NELSON PASS: The Patents Of Pass

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Brian Damkroger .....

If high-end audio were to carve its own Mt. Rushmore, whose faces would appear there—besides that of Stereophile founder J. Gordon Holt, of course? It's likely that no two audiophiles would ever come up with identical lists of subjects, but I wouldn't be surprised if they could agree on at least one name: Nelson Pass.

Pass's influence has now spanned three decades and shows no signs of stopping. Card-carrying audio junkies and the more-power-is-always-better crowd have long lusted after his megawatt beauties. The flea-power amp and open-baffle types drool over the Zen models of his First Watt line, and the true hard core, the DIY crowd, refer to him as "Papa" as they eagerly absorb his donated designs, wisdom, and general good humor.

Nelson Pass is many things, but most of all he's an engineer. He views the world as a puzzle to be solved, and immediately after figuring out how something works, he begins thinking about how to make it work better. He's approached audio design with this combination of curiosity and pragmatism, and the result has been a string of innovative, often brilliant designs, many of which have been based on technologies that Pass has patented. I tried to follow the evolution of his designs through these patents but was quickly buried in legalese, so I asked Pass to walk me through them chronologically. The result turned out to be a mini-history of his career as an audio designer.—Brian Damkroger

1976—US Patent 3995228: Active bias circuit for operating push-pull amplifiers in class-A mode: [Pass's first patent describes what's often referred to as "sliding bias," where the bias on the output devices varies with the signal to prevent them from shutting off, ergo avoiding crossover distortion. ]

There was a lot of interest in class-A operation, but there were practical issues, mainly their size and inefficiency. The maximum power output was theoretically limited to twice the bias current, and really more like half that. I modulated the bias to stretch the bend in the operating curve. Technically, this kept it operating in class-A, but at a lower bias level.

The design was successful; the amps sold like gangbusters and were copied immediately. Unfortunately, the approach ended up getting a bad reputation. Our first amplifier, the Threshold 800A, used about a 1:1 ratio, so that it idled at 150W for 150W output instead of 300W. That seemed reasonable and worked well, but some people took it to an extreme. Instead of idling at 150W, there were 200Wpc amps that idled at 10W, ran cool, and didn't have any heatsinks—all claiming to be class-A. In retrospect, we could have sued the copycats, but we were young and foolish, and besides, I had another design waiting in the wings.

1978—US Patent 4107619: Constant-voltage/constant-current high-fidelity amplifier: The patent application actually shows the circuit of the [Threshold] Stasis 1 as the example circuit. This is a kissing cousin to the Quad 405, injecting current into the output for error correction, but with way more current. We had a massive power supply and a huge bank of power devices that we used to support a smaller output-voltage source.

All distortions are variations in a device's characteristic with changes in voltage and current. Ideally, you want to ...

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