Interviews

We have gain devices; basic topologies; values of the many passive components, like resistors, capacitors and inductors; classes of operation (bias points and load lines); and a host of other details—but it's more about process. Because I personally do not have anyone dictating the desired results—lucky me—I often start with a consideration of the gain devices themselves, as their characteristics suggest the best or most interesting circuits that might play to their strengths.

I lean toward class-A because it makes the process smoother—again, no boss—and I work on individual gain stages with an eye toward an amplifier comprising three or fewer of them.

Of course, a lot of the work is just the day-to-day routine of layout, testing, refining, revising, documenting. Depending on the design, boutique parts are either first or last. Some of my transistors are boutique.

Austin: You're very open with your ideas—with what's often called "intellectual property." The DIY community is surely grateful. Can you explain your thinking about that?

Pass: I have more stuff than Pass Labs can possibly bring to market, much of it not appropriate to PL's audience anyway. So I created First Watt for "low volume" commercial product, and release most of the rest to DIYers.

The DIY thing has worked well since 1973, starting with The Audio Amateur, and currently mostly through www.diyaudio.com and the www.passdiy.com and First Watt websites. The most complete collection of articles is at www.firstwatt.com

It's still [my] intellectual property—and copyrighted—but it's available for noncommercial use. It has been argued that this favors our competition in some way, but I have not seen it.

Austin: Some of your comments, in this interview and elsewhere, suggest that you take a nuanced view of the connection between measurements and the perception of music. Can you expand on this?

Pass: Measurements and listening go hand in hand. There is a correlation between objective and subjective, but they're not strictly causal relationships. Clearly, there are some amplifiers that measure great with "standard" measurements but don't sound so good, and there are examples of good-sounding/bad-measuring as well. The discrepancies are interesting because they point to either things that have not been measured—more likely, misinterpreted—or aspects of perception and taste that don't correlate to measured flaws. Or both.

In the end, the subjective experience is what our customer is looking for. Our taste in sound may not appeal to everyone, but it's what we have to work with, and we only need a small segment of the market to be successful. I don't neglect the measurements; I put them to work.

Austin: In the brochure for the Pass Labs Xs-series amplifiers, there's a discussion of the influence of your First Watt SIT experiments on the development of the Xs series, via the "concept amps" you were working on earlier this decade. A key to your design process, apparently, was the subjectively excellent and easily heard sound of the SIT amps. You were, as the brochure says, "able to objectively identify the qualities of the sound—information which remains proprietary." As a former scientist and current subjective reviewer, I find this very interesting. Is there more you can say about "objectively" identifying "qualities" of sound?

Pass: When I had a custom batch of SITs made by SemiSouth, the first thing I did was test the performance of the part in the simplest possible circuit, which was just the one transistor in single-ended, class-A, Common Source mode without feedback. The SIT being very much like a triode, it is easy to make a single bias adjustment which affects the second harmonic distortion of the device, ranging from a relatively large amount [of] positive phase second [harmonic] through a null point with no second [harmonic], to large, negative phase second-harmonic distortion.

The result was so interesting that I released it as the SIT-1, with a knob on the front panel and a meter showing the voltage/current operating point of the SIT, so that users could make the adjustments and decide what they liked best. We floated some prototypes around and found that most people preferred a particular setting. There was a consistent subjective observation that there was a difference not only with the level of second harmonic, but phase also. Negative-phase second harmonic tends to expand the perception of front-to-back space in the soundstage, separating instruments a bit. Positive phase does the opposite, putting things subjectively closer and "in your face." I have heard this sort of comment from people who were not in a position to have expectation bias, so I treat it seriously.

Of course, there was also such a thing as too much second harmonic. It is a trick best done in small doses.

Armed with this, I went back to the still-under-development Pass Labs Xs amplifier, which has a massive, balanced, class-A push-pull output stage biased partially with single-ended high-current sources. Some work on the values of the bias currents gave us what we wanted, and finished products went to market. They were followed by a generation of .8-version X and XA amplifiers. They sound great, and they also measure well.

Both Pass Labs and First Watt have enjoyed success from the initial investment in the SITs. The big difference between them is that it's practical for First Watt to build anything without disrupting business at Pass Labs, and being all the same pockets, there is no conflict.

Austin: Earlier, you said, "I am centrally aware that all this is just entertainment, mine and yours. The objective needs of amplifier users are largely solved on a practical level, and as McLuhan perceptively noted, when that happens, we turn our technology into art." Are you saying that amplifiers already are—perhaps have long been—good enough, musically?

Pass: It depends on who the customer is. Except for the needs of a narrow segment of the population, the problems of amplifiers are largely solved. There remain those who want better yet, and devote themselves to that.

There are objectivists still working to improve the measured performance on the assumption that it will sound better—not a bad assumption, really. And there are people who are looking for a better subjective experience apart from specs. Maybe they like Kodachrome. I'm still exploring the territory, and when I find something interesting and different, I work with it and try to realize its potential. Apart from the commercial interest, there's a large audience in the form of do-it-yourselfers, so little gets wasted.

Austin: Thinking back on the Horgan book, in science—and probably other fields—there are explorers who seek out big, new ideas, and others who focus on getting the smaller things perfect. Would you say you're more the former or the latter?

Pass: Oh, I want perfection, all right, but I'm secure in the knowledge that I won't achieve it.