An interview with hifi legend Nelson Pass

Part-Time Audiophile talks to Nelson Pass

Nelson Pass formed his first audio company with a friend while studying physics at the University of California-Davis in the early 1970s. He went on to co-found Threshold Electronics and design the well-regarded Adcom GFA-555 amplifier. In 1991, he started Pass Labs to further explore his ideas about amplifiers and other electronics.

In his four-decade career, to paraphrase a line from “Lethal Weapon,” Pass likely has forgot more about building amplifiers than most engineers will ever know. Part-Time Audiophile contributing editor John Stancavage, after completing his review of the Pass X600.8, conducted this interview with Pass via email.

To read the Pass Labs X600.8 mono block review that John Stancavage wrote, and references in this interview, download The Occasional magazine Winter Edition interactive-online HERE.

John Stancavage:The first 50 watts of the 600-watt X600.8 are pure Class A before going to Class B. Many AB amplifiers generate far fewer Class A watts before switching. What was your goal in doing this?

Nelson Pass: Of course the goal is better performance. I wrote an article about this some nine years ago where I examined the performance of a push-pull output stage at various levels of bias current. The important point here is that the idle current has a strong effect on the performance at all power levels, from very small right up to clipping. Also, the transition from small-wattage Class A to larger power Class B (in an AB amplifier) is very much smoothed by higher bias current. So, larger bias gives you lower distortion and also fewer high-order harmonics.

 It’s always a trade-off between quality and hardware/efficiency.  For Class AB amplifiers, we simply set the bias as high as is practical – you will find that all our amplifiers run at about the same temperature, which is about 50 degrees C on the heatsinks.

 JS:In your listening tests – at normal volume levels – how close do you think the X600.8 comes to the performance of your fully Class A amps?

 NP: In my listening tests (and I am not the only one), they come fairly close, but then I only run a few watts. When I have to evaluate a higher-power amp, I use a network between the amp and my sensitive reference speakers to make them into a reactive 2 ohm load with 10 dB less sensitivity.  That works fairly well to achieve an “apples to apples” comparison.

Within a given power range, the Class A circuits take the prize by some margin, but often they aren’t practical.

JS:I’ve noticed more solid-state amp designers seem to be trying very hard to take distortion to the vanishing point. Some of these products can sound a bit sterile, however. Can you explain your philosophy about measurements, and discuss the role played by low-order harmonics in the Point 8 Series?

 NP: I like measurements, and I use them all the time, but they don’t get the last word.  Generally, there isn’t much conflict between what we measure and what we like to hear. I’ve spent decades working to correlate good sound with measurements, and we have a reasonable picture of what works, and you see that in our products. These represent our own listening tastes, and appeal to a large enough portion of customers to keep us in business.

With low-order harmonics, there is agreement that low-order distortion is much less offensive than high-order harmonics and IM (Inter modulated) sidebands, and this drove the development of the Threshold 800A back in 1975, where the nature of the distortion was considered as important as the size of the “single number.”

By 1991 when I started Pass Labs, I began to focus more on the specific character of second and third harmonics, and slowly settled into a character where negative-phase second harmonic dominates at low levels, segueing into symmetric third harmonic at higher power. The second fosters an illusion of expanded space and localization, and the third seems to improve dynamics. The distortion of these amplifiers is still quite low, but they are not sterile.

 JS:You’ve been creating amps for decades. What have you come to believe are the most critical factors in getting great sound out of a product?

 NP: It helps a lot if you understand the problems and solutions technically, but in the end it comes down to a lot of work, in which critical listening plays an important part. I have lots of prototypes that measure well and whose ingenuity is a source of some pride but which don’t sound special. And then there is the occasional piece which has conventionally abysmal measurements, but which many people really like – single-ended tubes and Static Induction Transistors (SIT) amplifiers being good examples.

I always keep in mind that we are in the entertainment business.

JS:There has been an explosion in high-end Class D amps in recent years. Some companies now market some fairly sophisticated – and expensive  – Class D models. Pass currently manufactures Class A and AB lines only. What are your thoughts on Class D?

NP: Personally I think it’s a miracle that they work at all, but then I think that about my phone, too. I have great respect for people who can make Class D amplifiers sound pretty good, and for delivering low cost, high power, and efficiency. They have earned their place in the industry.

That said, I am not tempted to go in that direction.

JS:You are famous for continuing to experiment, especially with your side brand, First Watt. How much better-sounding do you think amplifiers can get? Do you think there are still some major advances waiting to be discovered?

 NP: I don’t know how much better. I assume there are advances, but I think they will have more to do with progress in cognitive perception than in electronic parts and topology. I intend to keep plugging away, mostly because I still find it interesting and fun.