July 2005Audiopax Stereo Eighty Eight Stereo Amplifier
by Vade Forrester
If you have any doubt of the global appeal of the audio hobby, consider the subject of this review, Audiopaxs Stereo Eighty Eight amplifier. Designed and manufactured in Brazil, this brainchild of Eduardo de Lima follows his truly innovative Model Eighty Eight monoblocks, which have received widespread coverage and praise in hi-fi publications all over the world.
Although de Lima admires single-ended-triode amplifiers, he also recognizes their limited power output and frequency response, not to mention the still rather high (dare I say "artificially inflated"?) prices for the relatively limited-production triode tubes, especially the highly regarded 300B. To offset the cost problem, de Lima uses the KT88 beam pentode tube in all his amplifiers. KT88s are not exorbitantly expensive (about $25 each), and they are also quite easy to find -- just visit your local guitar store. My limited experience with KT88s tells me they're a good choice to replace triodes; the tube has a rich, juicy midrange reminiscent of triodes, and can generate some real power. After all, the KT88 is a pentode.
The 15Wpc Stereo Eighty Eight is an unconventional design with an unconventional appearance. Several finish options are available; the review sample was the $7990 USD Classic version -- a chromed brass box 13 1/2"H x 9"W x 14 1/2"D, with black lacquered sides and sitting on a black lacquered stand. It exuded elegance. Black-painted and anodized-aluminum versions are also available for $6290 and $6990 respectively. Two full-length handles on top of the amplifier made it easy to pick the Stereo Eighty Eight up and carry it around. These handles make so much more sense than front-mounted handles for amplifiers that can never be mounted in a rack.
The KT88s used in the review amp were highly regarded SED "Winged C" (formerly Svetlana) tubes, while the 12AT7 drivers were from Ruby Tubes, which are favored by guitar players. All the tubes are located on top of the amplifier. No protective tube shield is provided; my highly fallible aesthetic judgment says that a tube shield on this amp would be significantly ugly. The Stereo Eighty Eight ran rather cool for a tube amplifier. Although the output tubes arent self-biasing, there are no bias-adjustment pots visible from the outside, or instructions for replacing the tubes.
The Stereo Eighty Eights rather small output transformers are partly the reason for its relatively light weight. On that subject, the Audiopax website says:
Those are exceptional numbers for a single-ended amplifier.
Eduardo de Lima has conducted some very interesting research on why amplifiers sound the way they do. But his interest is not just academic; he actually provides an adjustment, called Timbre Lock, on his amplifiers that lets you tweak the amplifier for the best match to your system. Again from the Audiopax website:
I thought that this sounded rather vague and metaphysical, so I asked de Lima to expand on how Timbre Lock actually works. There is no significant measurable effect in the frequency response of the amplifier when you adjust the Timbre Lock. There are some small changes in the output impedance, and these will produce a very small change in frequency response (less than +/-0.2dB) with most speakers. The main measurable differences for different settings are in terms of the level, spectrum and distribution of distortion. The perception of changed frequency response comes from the different interaction of harmonic-distortion products.
Unpacking the Stereo Eighty Eight was a cinch. At 42 pounds, its easy to lift. When you remove the amp from its black cloth bag inside a double box, its easy to see how it survives shipping from Brazil.
Oddly for a tube amplifier, there is only one set of speaker binding posts. Because the posts are all metal rather than part plastic, if you feel the need to, you can tighten them down hard with a nut driver. Input is via a single pair of RCA jacks. Both the RCAs and the binding posts are by Cardas and among my favorite connectors.
Setting the Timbre Lock adjustment takes some time and experimentation, but once you find your preferred setting, you wont want to stray from it. Start out with the knobs for each channel around the 1:00 position and adjust them gradually until you find the setting that pleases you most (wont that drive audiophiles nuts). A ring of red LEDs around each knob shows you the current setting. Adjustments must be made with no music playing; the LEDs can be damaged otherwise. In my system, a setting at the 9:30 position, which means that the 9:00 and 10:00 LEDs were both on with equal brightness, seemed optimum. Settings clockwise from 1:00 seemed to emphasize the midrange, while setting counterclockwise from the 1:00 position seemed to promote a wider, flatter frequency range.
Input sensitivity is a low 2V for the rated output. That means your preamp needs to be capable of supplying the Stereo Eighty Eight 2V at input, so passive attenuators may not work with this amp. Thanks to the low input sensitivity, the preamp's volume had to be cranked pretty high to produce strong dynamic contrasts. In fact, the amp sounded dynamically lethargic until, on a whim, I reversed the output polarity of the speaker cables, connecting the positive cable to the negative amplifier terminal, and the negative to the positive. This improved dynamics quite a bit. The amplifier inverts phase, but so does my preamp. Taken together, they should work best with the "correct" (positive to positive and negative to negative) speaker connection. The manual says to experiment to see which way sounds best, and Im glad I paid attention for once.
The amps rubber feet might be OK on hard surfaces, but they allowed the amp to wobble on my carpeted floor. So I placed the Stereo Eighty Eight on an amp stand that was spiked through the carpet to the concrete slab underneath. Massive Mapleshade brass cones under the amp solidly coupled it to the stand. With this rigid mounting arrangement in place, the bass became tighter and better defined. I thought dynamics improved a smidgen, too.
The first thing you hear when you turn on the Stereo Eighty Eight is, well, nothing. Its as silent as a tomb! I like that in an amplifier. Musical detail emerges from a proverbial black background when you dont have to listen around amplifier noise.
When the music starts, its immediately obvious the Stereo Eighty Eight is blessed with a sweet tonal palette. Instrumental and vocal harmonics are rich and textured. Strings, in particular, are lush and full-bodied. Was this is a euphonic effect, or just a very accurate depiction of string tone? Whichever the case, its gorgeous. In Antonín Dvoráks Slavonic Dance #7 (Opus 46), performed by Vaclav Neumann and the Czech Philharmonic on Dvorák: Slavonic Dances [Apex 0927 48999 2], the string sections sounded unusually luxuriant, and the woodwinds were especially realistic.
Soundstaging is the area that usually shows the greatest difference between SET and push-pull amps and an area where the Stereo Eighty Eight lived up to the standard of a single-ended amp. In the title piece of the Tallis Scholars Allegri Miserere [Gimell 454 939-2], the positions of the voices within the main choir and solo groups were extremely precise, with no image wander. The purity of the voices, floating in the resonant space of a large church, was eye-watering. I expected to listen to a few minutes of this piece to check soundstaging, but I couldnt bear to turn it off. This is probably the Stereo Eighty Eights strongest area of performance -- the ability to cast a believable panorama of sound.
As you would expect from an amplifier with a -3dB point of 110kHz, the highs are extended, but they might fool you at first. Theres not even a faint trace of edginess or etch; the highs just extend up and out. The opening percussion section in "The Panther," from Jennifer Warness The Well [Cisco SCD 2034], exhibited loads of detail but not a bit of edginess. Warnes's voice sounded natural, unstrained, and very clear. That was at my preferred Timbre Lock setting of 9:30; if I rotated the adjustment clockwise, the vocals became more prominent, while the highs seemed to roll off.
"The Panther" also convinced me that the Stereo Eighty Eight's ability to convey inner detail was among the best Ive heard. The transparency of the sound made it possible to hear more information in musical phrases and nuances, but as a holistic part of the music, not isolated in a "look at that" manner. In other words, detail was integrated into a musical fabric and served the music, not the merely its reproduction.
Once the amp was solidly coupled to the floor, dynamics turned out to be decent, but arent what the Stereo Eighty Eight is about. The amplifier had to be played loudly to get a sense of dynamics, but the overall dynamic range (the difference between the loudest and softest passages) could have been wider. The microdynamic changes that musicians use to phrase their performances were clearly, but not vividly, presented. In Jordi Savall and friends "Folia: Rodrigo Martinez," on the CD La Folia [AliaVoxAV9805], the continually changing volume levels were somewhat compressed, reducing the sense of excitement that this piece normally exudes.
Lows were excellent, but different than I had expected from an amplifier with 15Wpc and such an impressive claimed -3dB point. On "Way Down Deep" from Jennifer Warness The Hunter [Private Music 01005-82089-2], the bass was tight and tidy, with particularly good detail and pitch definition; but it lacked the slam Ive heard from similarly (or lower) powered amplifiers. In other words, the Stereo Eighty Eights dynamic signature was consistent throughout the frequency spectrum. You need to push the Stereo Eighty Eight to get a hint of the music's visceral grunt, if it exists.
My $6500 Art Audio PX 25 amplifier is rated at "only" 6Wpc, but it doesnt roll off either the highs or lows. It also has a much higher input sensitivity (.7V for full power output), so the preamp doesnt have to work as hard to drive it, which could be an advantage if you want to use a passive preamp. Inner detail is amazing; yet that detail never jumps out at you, but is always presented in the context of the music.
The PX 25 sounded more dynamic and had better bass slam than the Stereo Eighty Eight. It presented the dynamics on "Folia: Rodrigo Martinez" better than any other amplifier Ive had in my system. This conveys the phrasing of the music as well as the rollicking good time the performers had playing it. The relatively subdued dynamics of the Stereo Eighty Eight masked some of the dynamic contrasts.
Eva Cassidys "Autumn Leaves," from Live at Blues Alley [Blix Street Records G2-10046], was very slightly smoother on the Stereo Eighty Eight than the PX 25. The PX 25 had stronger dynamic contrasts, although the Stereo Eighty Eight projected plenty of power during vocal climaxes. Which amplifier did the overall better job on this piece was a toss-up; both were excellent.
The Stereo Eighty Eight has a dramatic advantage when its time to replace tubes. Replacing the tubes of the Art Audio amp requires hunting for KR Audio PX 25 tubes that retail for $530 per pair, TJ Full Music 274B rectifiers for $250 per pair, and 6922 input tubes and 12BH7A drivers, whose costs depend on your appetite for new-old-stock tubes (although new-production tubes are quite reasonable). Ouch! In contrast, I could replace the Stereo Eighty Eights complete tube set for under $100 at my neighborhood guitar store.
The PX 25 provides a strong synergistic match in my system -- as it should, given that it's my amplifier of choice -- but the Stereo Eighty Eight could match up just as well in different systems. Thanks to its higher power output, it should work with a wider variety of speakers. I preferred the Art Audio amp, but its quite possible that some listeners would prefer the Stereo Eighty Eight's relaxed, serene sound to the sometimes "in your face" PX 25.
Paired with fairly sensitive speakers, the Audiopax Stereo Eighty Eight will provide many hours of relaxed, beautiful sound. In many areas -- soundstaging, vocal purity, inner detail, and frequency extension -- its as good as any amp Ive heard. Although I personally prefer a bit more dynamic punch, the Stereo Eighty Eight does a decent job in that area as well. Its unique design should be a good match for many audio systems, and dont forget the low cost to replace the tubes. I urge you to take a listen to this beautiful -- and beautiful-sounding -- amplifier.
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