Open Back Design Concept: Adjustable mid-scoop.

[matevana]

My self-imposed goal has been to build a new open back system each year for the past four, taking into consideration what I have learned through reading, experimenting and measuring throughout the year. For the first three years that meant a corresponding increase in complexity... from a full range set-up, to two ways with bi-amping, mini-DSP, etc. All of these progressions had a few things in common. (1) increased size and weight; (2) questionable cost-benefit analysis; and (3) over time the complexity yielded to inconsistency and eventually frustration.

Inspired by the common-sense approach of folks like Martin King, this year I decided to revisit what I found so appealing about OB design in the first place. The primary goal was as simple as the design itself; namely, to build a classic, room friendly system that could be driven by a single amp to reasonable levels. A secondary goal was to test and implement an alternate way of addressing dipole roll-off, through the use of a variable rheostat that enables the listener to dial-in a degree of “mid-scoop” that is then used to transition between drivers. Finally, the design must perform equally well in a 2-channel critical listening system and pull double-duty as the cornerstone of a multi-speaker home theatre system.

Configuration was the first part of my analysis. I had decided early on that a single full range driver would not likely provide adequate levels when asked to perform HT duties. I have also decided through earlier design that in my setting, I prefer the sound of dipole radiation only up to a certain frequency. To me, the perceived benefit of more natural sibilant cues in the higher octaves gives way to a degradation of the intended stereo image. This presented a bit of a design dilemma since most full range drivers are cones that freely emit a rear wave. I could attempt to absorb or deflect the rear wave, but that would only increase complexity and size. An alternative was a high quality chambered dome that could play low enough to mate with a variety of low frequency candidates. One such driver is the Usher 9950-20 which I have used in many designs elsewhere. With an Fs of 560 Hz, it can be crossed at 1800 Hz pretty effortlessly and even lower with a fairly steep slope. It is also to me, very natural sounding and uncolored and has an average efficiency of around 88dB. It requires little or no crossover shaping.

The selection of a good low frequency driver was a daunting task. Again I had decided through earlier experimentation that I much prefer the sound of flat panel suspension to either H-frames or U-frames (open or damped) when the driver is asked to perform above 200 Hz. Perhaps in a 3-way design this would be fine, but in keeping with the primary objective, this was not an option. Since no external signal processing was to be used, and only a single amplifier, the driver would need to be highly efficient so the mids could be attenuated resulting in a curve that favored the low-end. This would be done in an effort to compensate for the additional dipole roll-off. I have also discovered over the years that drivers with low weight/low moving mass do well in dipole arrangements, particularly where baffle vibration is concerned. The issues concerning a driver’s total Q in a dipole alignment have been thoroughly vetted on many websites. I completely agree that a high-Q driver yields a warmer low end, but I am not sure that should be the deciding factor here, particularly when other methods of LF compensation are sought. A well controlled motor structure (low Qts) on the other hand, may also contribute toward a greater linearity and corresponding lower distortion, particularly at high excursion. Finally driver size had to be considered. Having selected the Usher 9950 as the corresponding HF driver, I knew I needed a LF driver that was comfortable playing up to around 1800 Hz. B&C Speakers of Italy has been making high quality drivers since the mid 1940’s and have recently gained acclaim as the driver of choice in the Geddes line of speakers. They are extremely well made, have fairly smooth response curves, low moving mass, low weight, are highly efficient and have high excursion capabilities. After much analysis, I selected the 10NW64 10” neodymium woofer as a good candidate for OB. With a SPL rating of 96dB 1w/1m and a conservatively rated excursion of 8mm, they would mate well to the HF driver selected for this project.

The crossover topology is fairly typical for this project. The woofer is crossed at 2000 Hz (initially), 2nd order electrical LR-2. The tweeter is crossed at 1800 Hz, 2nd order electrical. There is an l-pad on the tweeter which yields a 2dB reduction in SPL and the polarity is not reversed with that of the woofer.

The apparent overlap of the woofers x/o is an intended part of the design. Unique to this circuit is a variable rheostat which is shunted at the woofer’s terminals and adds variable resistance to the crossover circuit as well as the load the amplifier sees. This affects the LF crossover point and effectively scoops-out the midrange, in a manner that can be dialed-in by the listener. In this design, it is an effective way to compensate for dipole roll-off, as there is an additional 8dB of SPL produced by the LF driver when compared to that of the selected HF driver to start. Keeping resistance at the 2 primary legs of the potentiometer between 8 and 30 ohms, will result in a total load of 4-6 ohms per side, which should be safe for many SS amps. Dialing in the resistance settings will effectively fine tune the LF crossover point somewhere between ~1000Hz by adding 4 ohms of resistance and ~1800Hz by adding 30, with an infinite range of points in between. In my environment, a final resistance setting of 18 ohms produced the flattest on-axis in-room measurements and yielded a total load of 5.6 ohms per side that the amplifier sees. The additional 2dB reduction of the tweeter as noted in the previous paragraph (level match) was determined only after a suitable crossover point was selected and measured. One can simply alternate between dialing-in a crossover point, then tweeter attenuation, in that order for an infinite array of points within the intended range of this design. I suppose the rheostats could ultimately be replaced by particular values of series and parallel resistors once a proper x/o is achieved, but I enjoy the flexibility as well as the ability to demonstrate my design concept on-the-fly.

Final baffle size is only 15” wide by 24” tall. The woofer cut out is chamfered in the rear and centered 14” from the bottom. The tweeters are mirror imaged; their centers are 3” from the top and 3” from each side respectively. The baffle sits on 3 sorbothane pads in a tripod configuration, the rear pad is situated on an adjustable foot which moves +/- 1.5”, allowing baffle tilt toward the listener and helping to align each drivers acoustical center.

How do they sound? Powered by a single NAD 30 watt integrated amplifier, these things are smooth. They also play loud at low volume settings and do not strain when you crank 'em. The B&C suspensions are very stiff and take about 200 hours to fully break-in. As they do, the midrange smooths-out and the bass gets more powerful, but remains well defined, weighty and clear. Listening to Brian Bromberg on acoustic bass sounds amazing, as does the piano work of Keiko Matsui and vocal presentations from Cat Power and Diana Krall. All-in-all sometimes simpler is better. I also like the ability to easily transport them and get great (and consistent) results from a variety of amps and venues.

[Nanook]

I like the approach taken: "less is more". Many times too many things can take away from the "purity" of a project. Adding complexity and/or better or more costly drivers is not beneficial. You've addressed some problems with OB design, and it does appear you are aware of the limits of OB design in general. This project results in a convenient size for almost any room. My OBs on the other hand require a lot of room, which unfortunately I do not have anymore. I've tried a mini OB with some success, but they need subwoofer help. This can be accomplished via a Kef 100 watt powered sub. The little OBs I usually drive with my main amplifier a 125 watt/ch McCormack DNA 0.5 deLuxe which is absolute overkill. They can be driven by as little as 10 watts/ch to a very satisfying result. Some time is required to set the subwoofer correctly, but the results are outstanding.

Thanks for the project. To bad more folks haven't looked these up or commented. They may be what many folks really need: an affordable, simple OB using (relatively) common ly available drivers. And the design is suitable enough for iteration to be done using various drivers, depending on your location and means.

[Bigun]

Looks nice - can you post a bit more information in terms of the frequency response achieved within the listening environment ?

[matevana]

Thanks Nanook. I agree with everything you mentioned. I have a later version called the Scamp+ which adds a dedicated sub to the same size baffle, driven by a separate plate amp and integrated cleanly with the help of a phase control. I've learned a lot about sub integration through experimenting over the past few months.

[matevana]

Originally Posted by Bigun

Looks nice - can you post a bit more information in terms of the frequency response achieved within the listening environment ?

Unfortunately I don't have access to test equipment right now, but I would say they are relatively strong down to the low 50's, and that would be a conservative estimate. The mid scoop allows the elimination of what was previously a hump in the 600Hz area, possibly due to some floor bounce.

The newest version has an 8" sub mounted below the woofer (plate amp driven), and its proximity to the floor lessens any adverse bounce. While the addition of the sub wasn't essential, it is somewhat better suited to certain styles of music. The "sub" driver used is actually an 8" Usher (8137A) which is a traditional kevlar woofer that happens to be very tough, and has really musical upper bass qualities, which translate into very fast, articulate bass. I tried several 8" subwoofers and wasn't satisfied at all. The variable phase is set at about + 10 degrees, a hair up from flat (0 degrees) for best integration.