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Reviewer: Steve Marsh
Analog Sources: Thorens TD-125 Mk.II turntable with SME 3009 tonearm, Garrot P77 mm cartridge, Cotter Verion phono cable; Rek-O-Kut TR-12H with Ortofon 309 tonearm, Ortofon SPU Classic GM and Ortofon Meister moving coil cartridges, Ortofon T1 step-up transformer, Partridge step-up transformer
Digital Sources: Vecteur L-4.2 CD player, NAD 5000 CD Player, Perpetual Technologies P3A DAC with Empirical Audio Turbomod, P3b Monolithic power supply
Preamps: Slagle Transformer Volume Control (TVC), Music First Audio Passive Magnetic, vintage McIntosh C22, vintage Marantz 7C, custom Japanese full-function tube preamp, Hovland HP100
Amplifiers: 6B4G triode-modified Dynaco ST70 tube amp, DIY Hifi Supply Lady Day 300B tube amps with Audio Note silver wire, silver foil coupling caps, silver binding posts [associated review loan]
Speakers: Bastanis Prometheus Mk. II speakers [this review]
Interconnects: Baton, Cardas Golden Cross, DIY made by Bill Allen [associated review loan], DIY made by Bill Allen digital coax interconnect [associated review loan]
Speaker Wire: Baton, WHG Grand Revelation [associated review loan], Bastanis [associated review loan]
Equipment Stand: Standesign 5, vintage wood equipment rack
Room dimensions: 22' long x 17' wide x 10' high, with eaves
Review component retails: $2,198 for driver kit with plans for building baffles, woofer cabinets | $3,750 for assembled speakers with baffles, woofer cabinets, painted black | $5,400 for assembled speakers with solid wood baffles, black woofer cabinets, additional charge for exotic woods | $800 for Gemini tweeter upgrade, includes high-pass filter cap upgrade to Jupiter beeswax
|While Prometheus tried to trick Zeus in Greek mythology, Prometheus the speaker had some tricks for me in this American odyssey of a review. It started with the arrival of the speakers in my driveway. Bill Allen, U.S. distributor for this German kit speaker, did indeed warn me about trying to lift the powered woofer cabinet out of the box on my own. However, my friend was late in coming over to help me and I got impatient. Thinking "hey, I used to be a competitive weight lifter", I yanked that sucker out of the box and turned to put it down. "Yeow!" went my back in complaint.
This was not an auspicious review beginning. Little did I know the trials and tribulations that were still to come. For those who skim over the meat of reviews for the final sizzle, don't worry. This story has a sizzling ending. But first, the usual background. The Bastanis Prometheus Mk. II incorporates two design features that are popular in the DIY underground circles (and increasingly above ground): a full-range driver and an open baffle (OB). As some readers may be aware, adherents of both design philosophies have gathered like bugs around a porch light at this website.
The allure of these designs is two-fold. The full-range driver runs straight off the amplifier and thus needs no crossover by definition. This eliminates the time and phase-related distortions that are often introduced by crossovers. Once you've heard the purity of a full-range driver, it's hard to go back to the typical multi-driver speakers that dominate high-end showrooms. Secondly and as panel speaker advocates know well, the lack of a speaker cabinet eliminates colorations associated with cabinet resonances and provides a more open and spacious soundstage.
Lest I miss a major selling point of the Bastanis Prometheus (and the rest of the Bastanis line of speakers), the efficiency of the open baffle drivers is claimed to be 100dB! Break out your flea-powered amps. From what I've been told, they run fine on single-ended tube amps using low-powered tubes such as the 45 and 2A3 (e.g. Yamamoto A-0S, Fi 2A3 amps). They are a SET lover's dream speaker then.
|In recent years, fullrange/open-baffle enthusiasts have been farming the past using vintage drivers from high-end European console radios of the tube era (e.g. Telefunken, Siemens, Philips, SABA). I heard a number of these and was very impressed with the natural, unboxy and very detailed sound. Friends of mine have also experimented with the vintage Altec Bi-Flex fullrange drivers like the 412A and 415A on open baffles and gotten good results.
Of course, OBheads pay a price for their brand of sonic bliss. Bass -- and often treble -- extension are sacrificed at the altar of midrange purity. If you try to use a larger fullranger to get more bass, you run into the problem of beaming as the driver enters its upper range.
Robert Bastanis, the designer of the Prometheus Mk. II and a complete line of speakers based on the fullrange/open-baffle philosophy, has analyzed this state of affairs and come up with a practical solution that in the Prometheus mirrors the Zu Definition our publisher has embraced with a vengeance. Robert uses a wideband driver manufactured by Eminence Speaker LLC to his specifications, with a voice coil designed more as a midrange driver rather than for the large excursion of a woofer. He then treats the cone to allow it to operate at higher frequencies. This treatment process is truly an artisan creation of Mr. Bastanis and at the heart of his speaker line. In a proprietary four-stage process, Mr. Bastanis applies his own mixture of oil and violin lacquer to the cone (anyone remember proponents of C37 lacquer application to Lowther drivers?). This makes the cone able to flex more in the upper frequency range by loosening the fibers and increasing their elasticity. He simultaneously stiffens the cone by baking it. Lastly, a ring of circular felt pads (floor protectors for chair legs?) is placed around the perimeter of the dust cap to dampen high frequencies reflecting off it. The treated Bastanis wideband driver is then able to be operated between approximately 100Hz and 12kHz, covering the critical frequency range of most music with a single driver. After these treatments, the driver needs about 400 hours of break-in so kit builders should be prepared.
Mr. Bastanis then adds a self-powered woofer with a 12dB/octave low-pass filter to meet the natural low-frequency roll off of the widebander. An Italian-sourced but modified compression tweeter enters above 12kHz with a single high-pass cap. The woofer's Titan plate amp is made in China to Bastanis specs and incorporates an EQ circuit to enable flat bass response in the somewhat undersized sealed woofer cabinet. Most importantly, there are no crossover parts between the amplifier and the wideband driver save for a single resistor to flatten the impedance curve for an easier amp load. An RF filter on top keeps out frequencies above 30Khz. Even this RF filter has been carefully selected to avoid, as Robert puts it, "negative effects on the music signal. There are lots of filters which work fine for what they are made for but add negative effects to the music signal. Ringing electrical echoes or time-delayed phasing effects are very common negative effects."
Robert markets these RF filters in another form dubbed Magic Twins where the filter is built into a T-shaped inline jack. He recommends inserting these at the input of the preamp and amp(s). While the filters weren't the focus of today's review, they were used during my time with the speakers and thus, were part of the overall sonic results. While I heard benefits, I would prefer to defer this discussion to another time when I can be more sure of my comments.
|Full-range driver purists might claim that adding a woofer and tweeter is heresy. After all, you've now got a three-way speaker. They certainly have a point. However, the wideband driver does cover the vast majority of the music, thus staying true to the original intent of the full-range philosophy.
|The Bastanis Prometheus Mk. II kit buyer gets the drivers, woofer plate amp, plans to build the open baffle and woofer cabinets for $2,200. An optional upgrade is a premium quality dipole tweeter for an additional $800 (more on this later). As Bill Allen readily admits, the number of audiophiles willing to build their own baffles and woofer cabinets is limited so he offers several options. You can order the Prometheus Mk. II with black baffles and woofer cabinets for $3,750 plus shipping. For non-premium wood finishes|
|(solid 2-inch thick wood, not veneers), think $5,400. For custom and exotic wood baffles (e.g. Bubinga), Bill refers customers to his woodworking associate Tony Landry in New Mexico.
DIY customers and owners of finished Bastanis speakers can get direct advice from Robert Bastanis or Bill Allen and also on Audio Asylum's Bastanis forum. The next four paragraphs are excerpted from Robert Bastanis' own comments there: "Open baffle speakers require drivers with relatively high Qts values, preferably over 0.5. These relatively low-damped drivers will make the required bass without reinforcement from a box or horn (they literally flop around in the air). By contrast, a highly damped driver will not make bass in an open-baffle speaker. PHY-HP offers drivers for open baffle speakers.
"The 12" widebanders are a special design. Together with the design of the OB, this creates a stable efficiency down to the predetermined natural roll-off at 12dB/octave, so there is absolutely no active/passive crossover needed. This allows maximum control of the power amp without loss of fine details. The common reason for using small wideband drivers is the beaming effect which starts earlier and is more problematic with big drivers. I use a unique method for making the cone elastic and flexible to turn portions of it into a flex cone. Beaming with the Prometheus widebander is a non-issue.
By using a big cone in wideband mode, you also get better impedance transfer to the surrounding air. This minimizes excursion requirements and accelerates response where, for a given SPL level, such a cone travels a shorter distance than a small one."
After my back recovered a bit, I set up the speakers as Bill Allen recommended it - about a third of the way out into the room, either facing straight ahead or slightly toed in. Since my upstairs listening room accommodates this proposed setup better than the downstairs space, that's where they went. Let me state upfront that while my downstairs system is relatively mature, my upstairs system is still under development. When I first moved into my house seven years ago, I set up a system in the upstairs room but had to abandon it due to a terrible RF-induced hum that I eventually tracked down to a local radio station. This station had then recently been taken over by Clear Channel Communications and they must have boosted their signal at the time. Since those days, I'm guessing the FCC (what's left of it) throttled them back and the room is usable now. Consequently, I'm in the process of building a high-end system there based around high-efficiency speakers.
The open baffles can be supplied in two versions. The pair I got was L-shaped, with the horizontal support for the head resting on top of the woofer cabinet. The other style is a flat board that extends fully to the floor to leave the woofer cabinet separate so it can be moved around to optimize placement for best in-room bass performance.
If you get the L-shaped baffle version, it is usually supplied in the teardrop shape as shown with my review pair. The baffles of my loaners were made from exotic and exceedingly expensive Bubinga. Bubinga is African Rosewood, harder and heavier than Indian or Brazilian Rosewood. An additional piece of Bubinga was also supplied to fit the front of the woofer cabinet, thus completing a very attractive -- if still somewhat acquired taste -- appearance to the finished product. Whether these would pass the WAF in your household is open to question. Additionally, the exposed fullrange driver is not friendly to children's poking fingers.
|Bill Allen supplied the baffles with the upgraded $800/pr Gemini dipole tweeter option. These tweeters are front- hornloaded and supposedly the only such commercial dipole tweeters in the market. They are built for Robert Bastanis in Bulgaria. He modifies their phenolic diaphragms with a mixture of violin lacquers and mineral powder.
The standard tweeters were also supplied and installed later in my listening as I was loathe to remove the Geminis. The standard units are monopole compression drivers sourced from an Italian manufacturer. If you consider all of the countries involved in the production of these speakers, the Prometheus seems truly worthy of a United Nations seal of approval. I started out using my Slagle TVC, NAD or Vecteur CD player and my 6B4G-modified Dynaco ST70 amp. Later in the review process, I was able to introduce other very high resolution sources and amplifiers. I used Cardas Golden Cross from the CD player to the Slagle, then Y-splitters from the Slagle output jacks with a pair of long no-name interconnects to the woofers' plate amps and Cardas Golden Cross to the Dyna amp for the open baffles.
|Something was grossly wrong right out of the gate. Very little bass was present. Rather than bore you with the|
|travails of diagnosing this problem, I'll just tell you that it simply turned out to be the pair of interconnects I used to the plate amps. Those cables were either defective or entirely incompatible in some unknown way. Whatever the problem was, I was able to move my equipment rack forward enough to use a two-meter pair of Cardas Golden Cross instead. Now all was well and I looked to the owner's manual for guidance in adjusting the woofer controls (phase, volume and crossover point). The manual is twelve pages long, with the first four pages in German. The remaining pages are in English and contain the kit parts list, building instructions, diagrams and one page on setup. While you can glean from it the general tips on woofer adjustments, I think a dedicated section in the manual for this would be clearer and better organized. In one of my phone conversations with Bill Allen, we discussed this and he said he generally tells customers to start with all three controls at around 12:00 o'clock, then work from there. This turned out to be good advice. My final adjustments had all three controls about 5 degrees to the right of high noon.|