Nest Theater Part One - DIY High Efficiency Speakers - The DRC BLHs 24Feb20

In a remote and esoteric corner of the audiophile realm lies a particularly rare combination of pursuits - high efficiency full range speakers AND do it yourself. I embarked on designing and building my own horn speakers a month or so ago and now have completed all but the staining and finishing (which I'll need to defer till the weather is warmer)! The speakers sound incredible and can be driven loudly with only a few watts. Bass output is amazing given just a single smallish driver - the power of horn physics. These things, while not too large, are beasts and weigh in near 100# each. This is the first stage of a new theater and listening room for the Nest. The new speakers are being driven so far by a NAD D3045 for testing but will eventually get their own amp and DAC of suitable unadulterated designs.

The design is a back-loaded horn, BLH, and depends on 20 feet of wrapped sound channel within the cabinet to produce the bass frequencies from the back of a full range driver. The Japanese make great transducers and I chose the Fostex 168E∑, an efficient, ~95dB/Wm, full range, 51Hz-21KHz, 2.5#, 6.5" driver. I perused as many cabinet designs as I could find appropriate for this driver and then created a variant of Nagoaka's D37. I refined the horn mouth with an exponential curve and adjusted several internal braces and dampening aspects. I used 9 ply 15mm Russian Baltic birch plywood throughout for the cabinet - the pair required four 5' X 5' sheets. I prepared cut plans for the sheets and had CO Lumber acquire the material and make most of the initial cuts. From a pile of parts I measured, verified, marked and organized the project; I'm building new matching audio shelving too.

The moment of truth comes quickly as one begins to "add value" to each piece with drilling, gluing and clamping. The first effort was to create a mounting for the driver - if I screwed up early here I would just need to cut a new panel with a driver hole. It went fine and the new WEN drill press made nice vertical holes. I used #10-32 screw-in nuts which will accept my #10-32 black machine screws and allow me to install and remove the driver easily. Fortunately the dry fit of the driver was successful.

My basic assembly technique was to use gobs of Titebond III (strong, long working time, water cleanup) spread on with a foam brush as appropriate. Then I'd clamp, measure, and reclamp or screw if needed. I did this for days. The first assemblies were a set of 3 stacked sticks cut into triangular sticks I'd use inside the cabinet for support. Then I built the lower horn mouths.

With my chops whetted I realized I'd need more glue ultimately and bought 2 more pints! The next steps were to methodically glue, align and clamp the outer and sound channel boards & assemblies to one side panel. First though I did a dry fit of all the parts to be attached to the side panel.

After completing the entire internal sound channel on each speaker I added the bottom and braces from internal horn parts to the back panel. I also filled the chamber below the driver with sand. I want the cabinet to be very solid and inert as to producing sound - I just want it to reflect sound, not vibrate and lose efficiency. I also obviously added the binding posts/jacks before buttoning up the second side.

I carefully joined and screwed the second side to each speaker and played the functional (but not finished) pieces for enjoyment and test. Test successful. I actually built two different lower horn mouth designs and can trial and measure each - kind of like tube swapping but actual horn physics altering. I like the expo horn most so far.

As mentioned, these are beasts... I completely covered the side panels each with additional outer panels (lots of glue) and clamped them. I beveled the outer panels for a more finished look ultimately - I'll carry this 45° bevel throughout the pieces in the Nest theater. I also added some base plates and sound isolating feet... again employing the screw-in nut scheme. I reinstalled the Fostex drivers.

The DRC BLHs are ready for listening, finishing and measuring. One can't buy speakers like these unless they were commissioned custom... and then the price would be exorbitant. I know they may have extended range issues at the highest and lowest ends of musical frequencies but wow most of my range of hearing is covered ever so sweetly.