Nelson Pass Threshold, Pass Labs, First Watt
By Greg Weaver
A case could be made that no other amplifier designer more clearly embodies the philosophy and spirit of simplicity of design than Nelson Pass. From the introduction of his first commercial product in 1975, he has continuously pursued the often flaunted but rarely realized “less is more” goal. Following a decidedly different direction than some other successful manufacturers of that time—companies such as Phase Linear, Harman Kardon, and Crown, who were revisiting the status quo (based on original published Class AB or Class B RCA circuits)—Nelson’s work started to blaze in new and uncharted directions.
Those early days of Class B and AB amplifiers were a time when measurement was king. Looking at the distortion of a Class AB amplifier on an oscilloscope, you could clearly observe that distortion actually increased as the output level decreased, where the crossover notch got bigger and bigger in proportion to the size of the diminishing signal. This was due to the failure of the plus and minus halves of the amplifier to mate up cleanly.
Most designers were using more complex circuits and large amounts of feedback to achieve better bench measurements, but the sluggishness of more complex circuits created problems with TIM (transient intermodulation) distortion. In addition, heavy feedback had a tendency to dry up an amplifier’s harmonic character, leaving it sounding a little sterile.
By the mid 1970s, Nelson recognized that as distortion numbers were driven down further and further through feedback, the sound was not seeing a corresponding improvement. He saw the inherent linearity of Class A amplifiers, whose traditional low efficiency had limited them to low power output, as an alternative. Since Class A eliminated switching, it removed the offending notch distortion of the waveform and allowed for a monotonic distortion character, diminishing as the level went down—the opposite of Class B and AB designs.
These insights would provide the jumping off point for what has been one of the most celebrated and illustrious careers in the industry. Pass founded Threshold with the strategy of developing a more efficient complementary Class A circuit. Even this early in his career, a pervasive theme had begun to emerge: select quality parts, put them in simple circuit, run heavy bias current, and use minimal (or no) feedback.
Threshold was extraordinarily successful on a number of levels, creating some of the first high-output Class A amplifiers, as well as delivering an incontrovertibly better sound than many other designs. Using bipolar transistors, Nelson pursued this line of development at Threshold throughout the 1980s, engineering one improvement after another—next building amplifiers with cascoded gain stages and then extending the concept to amplifiers having “current bootstrapped” output stages (and collecting a number of patents along the way).
By the early 1990s, Pass felt the urge to leave bipolar devices behind and explore the benefits of FETs, which offered output curves much like those of tubes, and sounded more musically natural. Over the next two and a half decades, his work at Pass Labs led to progressively simpler circuits and increasingly superior sounding amplifiers.
Pass has continuously advanced his craft with series after series of exciting and engaging products, including the breakthrough Aleph design in 1992 (the Aleph 0 was honored as “amplifier of the decade” by one magazine).
Whether making major advancements in circuit topology and performance with products like the revolutionary X (SuperSymmetry) and XA series, or simply refining and honing those already exquisitely performing circuits with revisions like the “.5” and “.8” enhancements, Pass has relentlessly employed minimalism in his pursuit of better sound.
A gifted and driven creator holding seven U.S. patents related to audio circuits, Nelson is likely not finished rewarding music lovers with his insightful and exciting work. Unlike many others in his field, he still believes that listening tests remain invaluable to advancing the discipline and that electrical measurements alone do not fully characterize the sound of an amplifier. His body of work demonstrates that just pursuing diminishing zeroes does not necessarily lead to better sonic performance, and positions him at the forefront in the Pantheon of High-End Audio Designers.