Build: 3-way Scanspeak towers

[MtBiker]

Background and Design Premise

I built my first pair of DIY speakers 22 years ago, and despite planning something much more ambitious and expensive just a few years later, I managed to instead build speakers for friends and family and indulge other hobbies in those intervening years. Now I have made up for lost time by building something overly ambitious. I made the cabinet construction more difficult than it needed to be, not only to get an nth degree of additional performance, but to make something visually striking and different, to experiment with new techniques and materials, and to confirm I am capable of doing all of this. It had a lot of educational and entertainment value. Deliberate hurdles include:

• Physical design impossible with pencil and paper; blueprints needed to be created using CAD.
• Complicated construction techniques beyond my initial skillset.
• Unproven materials and joinery.
• Shape requires painting, which is 100x as difficult for me as staining and finishing real wood.
• I have never been happy with the reliability of my measurements <500Hz, and this is a 3-way.

A few years ago, I built a dual mono subwoofer using a pair of 12” subs facing each other and housed in separate cabinets that were coupled together with threaded rods. This is push-push (not push-pull), so there is no cancellation of even order distortion, but there is no exposed magnetry and the force cancellation does work. The current project is a 3-way tower that offloads bass responsibilities to that sub.

Project Highlights

• Unique, sculpted appearance.
• Cabinets are low diffraction, high mass, highly damped.
• Creative construction techniques to account for my lack of tooling and skill.
• All drivers from Scanspeak. Tweeters and mids decoupled from cabinet with Sorbothane gaskets and anchors. Construction-wise, this worked as anticipated, but I have no measurements to prove it was worth the hassle.
• Active DSP crossovers.
• Hypex amplification with one amplifier channel per crossover region.

Cabinet Design

All 3D modeling was done in Sketchup, although there is additional routing and shaping I do not know how to do in Sketchup that I did on the completed speakers. I designed the exterior shape first, then worked my way inwards placing collections of parts on a separate layer so I could work on them in isolation. What I ended up with was the ability to explode the speaker and dimension all parts. Anything too complicated to build from a picture was printed 1:1 and adhered to plywood for cutting. For example, the horizontal bracing that is not perpendicular to the baffle has a different shape on the top than it does on the bottom, as the angle to the side of the brace changes several degrees from the front to the back. I had to first cut to the larger of the top/bottom profile for these pieces and then cut/sand at a variable angle to taper to the smaller profile. This worked reasonably well, but with ~1mm tolerance on all finished dimensions, it added up to a lot of slop during assembly, and I had to fill a lot of gaps with glue or resin.

3way_curved_5.0_front.jpg3way_curved_5.0_iso_front.jpg3way_curved_5.0_side.jpg3way_curved_5.0_interior.jpg

Project Details

TMWW + SS: 3-way active tower speakers plus dual driver mono sub with 8 channels of amplification.

Drivers:

• Tweeters: Scanspeak Illuminator D3304/6640 1” beryllium
• Mids: Scanspeak Revelator 18M/4631T00 7” slit cone
• Woofers: Dual Scanspeak Revelator 22W/8857T00 8” aluminum
• Subwoofers: Separate, part of previously completed project. Dayton Audio RSS315HF-4 12" Reference HF Subwoofer 4 Ohm

Cabinet:

• Portability of extremely heavy cabinets improved by building separate TM cabinets which sit on top of WW cabinets, isolated by Sorbothane sheets.
• 1-1/2” thick baltic birch baffle laminated from two sheets with silicone adhesive constrained layer. Exterior of baffle slightly curved horizontally, with 1-1/4” roundovers on vertical edges, expanding into additional shaping around tweeters.
• Driver recesses inside baffle enlarged with cove bits to improve airflow and avoid resonances.
• Mids and tweeters decoupled from cabinet with 1/8” Sorbothane gaskets, and are bolted to hurricane nuts which are also decoupled from the cabinet with Sorbothane isolation bushings.
• Woofers are rigidly mounted with T-nuts.
• Baffle tapers from the floor up, and is tilted back 5 degrees to improve physical time alignment.
• Skeleton built from dimensioned parts printed 1:1 and transferred to plywood using a light spray adhesive.
• Curved walls built using internal and external shells with a 3/4” gap and minimal use of spacers.
• Hollow cabinet walls filled with 60/40 mix of dry cement powder and epoxy to provide mass, strength, and damping. Exothermic heat is kept low via the mass of the cement and the use of a slow hardener. Experimentation has resulted in castings that are substantially more inert than baltic birch.
• Additional internal damping provided by aluminum-backed butyl sheets and 3M rubberized auto body undercoating.
• Internal stuffing is blue jeans insulation and pillow stuffing.
• Exterior seams sealed with shellac prior to priming to prevent showing through paint. This has worked well for me in the past, but did allow a few seams to show through this time.
• Painted with textured Rustoleum aerosol paint which provides a matte finish and some scratch resistance. However, getting an even texture on a large surface is difficult.
• All connectors are invisible, as is the star quad 14g cable run under the carpet.

Crossovers:

• Tweeter passive circuit:
  • 33uF Mundorf Evo Oil cap for protection, in series with
  • 2 paralleled 10.0 Ohm 12W Mills resistors to reduce hiss.
• Active: Pair of MiniDSP 2x4 HD, one per L/R channel
  • EQ each driver ~1 octave past each crossover point and apply textbook filters
  • Room correction of bass frequencies.
• Initial implementation:
  • LR4 IIR at 80Hz, 250Hz, 1.8kHz.
• Final implementation:
  • LR4 IIR at 80Hz, LR2 FIR at 300Hz and 2.5kHz made possible by well-behaved wide bandwidth drivers

Amplification:

• Tweeters: Hypex NC400 stereo amp with single SMPS600, already tested at ASR
• Mids: Hypex NC400 stereo amp with single SMPS600
• Woofers: Hypex UCD400 + HXR stereo amp with single SMPS1200A400
• Subs: UCD400 stereo amp with single SMPS1200A400

Sample construction details:

IMG_2537_1.jpgIMG_2638.jpgIMG_3068_1.jpgIMG_3208.jpgIMG_3687 (Edited).jpgIMG_6681_1.jpg

Full design and build galleries.

[MtBiker]

Sample results of completed product:

TV_room_7.2.jpgIMG_6839-Edit_1.jpg

XO415 FR 22in FR and phase.pngXO415 TMW 22in drivers XO targets.pngXO415 TMW distortion perc.png

425 Vector average step.png425 Vector average waterfall.png20200604 Horizontal Directivity (Normalized).png

Bass leveled via room EQ <100Hz
XO414 TMWS LP waterfall 20-200Hz.png

and spliced into off axis measurements to generate this
20200604 SPINORAMA spliced 200Hz.png

[YSDR]

Whoa! Looks gorgeous, even the measurements too.

[jamwat]

Wow what a nice pair of speakers.
You have some very lucky friends and family if the speakers you made for them are anything near these.

[H713]

Beautiful cabinet work... I'm very impressed. This is where DIY has an edge. You won't find a cabinet like that in a commercial offering without spending a huge amount of money.

Just out of curiosity, how do the MiniDSPs compare to the high end offerings from companies like XTA? I've always thought they were kind of interesting, but never really taken them all that seriously. You've clearly gone for a pretty high-end build though, so maybe it's time to reconsider.

[radule]

How much time delay was needed to tweeter section, to get that nice LR2 mid-tweeter crossover?
All your works looks perfect for me

[zman01]

MtBiker,

Superb execution and the results are something that you can be proud of.

[hifijim]

I am very impressed. Excellent measurements, and the aesthetics are top notch.

To fill the 3/4 inch gap between the inner and outer wall, how much cement and epoxy did you use? I assume by "cement" you mean Portland cement...

[MtBiker]

Originally Posted by H713

Just out of curiosity, how do the MiniDSPs compare to the high end offerings from companies like XTA? I've always thought they were kind of interesting, but never really taken them all that seriously. You've clearly gone for a pretty high-end build though, so maybe it's time to reconsider.

The only other DSP product I have experience with is a DCX2496, which had a flaky display I could never debug so I never really used it. I also did not like having to use RS-232 to connect and it doesn't provide FIR. The problem with both my 2x4HD units is the analog input creates a ground loop with amazingly loud hum and buzz, both of which are solved by either using the USB input (I have not tried Toslink) or line level Jensen isolation transformers. Even with excessive A/D/A and inline transformers, the flexibility and results make it worth the compromises.

What I need is either an 8 channel DSP with digital in (one box for convenience and volume control) and higher quality DACs, or 8 channel DSP with digital out to something like the Okto Research DAC. I am willing to DIY some or all of a replacement solution but am waiting for the DSP to advance one more generation.

[MtBiker]

Originally Posted by radule

How much time delay was needed to tweeter section, to get that nice LR2 mid-tweeter crossover?
All your works looks perfect for me

This is what I have in my MiniDSP now. I can't remember if it was based on measurements at 6ft outside or the LP indoors, but I know one was used for time of flight readings and the other for validating.
T: 0.05ms
M: 0ms reference
W: 0.01ms
S: 1.14ms

What is non-obvious is the FIR delay, which is I think was 3ms for TMW and not the sub. So add 3.0ms to the mid and subtract 1.14ms for the sub, and you get 1.86ms difference which is 24.8inches != the physical distance tower to sub. Unlike the delays for the drivers in the same cabinet (which were based on measurements and confirmed with step response), I arrived at the offset to the sub based on trial and error measurements for what blended the best with the towers. Using a tape measure provided terrible results.