Tip #30 (Posted 9/28/01)
This is the second in a series of articles focusing on Components of Exceptional Merit, many of which figure prominently in my current reference system.
As the name implies, the Art Audio PX25 amplifier is all about the PX25 directly-heated triode – a British tube that was originally designed and manufactured in the mid 20s by GEC. The tube stayed in production until the end of WW II when it became a casualty of a new breed of beam power tubes (e.g., KT-66) which were capable of more bang for the buck. The KR version is a copy of the original, but it cannot comfortably handle in excess of 370 V plate voltage versus the original's 450 V. Art Audio runs this tube at a bit less than it's maximum (around 360v) where the distortion level is at it's best. So what's the attraction here? According to Art Audio's Joe Fratus, this tube measures more linearly than most other "single ended" triodes (SET) and better than any new production tube that Art Audio has tested. The PX25 tube is said to feature a midrange sound similar to that of 2A3 but with more extension at the frequency extremes, and of course more power. The slightly higher plate resistance, relative to a 300B, allowed Art Audio to design an output transformer with an extended bandwidth - beyond what is typical for 300B or 2A3 based amplifiers. Throw in tube rectification and a 12BH7 driver stage and you've surely got a tasty tube concoction.
Is a PX25 simply a 2A3 on steroids? While I think that there's some truth in such a portrayal, the whole truth is not so simple. A good 2A3 is capable of astounding midrange clarity. There's an unmistakable you-are-there directness about its sound that is quite startling. No experienced triodephile is, however, likely to confuse the sound of the PX25 for that of a 2A3. For starters, its midrange clarity approaches, but does not equal the benchmark of the 2A3. In contrast, the PX25 is a warmer, more luxurious sounding tube. This tube gets much closer to my ideal of "velvet in a bottle" than either the 300B or the 2A3. It is the sonic equivalent of a rich milk shake. The PX25 baths the midrange with a lavish tapestry of harmonic textures, and it does this without sounding overly thick or by sacrificing detail. Here's an amp that satisfies my sonic sensibilities: a glorious blend of harmonic rightness and detail.
Because subjective tonality is affected by the amplifier's bandwidth, it's difficult to make comparative tube judgments without also taking the output transformers into account. Some SET designs currently on the market have hard time exceeding a power bandwidth of 20 kHz. As the bandwidth shrinks, stage lights turn more mellow yellow. Midrange tonality is emphasized and textures are perceived as softer, more liquid. This is the trap many other SET amps fall into, trading transient speed for an overly soft and sweet sound. This was not at all the case with the Art Audio PX25/Lowther BassZilla coupling. The combination of Art Audio's extended bandwidth (60 kHz) and the Lowther BassZilla's DX4 full range produced exemplary transient speed and control. The remarkable point, that bears emphasis, is that even with all this inherent speed and treble extension, the PX25's midrange sounded gloriously lush and natural.
The other hugely enjoyable aspect of this amplifier is its rendition of spatial perspectives and image outlines. It paints a truly believable 3-D soundstage. The depth and width perspectives are especially noteworthy, and are responsible for the soundstage's breathtaking vista. In this regard, the PX25 sets a new standard, at least in the context of driving the BassZilla loudspeaker. Acoustic signatures of various halls, and even artificial reverb added to individual channels in a multi-channel mix were clearly resolvable, down to the noise floor of the recording. No other amplifier to date, regardless of cost, has equaled the PX25 in fleshing out a believable soundstage. Image outlines were palpably resolved with exceptional resolution. It was easy to delineate the spatial coordinates of individual instruments in a mix or to isolate a particular voice in a chorus. In contrast, lesser amps homogenize massed voices, in effect causing spatial cues to bleed together as in a runny watercolor. The PX25 snaps image outlines into focus for a 20/20 view of the orchestra. In addition, image size was more realistically proportioned than with say the Volksamp Aleph 30. The dipole mounting of the Lowther DX4 with its large open baffle provides for more realism with regard to the emulating the wave launch of individual instruments, and the PX25 pushed the BassZilla to the limit. The chest, sounding board, body, or whatever resonant structure you can think of was effortlessly resolved.
Volume and pitch nuances, the domain where microdynamics and emotions reside, is given full bloom by the PX25. Taken in combination, its virtues of harmonic integrity, emotional intensity, and spatial incisiveness breath life into the soundstage. In the process, my favorite instrument – voice - benefited dramatically. Over the years, I have struggled mightily in all of my designs and audio system integration to get one thing right; namely, my spouse Lesley's voice. Her 1992 recording session in the old VTL studio (sadly, now defunct) by David Manley (Lesley Olsher: Jazz Me, Black Dahlia Music BDM 78003) gave me a lasting reference. The live-to-two-track recording session, the analog master playback in Manley's listening room, the digital master, and ultimately the production CD – my ears were in the loop every step of the way. It's fair to say that I have a excellent idea of what this CD should sound like. After auditioning numerous systems using the Jazz Me CD, both at trade shows and elsewhere, I'm flabbergasted (and depressed) at how rarely expensive audiophile systems do it any justice. Dynamics are typically compressed, which is equivalent to letting the air out of a tire; the drama gets sucked right out of the soundstage. Spatial resolution may also suffer, but I can deal with it, as long as rhythmic drive and emotions are present and accounted for. Finally, the timbre of Lesley's voice is usually pummeled by multi-way speakers, whose disparate sounding dome tweeters and midranges, and inevitable crossovers in the range of 2 kHz to 4 kHz harm coherency and harmonic purity. It's actually an amazing experience to walk away from a $100K system, that failed to resurrect Lesley, shaking your head in disbelief. How can something so simple, be so difficult to reproduce?
This should give you a rational basis for my current love affair with full range drivers such as the Fostex FE208 sigma and the Lowther DX4, and my current minimalist design approach. Driving the Lowther BassZilla, the PX25 did Lesley justice. David Manley was so convinced that Lesley could not overload his Gold reference mic. Nobody, especially a diminutive soprano was going to do that to his mic. Well, he was wrong. Therefore, there's no close miking on the Jazz me album. Lesley was deliberately positioned about a meter away. Still, in spots she still challenged the recording electronics. These dynamic peaks were easily handled by the PX25. Take Bill Ebanks' "Lay All Your Love on Me," which absolutely rocked during the recording session. Listening to the live feed, this track left me breathless, yet over the years it proved elusive to recapture the magic of the moment. Joy at last!
Male voice was also given full scope for expression. Let me mention just one example. Keb' Mo' (aka Kevin Moore) with a slightly raspy take on original blues, is crisply recorded on his 1999 self-named album (Okeh/epic EK57863). Performing triple duty on guitars, harmonica, and banjo he lets loose with lightening fast transients and benefits from excellent rhythm and bass support. Timbre accuracy with plenty of natural detail was how the PX25 facilitated the music. I certainly have not enjoyed reproduced music at this level before. That sense of anticipation that accompanies a live concert is back in my listening room.
All of the above gets me to the business of system considerations and synergy. Occasionally, perhaps a handful of times in an audio career, there comes together an assemblage of components that transcends ordinary Hifi. This is just such a happening. To tout an amplifier out of context makes about as much sense as an engine review without reference to a particular car. So let me sort through the compatibility issues. The PX25 is not a bright sounding amp. In fact, based on auditions with both the Diva M7 and Air Tight ATC-2 line stages, I inferred that the PX25's voicing is slightly laid back, and benefits from a lively sounding preamp. The Air Tight, itself being organic and very relaxed, did not complement the PX25. Instead, the Diva M7, a purist reworking of the Marantz 7, proved to be an ideal match for the PX25, filling in the upper octaves and stepping up the sensation of presence. It's true that only a handful of amps mate well with the Lowther's propensity for excess presence region. So getting the right mix of spices is not an easy thing at all. In general, I would expect the PX25 to work well with bright sounding speakers, since it has some cushion to soak up excess energy through the upper octaves. From my perspective, the question isn't so much about the PX25 being the perfect amplifier, but rather it is about nailing down the perfect system.
To be sure, the PX25 is not a perfect amplifier. If falters a bit in negotiating the bass range. The deep bass lacks the extension and punch afforded by the Volksamp Aleph 30. Performance relative to the Aleph 30 improves as we climb the frequency ladder though the mid and upper bass bands. Still, the PX25 lacks the Aleph's iron hand control in dishing out bass detail. Bass territory is Aleph country. With its current drive it simply bulldozes over the PX25. Yet, relative to other SET amps, the PX25 rates highly. It is especially adept in reproducing the full majesty of a cello.
Note that my numerical ratings of the Art Audio amplifier below are based on substituting the Philips Bugle Boy 6922 for the stock Tungsram NOS provided by the factory. In all honesty, the Tungsram is not the cleanest sounding 6922 to grace this planet. The Philips Bugle Boy really does bridge the gap between the PX25 and the sound of a good 2A3 in terms of midrange clarity and incisiveness.
Would you like to look and feel 10-20 years younger ? Would you be interested in increasing energy levels by 84% ? How about Increasing Sexual Potency Frequency by 75% ? Would you like to increase your Muscle Strength by 88% while... at the same time... reducing Body Fat by 72% and Wrinkles by 51%? Well, guess what, the Art Audio PX25 can't do all that for you (try Human Growth Hormone instead). But it can absolutely transform your musical experience in the home - providing you choose a mating loudspeaker carefully. Life is presently good: I'm having the time of my life driving it into the Lowther BassZilla. In the realm of low-power SET amps, it represents an essential audition for anyone with access to a high-efficiency loudspeaker.
Kudos to the Art Audio design team, Joe Fratus, Tom Willis and David Gill, who invested 17 months in perfecting this design. The audio Gods are very pleased. At last, a sublime auditory delight without sticker shock! And finally, a word of thanks is also appropriate to KR Enterprise's Riccardo Kron for keeping the flame and spirit of the PX25 alive.
US Retail: $ 5495 ($ 5000 w/black chassis & chrome plated accents; $ 200 additional w/ volume control)
Power Output: 6 wpc
THD: less than 1%
S/N Ratio: >85 dB
Output Impedance taps: 4 and 8 Ohm
Input Sensitivity : 400mV
Input Impedance: 180 kOhm
Frequency Response: 10 Hz to 20 kHz +/- 0.5 dB
Weight : 60 lbs.