Dutch Duplex build log and impressions

Hi all,

After the warm welcome in the introductions section I decided to post something that resembles a build log in this section of the forum.
This thread is meant to document the process of a recently finished build, but since this is my first rodeo with this driver I would also like to compare notes with those of you who have experience with the GPA driver or Altec predecessors. Also, please feel free to ask me questions about the process or result.

The idea is to keep updating this thread to reflect changes made to fine tune the cabinets.

A summary of build details:
  • The cabinets have a net volume of about 349 liters or roughly 12 cubic feet. (Calculated the net volume displaced by bracing and driver to the best of my abilities which is more of a guesstimate than anything else)
  • They were designed by Scott Lindgren by my request. He was very patient with me and instrumental in turning this duplex fantasy into a reality.
  • All panels are doubled 18mm baltic birch and I copied the bracing scheme from another one of Scott's designs, the Bison cabinet.
  • The driver is mass-loaded by the bracing structure and consequently by the entire enclosure. The magnet brace construction is based on these pictures @shoshin posted some time ago.
  • For lining the cabinets I used 20mm thick denim insulation. I lined the top, one side, and back panel down to about 70% of the length of the cabinet.
  • I finished the cabinet in walnut veneer and used Rustins Danish Oil to finish. It was my first serious attempt at veneering and overall I'm pleased with the result. I used the iron on method with PVAC wood glue.
  • Crossover is the Werner Jagush one with autoformers for the HF driver.
  • I'm using teflon coated 16 AWG silver plated stranded copper wire. Double runs from amp to crossover and single runs from crossover to both LF and HF section.
Some first thoughts:

  • At first I was a little bit underwhelmed by what I was hearing. Everything sounded pretty flat and lifeless and double bass and bass guitar as well as some kick drums had no meat on their bones. Besides, they had to compete with unreasonably high anticipation that had been built up over the past few months (years if you count the time from first design pass to finished result). I attributed this to sub optimal speaker placement, to driver break-in, and everything else settling in.
  • I started messing around with speaker placement and listening position and measuring the changes with REW. Moving them closer to the front wall has made a significant difference in the low end response of the speakers. They come in strong at slightly below 30Hz and because of the width of the baffle they sound anchored in the lower mids which I really like. Mid bass is very satisfactory, too, however I feel like this can be improved upon with room treatment.
  • Subjectively some soundstage width and imaging clarity was lost when moving the speakers against the front wall and closer into the corners. I feel the resulting improvement in tonal balance is definitely worth it in my case.
  • The fact that it sounded rather dull and somewhat lifeless at first puzzled me. Not very Altec/GPA like from what I've read. Placement has made a lot of positive difference in this area and simply getting some hours on the system definitely improved things furthers. I'm very happy with how things are right now.
  • I feel like I've might have over damped the enclosure and driver slightly so experiments are in order. Starting with an empty cabinet to hear its effects and then slowly building up an optimal compromise. As I've read this might help the mid-bass fequencies too. I'm also thinking the 24 feet loop runs of 16 AWG wire from crossover to LF driver might impose some unwanted series resistance. I will experiment with shorter cabling and perhaps doubling the runs, too. The above experiments might bring improvement and will post my findings with the changes.
  • This might sound like a negative first impression but that's not the case. I'm simply trying to understand its minor flaws and figuring out ways of optimizing the performance. On an absolutely positive note; I'm in love with these drivers! Driven by the DHT 2a3 Stereomour II amp the sound is so eerily real at times, it's startling. They sound absolutely coherent, as if you're listening to just one driver. Very cohesive and non fatiguing, no horn shout to speak of. In short: they put a smile on my face!

Well, this is about it for now. Hope I didn't bore you to death. Will keep listening and enjoying for now!
 
Last edited:
Glueing the doubled panels together. I couldn't really figure out how to apply even pressure to such large panels so I decided on screwing them together. As you'll notice, these cabs were built for the most part inside the room they now reside in. I had no choice as it's in the attic of my house and they are too big to fit through the stairwell. :)

IMG_0342.jpg

Again, the cabinets are so large that I really had no clue how to glue and clamp them in one go.
I went panel by panel and carefully aligning them using homemade right angle jigs. The joints were glued and screwed together.

IMG_0350.jpg
Almost done
IMG_0373.jpg

Cleats glued in place. Clamping wooden blocks in de corners to increase joint strength.
IMG_0466.jpg

Wooden blocks visible and installed nuts to screw the back panel into.
IMG_0476.jpg

IMG_0479.jpg
 
Routing the driver openings. I wanted to recess the drivers into the baffle and had made test cuts into scrap pieces of MDF beforehand.
IMG_0535.jpg
IMG_0534.jpg

I decided on cutting the holes in the two panels separately and glueing them afterwards. I would dry fit the two panels and screw them in place in 2-4 spots. Then fit the drivers and adjust if necessary. Luckily they fit perfectly first try. The screwed holes are now useful as guides during the glueing process so you get the exact same placement and the panels won't slide.
IMG_0540.jpg
IMG_0542.jpg
IMG_0550.jpg
 
Cutting the braces was quite a daunting task and really took about as long as the entire process until this point.
I'm happy with how they turned out.

I printed out the drawing on the GPA 604 spec sheet onto some foam and cut it out tracing the outline of the back of the driver. Since the vertical back to front brace would have to make contact with the driver I wanted this to be reasonably close in shape and size to have a good starting point. To remind you, I'm no woodworker.. I thought this would be the easiest route. :rolleyes: In the end I needed to cut it a bit larger than the printout. I suspect the drawing wasn't 100% 1:1 to scale. No big deal though :-)

IMG_0696.jpg
IMG_0697.jpg

Decided on a layout for the holey brace.

IMG_0716.jpg

Fast forward 3 days, 2 braces done!

IMG_0728.jpgIMG_0735.jpg

And another 3 day or so and the side to side braces were done.

IMG_0805.jpg

Glueing the braces was actually very difficult to do in this manner. Next time I would probably build the bracing together with the cabinet just so you can get it to fit nicely in one go. This way it was very tight in some spots, but eventually everything went OK. Glued and screwed, again.

IMG_0812.jpg

A little shim needed on the bottom, but the fit worked out great.

IMG_0881.jpg

When dry fitting the back I noticed a slight gap between the brace and the back panel. I thought it would be good to have a solid connection so I added a strip of 1mm ply to fill the gap. The back now fits perfectly.

IMG_0840.jpg

The basis for the driver brace/clamp. The vertical brace to which this clamp attached is off centre from the driver so I needed to take this into account with the clamp as well. One side would be smaller, the other side slightly bigger.

IMG_0743.jpg
 
Veneering and finishing was a very time consuming but equally rewarding process.

Below the first baffle just finished trimming the veneer edges with a router trim bit. The router made life very easy many times and this was one of those times.

IMG_0979.jpg

Next I cut a rabbet edge on both baffles. I was inspired by the look of the Model 17 / 620 Altec cabinets. I really liked that black gap and decided to try it with this build.

IMG_0991.jpgIMG_1032.jpg

After the baffle were glued to the rest of the cabinet I edge banded the plywood visible from the front. I tried the faux miter and succeeded maybe twice. :tongue: The rest of the cabinet was already covered in veneer at this point.

IMG_1042.jpgIMG_1043.jpg

Sanded to 320 grit and then applied 3 coats of Danish Oil. That stuff works beautifully with this wood type. I love the way it makes the grain and colour pop in a natural and understated way.



Quick check to see how they fit in the room. Really happy with how they turned out.
 

Attachments

  • IMG_1083.jpg
    IMG_1083.jpg
    444.7 KB · Views: 41
  • IMG_1084.jpg
    IMG_1084.jpg
    353.7 KB · Views: 54
  • IMG_1117.jpg
    IMG_1117.jpg
    544.8 KB · Views: 58
Installing drivers and insulation.

IMG_1146.jpg

Driver clamp with 5mm EPDM gasket material

IMG_1127.jpg

Insulation fitted to back panel. Leaving the port area free of acoustic damping material

IMG_1095.jpg

The rest of the guts and clamp fitted.

IMG_1149.jpg

Wiring through the port for now, will tidy up (maybe) once tweaking has finished.

IMG_1156.jpg

Done!

685452BE-AC03-4AA5-9B7F-B0A3C43E1660.jpg2899B353-01B3-4CE6-B7FC-B060980EC81D.jpg

That's about it!

Cheers,
Thijs
 
Last edited:
I really like the black painted rabbet around the baffle's perimeter. It helps break up the baffle, aesthetically, and makes it 'pop' forward visually, imo.

Looks like a very well executed build. When I first saw the braces, I thought for sure you had them CNC'ed. I've cut quite a few like that 'by hand' as you did, and I know that process can be tedious, to say the least.

Now you get to do all the fun tweaking, tuning, room placement, room treatment stuff!

Cheers~
 

Audionut

Next Round Is On Me
At 36mm; 1.5”, this cabinet is pretty solid and more damped than most (mine is half the thickness panels (18mm or 3/4). Some may disagree with me but I’d remove some of the bracing or all and add back as needed. Insulation is spot on.
Great build thread too!
 
At 36mm; 1.5”, this cabinet is pretty solid and more damped than most (mine is half the thickness panels (18mm or 3/4). Some may disagree with me but I’d remove some of the bracing or all and add back as needed. Insulation is spot on.
Great build thread too!
I get the reasoning behind the super-rigid cabinet, but I think the Altec engineers knew how to make their drivers shine. Mine will get the floor moving well before cabinet resonance of any significance can be observed.
Mad props to the OP, though. I can’t even stand up in my attic, let alone build anything up there. Beautiful work Thijs!
 
Thanks for the kind comments everyone!

I really like the black painted rabbet around the baffle's perimeter. It helps break up the baffle, aesthetically, and makes it 'pop' forward visually, imo.

Looks like a very well executed build. When I first saw the braces, I thought for sure you had them CNC'ed. I've cut quite a few like that 'by hand' as you did, and I know that process can be tedious, to say the least.

Now you get to do all the fun tweaking, tuning, room placement, room treatment stuff!

Cheers~

The braces were indeed tedious to cut, to say the least. Especially since I could cut maybe 2.5 holes before the vacuum hose and power cord got twisted and stuck. 😅

At 36mm; 1.5”, this cabinet is pretty solid and more damped than most (mine is half the thickness panels (18mm or 3/4). Some may disagree with me but I’d remove some of the bracing or all and add back as needed. Insulation is spot on.
Great build thread too!

Hey @Audionut, good to see you here!
I definitely see where you're coming from. Unfortunately it's not really possible for me to remove any of the bracing. They're glued and screwed in place, so I'm kinda stuck with them. 😅 Thanks for your kind comments!

I get the reasoning behind the super-rigid cabinet, but I think the Altec engineers knew how to make their drivers shine. Mine will get the floor moving well before cabinet resonance of any significance can be observed.
Mad props to the OP, though. I can’t even stand up in my attic, let alone build anything up there. Beautiful work Thijs!

Hi @UncleBingo, thanks for chiming in! I definitely agree with you that they knew what they were doing. I might even explore that path, who knows! It would be fun to compare this build to a tried and true design like the 620 cabinets.

Cheers,
Thijs
 

Audionut

Next Round Is On Me
Thanks for the kind comments everyone!



The braces were indeed tedious to cut, to say the least. Especially since I could cut maybe 2.5 holes before the vacuum hose and power cord got twisted and stuck. 😅



Hey @Audionut, good to see you here!
I definitely see where you're coming from. Unfortunately it's not really possible for me to remove any of the bracing. They're glued and screwed in place, so I'm kinda stuck with them. 😅 Thanks for your kind comments!



Hi @UncleBingo, thanks for chiming in! I definitely agree with you that they knew what they were doing. I might even explore that path, who knows! It would be fun to compare this build to a tried and true design like the 620 cabinets.

Cheers,
Thijs
I may be going back on my own word here; but having a dead quiet/properly braced cabinet allows you to get an un-colored picture that only your speaker paints. I think you are ahead of the game and like @gable noted you just need to perform minor system/room tweaks to really let these speakers shine!
 

Fran604g

Just Call Me Junior
Stunning! I'm impressed with the accuracy of your fabrication and fitting of the various interior bracing you attained - all by hand. Very impressive work, and a beautiful result. Congratulations.
 
Solid looking build @Thijs! It sounds to me like you know what you need to do. Have you experimented with damping yet? It seems you have used more than what is used in my Bisons, though the materials are different. Did you change the runs from crossover to speaker? What crossover are you using? Cabinets and crossover make or break these drivers.

I agree, the GPA604 drivers are very nice indeed, when implemented well, which might not be easy. I've been looking for alternatives and a "better" set of compromises, for me, are not easily found. The midbass driver is especially good for SETs - better on paper than the 414 and 416 IMO, if you can put them in big enough cabs. Still, I'd like to try the 414 with an AMT mounted in a waveguide with minimal crossover...

I have not had to tweak my Bisons much at all. Perhaps my expectations are different and/or my points of reference fewer and I (happily) don't know what I'm missing. The builder of my Bisons was familiar with the MLTL alignment and its tuning so maybe that is part of it - they do sound decent to me.

Altec engineers designed their cabinets at a certain point in time, for a specific purpose, with perhaps different constraints to our current builds for domestic use. Classic Altec cabinets/methods can probably sound quite good. When planning my build I started with what I wanted to achieve then collaborated on how that might be achieved within various constraints - it didn’t lead to an Altec solution.

That is JMO based on limited experience.
 
Last edited:
Top