First Build: Pensil P11

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Hello Everyone,

Recently I stumbled on an ad for the Wrensilva's audio consoles, and the idea of making a similar piece stuck in my head. I'm quite competent making cabinets and have machinery and some materials available. On the other side, I'm an utter neophyte regarding audio; so I started lurking here.

Since I know next to nothing about electronics I opted to go with a full range design. I got a good deal on a pair of Pluvia 11s and so, I have the drivers in my hands. I later opted to make a pair of speakers instead of the audio console, since don't really have room for it.

I usually listen from Spotify via Bluetooth, have a number of classical music CD's which I listen from time to time too. I bought an Onkyo TX8220 to power the speakers, probably arriving later this week. Not a conossieur, as you may notice.

Initially I was thinking into making a folded horn design, but found a review of the Pensil 11 on HiFi Pig that changed my mind. Also it looks easier to build.

Now regarding the build, I'm going to list some hard constraints, some good things, and some questions in no particular order. Would appreciate your input.

  • First of all, I'm using the Pensil A11/P11 plan as base for this build: https://frugal-phile.com/boxlib/pensils/PensilA11-P11.pdf
  • Something not good: I don't have a source for good plywood. On the other hand I have plenty of MDF. I made a preliminar design using 15 mm MDF keeping the internal dimensions untouched. Could use other thicknesses, but I'm used to work with 15 mm and have more finishing options with it.
  • Another not so good thing: I have no clamps. I will "press" the panels using Minifix fittings while the glue cures. I've done that before with furniture, let's see how it works on speakers.
  • Something positive: I have plenty of dampening materials: delcron, felt, PU foam, eggshell foam, etc. Also, have access to some well tooled CNC routers.
  • A question: If not using internal bracing, how do you keep the fiber stuffing from going to the bottom?
  • Regarding the terminal cup: Is there a prefered place to put it or it doesn't matter as much?
Thanks a lot.


Rodrigo
 

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A couple of points:

-You are asking for trouble if you use unbraced, single-thickness 15mm MDF in an LF enclosure of this size.

-Do not use egg-crate foam. I stress this point for every single pensil: I did not design them to use it, and it doesn't (can't) provide the correct damping characteristics. It has uses, but not for these speakers. The default is dacron (hollow-fibre stuffing).

-The stuffing is uniformly distributed throughout the box, keeping it a few inches away from the rear of the driver. Generally it's self-supporting at the densities in question, but you can put a dowel or two across just above the vent. However, if you're going to use MDF, above all insufficiently thick MDF, bracing of some kind is essentially mandatory, with the longitudinal bracing of the type shown in the [Super]Pensil 12P pdf being the most effective.
 
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Having built a few pairs of Pensils and many of Scott’s other designs, I’ll strongly second his remarks. In my past active building career, I had access to more than sufficient free off-cuts or wholesale priced MDF, but after a test build of identical enclosure and driver design in the two materials (3/4” MDF and 18mm Baltic birch) in the early 2000’s, I used MDF only for templates and base plinths.
Aside from the vent opening, it’s critical that all joints are air tight - something that’s certainly not the case for furniture or kitchen cabinets. If you have access to CNC machining, I’d suggest at least 18mm MDF, and the use of dadoes/rabbeted joints - glue lock joints if you can manage them, but note that MDF can be both become fragile when so machined, and thanks its heat tempered surface hold a sharp enough edge to cause physical injury.

Cue the Greek chorus of debate on the merits of different material types and construction/joinery techniques.

As to to the location of terminals, I’d personally opine that it hardly matters diddly squat - just make sure that the back panel is both removable for installation / adjustment of damping fill, and well sealed with something like closed cell foam weather stripping tape.
 
frugal-phile™
Joined 2001
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If MDF is all you have, 15mm is too thing for this box. You could get away with well braced 15mm quality plywood, but you will need to use at least 18mm MDF and it will likely need more bracing than the 15mm ply.

856928d1593560582-build-pensil-p11-pensil-11-cad-preliminar-jpg


From this drawing: the braces that are pararllel to the bffle should not extend to the point that they are behind the driver (you could probably make then in a bit taller.and i would not put any holes underneath were the braces cross. Make sure the braces are off-centre. With 18mm MDF you may need more bracing. Probably another side-to-side set. Account for the volume of th braces when doing the box size calculation.

dave
 
Thank you for your valuable feedback.

I made the pertinent changes: I'll go with 18 mm MDF for the sides, top and bottom, and 27 mm (15 + 12) in the front. Will use dadoes in most unions.

Next thing I will do is take into account the volume substracted by the braces and back mount.

I'm probably using 2 faces finished MDF for most parts, does the absence of porosity affects substantially the outcome?

Thank you again,


Rodrigo
 

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I am planning to build Pensil 11 with Alpair 11ms. I have the plans but nowhere can I find plans with exact dimensions for the recommended bracing. I am also not clear whether the driver should touch the bracing. Is that why there is the neoprene strip in KFJ Audio kits?
 
There aren't any plans with exact dimensions for the bracing to the best of my knowledge. It's non-critical other than aim for c. 50% stock removal -Dave will no doubt be able to provide further guidance. In an ideal world you'd have a perfectly rigid couple to the rear of the driver, but we don't live in an ideal world so a thin rubber or equivalent gasket usually works well in practice. The emphasis there is 'thin'.
 
frugal-phile™
Joined 2001
Paid Member
…aim for c. 50% stock removal...

33-40%. above that you start to lose brace integrity.

Brace should be firmly against the baffle, top, bottom and back. A notch out for the driver magnet should be sized such that it just supports the driver, form, but not so much as to stress the basket. The holey brace behind the driver to the back panel should remain solid.

The box width should increase by 2/3rds the thickness of the brace (and the vent height teaked to compensate for the extra width)

The back screws into the battens and the holey brace

Brace should be just off-centre. if the drive is centred on the baffle, line up one edge of the brace to the centre-line.

Additional braces can be extended from the sides of the holey brace to the box sides (only below the driver). Or as shown in this “small” pencil, an X can be used.

pensil7-braces-free-benefit.jpg


The brace(s) can be more work than the entire rest of the box so we were asked to leave it out of the drawings so as not to discourage people from building them.

dave
 

GM

Member
Joined 2003
A couple of points:

-You are asking for trouble if you use unbraced, single-thickness 15mm MDF in an LF enclosure of this size.

All things considered, wouldn't just doubling up the 15 mm be sufficient for such an inefficient driver since stiffness increases at the cube of thickness? [still no calculators loaded to answer by own Qs :(]

Then all that's needed is a simple cross plank for the mass loading motor support.

GM
 
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Agreed, I just couldn't remember what I'd posted on several occasions was the MDF thickness required to match up to the 19 mm no void plywood's 1.8 mil psi MOE spec; finally remembered it is 1.25"/31.75 mm, so two 15 mm bonded together with a hard setting adhesive is close enough to not need any bracing for a small 'FR' driver.

GM
 
Well the SuperPensil build is underway; much learned on my first build, operative word being first.
I attempted, well I might add, to miter everything vs veneer. Don’t do it unless you’re skilled, a lot of waste and frustration.
With that said, I dry fit them last evening and only had a few areas of correction. Once I finish the bracing and final assembly I’ll post some pictures. I don’t know how I will finish them, form to follow.
 
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