Failed Building, Powering & Measuring The DRC BOnken Center Channel 18Oct23-18Nov23

With the unmitigated success of the small full-range DRC Onkens in the NestMT as a center channel using the Dolby DP564's derived signaling and Willsenton amp, I am moved to take the next step and realize a full 5.1 system. I built a new center channel speaker and will redeploy the original DRC Onkens as L&R surrounds... it was always a possibility in the back of my mind. You may be thinking I am straying from "music first," but I'm not; the DP564 did so well in deriving a center channel I just "have" to see what it does for surround sound ~ and this is an experiment. So, I designed an all new Bigger Onken (that I call the DRC BOnken) which employs the same 6.5" full-range Fostex FE168e∑ as in the back loaded horns I use as mains. The Theil small parameters of the driver really required a much larger Onken box ~ and that makes sense, the 6.5" driver has 226% more radiating area than the 4" driver and Onken designs in general need 85%-90% of that area in port cross section. The front soundstage is now formed by all the same drivers, and yields a very consistent timbre. While I first did a dive deep into horn alternatives and the associated enclosure math &  simulations, all my ideas were just too big for a center, especially too tall. There are three primary aspects to this upgrade: the design & build of the DRC BOnken, the relocation & install of the DRC Onkens as surrounds, and the signaling & control to enable all these speakers to play great music in the Nest Music Theater. Of course I'll offer final impressions, observations, and measurements.

I started by iterating models & simulations for the new Onken with different drivers and cabinet & port dimensions, to get some relevant bass. I stuck to my guns and selected the bigger FE168e∑ for reasoning prior given. It is on the hairy fringe of applicability in an Onken-like cabinet and required very long ports (hence the 20" depth on the DRC BOnken) as a result.

I just hand-sketched the cabinet idea and then went to work on the detailed dimensions and cut sheets for the 15mm Baltic birch material I've preferred before. Mainly, the 11 layers make this stuff very stiff and inert, plus the birch is pretty in good grades. Often in the US our plywood has many fewer layers and is comparatively flimsy. I iterated the design some more before finalizing the precision cut sheet. CO Lumber's Matt again did the cutting on two 5' x 5' panels for a reasonable price. I was pleased that the Ukraine war sanctions had not limited availability of Russian plywood to me ~ plenty was purchased in advance I was told, and is still in US warehouses. But if you need some, buy now. Yeah, the BOnken is big with an OD of about 12" X 20" X 35" with a main cavity of 9 liters (~14 liters all up externally), but it all fits OK in the NestMT center channel spot. I don't know whether I will bevel the port areas on the new BOnken as I did on my originals or not; I don't "need" to so do for this construction so will measure audio results first and do some listening. Without the bevel there is a chance some of the sound waves from the ports will mix poorly with the directly radiated sound of the driver. I read a lot on whether offsetting the driver to one side or another would make a difference in performance ~ I concluded it would not so I centered the driver.

I received the cut lumber from Matt/Colorado Lumber and verified dimensions then transported the whole lot to Summit Co for the build.

Step one was to build the channel boards and stain them and the interior of the sides in the right spots before securing to the outside sides of the BOnken.

Next I secured the front and back to the completed side/channel pieces. To reinforce the structure, I added rib pieces down each corner of the interior of the BOnken. This also helped register the front and back pieces evenly with the sides... I did screw through the ribs to the sides, as well as glue them. The BOnken is taking form!

I had to roust all my longest clamps from COS to attach the top and bottom in place. I might need to get back into the cabinet for adjustments so I secured the top and bottom using black flathead wood screws. But first I added 3/4" felt damping material (secured with spray adhesive) to the main cavity as it is not easy to get in there after it's buttoned up. I installed the binding posts too before putting on the top and bottom. The clamps here are not to secure glue but to pull the pieces into alignment before securing the top & bottom with wood screws.

I wired up and then secured the driver in place with a thin gasket and installed the BOnken in its rightful spot atop the center channel shelf.

The DRC BOken is a pretty efficient speaker at -91dB/w, but it sill needs something to power it and I will not overpay. The requirement for a mono amp of ~20w is a bit odd. All the single ended EL34 amps were 2 channel. I dipped into my vendor bag and have chosen the brand new, and lauded, Schiit Gjallarhorn solid state amp to drive the BOnken. It is unexpected for me to not employ tubes for this application and we'll see how long it holds its position ~ good so far; it rocks. This odd new offer from the capable Schiit team is 2X10w into 8Ω or bridged mono for 1X30w into 8Ω. It also has the advantage of employing an XLR connector at the signal level ~ this will connect well to my Dolby DP564 for the center channel signal. It's also odd for Schiit to just offer female banana outs for the speakers - but that's fine with me.

The BOnken measures fine; it does have a predicted "shout" in 1K-2K range which many FE168e∑ speakers face... I will tame that later. Nonetheless, without further EQ, the BOnken sounds good as a center channel for the NestMT. I am not concerned with the low end runoff for this vocals-oriented channel.

However, as you may have noticed from the "installed" image above, it is not only big; it is too big. Maybe it is just my sense of what I want ascetically as well as sound-wise but the BOnken is enormous and not what I want. I knew the size and measured it out in place before building it but the reality is stark evidence. As much added value as there is in this new cabinet, I have decommissioned it and returned the NestMT to a 3.1 system for now. Back to the drawing board. Grrr...

On top of the "too big" aspect of the BOnken, the Schiit Gjallarhorn amp's left channel failed when attempting to have it drive the DRC Onkens just for grins. It is going back to Schiit while I figure out my next steps for the center channel. Too, the Fostex FE168e∑ will be sold on eBay.