one who has to move those speakers :) they look really nicely made/finished
Tell me about it. I haven't weighed them, but they're definitely north of 50kg each and I haven't added the 18mm oak to either side yet....

I actually bought some cheap furniture castors that screw into the leg threads so at least they can be pushed around. I have no idea how we're going to elevate them sufficiently to measure the FR.
 

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