[DIY design] AK OB1 open baffle loudspeakers

Product: AK OB1 open baffle loudspeakers
Manufacturer: not for sale, DIY design
Author: Arvind Kohli - TNT USA
Published: December, 2020

I've been scratching this itch for a good three decades, the audio gear itch that is - the search for better sound. And along the way, I've experienced a few things that have made a significant difference for the better, almost every time. These features have become part of my short list to look for in gear. I now know enough, to not claim these features are universal improvements, but they seem to more often than not provide a better sound. Here is the short list:

The journey has definitely resulted in the sound in my parlour, getting better and better over time. But it also has resulted in the products that have any appeal to me becoming fewer and fewer. This phenomenon has gotten to the point, that my latest speaker itch left me with very few possibilities that appeal to me, and vastly fewer that would be in the price range I'd consider. I was willing to review a series of speakers that piqued my interest, but I thought to first ponder a DIY pair. Also, the constriction of choices, and plentyness of time at home due to the Covid19 pandemic brought me to consider a DIY project.

The game's afoot, Watson

Step one, was to decide on the design priorities of the speakers. They may eventually compete with each other, and not all be entirely achievable. I thought it critical, to start with the list of all the ideals and then see how many and how much could be accomplished. Robert Browning said it best - “Ah, but a man's reach must exceed his grasp, or what's a heaven for?”.

The center piece

Step two, was to select the main driver. I wish I had the time, resources and energy to try out each driver in the world. But, since I unfortunately have to live in the real world - job, family, house - I settled for selecting on spec. After looking over the spec sheets of at least a couple hundred drivers, I short listed about 3 dozen. Of those the winner was this 10" doped paper driver by Faital of Italy, the FaitalPRO 10FE200.

What was so special about this guy? For one, the measurement curve supplied by the manufacturer shows the frequency response was 70 to 6,000 Hz within about 5dB. The lower end is not as extended as I was hoping for, but I was willing to see how well I could mate it with my subwoofer. I've established a long standing preference, to set the subwoofer's crossover below 50 Hz. In the past, I have been able to localize the subwoofer when set somewhere above 80Hz, and that is an unacceptable compromise to me. But now, maybe it was time, to find exactly where that bent for me.

This driver also sports a Qts of 0.70 - check. Cone material is treated paper - check. Price is only $57USD each - check check. Sensitivity is 96 dB 1W/1M - check check check.

But, perhaps the most exciting aspect of this driver was in its frequency response curve. I felt, it fell off cleanly enough that I'd risk using it with no crossover components whatsoever!!! I could not wait to hear what that sounded like.

The rest

After spec checking a few dozen tweeters I settled on this model - Dayton Audio DSN25F-4. The sensitivity was just 1 dB lower than the main driver; dome material is silk; the price just $17USD each and also sold by Parts Express, who happen to be in Ohio where I live. Sadly, this would not be dipole, but I was willing for that compromise on the first iteration. For a crossover I was only going to put on a capacitor for a high-pass filter; so I picked up a pair of 4.0, 5.0 and 6.2 uF values for around $3USD each, and play around with where I liked the crossover point the best. Pricier crossovers could come in later, if the DIY was generally a success and worthy of further investment.

Subsequently, I picked up a pair of the ESS Heil AMT III. This is the smallest of the three tweeters they offer. I picked this version on the assumption that the smallest unit would have the best dispersion characteristics, sadly ESS Heil did not have measurement data to prove or refute my theory. Nonetheless, it is a dipole design and has a sensitivity of 95dB/W/m, so it met my technical requirements. And I have always loved the sound of the Heil tweeters, they seem to render all the detail but with not an iota of harshness.

For cabinet material, I landed on 2" nominal thickness Southern Yellow Pine. It is relatively cheap, easy to work with, easily available. So, overall a good candidate for the first iteration. A sizing prototype was made using an oversized cardboard box, this was useful to understand what the general dimensions should be.

The cabinet design was fairly simple, yet a lot of work. The front baffle was 11" wide; the sides, top and bottom were 18" wide, with their sides rigidly mounted to the back of the front baffle with glue and pocket screws. Thus, the front view only shows the front baffle, and the left and right most mounts of the main drivers were coupled thru the front baffle into the sides using extra long screws.

There of course was no rear panel, making this an open baffle design. Early listening tests revealed the back waves were bouncing against the side panels, so I covered the inside walls with multiple layers of batting, wrapped in fleece. This mostly solved the issue, but I have decided that I will later on build another design with a much wider front (20-24") and very shallow sides (6-12").

All edges were substantially rounded, and the front baffle was tapered off to the sides; all to eliminate diffraction. The Dayton Audio tweeter was mounted in its own housing on the top wall, time-aligned with the woofer. Later when I switched to the ESS Heil tweeter, I mounted it directly on the top wall, also time aligned. Finally, 3 brass cones were mounted onto the bottom of the cabinet, one in the front and two in the rear. The resulting height was such that my ears aligned with the top third of the main driver.

Listening

"Moon River" (Andrea Motis & Joan Chammoro; Feeling Good; Bucbonera Records)
The OB1 easily had more detail in female vocals, especially in terms of longer decays and much better separation of sounds. The OB1 also had better integration between the treble and mid/bass regions. This likely is because of design rather than drivers. I have the AMT tweeter mounted atop the Quad panel, the acoustic centers of the drivers are roughly 23" apart. I hadn't much noticed the "separation anxiety" this was causing till now. The OB1 drivers are much closer together in the prototype and will be closer yet in the next cabinet design - moreover the tweeter and woofer axis are aimed to converge around the listening spot.
The Quads were noticeably warmer and more laid back than the livelier and more detailed and more dynamic OB1. I could not choose between the two, albeit they were quite different.
The Dynaudios were again amazingly box free sounding. There was a bit of resonance at a very specific point. Dynamics and openness were a smidge less, but I'd gladly live with these for a long time.
With the PSBs the vocals were constrained physically by the cabinet, and a touch of resonance at the same point as had occurred with the Dynaudios.

"Long Distance" (Muddy Waters; Folk Singer; MCA Chess; 088112940-2)
The Quads again had a relatively much warmer vocal presentation, again I had no preference between the two speakers although they were considerably different in this aspect. The OB1 definitely was way superior on vocal dynamics and detail. There was much more transparency into the recording than with the Quads. The Quads, however, did plumb a fair bit deeper with the Double Bass, I would not consider the OB1 without a subwoofer. Treble detail was also considerably better with the OB1, the shimmer of the steel strings was barely noticeable with the Quad/AMT combo, and undeniable with the OB1 - the increase in transparency into the recording was incredible.
With the Dynaudios the separation of instruments was quite remarkable. Male vocals were not quite as dynamic as the OB1, but you wouldn't know the difference unless you were doing a direct comparison. I also keep being impressed by how these speakers do not sound boxy at all.
The PSBs had relatively much better bass response, the acoustic guitar was presented in full range whereas the OB1 was a bit lacking and a subwoofer would be essential. Male vocals seemed a bit constrained by the box instead of free, airy and open with the other 3 speakers.

"Scherzos No.1, Op. 20 in B Minor" (Arthur Rubenstein - Fryderyk Chopin; The Chopin Ballades & Scherzos; Living Stereo; 82876.61396)
With the PSBs, the treble fortissimo lacked in the level of detail and decay that the OB1 could render. The fortissimo in the bass region seemed to fall apart, and instead a morass of sound was delivered. Whereas the OB1 was easily able to reproduce the attack and the complex decay tones that resulted.
With the Dynaudios, the Esotar tweeter was a champ at the treble section, the attack and decay were beautifully rendered. The OB1 did take a back seat here, but for the price of this tweeter the difference was very small indeed, I am very impressed by this unit from Dayton Audio. On the bass fortissimo, the Dynaudio could not compete with the OB1. The energy of the attack did overwhelm the Danes, and the decay was lacking as well.
With the Quad/AMT combo, the treble portion was very well rendered, with perhaps a bit less decay. The fortissimo in the bass region was presented with good control on the attack, the speakers did not get overwhelmed like the PSBs and Dynaudios did. However, the decay was quite a bit lacking in comparison.

"Raag Bilaskhani Todi" (Shujaat Khan; Raga Bilaskhani Todi/Bhairavi; India Archive; B00005BI1G)
With the PSBs, the tabla was not nearly as natural sounding, and also seemed constrained by the cabinet.
On the Dynaudios the tabla was outstandingly natural sounding, with a bit more warmth than the OB1. No cabinet colouration whatsoever. Dynamic range was perhaps a bit less. The OB1 definitely need the subwoofer to complete the instrument.
On the Quad/AMT combo the sound was quite a bit laid back compared to the livelier presentation of the OB1.

"Guitar and Percussion " (Tonian Labs demo)
The decay and separation on the treble section of the guitar were far superior with the OB1, as was separation of notes and instruments. The Quad/AMT was again quite a bit warmer in presentation. I think some folks would prefer the warmer presentation of the Quads, and feel it is easier to listen to for long periods. By this point I was starting to prefer the very transparent, responsive and natural sound of the OB1 - I find those qualities make me want to listen to more music.

"All or Nothing at all" (Diana Krall; Love Scenes; Impulse; IMPD-233; CD)
This track features female vocals with a pretty wide dynamic range and a prominent double bass.
The PSBs had much deeper bass response, albeit a bit boxy sounding. Female vocals not as airy and open, and noticeably lesser in rendering micro-dynamics and a sense of realism.
The Dynaudios had a much deeper bass response and not at all boxy sounding, like the PSBs. Female vocals were a bit more open and dynamic than the PSB, but still less so than the OB1.
The Quad/AMT combo bass response was very similar to the Dynaudio, fairly deep and not boxy at all. Female vocals were a touch warmer and a bit laid back compared to the OB1, the OB1 had a smidge more detail and dynamics. The Quads would likely be easier to listen to over longer periods, whereas the OB1 would be more exciting and involving.

More listening

"Raag Bilaskhani Todi" (Shujaat Khan; Raga Bilaskhani Todi/Bhairavi; India Archive; B00005BI1G)
The ESS Heil decays were much longer, distinct and natural sounding. The attacks on the Dayton seemed a bit more prominent, maybe a tad strident even in direct comparison, whereas the ESS were relatively a bit laid back yet not missing any detail.

"Guitar and Percussion " (Tonian Labs demo)
Again the ESS were champs at decay. It is remarkable, how much more natural a reproduction of a cymbal sounds with a more complete decay.

Conclusion

With all the challenges 2020 has brought, I am still thankful for the opportunity it gave me in the spring to take on this project. Not just because I built it, but I'm extremely pleased with what this speaker can do. The transparency, dynamics and detail are at a level I've never heard in my setup. I look forward to the revised cabinet next, I'll keep you posted.

REFERENCE GEAR
Gear	Manufacturer and Model
Digital Disc Players	•Oppo BDP 105
Phono Cartridge	•Ortofon Quintet MC Black Goldring Elite(MC) Audio Technica OC9 MLII(MC)
Turntable	• Technics/KAB SL1200 MkII, with tonearm damper
Phono stage	• Pass Labs XP-15
Pre Amplifier	• Promitheus Audio Reference TVC
Power Amplifiers	• Monarchy Audio SM-70 Pro, in monoblock configuration. • Classe CA-100
Integrated Amplifier	• TBI Millenia
Speakers	• Quad ESL 988 • Dynaudio Contour 1.3 MkII • Triangle Electroacoustique Titus 202
Subwoofers	• ACI Force Velodyne F1500R
Headphone Amplifier	• Practical Devices XM3
Headphones	• Stax SR-40, with SRD-4 adaptor AKG K701
Connectors	• Various

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