The Aleph Null, or 0, represents Nelson Pass’s maiden product under the Pass Laboratories banner. When he left Threshold several years ago, Pass had the luxury of starting over with a clean slate, and decided immediately that he wanted to design a single-ended MOSFET amp. The result is aptly named after Georg Cantor’s first transfinite number: Aleph Null, the gateway to higher-order infinities. Just as Cantor’s transfinite mathematics stretched minds with its novel conceptual view of the infinite, the Pass Aleph 0 tantalizes the imagination with a new dimension in the future of solid-state amplification: a single-ended output stage.
The sound of one transistor clapping
If only objective measurements are used to assess the performance of a power amp, then complex circuit topologies will be seen to excel compared with much simpler designs. The problem with measurements, of course, is that they describe performance not in the perceptual but in the physical domain. What something sounds like does not always correlate with how it measures. I’m not advocating or excusing poor measurements—decent measurements serve to validate a given design’s veracity and, by implication, the designer’s competence. Rarely will an amp that has gross test-bench measurements sound good. Conversely, I’ve heard many test-bench wonders sound downright ugly. There’s a point beyond which measurements fail to convey the musical potential of a device. If it were true—as some mainline reviewers and magazines maintain—that the story begins and ends with measurements, then a Japanese receiver ought to sound superior to almost every expensive high-end tube design.
As Nelson Pass is fond of saying, audiophiles have voted with their pocketbooks: on the used-equipment market, a Marantz 9 is highly coveted, while the early solid-state Dynaco 120 is largely unwanted. Looking at the last 50 years of audio history, I’ve found that simple designs (tube and transistor) have come closest to capturing the textural purity and intimacy of live music.
SE designs aren’t exactly new to Pass, who published a 20W single-ended DIY design in 1977 in the pages of Audio magazine. To paraphrase his words, the Aleph’s design rests upon the philosophical tripod of simple circuitry, transconductance gain devices, and maximally linear operation.
The Aleph uses only three gain stages. The differential input stage accepts both balanced (XLR) and unbalanced (RCA) inputs, the former tied directly into the feedback loop without additional active circuitry. A cascoded voltage gain stage drives an n-channel MOSFET output stage, which is biased at 2.5 amps quiescent current. Out to about 40W into 8 ohms—that is, almost up to 2.5 amps output current—this stage operates single-ended: both the negative and the positive signal swings are accommodated by paralleled n-channel devices. Beyond this point, p-channel power MOSFETs kick in to handle the positive signal swings. The amplifier therefore operates in class-A push-pull out to its rated power-delivery limit. Pass believes that defaulting to class-A push-pull is sonically superior to simply clipping the SE stage at a lower power level.
The MOSFET, Pass’s current gain device of choice, is a transconductance device, like a vacuum tube. However, it doesn’t require transformer-coupling to the low-impedance load—the Achilles’ heel of tubed SE amplifiers. Also, like a triode tube, a MOSFET’s transconductance, or gain, tends to increase with current. Pass believes that the bad rap many MOSFET-based designs have received is attributable to two factors: most designers simply dropped MOSFETs into the same (read: complex) topologies optimized for bipolar transistors, thus failing to capitalize on the MOSFET’s full potential. And MOSFETs perform poorly at low bias currents, exhibiting low-level non-linearities in traditional class-AB designs. Nelson’s bottom line is that a MOSFET must be operated in class-A—where maximum intrinsic linearity is achievable—to fully realize its benefits.
US-made, International Rectifier, HexFET power MOSFETs are used exclusively for all of the Aleph 0’s gain stages. Input devices are matched to within 0.2%, output devices to within 2%. Each output MOSFET is capable of handling peaks of 25 amps; two banks of eight MOSFETs are run in parallel. Since no current-limiting circuitry of any sort is used, peak currents of 50 amps can be delivered before the fuses blow. The circuit’s inherent linearity means that only some 20dB of global negative feedback need be used to tailor the overall voltage gain and increase the damping factor to about 800—equivalent to an output impedance of 0.01 ohms.
Is that you, Mother?
So runs the punch line of the joke that begins: What does the baby porcupine say after he backs into a cactus? The Aleph 0, with heatsink fins on all four sides, reminds me of a porcupine. So it doesn’t look very French or German—an American original, indeed! The machined-aluminum chassis is grained and anodized for a cool metallic look.
In 1974, Nelson Pass ushered in a new era of “cool-running” class-A amps with his patented, dynamically biased “Stasis” output stage. Pass now admits that he hasn’t used this technique for the past 15 years, because he’s found that operation at reduced bias levels degrades sound quality—primarily soundstage depth and textural liquidity.
So, no surprise, the Aleph 0 runs hot, its heatsinks warming up to 120–130°F after one hour of operation. When the Aleph’s internal temperatures exceed 160°F, a thermal protection system in the unit shuts the amp off until the thermal sensor has cooled. With adequate ventilation (at least 6″ of clearance is recommended on the sides and top), and ambient temperatures of up to even 90°F, chassis temperature is supposed to remain below the trigger level of the protection circuit. Of course, the chassis’ rate of warmup is also a function of the type of signal used, test signals being harder on the amp than music.
So when one channel went mute one hot summer evening, it took me a while to realize what had happened. I checked all cable connections and scratched my head; and when the amp came back to life some ten minutes later, it dawned on me: thermal protection! This prophylaxis is aimed at protecting the output transistors, but I found it inconvenient at the time. A few minutes after the first channel came back to life, the other channel played dead. The final solution was to shut the amps off for a while. Although it was a hot New Mexico day and the amps had been running for several hours, they were well-ventilated, and ambient conditions were within the envelope specified in the owner’s manual. The thermal-protection system needs fine-tuning.
The Aleph 0 is DC-coupled from input to output—there are no capacitors in the signal path. It takes an hour for the chassis temperature to stabilize, during which offset levels are a bit higher than 50mV. After the first hour, DC offset at the Aleph’s output drops to around 50mV. Such DC offsets do not interfere with loudspeaker performance. DC offset levels could, however, be exacerbated by the preamp—when the preamp’s own output stage is DC-coupled. Another demand the Aleph places on the partnering preamp is its ability to drive a rather low input impedance, which means that, in general, the preamp’s output impedance should not exceed several hundred ohms.
NEXT: Page 2 »
COMPANY INFO
Pass Laboratories Inc.
13395 New Airport Road
Suite G, Auburn, CA 95602
(530) 878-5350
passlabs.com
ARTICLE CONTENTS
Page 1
Page 2
Aleph 0 and 1.2 Compared
Specifications
Measurements
Click Here: state of origin merchandise