Pass Aleph P
Pass Aleph P
(ขายต่อแล้ว เมื่อวันที่ 1/8/2559)
|Stereophile, February 1996||Steven Stone|
|What is black, has four knobs, and could be dropped off a second-storey window without sustaining any major damage? If you say the Pass Aleph P preamplifier, you’ve been reading the boldface type at the top of the page instead of jumping immediately to the summary section. How gratifying. Nelson Pass, never content to sit on, stand on, or do whatever he does with his laurels, has produced a preamplifier to go along with his Aleph series of amplifiers. This component shares more than just the name Aleph with its beefier brethren; it even uses the same output devices. But that’s the sort of technical detail reserved for another section of this review. To whet your appetite for further semiliterate exploration, I’ll just state that this is a preamplifier worthy of the Aleph moniker. Doesn’t this Pass feller ever quit?
DESIGN & ERGONOMICS
The Pass Aleph P design philosophy can be summed up in one word: simplicity. Its exterior styling is positively austere. Only the words “Pass Aleph P.” “Source,” “Right,” “Left,” end “Volume,” deeply engraved in heavy block lettering, grace its front panel. The rotary selector on the left side has just four dots to delineate each of the four possible input selections. The master gain control knob on the right has only a slit for a setting mark. The front-panel center is populated by a glowing blue light and a pair of knobs to control the gain of each channel. That’s all there is. A bale of hay has more visual interest.
The rear panel is equally minimalist: four inputs, each featuring single-ended RCA and XLR connections. To choose single-ended inputs, you insert a gold U-shaped shorting plug into the corresponding XLR input. Find a nice, safe resting place for the connectors; my cats think they’re the best play-toys this side of a catnip mouse. There are two RCA outputs, one with fixed gain for supplying a tape deck and one with variable gain to connect to an amplifier. Two XLR outputs provide the same functions for balanced devices. The final item on the rear panel is an EIC connector for the power cord. To eliminate nasty feedback howls, input 4 is intended to be used as a tape input, so a relay shuts off the tape outputs on the back of the preamplifier when it is engaged. Safety is a wonderful thing.
The Aleph P chassis is constructed of 1/2″-thick solid pieces of machined aluminum on the front and sides, and 1/4″ thick pieces on the back, top, and bottom. Don’t drop this baby on your foot, or you’ll be hopping all the way to your nearest health-care facility.
The instruction book is an informative “good read.” It contains everything a normal person needs to know about the Aleph P and Nelson Pass’s design philosophy, including his estimate of the sun’s longevity.
The test sample was a manual unit, sans remote control. By the time you read this review, the Aleph will be available with a remote control for a paltry $500 more. All older preamplifiers without remotes will be retrofittable; just a short visit to the Pass Laboratories’ sumptuous corporate facility will be required. No firm price has been announced, but I have it from the horse’s mouth that the modification will probably cost slightly less than $500.
The Aleph circuit is as simple as its exterior. Each input line is amplified by a single International Rectifier power MOSFET operated in single-ended class-A mode with zero feedback. With a maximum power rating of 125W and peak current capability of over 50A, this MOSFET is the same device used in the Aleph 0 amplifier. In the Aleph P circuit, they’re on semipermanent vacation. Upon entering the Aleph P. an input signal is patched almost directly to the gate of the MOSFET1. Only the source selector relays, one 200 ohm resistor (to prevent parasitic oscillations), and one metalized film capacitor (to block DC) are in the path in front of the MOSFET. Input sources aren’t directly coupled, since the MOSFET uses a DC-biasing arrangement that could pass DC back to an input source if not blocked.
Once the signal reaches the gate of the MOSFET, the circuit design specified in US Patent No.5376899, “Amplifier with gain-stage coupled for differential error, correction,” goes into effect. The output from the source connector of the first MOSFET—let’s call it A—is routed through a variable resistor before going to the source of a second MOSFET, which we’ll call B. The resistance between these two sources helps to set the intrinsic gain of the circuit. MOSFET B also receives the negative-polarity signal from the balanced input at its gate. When a single-ended source is used, the little U-shaped shorting plugs ground the MOSFET B’s gate.
The elegance of this circuit is that the variable resistor that controls AC voltage from MOSFET A’s source pin to MOSFET B’s source pin also passes any distortion created by MOSFET A to MOSFET B. where it is canceled out by identical inverse-polarity distortion at MOSFET B’s drain. This same mechanism occurs with distortion from MOSFET B that is transferred through the resistor to MOSFET A. This is the patented differential error correction. A further bit of cleverness is that differential distortion-cancellation occurs whether you use a balanced or a single-ended input or output.
The master volume control for the four outputs (one positive-polarity and one negative-polarity per channel) is clone via four-pole double-throw relays controlling precision Dale metal-film resistors placed just before the output. The front-panel volume knob actually supplies a DC voltage to an analog/digital 6-bit converter with 64 levels. Let me emphasize that the audio signal is not transposed into the digital domain. The A/D is simply a far more accurate way make a volume control—even the most expensive off-the-shelf four-way potentiometers have a much higher tracking error. With anything other than a digital controller, the preamplifiers CMRR (common mode rejection ratio) would be far less than optimal.
Through careful matching of resistor values, this master volume arrangement makes it possible to achieve CMRR of at least 60dB. Since the Pass Aleph operates without feedback and is buffered by the master volume attenuator, severe loading shouldn’t produce distortion. The Aleph P will drive any impedance load demanding less than at least 20mA peak current (equivalent to +20dB into 600 ohms).
The Aleph P’s power supply uses a toroidal power transformer that delivers 85V. This unregulated power is then filtered before it reaches active discrete regulator circuits, which employ both passive and active systems. The regulated power is then sent to each channel through a double set of passive filters. Using one active and six passive filters reduces power-supply noise to a level of about 5mV. When the Aleph P’s balanced outputs are used, even this small amount of noise is differentially rejected.
The following equipment was used for thus review: Analog sources were a VPI TNT Jr. turntable with cost-effective upgrade and outboard flywheel on a Bright Star base and Townshend Seismic Sink. Tonearms mounted on the table were the Graham 1.5 TC and Clearaudio/Souther TQ-1. My small room has a VPI HW-19 Mk.lV with SAMA sitting on a Bright Star J-7 base mounted with a Clearaudio/Souther TQ-1. Cartridges included the van den Hul MC-1 Super, Dynavector XX-11 low-output MC Fidelity Research/van den Hul FR-1 Denon 103/van den Hul, and a Denon DL-SI. Digital front-ends were PS Audio Lambda and C.E.C. TL 2 CD transports, and a Sony D-7 portable DAT recorder connected via coaxial, AES/ EBU, TosLink, and AT&T optical connectors to EAD DSP-7000 Mk3 or DSP-9000 Mk.3 D/A processors.
Other preamplifiers in-house were the Threshold T-2 and Carver Research Lightstar line-level units, with Vendetta SCP-2C, Audio Research PM-2, and Gold Aero dB-45 outboard phono units. Power amplifiers used were the Rowland Model 6, Manley Reference 240, and Pass Aleph 0. Speakers were the Dunlavy Signature VIs in my large room, and the Avalon Eclipse speakers in my small room.
Interconnects included Audio Magic Sorcerer, Synergistic Research Kaleidoscope, and WireWorld “Eclipse.” Only balanced lines were employed.
Speaker cables used were Dunlavy Labs DAL-8Z, Audio Magic Sorcerer (with the Dunlavys), and Synergistic Research Signature 2 and 3 (with the Avalons), in 8′ lengths. Digital cables used were Mod Squad Wonder Link 1 coaxial, Audio Magic Sorcerer coaxial, TARA Labs RSC Master AES/EBU, AudioQuest, Sony, and Parasound optical cables.
Other accessories included Room Tunes CornerTunes, EchoTunes, and Ceiling Clouds, Acoustic Sciences Tube Traps and Shadow Casters (in small room), Arcici Levitation stand (in large room), RoomTunes Just-a-Rack, Arcici Superstructure II, Soundstyle X503 and Billy Bags amplifier stands, with all major components on Bright Star Audio Big Rock bases and Little Rock top-plates (in small room), Shakti Stones, Fluxbuster, PAD break-in disc Music and Sound ferrite beads, AudioQuest ferrite damps, NoiseTrapper Power Strip, Synergistic Research power cords, TARA Labs RSC Master power cords (with Pass Aleph 0), Coherent Systems EAU-1 Electroclear AC line conditioner, AudioQuest record brush, Gryphon “Exorcist” conditioning tool, Nitty Gritty record-cleaning machine, Radio Shack Sound Pressure Meter, Kleenmaster Brillianize CD cleaner, and a 1946 Gibson Southern Jumbo guitar.
The Pass Aleph P is easily the best solidstate line-stage preamplifier I’ve ever had in my clutches. Is it perfect? Is it the fabled Holy Grail of a straight wire with gain? No. Who do you think Nelson Pass is— God? It’s still an active preamplifier possessing some of the small subtractive colorations and variations from absolute neutrality that seem to plague all active devices. I’m still waiting (and listening) for the preamplifier that’s as good as no preamplifier at all. So far, no luck, but I’m young (relatively) and incredibly patient. ..
The differences between the Aleph P and no preamplifier at all were quite noticeable, On Susan Werner s cover of the Paul Simon song “Something so Right,” from her Last of the Good Straight Girls (Private Music 82126-2), low bass was tighter, there was greater dimensionality and less grain on Susan’s voice, a slightly wider soundstage, better lowlevel detail, more sizzle and air on the ride cymbals, and much better delineation of decays without Pass preamplifier in the circuit. Even though the Pass is easily the fastest active preamplifer I’ve heard, it still slightly smeared decays.
Without any preamplifier, the delineation between the presence and absence of music was more distinct, so sound faded away in a more natural and realistic way. On Joan Osborne’s song “Spiderweb” from Relish (Mercury/ Blue Gorilla 314 526 699-2), the Pass was less fast on the transients. With this dense mix of percussion instruments and guitars, individual parts weren’t as easily individualized as in bypass mode. Again, there was a noticeable increase in air and top-end extension when the preamplifier wasn’t in the circuit. Cymbals have an airiness that evaporates through the Pass. Microdynamics were also ever so slightly compressed by the Aleph, resulting in a loss of vibrancy and rhythm.
I know this all sounds pretty grim for Nelson’s new baby, but every active preamplifier I’ve ever heard produces sonic anomalies. The Aleph P is the least colored of the bunch. Not to denigrate any of my fellow reviewers or audiophiles, but I’m amazed that they can argue and obsess over the minutiae of tubes, Tiptoes, and wires when so much sonic degradation is caused by their active preamps. It’s like worrying about whether your underwear is clean just before you’re hung by the neck until dead.
ANALOG LISTENING COMPARISONS
So that no one confuses me with “Digital Lad,” it’s time to use up some ink on the Pass Aleph P’s performance with analog sources. I just received Classic Records’ new release of Mahler’s Symphony 4 performed by the Chicago Symphony Orchestra directed by Fritz Reiner (Classic Records LSC-2364). One of the most treasured records in my +4500 LP collection is a “White Dog” copy of RCA’s original release given me by Harry Pearson of The Absolute Sound over 10 years ago. Even through a Dixie cup and a string you could hear the differences between these two records. The Classic Records reissue has far greater low bass information, with more distinct timpani and double-bass parts. The Classic reissue also has a plethora of top-end air—lacking on the original—on flutes, violins, and triangle. Inner detail and low-level information are superior on the reissue; you can even hear Maestro Reiner stamping during a few passages on side one.
While in most ways the reissue is superior to the original pressing, it’s not a hands-down, grind-its-face-into-the-dust winner. Despite its fidelity to the 45-year-old master tape, the Classic release doesn’t have as true an overall gestalt of the flesh-and-blood performance as the original release. The main problem is that you don’t have a feeling for the forest because of the trees; the blend of the original is sacrificed for the finer details in the re-release. While the original gives you a wonderful feeling of the whole orchestra as a unit on stage, the reissue separates the orchestra into its individual parts.
This effect reminds me of the difference between the sound of my 1946 Gibson Southern Jumbo and a 1995 Gibson Advanced Jumbo. The old guitar has a wonderful resonance and homogenization to its harmonic character that’s lacking on the new guitar. The 1995 guitar sounds tighter, with a clearer, more distinct differentiation between the strings, but is less harmonically cohesive. You’re more aware of the individual notes, but less wrapped up in the music.
The Classic Records re-release is just like this. The parts are clearer, but the harmonic blend isn’t fully realized, so your feeling for the music is diminished. Too little center-fill on the reissue could be the problem. While there isn’t a real hole in the middle of the soundstage, sound does recede slightly in the middle. Reflected sound from the rear walls is lost, while reflected sound from the side-wall seems exaggerated. This increase in sidewall reflection also makes the stage seem wider, but not as deep as on the original. It sounds as if some giant grabbed the soundstage and pulled on either side, stretching it like Silly Putty from a ball into an egg shape. Compared to the Audio Research PH-2 running straight into power amps, the Pass Aleph P sounded a wee bit “smoky.” That old phrase “veiled” sprang to mind. Transient information wasn’t quite as fast-sounding. Through the Pass, both the top-end air and low bass energy were slightly truncated. Obvious differences still exist between the two Mahler releases, but resolution was reduced so that low-frequency information like the foot-stomping on Side One was less obvious. Though the Pass’s differences in depth, center-fill information, and other sonic subtleties were still quite noticeable, a bit of clarity was lost. Something more was added between the listener and the music.
UP AGAINST THE CARVER LIGHTSTAR
Compared to the remarkable Carver Research Lightstar Direct preamplifier used in its passive balanced-output mode (see review elsewhere in this issue), the Pass Aleph ran a very close second. On Sara K’s cover of the Allman Brothers Band’s blues chestnut “Whipping Post” from Tell Me I’m Not Dreamin’ (Cheeky JD 133), the Pass was slightly darker harmonically, losing some of the “plastic pick sound” from Bruce Dunlap’s exceptional guitar and some of the sibilance from Sara’s s’s. Microdynamics seemed slightly compressed on the Pass, with some of the subtle vocal shadings on Sara’s voice reduced Depth was somewhat truncated by the Pass, as was the dimensionality of the reverberant field in St. Peter’s church.
On a spatially precise system you’ll notice that Sara’s voice moves around the soundstage during the song. Preventing emotional singers from putting some body English into their work is impossible. With the crossed-figure-8 mike arrangement used on this disc every move seems like a hike across town. On the plus side, the Pass is was silent as the Carver, and added only the very slightest amount of grain to the sound. On Terrell’s song “Just Give Me Some Time” from Angry Southern Gentleman (Pointblank 40099 2), the Carver was a bit faster-sounding, with better transient rendition, especially on the big, boomy stand-up bass and acoustic dobro. The Carver delivered more of the double-basses’ low-frequency impact without sounding bloated, fat, or slow. The Pass’s bass resolution was good, but lacked a smidgen of the impact delivered by the Carver.
On Patty Larkin’s latest release, Strangers World (High Street Records 10335-2), the song “Johnny was a Pyro” features exceptional drumming by Shawn Pelton. The neat rimshots sounded more like real rimshots through the Calver. With the Pass they were duller with an excess of thud and less leading edge twack. The dense guitar textures created by Patti and her producer, guitarist-extraordinaire Jon Leventhal came through the passive Carver with less homogenization.
Again, the difference in low-level detail between the passive Carver and the active Pass made the music easier to decipher through the Carver. Fortunately, the Aleph P’s harmonic balance is remarkably close to that of the Carver, lacking only a speck of top-end air on cymbals and low-bass resolution on synthesizer, percussion, and bass guitar parts.
However, when the Aleph P was compared to the Carver’s active single ended output, the results weren’t even close.. . the Aleph clearly outclassed the Lightstar. There wasn’t a single sonic parameter where the Pass wasn’t vastly superior—dimensionality, grain, transient response, transparency, bass extension, top-end air, basic noise level, lowlevel detail—you name it, the Pass ruled.
UP AGAINST THE THRESHOLD T-2
For a final mano a mano I moved the Pass Aleph P into my small room, where I pitted it against the Threshold T-2 preamplifier. At last, a fair fight. On the song “Bate-coxa” from Badi Assad’s new Rhythms CD (Cheeky JD137), the Pass produced a more natural timbre on Assad’s classical guitar. The Pass also did a better job of preserving the three-dimensionality of the recording. This added dimensional fidelity is especially noticeable on percussion instruments.
The natural acoustics of St. Peter’s Episcopal Church in NYC were more convincingly rendered by the Pass; its faster rendition of transient information made the reverberant field of the recording venue more palpable and real. The Threshold and Pass have very similar harmonic signatures except for the slightly brighter upper midrange and warmer upper bass on the Threshold.
On “No Curb Service,” performed by Liz Meyer on her Womanly Arts (Strictly Country Records SCR-37), the dobro sounded less metallic and more natural through the Pass preamplifier. Through the Threshold T-2, the Dobro’s timbre sounded different from that of the acoustic guitar, but not as different as through the Pass, or in real life. Low-level detail was also better through the Pass, with more precise individualization of each instrument. Even when everybody is really wailing during the last verse, each part could be followed through the Pass with greater ease than with the Threshold.
In my renew of the Threshold T-2 last July (Vol.18 No.5, p.112), I noted that it was an exceptionally quiet preamplifier, with only the slightest amount of white noise at the listening position when supplying +5dB of gain. The Pass is even quieter. With every gain control maxed-out (+20dB of gain) I heard absolutely no noise from the Avalon Eclipses at my listening position. The Pass Aleph P is simply the quietest active preamplifier I’ve ever reviewed.
Listening to a version of Shawn Colvin’s “Cry Like an Angel” from a live E-Town performance on DAT, I was aware of the Pass’s slightly larger soundstage, especially during the applause at the end of the song. The Pass also did a better job of layering the different parts of the music—Shawn’s voice is in the foreground, with her Lowden guitar behind, followed by Bruce Hornsby’s background vocals, and finally Hornsby’s piano along the back of the soundstage. Let me emphasize that while the Pass did better the Threshold in most sonic parameters, it didn’t make the T-2 any less enjoyable.
Even after these extensive A/B sessions, listening to music through the T2 is an enjoyable, involving experience. Considering the T-2’s far more extensive features, it’s remarkable that its sonic performance is even dose to the Aleph P’s. —Steven Stone
MEASUREMENTS FROM TJN
The balanced output impedance of the Pass Aleph P at its line output measured 1497 ohms (slightly less in the left channel); the unbalanced output impedance measured 744 ohms. Both measurements were at the maximum setting of the main, left, and right level controls; as the main level control was reduced, the output impedance also decreased significantly (to 324 ohms at 3:00, 121 ohms at unity gain—about 12:00—and 79.8 ohms at a 9:00 setting, all balanced readings).
This may create minor matching problems at playback levels near the main level control’s maximum setting with power amps having low input impedances; ironically, Pass’s own Aleph O has an input impedance of just over 7k ohms (balanced). However, the relatively high gain of the Aleph O (22.3dB balanced and 15.8dB unbalanced) should minimize this problem in a system with an otherwise typical gain structure, as the volume control will be used in a position where the Aleph’s source impedance is respectably low.
The Aleph P’s input impedance measured 27k ohms balanced and 12.8k ohms unbalanced, and was unaffected by the setting of the main level control. The output impedance at the tape output (balanced) was 50.5 ohms with a 50 ohm source impedance and 583 ohms with a 600 ohm source impedance, indicating unbuffered tape outputs.
The DC offset at the Aleph P’s outputs measured under 1.3mV either channel, balanced or unbalanced. The preamp is noninverting from its inputs to its main outputs, and in the balanced mode pin 2 is positive, pin 3 negative. S/N ratio (unweighted, ref. 1V out) measured 87dB over a 22Hz-22kHz range, 84.6dB over lOHz-500kHz, and 89.3dB A-weighted.
The Aleph P’s frequency response is shown in fig.1. Note that as the level control is reduced, the small rolloff at the high end flattens out. The Aleph P’s channel separation is shown in fig.2 (100mV input, full gain on all controls). Though there’s a dissimilarity between the channels—and one balanced channel is notably better than the others— the result here is an inconsequential level of crosstalk at any measured frequency. As is typical, capacitive coupling between channels causes the crosstalk to increase at higher frequencies.
The Aleph P’s variation of THD+ noise against frequency is shown in fig.3 (again, 100mV input, full gain on all controls). Distortion is very low across the full range, even with the commonly encountered slight rise at higher frequencies.
Fig.4 shows how the THD+noise percentage varies with output voltage into 100k ohms. (The test frequency was 1kHz.) The rise at lower output levels is due, as usual, to noise. The 1% “clipping” point is reached at a massive 20V output! Finally, the distortion spectrum of a 50Hz input at a high output level of 5V is shown in fig.5. The third harmonic predominates, but at -69dB (about 0.035%) is still quite low. At a more typical output of 1.3V (not shown), all artifacts remain under -80dB (0.01%).
With the exception of the moderately high output impedance at maximum settings of the level controls, the measured performance of the Aleph P is first-class. —Thomas J. Norton.
CONCLUSION FROM SS
The Pass Aleph P is a very well-built preamplifier, with a neat, well-laid-out interior architecture and a high level of fit’n’finish. Even its deepest interior recesses show pride of workmanship and attention to detail.
Ergonomically, the Pass Aleph is simple yet quite functional. A polarity-inversion switch, a mono switch, and a real tape loop would be useful additions, but this preamplifier is designed to be an enthusiast’s product, not a fullfeatured soup-to-nuts device. It is an exercise in minimalism: How much can be eliminated to ensure maximum fidelity?
I realize I’ve been very hard on the Pass in this review, detailing its shortcomings against no preamplifier at all, and the Carver Lightstar preamplifier used in passive balanced mode. Sorry, but that’s my job. Anything less than absolute and utter sonic neutrality is a reduction in fidelity, and ultimate fidelity is what the High End is all about. Gear that is euphonic, musical, involving, and musically satisfying may be good enough for audiophiles and reviewers who listen only for their own personal pleasure, but ultimate fidelity is not about “good enough,” or even about personal preferences. Complete transparency must be the ultimate goal of any piece of high-fidelity equipment, especially a preamplifier.
For many audiophiles, using a passive balanced preamplifier or no preamplifier at all is impossible. This is unfortunate. For everyone who must use an active line-level preamplifier with 20dB of gain, the Pass Aleph P is a great way to go. It’s the least expensive solid-state preamplifier on the market that will deliver Class A sound Its sonic performance is a tribute to the philosophy that simpler is, indeed, better.
The Aleph P is an exceptional-sounding unit. It is quiet as death, with only a bit of subtractive coloration separating it from absolute neutrality. I believe the P to be sonically competitive with any active preamplifier on the market, regardless of price. While it’s too soon to call the Pass Aleph P a classic component, and prognosticating is best left to gurus and idiots savants, still I feel sure that time will prove the Aleph P to be a breakthrough product. Need I say more? —Steven Stone