Nest Theater Part Twenty Six ~ Making The DRC Onken Speakers 15Jun23-30Jun23

Sticking to my over-arching design principals for the Nest Theater of music first, DIY, high efficiency, low power, full range, and inexpensive, I sought speakers for the center (and surrounds?) which were built from the same family of high efficiency Fostex drivers as the originals in the back loaded horns of my L&R mains. But BLHs are too big to add three more to this room ~ the center would block the TV screen and the surrounds would block the walkways. I needed to find a strong bass reflex idea to compliment the BLHs, and I did so with an Onken-derived design loaded with the Fostex FE108ES (∑) full range driver. The Fostex FE108ES is a 4" version of the 6.5" FE168ES drivers in my main BLHs and are themselves very efficient with sensitivity of 90dB. As implied, I chose them for space considerations in the Nest Theater room. I was pleased to find some new ones on eBay as they have been discontinued.

The Onken originated in the 60s in Japan from a company bearing this name. It became fairly successful in its own right in Japan but mostly through Japanese amateurs building their own. It is most often a nearly aperiodic bass reflex enclosure which is heavy, braced, felt lined, and inert. The bass ports are typically along each side of the front of the cabinet and equally sized ~ not perfect alignment for bass, but cool. Altec Lansing in the US adopted it and made a number of speakers on the Onken idea, as did Dynaco for aperiodics, like their A25.

An aperiodic enclosure is one with flow resistive ports which enables a smaller box to contain a driver which might otherwise deliver unwanted peaks in some bass frequencies. The core port design is for bass production and while I need some mid-bass from the center channel I am not too concerned with deepest bass response from these speakers (always elusive with small drivers) as I have a subwoofer. The average sensitivity/efficiency of an Onken is 10dB higher than average speakers ~ and I do want that. The Onken looks keen and will compliment my DRC BLHs well as to aesthetic and performance and efficiency. My enclosures respect the look of the originals but are smaller and have beveled fronts over the bass ports, to disperse the driver's direct sound more. I couldn't reach Dave at Planet-10 to sell me plans, as he's done the CAD for many smaller Onkens, so I built my own with CAD and modeled, simulated, and analyzed results till I was satisfied "on paper."  I created a "cut sheet" for a new piece of 9 ply 12mm Baltic birch to guide CO Lumber in preparing all the pieces I'll need to build two 9"x12"x14" DRC Onkens and some shelving for gear.

20Jun23... I picked up my cut pieces for the DRC Onkens today and transferred everything up to the Nest. I verified the dimensions cut by CO Lumber as I've done prior... I can strongly recommend 12mm or 15mm Baltic birch for speaker builds and have found CO Lumber excellent for precision panel cuts ~ talk to Matt. So, on with the build. The first cuts are the deepest cuts and Kaizen taught me to root out quality issues early ~ I marked, drilled, and screwed in the 8 driver screw-in nuts first as I wanted to remake the front baffle panels early in the process if I messed up anything. I didn't, and the drivers mounted/unmounted sweetly to the front panel on my dry fit.

I next built the base reflex ports area by gluing my spacers to the "channel boards."

And I built out the new rack for the DP564 and the R8. I used the drill press to drill for screw-in nuts on the rack standards and legs. I used my table router to bevel the shelves as I did in the originals I am mimicking. I did have to buy more screw hardware. I also built a single center channel speaker stand, in a similar style, to hold the DRC Onkens.

Back to the speakers ~ I next took the port channel boards and speaker side panels and stained the areas I won't be able to reach once assembled. Black is always messy. I then glued and clamped the sides to the port boards. There are 6 ports in each DRC Onken, 3 on each side, and the channels go almost all the way to the back of the speaker.

It's the next day again to allow glue to dry fully. I squared flush the port side assemblies with a belt sander. Then I built the first DRC Onken except for the top. Clamp City... so many required that I had to buy more and then didn't even have enough to build the 2nd one simultaneously ~ that added a day.

Once dried, with the top still off and the internals fully accessible, I drilled the back for the speaker posts. I chose heavy duty rhodium-platted copper binding posts; they are sturdier and longer lasting than gold-platted versions. Remember "flow restricted ports" for aperiodics above? ~ I lined the speaker interior with 3/4" wool fiber pad as damping, covering the bass ports. I used spray adhesive to secure the wool to the interior plywood and cut holes in it for the binding posts. I then completed wiring from the binding posts to the eventual driver spot. And finally glued the top on the speaker.

I took a wide shot of some of the tools I used to make this stuff... left to right: a hand drill, a table router, a vacuum, a table saw, a drill press, some squares, and a ton of bar clamps. Both boxes are assembled and are finishing drying.

You might be wondering where those sexy custom-designed bass ports are on the front, since my front baffle covered them right up. That's the next step ~ two challenging bevel cuts on the entire speaker assembly that will reveal the ports on either side. It's surely a nervous time to make this cut with a home table saw on a speaker that has so much added value in it. It's a cut that will even require I add a custom rip fence to the saw and remove flyback protection stuff. Here goes...

30Jun23... Hell yes! The DRC Onkens are born 30Jun23 11:30AM MDT. I'll load the drivers and make some measurements in their home spot before final finishing work.

OMG, I measured the new DRC Onkens driving through the DP564 & R8 as I'd hope they play. They measured very flat from 80Hz-16KHz IMO; the small dip at 180Hz is no problem as that is kind of a muddying frequency for voice anyway. They are so flat in fact that I will not be applying any corrections to integrate these into the NestT room & soundstage. The measurement pic shows the DRC Onken center channel freq sweep compared to the stand-in NHT speakers, and my house curve. After measuring I took a little time to listen to just the DRC Onkens in stereo and the new full NestT system. So sweet.

Frankly I was worried the DRC Onkens might not perform well enough or that the Fostex drivers would be finicky in a bass reflex cabinet or I couldn't otherwise improve on the NestT 2.1 system at the end of the day. But, the new 3.1 version is significantly better; it's spectacular. The DRC Onkens and their associated source and amp are an excellent addition... the front sound field is indeed coherent now as well as wide & deep. On with the finish work on the speakers and shelving. To match the Nest Theater's other gear I stained the new audio shelving and the DRC Onkens in True Black and then sprayed them with multiple coats of polyurethane. Looking good.

04Jul23 Final NestT Center Channel Setup... I received some missing hardware and was able to assemble the final shelving for the center channel at the NestT. Gear recap for the center includes: Dolby DP564 deriving a center channel signal, Willsenton R8 loaded with EL34s in triode mode amplifying, and the keen DRC Onkens as transducers to sound. Tests and demos over the holiday days with the Browns, BK, and Kala & me were very successful... sounds and looks great. I've been diggin' listening to Abby Holliday's drop on Audiotree Live recently but prefer her voice on the tunes where it's unadulterated.

Here's the final frequency sweep sound pressure levels of the DRC Onkens, the DRC BLHs (w/ super-tweeters & corrected by Dirac), and the Hsu sub, all discretely measured. Basically when listening you are adding all this together into one glorious sound. I just thought it keen to show each element. The Nest Music Theater is very clean from 15Hz to 20KHz ~ the sub does it's thing from 15Hz-150Hz, then the BLHs & Onkens are superb through 16KHz, after which the supertweeter keeps going till ~20KHz for cymbal crashes and the like. There are no crossovers or feedback in this system; its mostly natural and I love full range dynamic drivers. Its sound is better than at most clubs, theaters, and ballrooms at which I’ve seen live music… a number of radio studios’ live feeds are excellent sources and vital for me. Sure I’ve been to a dozen concerts already this year but most often my home setup sounds better.