Troels Gravesen's PMS (Poor Man's Strad) DIY build

[The Mad Scientist]

I've been running a pair of John Krutke's (Zaph Audio) ZRT 2-ways in sealed 23 litre cabinets for the last couple of years (which are very, very good IMHO) but I have always had a hankering for a good 3-way design.  I like the idea of a wide baffle - it takes baffle diffraction effects well out of the mid-range, improves efficiency and you can build a moderately wide, relatively shallow speaker with a high WAF (wife approval factor), so I'm off down the rabbit hole of the PMS, a design inspired by the Sonus Faber Stradivari speakers.

 

Build will be Baltic Birch ply internals and bracing and I'm going down the route of a dual 12mm birch ply outer shell with a layer of Dodo Mat sandwiched in between with a suitable adhesive - a sort of "constrained layer" construction which should be heavy and highly inert. Cabinet finish will be a carbon fibre wrap - a 2/2 3k twill epoxied on and built up with epoxy and sanded/polished to a high gloss finish. Plinths will be 3mm thick 304 stainless in a satin finish, or maybe even 5-6 mm aluminium (not got the design clear in my head yet), plasma cut to size and stiffened with TIG welded ribs with welded-on, drilled and threaded bosses for the spikes with appropriate dimensions to give the required 5 degree back-tilt. There are lots of technical challenges - ply will need to be kerf bent to achieve the cabinet curvature and I reckon the port and drive cut-outs on the outer layer will be best cut after the carbon fibre is applied, which will probably be best achieved by embedding 3.2 mm (~1/8") guide pins in the cabinet for my router circle jigs before wrapping the carbon fibre. I have lots of 3.2 mm zirconiated tungstens I can use here - I was recommended these for Alu welding, but 2% lanthanated seems to work sooo much better (at least for welding 5 mm thick Alu @ ~200 Amps!)

 

First cabinet is under construction (pictures to follow). As is usual with these types of projects (for me anyway), the design is being refined (learning from my mistakes! ) on the hoof.

Edited February 23 by The Mad Scientist

[The Mad Scientist]

Finally, a little progress. First "skeleton" assembled, glued up and sanded:

 

 

 

 

 

Onto the second one now......

[frankn]

Nice to see the progress, and, the snow has dispersed. 

[The Mad Scientist]

There wasn't really much snow to speak of down here in Maidenhead. Some of the more elevated areas in Bucks had a bit more, but nothing like that in some parts of Derbyshire and Northern Scotland. I don't mind snow at all - grew up in Scotland and later on lived in Austria for a couple of years. I run on winter tyres from November to March/April and have a set of snow chains to hand just in case. Anything more than a centimetre on the roads down here though, and it's carnage!

[The Mad Scientist]

Both skeletons now assembled. Tops and bottoms of the cabinets will be mirror polished stainless. Plinths will be a sandwich of 3 mm thick 304 stainless steel with a 12 mm edge depth insert, mirror polished and filled with sand. The black sections will be 308L TIG filler wire.

 

 

Stainless plate blanks all marked up ready for plasma cutting. Nearly £400 of stainless here:

 

 

 

 

[The Mad Scientist]

Progress has been slow lately. Very busy with work, family stuff and an 11th wedding anniversary which is "steel". Good way to refresh my stainless TIG welding and plasma cutting and learn some engraving by turning six coupons of 304 SS into a cube desk ornament:

 

 

 

 

 

 

Anyway, back to the build. Top and bottom of the cabinets will be mirror finish stainless plate. Had to cut the holes for the ports in the bottom plate and as I feared, the smallest hole I could cut with the plasma circle jig was a smidge (~1.5 mm) too big. Couldn't hole saw it as I don't have a carbide hole saw in the right size. Carbide hole saw set on the shopping list!

 

 

Only one thing for it, and that was to make the hole smaller:

 

 

 

I love working with metals, they're so forgiving. If you make a mistake, you can just weld on more metal!

 

 

Aluminium backer is a chunk from a 110 kW inverter drive heatsink bought from eBay.

 

A bit of finishing off and the hole will be the perfect size....

[The Mad Scientist]

After grinding down with an 80 grit flap disc and tidying up, now have 68 mm rather than 71 mm holes:

 

 

With hindsight a 68 mm carbide hole saw would have been much more straightforward, but hey, I've learned how to successfully reduce an oversized hole in 3 mm stainless plate!

 

 

 

[The Mad Scientist]

A little more progress. The weather's been better, and the days are growing longer, so I'm squeezing in a couple of hours each day after work.

 

By far the hardest part of this build has been figuring HOW to undertake the various elements of the construction. The actual execution has been relatively easy, albeit very time consuming, especially when taking into account the time to remedy "mistakes". The 'oversize' port apertures probably took me 4 hours to fix with all of the clamping up, welding, grinding and finishing. Could have cut all 4 of them them in about 10 minutes with this:

 

 

Top and bottom stainless plates fitted and mounting studs for plinth in place:

 

A little grinding with a Norton rapid blend disc and an 80 grit sanding to start. A fair bit of sanding still to do to remove the imperfections caused by welding the studs to the underside.

 

Top plate bolted on with said studs and bonded to ply with polyurethane  adhesive:

 

 

Still some rough finishing to do on the bottom stainless plate, but the port apertures all line up perfectly and the port tubes will fit very snugly:

 

 

 

Bottom plate will be trimmed flush with the cabinet before the carbon fibre wrap so won't be visible from the front. Top plate will be trimmed flush with the carbon fibre wrap once it's in place, so the finished edge will be visible.

 

Both top and bottom plates are very subtly curved to improve the aesthetic finish. Surprisingly easy to achieve with a panel beater's hammer, even in 3 mm stainless plate:

 

 

Bottom plinth mount studs are welded to an internal stainless plate and the two plates sandwich the bottom plywood layer with polyurethane adhesive. Not my tidiest TIG welds by a long way. There was limited space for the torch and filler once everything was clamped down onto an aluminium backing plate to keep the plate cool to minimise warp and to avoid the need for back purging. Stainless oxidises badly on the back side when welding (known as sugaring) unless it's in an inert atmosphere or clamped to a copper or aluminium backing bar.

 

 

The blackening isn't burning - it's spray paint from where I tried to mark the drill holes on the bottom stainless plate after drilling the holes in the ply..... Didn't work, so I opted to use blue layout fluid painted onto the steel and marked out through the holes with a very sharp 1.6 mm tungsten.

 

 

Midrange driver rear chamfers in the rear half of the front baffle sandwiches routed in.

 

 

Next job is to sandwich the front baffle sections together with Dodo MLV. More pictures to follow.

[The Mad Scientist]

Kerf bend cutting underway:

First half of the rear of the first cabinet clamped in place. My apprentice casting his beady eye over my workmanship:

 

 

 

 

[Mr 57]

Your talents are endless… how did the SS cube go down as a gift ? I would guess it was well received

This is a serious diy build, many eyes are watching, and we appreciate you allowing us to peer over your shoulder…..

Well done !

[The Mad Scientist]

29 minutes ago, Mr 57 said:

Your talents are endless… how did the SS cube go down as a gift ? I would guess it was well received

This is a serious diy build, many eyes are watching, and we appreciate you allowing us to peer over your shoulder…..

Well done !

Thank you! I'm an avid tinkerer and my comfort zone seem to lie somewhere between biting off more than I can chew and nigh on impossible!

 

Katy was well pleased with the cube. It adorns her office desk. The thought and effort that went into it is appreciated much more than the gift itself, I suspect.

 

I'm really looking forward to finishing the speakers and having them in the lounge, but then I'll have to find something else to make!

[crtexcnndrm99]

1 hour ago, The Mad Scientist said:

I'm really looking forward to finishing the speakers and having them in the lounge, but then I'll have to find something else to make!

So it goes!

 

Most recent pics of the speakers, they are coming together now. Next level diy build and as above, we’re all looking forward to seeing them in the listening room.  

[The Mad Scientist]

A little more progress:

 

I had a few pieces of Korff Hawaphon (https://www.korff.ch/en/hawaphon-en-2/) leftover from a previous project, so I'm using them on the rear panel of the bass and mid range compartments under a layer of eggcrate foam and probably on a few of the larger panels:

 

 

 

 

 

 

All of the other panels will have a layer of Dodo Mat MLV plus a layer of 8 mm felt.

 

With it being such a wide baffle cabinet, I want to keep cabinet resonances to the absolute minimum.

 

The hole in the midrange compartment is there so I can fix a guide pin for routing out the port orifice once the carbon skin is in place.

 

Crossover  components are also mounted and wired up ready for installation before the front baffle goes on:

 

 

 

 

I've also roughed out my sketches for the plinth design:

 

 

 

 

Having to reacquaint myself with long-forgotten trigonometry!

 

Excuse the greasy marks on my graph paper

 

10 m of Canare 4S11 Starquad cable on the way from Blue Jeans Cable to make up the speaker cables.

 

I have all of the Neutrik Speakon connectors needed for the cable and the speakers (my Benchmark AHB2s already have the requisite Speakon outputs):

 

 

 

Edited April 25 by The Mad Scientist

[ICUToo]

On 26/04/2023 at 6:12 AM, The Mad Scientist said:

the carbon skin

WHOA!!!

[The Mad Scientist]

5 hours ago, ICUToo said:

WHOA!!!

You probably missed that bit amongst my ramblings ! Will be skinning the cabinets with 210 g 2x2 twill 3k carbon fibre cloth::

 

[The Mad Scientist]

Slow progress this week. Almost all of the Dodo Mat MLV (mass loaded vinyl) and dampening felt is done on the inside of the cabinets. Every solid brace has a layer of Dodo and a layer of felt. The larger panel areas have Hawaphon and eggcrate foam or Hawaphon and felt. Felt is 8 mm from Jantzen Audio in Denmark, distributed by Hifi Collective in the UK. The "knuckle rap" test results are very encouraging!

 

 

The Dodo Mat has presented some challenges. As it's supplied in a roll, it has a tendency to "set" into a curved shape, which can make sticking it to panels a tad tricky:

 

 

I had a brainwave last night and decided to try warming it up. A hot air gun, set to 160°C did the trick. The hot air gun is a cheap one from Amazon (< £40), with digital variable heat and fan control. Came with a range of different nozzles too. Great for applying heat shrink and all sorts of other stuff. A bit of heat and a little flexing in the opposite direction got the vinyl nice and flat:

 

 

The residual heat makes the adhesive tack up very quickly too, which is an added bonus.

 

The adhesive I use is a Vinyl flooring adhesive from Mapei:

It bonds pretty quickly, and although it's described as setting hard, it actually sets to a sort of firm, rubbery texture. For flooring, it's intended for trowel application, but for sticking the Dodo MLV, a brush works well.

 

The cabinets are growing heavier by the day and are already pretty hefty. I reckon there's another ~17 kg of plywood and Dodo Mat to go on each cabinet, plus drivers and crossovers and then the carbon fibre and resin. Stainless plinths filled with steel shot will be ~11 kg, so I reckon each speaker will be getting on towards 50 kg.

 

In other news, 10 metres of Canare 4S11 Starquad speaker cable arrived from Blue Jeans Cable today:

 

 

43.5 hours from order until delivery. Seattle, WA, USA to Maidenhead, Berkshire, UK. Phenomenal!

 

Edited April 27 by The Mad Scientist

[ICUToo]

The carbon won’t add too much weight

Great project, I am enjoying it very much, thank you 

[kiwilistener]

Nice work Graeme, I have been watching your progress with interest. I will be interested to hear what you think of them once you have them running for a bit. Suffice to say for now I was surprised at how they sounded compared more traditional box speakers.
I have a thread here detailing my build of these speakers, unfortunately the images don't appear to have survived the change servers a few years back, however this link has a few images of the pair I build 10 years back. It will be interesting to compare notes. Introducing the PMS speakers - Techtalk 

 

[The Mad Scientist]

Thanks. I'm looking forward to getting them up and running.

 

In my current setup, the "sweet spot" is a very small area. I'm hoping the wide baffle of the PMS changes this.

 

I saw from the link, you had some fellow enthusiasts coming round for a listen. How were their  listening impressions compared to their experiences of conventional boxes?

 

Do you have a link to your original thread? I've not been able to locate it with the search function.

Edited April 30 by The Mad Scientist

[The Mad Scientist]

A little more progress. Second layer of ply on the first cabinet rear is now in place.

 

Adhesive applied to the first layer in readiness for the Dodo Mat. I had to run out and buy another bucket. This layer used nearly half a tub.

 

 

Dodo Mat applied. I used a scraper to really work the mat down onto the adhesive, squeeze out air and any excess adhesive:

 

 

After applying the second layer of adhesive an the outer layer of ply, it's all clamped up. Pieces of 1/8" aluminium to prevent the ratchets from denting the plywood.

 

 

The drive unit portions of the front baffles are all trimmed up ready to fit. Layer of Dodo Mat visible in the middle:

 

 

It's been a bit of a faff installing the Dodo Mat, but the effect on the cabinet resonance by sandwiching it between 2 layers of 12 mm birch compared to using even a 36 mm laminated layer of birch plywood is staggering. They're completely dead.

 

By the time all of the panels are fitted, I reckon I'll be needing my wife's assistance to move the cabinets around. They're becoming very heavy.

[ICUToo]

7 hours ago, The Mad Scientist said:

It's been a bit of a faff installing the Dodo Mat, but the effect on the cabinet resonance by sandwiching it between 2 layers of 12 mm birch compared to using even a 36 mm laminated layer of birch plywood is staggering. They're completely dead.

 

i love this project.

Nothing succeeds like excess

[kiwilistener]

9 hours ago, The Mad Scientist said:

A little more progress. Second layer of ply on the first cabinet rear is now in place.

 

Adhesive applied to the first layer in readiness for the Dodo Mat. I had to run out and buy another bucket. This layer used nearly half a tub.

 

 

Dodo Mat applied. I used a scraper to really work the mat down onto the adhesive, squeeze out air and any excess adhesive:

 

 

After applying the second layer of adhesive an the outer layer of ply, it's all clamped up. Pieces of 1/8" aluminium to prevent the ratchets from denting the plywood.

 

 

The drive unit portions of the front baffles are all trimmed up ready to fit. Layer of Dodo Mat visible in the middle:

 

 

It's been a bit of a faff installing the Dodo Mat, but the effect on the cabinet resonance by sandwiching it between 2 layers of 12 mm birch compared to using even a 36 mm laminated layer of birch plywood is staggering. They're completely dead.

 

By the time all of the panels are fitted, I reckon I'll be needing my wife's assistance to move the cabinets around. They're becoming very heavy.

Sorry   Grame the original thread I did here for the PMS build has gone the way of the dinosaurs. It was one of the many threads that seemed to disappear when the websites amalgamated, and I never kept a local back up either 

Yep, they are gonna be heavy, mine topped out at 65 kilos.
I can't say I'm a true advocate of having completely inert enclosures. My preference is for a small amount, of resonance, in much the same way as a musical instrument has resonance. Typically, I like to aim for the 70hz or less zone. Cabinets that resonate in higher frequencies tend to distort the sound too much or create very disagreeable coloring to the sound that becomes fatiguing. 
 

[The Mad Scientist]

It's a shame the original thread is gone. It's always good to be able to read someone's build progress on a project. We all makes mistakes and incorporate learnings and refinements as we go!

 

I'll have to try to measure the resonant frequencies when the cabinets are assembled. I'm sure there will still be a small degree of low frequency resonance. The constrained layer damping certainly kills the higher frequencies. There's no ringing from a knuckle rap on any of the panels, just a dull thud.

[rand129678]

On 26/04/2023 at 4:12 AM, The Mad Scientist said:

Crossover  components are also mounted and wired up ready for installation before the front baffle goes on:

Nice thread.

Any reason you wanted to go with passive XO instead of DSP?

I guess one (good) reason is not having to buy another one or two Benchmark amps

[The Mad Scientist]

8 minutes ago, rand129678 said:

Nice thread.

Any reason you wanted to go with passive XO instead of DSP?

I guess one (good) reason is not having to buy another one or two Benchmark amps

Thanks.

 

Troels generally does his crossovers really well, so the passive one will work well. I did consider DSP, but there's a lot more to it than just getting the frequency response of the drivers right. There's an awful lot of work involved in getting good driver phase and timing alignment too. It's not a rabbit hole I'm quite ready for yet. Though in the future, who knows? One more AHB2 is all I would need for the power side, but I'd need a multichannel DAC with comparable performance to the DAC3 and enough output signal level to let the AHB2s perform at their best.

 

Maybe a project for my retirement in a few years!