12 Loudspeaker Kit
Approximately three years ago I first became aware of Dr. Earl Geddes and his research on distortion perception and his Summa Loudspeakers. I was very intrigued by his research and design choices, making the Summa a speaker I really wanted to hear but with direct only sales and my living a long way from Michigan that was just not going to happen. Fast forward two years and Dr. Geddes had started a thread on diyaudio.com discussing the offering of a kit of his 10-inch version of the Summa called the Nathan. A few months later plans were made to offer a 12-inch version called the Abbey. At that point I put down a deposit and took my place in the queue for the Abbey. I fully expected delivery would take six months as Dr. Geddes had not even produced a prototype of the 12-inch waveguide at that time. Almost exactly six months later I had all the parts for my Abbey kits.
All of the Geddes designs use a OS (Oblate Spheroid) waveguide mathematically designed to produce the fewest HOMs (High Order Modes) possible and a patent pending foam insert to control the HOMs that cannot be eliminated. A high efficiency B&C compression driver employing a Mylar diaphragm is used with the waveguide. The woofer is also from B&C with a sensitivity of 95dB/W/m and nominal power handling of 1000 watts and continuous rating of 2000 watts. Yes, that is correct, B&C lists those as the power handling levels. Suffice to say power compression is a non issue with this driver.
Time To Get Dirty
For my tastes the cabinets did not have enough bracing with only a 0.75-inch square brace from the front to rear panel and the same from side to side. I replaced the front/rear brace with 2-inch wide oak and the side brace with two of the 2-inch oak braces. Additionally I added 18 oak stiffeners to the side panels. I have no measurements of the results since I did this while building the cabinets but I can say that they sound much less resonant on the knuckle test than before I added them. The cleats attaching the rear panel were also replaced with oak as I wanted the extra stiffness and don't care for wood screws into MDF.
The capacitors supplied with the crossovers are Solen polypropylene While there is nothing wrong with them, I chose to use Jantzen Z-Superior capacitors for the two in the tweeter signal path. I used the supplied Solen caps and Madisound air core inductors for the LCR networks. I assembled the crossovers on peg board so I could use zip ties to secure the caps and coils and to allow easy installation in the cabinet. I used a high purity 16ga copper wire inside the speakers and WBT NextGen binding posts that I had on hand. The binding posts fit perfectly with no modification.
The rear panel is secured with 44 pan head wood screws and sealed with 0.25-inch thick closed cell foam rubber. The screws are spaced every two inches around the perimeter and two in the front/rear brace. I'm sure fewer screws would have been fine but I wanted to ensure a total seal and lack of vibration.
Lastly I built a pair of stands since every pair of speakers in my main system have always been floor standers. The stands were built from 1 1/2" maple butcher block, 4" ABS pipe, some 3/8" threaded rod and filled with sand. They are essentially the same as the "Stubby" described by Scott Faller some years ago.
The highs are clear and well defined but lack the last bit of air that some high end speakers provide. My main speakers for the last four years have been Meadowlark Blue Heron 2. They employ the Scan Speak R2904/7000 tweeter, a very nice piece to be sure. In some ways the compression driver/waveguide combo is superior to the SS ring radiator, particularly in dynamics and apparent freedom from distortion. However the SS tweeter has a greater sense of delicacy and greater top end extension. On a whim I decided to bypass the two tweeter caps with some high end Mundorf caps that I received from a friend. I had one pair of Mundorf Supreme Silver/Gold & Oil .01uF and one pair of Supreme Silver & Oil .1uF. I bypassed the large 8uF cap with the Silver & Oil and the smaller 1uF cap with the Silver/Gold. I let them play for a few days before any serious listening as I wanted to allow for any changes that might occur. The addition of the Mundorf bypass caps opened up the top end a bit and improved the imaging as well.
The midrange is notably less colored than the midrange of the Meadowlarks. The midrange in the Blue Herons was always just a little shouty, having a slight resemblance to cupped hands. The Abbeys just don't have that. Piano, vocals, horns, acoustic guitar; they are all just clean and clear. I find it very difficult to listen passively with the Abbeys, I can't sit and read while music is playing, I just keep getting sucked into the performance by the sound.
One often reads about discovering new things on familiar recordings. I pulled out Pink Floyd's Dark Side Of The Moon primarily to size up the sub woofer integration (more on that in a minute), and was grabbed by just how clear and intelligible the spoken words at the beginning of track 1 were. I've owned this since it came out and while the words were not new, the clarity with which I heard them was. I have heard this countless times on many systems and never have I been so struck by the delivery of those two sentences.
Then we have the bass, or lack thereof. Even though a 12-inch woofer is used the Abbey does not play low. The response drops off quickly below 60Hz, requiring the use of one or more sub woofers. Dr. Geddes suggests the use of multiple sub woofers, preferably three or more, to distribute the bass modes in the room and promote a smoother response. He is not the first to suggest this but has written a great deal on the subject and offers excellent information on how to set up a multi sub system. I'm currently using two REL Storm III subs on loan from a good friend. Not quite the three suggested but certainly better than one. Over the years I have developed a dislike for ported speakers as I can usually hear the port resonance and find it very distracting. The transmission line design of the Meadowlark's did not have this problem in my old listening room but I was never 100% satisfied with the bass in my current listening room. With the two subs in the system I am quite happy with the bass of the Abbey's. My room is a dedicated space so I have the luxury of full room treatments and placing things where they sound the best. I'm sure over time I can get things even better but I think I'm fairly close.
While the sensitivity is quite high at 95dB/W/m the speakers really like some solid power, perhaps they need an amplifier with good current delivery to sound there best. I experimented with a couple of different amps, primarily my R.E Designs LNPA 150 mono blocks and a custom 100WPC OTL amplifier. The LNPA's will deliver 75W into 8R and 150W into 4R and about 260W into 2R while the OTL will manage 100W into 8R and about 70W into 4R. This is quite good for an OTL and the Abbeys were very nice with the tubes but they really come to life with the higher current capabilities of the R.E. Designs amps. Dynamics are simply amazing when combined with these amps. I also tried a little Rotel 1080 I have been using for the woofers in my active bi-amp setup. Though it has more than double the power of the LNPA it cannot deliver the dynamics and effortless sound of the LNPA and is not even close in terms of sound quality.
Lastly the dynamic capabilities of the Abbey are shocking, being very similar to big horn speakers without the coloration that I hear in horn speakers. They will play at any level that I care to listen at with no hint of strain or compression. Even at low volumes the music has dynamic swells that are just not present with most loudspeakers and at high volumes some recordings are just mind blowing.
Another cost is that of finishing. They are raw MDF along with the stark white plastic of the waveguides and require some type of finish. The use of 1-inch radii on the front, top and bottom edges along with the waveguide makes veneer impractical or at least very difficult. Most folks will opt for some type of paint finish. I have not started on the finishing process yet since it's simply too cold for finish work in my garage and I am unwilling to use nasty primer and paint in my basement. In Colorado the humidity is generally very low and my listening room is kept at a constant temperature, I an hoping the bare MDF won't absorb too much moisture by spring when I can begin the finishing process.
Some of the other builders have reported taking time to become accustomed to the sound. I found the sound immediately appealing as did one of my audio pals who's hearing I trust completely. The other audio pal who has listened to these also liked them very much. Will you like them? If you value lifelike dynamics, great clarity and resolution with a smooth though slightly recessed treble all at a reasonable cost, then yes, you may very well enjoy these as much as I do. They get me closer to the music than any speaker I have owned in the 35 or so years I've been tinkering with audio reproduction.
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