The Artemis Labs PH-1 Phono Stage
Tip #52: by Dick Olsher
It is ironic that 25 years after the launch of the standard CD by the Sony and Philips digital consortium vinyl LP sales have recently eclipsed those of the two high-resolution digital formats. The statistics are quite humbling. During 2004, vinyl exceeded the combined sales of DVD-Audio and SACD! If Sony had pinned its SACD hopes on the audiophile market they must be bitterly disappointed by now. Neither hardware nor software sales could be sustained by the high-enders. The mass market appears more interested in convenience per se rather than sound quality, opting for the ultra convenience of MP3s and the iPod. The public has spoken with its pocket book; one can only conclude that the vinyl LP is presently the leading consumer high-resolution audio format.
If you are reading this, you obviously must have more than a passing interest in good sound, and in particular, upgrading the performance of your vinyl playback system. So please stay tuned, the following is required reading for anyone interested in a remarkable phono stage at a reasonably affordable price. Featuring a nominal midband gain of about 50 dB, the PH-1 should be able to accommodate even low-output (but not insanely low) moving-coil cartridges when used in conjunction with a line preamplifier of 10 to 20 dB gain. I had no trouble at all obtaining more than enough gain with my Symphonic Line RG-8 gold cartridge, a low-output moving coil cartridge hand crafted by A. J. van den Hul.
The Artemis Labs PH-1 is a tube-based, made-in-USA, all-triode, phono stage designed by John Atwood of One ElectronTM fame. John has been involved in some high-powered tube design projects. For example, his work on the now defunct Fourier Components Sans Pareil and Triomph OTL amplifiers is well know to me. The PH-1 represents his latest thinking on subject - the result of several years of perfecting a simple yet extremely high quality phono stage, optimized for medium to high-output cartridges, either moving coil or moving magnet. While many affordable phono stages tend to be rather basic in conception and execution, this product features such a veritable cornucopia of interesting design aspects that I feel honor bound to do all of them justice, so please bear with me.
The Artemis Labs PH-1 takes its cue from the 5687 used in the output stage. This is a medium mu dual triode from the tube design team at Tung-Sol that was intended for industrial and military applications. It runs at a much higher filament current relative to other 9-pin miniatures, which is partially responsible for its linear response, and in its mil-spec version, is known for unit-to-unit consistency. Fortunately, it has seen little use in high-end tube gear and thus it is readily available at very reasonable prices. This is a tube that I also like a lot; it just sounds good even when driven hard.
A large (500 Hy) Lundahl choke is used for the 5687 plate load instead of a “Plain-Jane” resistor. It is a high-cost solution to the age-old quest for the ideal plate loading. A large plate resistor makes the tube behave like a current source, provides excellent power supply rejection, and generates the lowest distortion possible. Unfortunately, because the tube and plate resistor act as a voltage divider with respect to the high-voltage bias supply (B+), there is a practical limit to the plate resistance that may be used: the larger the plate resistor value, the higher B+ needs to be. Recall that a choke features a large AC impedance but a low DC resistance. Thus, from the tube’s perspective, the choke appears as an ideal “infinite” resistance load to the audio signal without a significant drop in B+ voltage. Choke loading makes it possible to have your proverbial cake and eat it too: current source operation with a moderate plate supply of only 160 VDC. Another benefit results from the fact that the choke is also an energy storage device, permitting plate voltage to swing to almost 100% above the B+ plate supply, doubling the 5687’s headroom and accounting for its huge output voltage of over 40 V rms before clipping. The downsides, in addition to cost, are the need to shield the choke from magnetic fields (hum) and the required core size to avoid low-frequency distortion. The weight of the two chokes is reflected in the unit’s weight, being quite substantial at 24 pounds.
The output stage is actually a two tube affair with a 12AX7 high-mu triode feeding the grid of the 5687. A modest amount of feedback (6dB) is used between the plate of the 5687 and cathode of the 12AX7 to stabilize the gain of the output stage and reduce the frequency response sensitivity to loading. In my opinion, this degree of inter-stage feedback is a good thing, and purists need not be concerned.
To quote Atwood, “the first stage is a low-noise "preamp" to bring the signal level up before hitting the passive equalization network. A high-transconductance tube with reasonably high gain is needed. I used to use 6DJ8s for this position in my old designs, but have come to dislike the sound of that tube. The Russian 6N1P sounds much better, and is quite low-noise, too.” Being tasked with amplifying microvolt level signals, the input stage’s design and execution are critical to the overall success of the phono stage. Low-noise operation is imperative. Passive parts are said to have been chosen carefully for low noise and minimum low-level nonlinearities. To get around the problems of cathode biasing and the typical electrolytic bypass capacitor, a single “N” alkaline cell is employed in the grid circuit to provide fixed bias. This allows the cathode of the 6N1P to be connected straight to ground. Being in the grid path, virtually no current is drawn, resulting in many years of battery life. It is important to note that any battery non-linearities are not manifested by the tube’s current flow, which would have been the case had the battery been inserted in series with the cathode.
Atwood’s philosophy is to use feedback very carefully. He says that phono stages with feedback-type equalization are especially prone to slew-rate distortion, due to high-energy high-frequency crud coming off the record. Thus, the PH-1 uses fully passive equalization, followed by a two-stage amplifier with limited feedback. This means that little high-frequency energy interacts with the feedback stage. EQ accuracy is said to be within +/-0.2dB. In order to achieve such an EQ tolerance, network capacitors are hand-selected on a precision impedance bridge to better than +/-0.5% accuracy.
As the number of active amplifying tube stages is odd (three to be exact), the PH-1 inverts absolute polarity. There is fundamentally nothing unusual about polarity inversion – it occurs frequently in a complex recording chain and at the output of a variety of gain stages. What matters is that during playback that the speakers “see” an even number of polarity inversions. However, a faction of audiophiles possess an irrational fear of things they don’t properly understand. All that is required to ensure correct polarity at the speakers in this situation is a reversal of the speaker lead polarity at either the amplifier outputs or the speaker inputs. It is as simple of that. I just wish that the PH-1 manual clearly stated that the unit inverts polarity. To quote Atwood yet again: “I usually tell people who are concerned about maintaining polarity throughout their system is to swap their speaker cables to correct for an inversion. However, since the polarity of the source material is ambiguous, on-the-fly switching is what is really needed. The problem is that implementing this (using, for example, phase inverters or transformers) usually compromises the sound. This is a problem I am still working on.”
A totally cool feature of the PH-1 is its Cool-Swap technology that facilitates cooler running tubes and provide a built-in spare. Since many dual triodes, including the 5687 and 12AX7 (but not the 6N1P), have split heaters it is possible to operate only a single triode section. In the Cool-swap configuration, one 5687 and one 12AX7 are used per channel, each with only one half heated. The right channel uses triode #1 and the left channel uses triode #2, the unused triode in each tube being essentially a spare. After a couple of thousand hours of use, you may want to swap the positions of the two 5687s and 12AX7s, thus bringing the unused spare triode into use. In other words, the left channel tube changes places with the right channel tube. You might say that Cool-Swap effectively doubles tube life, and since only half the tube is heated, these tubes run cooler, an important factor in the typically hot running 5687. Tube life is further enhanced by a soft heater turn-on and delayed application of high voltage. The B+ plate supply is delayed by about 40 seconds to give the tube heaters ample chance to warm-up without the chance of “cathode stripping.” In addition, a muting relay shorts the audio outputs for about 4 seconds after high voltage is applied to allow transients to die away. Note that all time delay and regulation circuits use proven circuitry based on discrete semiconductor devices.
The power supply uses high-speed solid-state rectifiers for both the plate and heater supplies. A MOSFET based voltage regulator is used for each channel B+ rail to maintain a stable operating point, independent of power supply fluctuations.
All critical circuits in the audio path are wired point-to-point on military-style terminal boards which feature silver-plated turrets. According to designer Atwood, fiberglass FR-4 material has odd dielectric behavior, something that Tektronix found out in their oscilloscope designs. By using military-style terminal board construction and point-to-point wiring in the areas where high-impedance audio signals are present, the sonic effects of the PC board is minimized. Conventional fiber-glass PC boards are used for the power supply and for the rear panel jacks and switching. All film capacitors in the signal path are either polypropylene or silvered-mica types.
While moving-magnet phono cartridges are typically designed to work into a 47 kOhm load resistance, most moving coil cartridges sound their best into a much lower resistance and may even benefit from additional load capacitance. With almost all phono stages one is pretty much stuck with a fixed loading. Thus, unless you are handy with a soldering iron and probably willing to violate the product warranty, you have no chance to experiment with cartridge loading to squeeze out the best sound possible. Not so with the PH-1. In fact, it facilitates experimentation. A 47.5 kOhm resistor is hard-wired across the inputs of the PH-1. That’s the starting point, but a 3M TexTool® ZIF (Zero Insertion Force) Socket, originally designed for the semiconductor industry, makes it very convenient to add shunt resistors or capacitors. Simply take the cover off and flip the lever out to open the socket. The top three positions of the ZIF socket are available for the right channel and the bottom three positions are for the left channel. Be sure to shorten the resistor legs and bend them to fit into the apertures. Closing the socket clamps the resistor/capacitor leads between gold-plated blades for a secure electrical connection. Note that any added resistance is in parallel with the fixed internal resistance of 47.5 kOhm. It was a good old-fashioned analog blast experimenting with my Symphonic Line RG-8 Gold cartridge, eventually settling on 681 Ohm shunt resistors (1/4 W, 1% Vishay/Dale metal-film), which give an effective load resistance of about 670 Ohm.
For the past several years my phono system has evolved and crystallized to near perfection. Its foundation remains the Kuzma Stabi Reference turntable. Outfitted with the Graham Engineering model 2.2 tonearm and the RG-8 Gold MC cartridge, I’ve been enjoying the best analog sound ever. It was into this exalted front end that the PH-1 made its grand entrance. To be sure, my expectations did not run particularly high in view of its relatively modest asking price – at least in high-end terms. Holy Cow! Speak about a strong first impression, my head turned in its direction with new found respect. Only once in a great while have I established an instant “bond” with the sound of an audio component. My first such love at first listen was the QUAD 57 loudspeaker. Spacious, out-of-the-box, pristine midrange to die for. My conception of a loudspeaker’s potential was changed for ever by that first close encounter. It is a shame that QUAD has never sought to resurrect that classic. Contrary to many published opinions, I still regard the original as superior to later editions. Add the PH-1 to that select list. Audio suaveness is hard to define, but I know it when I hear it, and the PH-1 has it in spades.
Imagine tube heaven: tube smoothness with the transient agility and control of solid-state amplification. Throw in strong bass lines and a sure hand in unraveling microdynamic nuances and you have a pretty good idea of what I mean by suaveness. OK, it was time to settle down for a set of extended listening sessions. One of the sonic attributes I like to get an immediate handle on is tonal balance. It is not that I am fixated on tonal accuracy. To confess, I have always had a preference for a warm, full-bodied presentation. It’s a question of quickly ascertaining a component’s tonal color in order to be in a position of accommodating its personality. And the best method of deducing a front end component’s impact on the overall system sound is via substitutions downstream. I happened to have on hand three fine line stages, whose sound I was quite familiar with: the deHavilland Mercury (review pending), my own Blue Velvet, and Audio Consulting's Silver Rock Transformer Potentiometer. Listening to the sound of the PH-1 through each of these line stages allowed me to gauge the PH-1’s intrinsic character. In essence, using each of them as a sonic mirror to determine if the PH-1 added or subtracted from the sound of the line stage.
With the deHavilland Mercury, the impression of neutrality and timbre fidelity carried through the rest of the chain. Two albums that are always near the top of the pile are Cleo Laine “Live at Carnegie Hall” (RCA LPL1-5015) and Mendelssohn’s Violin Concerto with Itzhak Perlman (EMI ASD-2926). Cleo Laine, no less than a Dame Commander of the British Empire, sounded spectacular on this occasion possessing exceptional timbre accuracy and focus. Perlman’s playing was also a highlight, lyrical, poised, without excess. Violin overtones shone with just the right measure of sheen and sweetness. Another example, “Lesley” (VTL recording by David Manley) impressed with its effortless enunciation of musical lines, image focus, and dynamic range.
Switching over to the Blue Velvet brought about a much different midrange voicing. The big-tone sound of the RCA VT-231 was very much in evidence. Textures were coated with a “taste of honey.” Orchestral foundation remained strong and well defined. Midrange textures were sweet and pure sounding. The soundstage perception was of an integrated organic whole with excellent width and depth. There was always plenty of low-level detail that helped to flesh out the ambient signature of each recording. Massed voices were easy to resolve. The listening perspective was neither forward nor too distant, being approximately Row M, to quote one of J. Gordon Holt’s useful analogies. Complex musical passages were unraveled with ease. The dynamic scale from soft to loud unfolded as if shot from a catapult.
It’s time to mention another couple favorites of mine. First, the self-titled “Joan Baez” (Vanguard VSD-2077). If you are looking for a recording of a clear and enchanting soprano voice that can capture the emotional states of her folk material, then look no further. Aided by a simple acoustic arrangement, Ms. Baez’s soulful voice resonated with meaning. Second, Taj Mahal’s “Recycling the Blues & Other Related Stuff” (Columbia 31605), and in particular, the “Sweet Home Chicago” and “Texas Woman” tracks delivered plenty of satisfaction. There was plenty of testosterone in evidence. Taj Mahal’s National steel-bodied guitar cut though the mix at Formula 1 speed and precision.
It was time for the Silver Rock. It too generated smooth and sweet harmonic textures, while managing to sound slightly less electronic than the Blue Velvet. However, without the latter’s midband richness and bloom. It was time to jot down the conclusion that the PH-1 failed to impose a vintage tube personality on the sound. While possessing tube virtues such as sweetness of texture, it nonetheless did not tilt the Silver Rock toward an overly lush or romantic presentation. It clearly allowed the personality of the line stage to assert itself, as was clearly the case with the Blue Velvet. Therefore, should you desire a thicker more vivid harmonic palette than that offered by the PH-1, you have the option of inserting your favorite vintage tube stage into the chain. Bottom line: the PH-1 is tonally neutral. It sound is even without upper octave brightness, midrange bloat, or bass heaviness. But, as you may have guessed, suaveness is all about balance, detail, and musicality.
You know, I have done some crazy things during my audio career, like wrestle full-size helium gas bottles into my listening room to feed the ever helium-hungry Plasmatronics plasma loudspeakers. On the other hand, recommending the Artemis Labs PH-1 phono stage is a no-brainer and light years removed from all that high-end madness. It is an exceptionally well designed and crafted component, and quite affordable by high-end standards. Its sound quality is in fact bordering on the amazing relative to its asking price and it easily makes my all-time top-three short list of phono preamplifiers. Simply put, the PH-1 dishes out an exquisite analog listening experience – a blend of tube strengths and the sort of bass control and detail resolution traditionally associated with solid-state amplification. It would be fair to dub it as an all around performer, innately sweet sounding, but when called upon, it proficiently delivers precise forceful bass lines. The beauty and the beast under one hood – what more could you ask for?
Mid-Band Gain: (47K load, 1KHz): 52dB +/– 2dB