Nelson Pass

Nelson Pass

Nelson Pass (born June 27, 1951) is a noted and widely respected designer of audio amplifiers[citation needed].

Unlike some audio engineers, Pass remains vocal that listening tests remain valuable and that electrical measurements alone do not fully characterize the sound of an amplifier.[1]

Mr. Pass holds at least seven U.S. patents related to audio circuits.[2]



Studies, PMA and ESS

1974 he received his BS in physics from the University of California-Davis.[3] During his studies he and Mike Maher founded the small speaker company PMA.[4] From 1973 to 1974 he was employed at ESS to design speakers.[3]


After graduating in 1974 he and René Besne founded high-end amplifier company Threshold Electronics on December 5, 1974. Later Joe Sammut became the third partner.[4] Threshold is perhaps best known for the "Stasis" amplifiers (a design later also produced under license by Nakamichi). He sold Threshold Electronics in 1997. The company continued without Nelson Pass under the name Threshold-Audio.


In the mid 1980's, Mr. Pass designed the well-reviewed Adcom GFA-555 amplifer. [5] [6]

Pass Labs

Pass founded Pass Labs in 1991. For many years a notable and highly regarded design / product by Pass (and his companies) was the Class A "Aleph" series of single ended amplifiers. Pass Labs produces the powerful and highly regarded X series amplifiers[attribution needed], which make use of the "supersymmetry" topology patented by Pass, to give extremely low distortion levels, and more recently the XA series of amplifiers, which advantageously combines aspects of the Aleph design with the supersymmetry technique. Recently Pass Labs has introduced a loudspeaker, and Pass DIY has increasingly explored the field of high efficiency / full range speakers as a complement to minimalist amplifier designs.

First Watt

In parallel with Pass Labs, Pass also runs First Watt, a self described "kitchen table" commercial venture where Pass hand builds (in very limited numbers) some low power / minimalist designs he chooses to not series produce through Pass Labs.


Unusually for a leading figure producing commercial equipment, Pass has also long been very supportive of the DIY audio community, by way of published articles (Notably in "The Audio Amateur") and schematics of out of production models on the Pass Labs site and more recently the First Watt site. Unusually for such a noted figure, he frequently interacts directly (if tersely) with audio hobbyists individually, all of whom are extremely grateful.[7][8][9] His nickname among the DIY audio community is "Papa".

Pass' Design Characteristics

Typically using MOSFETs as the gain devices, (often as current sources rather than voltage sources), most of his designs operate in Class A, with all the low distortion of small signal benefits generally associated with that. He is also known for his advocacy of "minimalist" designs which reduce the circuit complexity / number of components through which the audio signal must travel, resulting in any residual distortion having a very simple distortion spectra. His more powerful designs that go beyond what can practically be realized in Class A (such as the Threshold and Pass Labs products) operate in Class AB1 but retain a heavy bias so that they operate in Class A during quieter passages, and great emphasis has been given to avoiding / minimizing switching effects (e.g. the Stasis concept)


  1. ^ Friend or Foe? High End Designers Reassess the Importance of Audio Measurements
  2. ^ Patents with "Nelson" and "Pass" in the inventor list
  3. ^ a b NP@diyaudio
  4. ^ a b Interview with NP
  5. ^ Stereophile review of GFA-555
  6. ^ Discussion of Nelson Pass work at Adcom
  7. ^ The Pass Zen Amplifier
  8. ^ The Pass Labs DIY site
  9. ^ The DIY Audio site

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