PX-25 | REVIEW | THE AUDIOPHILE VOICE

EVEN BEFORE the Art Audio PX-25 power amplifier arrived for an extended listen for this review, this electronics line had already begun to trigger my curiosity during informal encounters at shows and a local dealer. When the unit arrived, it was immediately obvious that the PX-25 is no ho-hum knock-off of other single-ended, triode tube power amplifiers. The PX-25 is obviously lovingly crafted and beautifully implemented. It is Art’s first entry into the ultra low power realm, and with the resurgence of high sensitivity loudspeakers, helped along by the continuing popularity of vacuum tube electronics, the PX-25 is a sweetheart that should do nicely in this niche. In this case, she’s done up in a stylish, brightly polished, heavy-gauge, stainless steel chassis, with chrome accents and open-mount tubes in ceramic sockets. She’s a very sexy dame, indeed.

For a number of years, I have spent a good deal of listening time appreciating how involving music can be when presented via lowpower, single-ended vacuum-tube power amplifiers, typically 10 watts or less. When paired with sufficiently efficient loudspeakers, amps using either 300B or 2A3 tubes seem to be particularly faithful to the music. They always seem to be especially ready references, regardless what other solid-state or tube gear isrunning in the listening room.

Therefore, I approached this Art Audio amplifier with strong curiosity about its sonics, yet admittedly I had a bit of skepticism because of the reputation of the Czech PX-25 power tube and its acclaimed performance beyond that of competing tubes in the similar power range. This is a tube that some folks swear bests the famed 300B. It is also a tube with which I was only slightly familiar. All this really twitched my eyebrows, because I rightly hold my WE 300Bs in very high regard.

I had also assumed that the current crop of designers had likely squeezed about all there was out of single-ended circuits and esteemed triodes, at least from those units priced within the atmosphere, i.e. my own budget and that of other “working” audiophiles. Subse-quently, my encounter with the attractive PX-25 actually caught me off-guard. Then again, I should have known better. The acclaim that this product line has garnered in just a few years from valid sources, is now validated by my own experience. I’m here to say, right up-front, this has been an intriguing eye- and ear-opener.

Art Audio-USA founder Joe Fratus, a devoted audio designer for over 20 years, is based in Cranston, RI. He and his associates there, in Raleigh, NC, and the UK have developed a line of several other amplifiers including the famed Jota, a 20-watt-per-channel, single-ended stereo unit. The PX-25 covered here is one of their newest products. Its output is rated at a very modest six watts per channel, yet one gets a quite robust audio presentation from these high-current power tubes. Monoblock and push-pull power amps are also offered in the Art Audio line, as is a moving-magnet / moving-coil phono preamp. All of Art Audio’s products are tubebased, and from my observation, they feature top-quality parts and construction. I’m told that personal consultations with customers to match the right amplifier with the right loudspeaker(s) are integral to the firm’s approach to the manufacturer- customer relationship.

The PX-25 derives its name from the early 1920s-vintage, direct-heated triode tube that was originally manufactured in England by GEC and which remained in production through WW II. This was also the heyday of efficient compression drivers in speaker systems that were well served by low-power electronics. Thereafter, a range of more powerful tetrodes and pentodes emerged and pretty well dominated audio applications until today. In the new millennium, KR Electronics (formerly KR Enterprise) of the Czech Republic has reissued the PX-25 and is applying modern manufacturing techniques and internals of the tubes. KR is noted throughout Eastern Europe for its manufacturing of a broad line of commercial and consumer vacuum tubes. As powerful and versatile as the new multi-element vacuum tubes have evolved to be, it was and still is thought by many that the simpler three-element triodes produce the purest, most-faithful sound across the audio spectrum. It will be interesting, then, to see and hear what current practice and materials have done with triode sound.

The PX-25 is a high-Mu (gain), low grid-to-anode capacitance, high-anode impedance (higher than the 300B’s), 5-inch tall, vintagestyle, bulbous envelope tube (whew!). Its frequency response is specified as 9 Hz to 60 kHz, +/-0.5 dB. Furthermore, the factory’s measurements say that this tube will maintain this linearity out to 100 kHz where it will still have less than 1 per cent distortion in an active circuit. These characteristics allow use of an output transformer with an extended bandwidth, one with response beyond what typical 300B or 2A3 tubes allow. Parts counts in these designs are remarkably low. In the ART circuit, the PX-25 B+ plate voltage is held to 360 V d.c., a bit below the design maximum, not only to assure linear operation, but extended life as well. My listening tests reveal that this tube is truly capable of maintaining faithfulness to the input signal. Both high and low 3 outputimpedance versions of the PX-25 amplifier are offered at no extra cost. Appropriate selection will provide better voltage-gain matching with either super-sensitive (100- dB SPL+) or medium-sensitive loudspeakers. The manufacturer recommends loudspeakers of 90-dB SPL or greater sensitivity, though I believe some rated below this level will perform just fine. Upon request, the ART staff will make recommendations as to loudspeaker selection. According to Art’s Joe Fratus, the total project and combined development by himself, Tom Willis and David Gill required about 17 months. This includes time spent researching the actual tubes from Riccardo Kron of KR Electronics.

Faithful transmission of the audio signal to the loudspeakers requires an equally important element, the output transformers. Single-ended operation requires only one per channel, yet it is asked to do quite a lot, and every element must be carefully selected and tuned for optimum performance of these circuits. There is really no room for even a small error at this performance level and little (if any) correction circuitry could compensate for shortcomings. Both primary and secondary output transformer windings are commonly hand-wound by skilled assemblers, typically patient women with many years of experience. Selection of the correct mass and configuration of the high-silicon, grain-oriented steel in the core has been found to be equally important. Such attention to detail helps assure a linear transfer of any complexity of audio signal, however wide the band, with minimum phase shift, from primary to the secondary and then to the speaker load, whatever the frequency, voltage, current and operational temperature.

All this while dealing with the often-complex loudspeaker impedance! These custom transformers must carry the full voltage the audiophile voice and current swing without any help from a companion as in push-pull designs. It’s all or nothing with single- ended circuits, and it better be right. The result is that the PX-25 is not an inexpensive, mass-market amplifier, yet the designers appear to want to give a lot in return. I see obvious attention to quality and detail, which bears this out.

Use and Listening Test
The amplifier I received was already broken-in, so I did not have to “season” it for a couple of months for the components to settle as the manufacturer recommends for totally new units. Thus, right out of the box, the PX-25 conveyed sweetness in the strings and faithfulness to vocal intonations that immediately made me pull out my favorite CDs and LPs, especially several reissues and test pressings from Classic Records. Included here is a special 45-rpm test pressing of an early Miles Davis recording. On replay, I was offered rich and speedy dynamic contrasts that only the best tube designs achieve, plus a resolution of detail and nuance that sets this amplifier apart.

With as much esteem as I hold for well-implemented solid-state designs, it still appears to me that basic triodes capture the harmonic structures and timbres of music as no other design does. For me, a special treat occurred when the PX-25 was paired with an extraordinarily good LP source; the synergy was truly magical. It’s amusing to me that today’s best solid-state offerings are often touted as having “that tube sound.” It’s remarkable how far around this circle we’ve come, just to get back to basics and rediscover that great analog foundation laid down in the 1920s.

We reviewers and critics have developed an ample set of superlatives to describe the best music and
audio gear. Accordingly, some of us say that single-ended circuits, such as the PX-25, add their own harmonic textures to the music. Maybe so, yet these eras find no fault. In any case, what we do know is that these difficult-to-discern, low-level harmonic distortions are largely even order in content, as are their clipping products. The PX-25 behaved admirably throughout its stay in my home, and even my spouse Joan thought it was a remarkable piece. This amplifier will only lose its composure when it is overdriven with slamming low bass notes into loudspeakers that have insufficient sensitivity or present impedance mismatches.

When all is said and done, what’s really important is how the music makes you feel, how involving and engaging is it? Is it true to life? Does it seem near to the real thing, heard live in a decent concert venue? Does it stir the blood? Are you enveloped by the wonder of it all? Is your acceptance heartfelt?

The Art Audio PX-25 is, indeed, capable of invoking these emotions in me, and I was happy it did so at my listening room for several months. The ART PX-25’s character is neither euphonic or more “vivid” than life, nor does it over-embellish musical content. Its presentation seems just right, with a mid-stage presentation and first-rate near-surround imaging with the best recordings. All the while you’re offered ample enough dynamics to follow the most complex musical lines. Yes, you know its tube-based amp from its touch of warm sweetness that envelops the room, yet the PX- 25 does not overly call attention to itself, despite its uniquely pleasant sound. You soon forget that this amplifier is outside the consumer mainstream; you just kick back and enjoy the music.

One nice operational touch: The PX-25 auto-bias circuit relieves the owner of performing this periodic task, just to keep the amp operating optimally or when installing new output tubes. And that, to me, is the way it should be, no worrying over the technology, just enjoy the outcome. A memorable case in listening to the PX-25 is how the amplifier floated images of Heifetz’s dynamic reading of the Bruch “Concerto in G Minor” (Classic Records RCA LP LSC 2652). The PX-25 reached heights of realism rarely visited in home audio. The character of the bow on strings, biting in one phrase, contrastingly delicate and subtle in the next, was as it should be. Guest listeners often were near applause after such a spin of this faithful analog source. And, yes indeed, I do still love spinning those glorious LP recordss. This lovingly crafted sixwatt wonder opened windows into the music that were quite remarkable. Several of the tunes I played are still stirring in my memory. Much of this character I attribute to the superb PX-25 triode output tube, resurrected to modern manufacturing standards, and its associated circuitry.

During much of my listening, the Von Schweikert VR-6’s were my main reference loudspeakers. They gave the amplifier quite a work-out, what with their nominal 4-ohm impedance and sensitivity demands, yet the PX-25 handled this loudspeaker with remarkable composure. Another capable loudspeaker I paired at some length with the PX25 was the Klipsch KLF 20, a 100-dB SPL sensitive, 8-ohm, horn-andcone tower loudspeaker of reasonable cost which I came to admire during the preparation of a TAV review I wrote a few years past. The PX-25’s amicable character took to the KLF 20 like a duck to water, and held me and guest listeners in awe with its speed, dynamics, and tonal accuracy with every type of music we threw at it. In this case, the amplifier appeared to have limitless reserves of power, invoking exclamations such as, “Is that really only six watts?” Oh yes, indeed folks, surprise, surprise!

Another loudspeaker, the Soliloquy 6.5 towers which I have listened to at some length was also found to mate well with the PX-25 despite a lower 92-dB SPL sensitivity. This speaker and amp combo offered an easy, smooth and detailed presentation, confirming this amplifier does offer the versatility to handle many sorts of loudspeakers.

If you don’t feel enough pantsflap bass with these lower power amplifier, then one might cautiously apply a powered subwoofer for the lowest bass (I did not use a subwoofer myself). I only recommend this if perfect integration can be achieved. The PX-25 is simply too good that you should allow it to be smeared by poor subwoofer or crossover integration. Otherwise, sit back and enjoy the sweet pleasures offered by the PX-25.

Conclusion
If my experience with this amplifier is any indication, it is likely that the Art Audio PX-25 may well take its rightful place among the classic single-ended power amps of the new millennium. It may well become a legend in its own time. It offers enormous retro appeal, yet features modern design, fit and finish with carefully selected 21st- Century materials and parts. The sheer pleasure this amplifier will provide the listener who takes the time to mate it with an appropriate loudspeaker, confirms why we pursue this engaging pastime in the first place, at once discovering a unique blend of art and science which invokes deep emotion, which allows us to plumb the true depths of our favorite music.

The PX-25 will support your enjoyment of both CDs and LPs, while not favoring intimate jazz or chamber ensembles over large classical orchestra or opera. No matter your preferences, even they run to classic rock such as the Eagles or the early ‘70s folk of James Taylor, is appropriately conveyed. And one ought not ignore the fact that Willie Nelson’swarm and sweet Texas country twang is lovingly and accurately reproduced by this amp as well.

Sit back and enjoy this fine power amplifier, I feel sure it will not disappoint you.