Pass Labs Aleph 3 power amplifier

Everyone's going crazy for single-ended power amplifiers. What's the big deal? What is it about these relatively low-powered contraptions that could make everybody so nutso? And has Pass Labs' Nelson Pass completely lost his marbles, selling a 30Wpc amplifier for a price that can buy a high-quality 200Wpc amp? Isn't that 200W amp seven times as loud—and seven times as good—as a 30W amp?

First of all, while a 200W amplifier is 8dB more powerful than one rated at 30W, that 8dB sounds only about twice as loud to the human ear, not the seven times you'd expect. For an amplifier to play music that sounds seven times as loud as 30W, it would have to be about 24dB more powerful, which equates to 7500W! So you see that an amplifier with only a fraction of the industry-standard 100W or 200W can still actually produce volumes that are fairly close to those other amplifiers, because of the logarithmic way that our ears perceive loudness.

However, losing even a small amount of volume capability is still undesirable, so there must be something special about single-ended operation that makes the power tradeoff worthwhile.

Let's see...what other things that affect music are single-ended? Well, there's air. The air pressure increases and decreases to propagate the sounds, but the pressure never goes below zero. Considering that it's the very thing that allows us to hear music in the first place, perhaps air is a very good thing for an amplifier to emulate.

Another interesting thing about air: it's very simple. As is the Aleph 3: it uses just two gain stages, compared with up to eight gain stages in conventional power amplifiers. The Aleph 3 also requires no adjustments. All operating points are determined by the properties of the components, and will remain stable over time. This amp is also said to have a monotonic distortion characteristic, meaning that its harmonic distortion decreases smoothly with decreasing level. Air, too, is monotonic, although its distortion is generally not a concern at normal sound levels.

The MOSFET transistors used in the Aleph 3 have a transconductance that increases smoothly with current, which results in the amp having primarily second-harmonic distortion. Second-harmonic distortion tends to be the least offensive kind. So the Aleph 3 (footnote 1) seems to share many of the traits of the one medium that always connects the audience to live musicians. But does it sound anything like that perfect medium?

Wherefore art thou single-o?
Nelson Pass is the type of designer who is rarely content with any of the widely accepted ways of doing things. He has moved on to forsake most of the "conventional wisdom" of audio amplifier design, some of which are, ironically, remnants of his own past innovations.

Why not design amplifiers that use high feedback so that all of their standard measurements are excellent, have complicated biasing schemes that reduce heat output, and provide high gain and high power? In the "Product Philosophy" section of the Aleph 3 owner's manual, Pass answers that "The history of what has been done to the music is important and must be considered a part of the result. Everything that has been done to the signal is embedded in it, however subtly." I hold the belief that eventually all audiophiles will catch on to this idea. The best components are those that do as little as possible to the sounds passing through them. The music has already been made, so the reproducing system should leave it alone; it should make like air and disappear.

Unlike the older Aleph 0, the Aleph 3 and the rest of the new generation of amps from Pass Labs do not include an output "pull" stage to take the amp into push-pull mode during brief peaks. The 3 runs single-ended only, with the bias current keeping the output devices turned on at all times, something that actually improves the sound of the amp, according to Nelson Pass. He refuses to run his new amps in any mode other than wide open, accepting no sound-quality compromises for the sake of power ratings.

The amp is amazingly bulletproof, both physically and electronically. One may plug and unplug both the inputs and outputs while it is on without any fear of damaging the amp (don't try this with other amplifiers). Still, in the interest of kindness to your speakers, the 3 should be turned off for input changes. Speaking of speakers, this baby appears to be stable into any load, all the way down to a dead short across the speaker terminals!

Footnote 1: For more of Nelson Pass's single-ended MOSFET philosophy, the interested reader is referred to Dick Olsher's excellent review of the Aleph 0 monoblock, the precursor to the Aleph 3, back in the March 1995 Stereophile (Vol.18 No.3). The Aleph 0 was Stereophile's Amplification Component of 1995.—Muse Kastanovich
Pass Labs
21555 Limestone Way
Forest Hill, CA 95631
(530) 367-3690
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