Pass Labs Aleph 5 Amplifier
Tip #107
Dick Olsher (July 2024)

Pass Labs came into being in 1991, after Nelson Pass had left Threshold Audio. He created an instant sensation by unveiling the Aleph line. Its essence was the pursuit of lower power and simple FET based circuits that offered a closer approach to live music. The inaugural product, the Aleph 0, was a 70 watt monoblock operating in single-ended Class A with only three gain stages. By 1995, the gain stage count was reduced to two with the Aleph 5, a 60 wpc into 8-Ohm stereo amp.

To quote from the Aleph 5 Owner’s Manual:

“The Aleph 5 is unique in a number of ways: Unlike the early Aleph amplifiers, it operates in pure single-ended mode over its entire operating region, and does not incorporate a “pull” stage for additional current beyond its bias point. When the single-ended output Mosfets (four sets of three in parallel) reach cutoff, the amplifier clips. Most amplifiers on the market have between five and seven gain stages in series between the input and the output. The Aleph 5 has but two, and enjoys a very direct path from input to output, further enhancing the purity of the circuit and the resulting sound. Early Aleph amplifiers have three gain stages.

The output stage of the Aleph 5 is a unique blend of traditional design and innovation addressing the unique requirements of loudspeakers. Previous methods of loading the output stage have used networks consisting of resistors, coils, transformers, and active current sources, all of which offer an optimal load line based on a resistive load. The Aleph 5 has a current source topology which optimizes performance for a wide range of impedance and reactance in the load, improving all aspects of performance into real loudspeakers. Pass Labs has a US patent pending on this output stage topology.

The Aleph 5 is unique in that there are no adjustments of any kind in the circuitry. There are no potentiometers to adjust. The operating parameters of bias currents and DC offset and so on are defined by physical constants, and will not go out of adjustment.”

I picked up a used Aleph 5 about three years ago and sent it to Pass Labs for refurbishing. It underwent a full recap together with some additional touches and ended up being even better than new production. In the post test it slightly outperformed the original specs, e.g., noise measured under 300 uV, whereas original production was no better than 350 uV.

A power amp is forever in search of a compatible speaker load. Its performance can only be assessed subjectively when hooked up to a speaker. And factors such as the speaker’s sensitivity, reactive impedance, and distortion spectrum affect the amp’s perceived performance. A speaker’s total harmonic distortion is usually not known, it could be under 1% at low volume levels and several percent at high volume; usually much higher than that of the associated amp. So dropping an amp into a review system and opining about its sonic character at best results in anecdotal rather than definitive findings that should be taken with a grain of salt.

It’s only when many users chime in with findings in different system contexts that a true picture of an amp’s performance can emerge. And that turned out to be the case with the Aleph 5. The online consensus that materialized was of a sweet sounding, transparent, and spatially incisive amplifier. Some folks were not enamored with its looks. And I agree that it looks a bit like a large porcupine by virtue of its 360-degree wrap-around heat sinks. It runs hot because of its Class A operation, idling at 300W, and takes up to an hour to reach thermal equilibrium and sound its best.

I had listened to the Aleph 5 previously with the Analysis Audio Omega loudspeakers and found it to be incredibly liquid sounding but not sufficiently powerful in this application. With the arrival of the Fyne Audio F703SP loudspeaker (94 dB sensitivity) the Aleph 5 found a perfect match. The Aleph 5’s input impedance is only 15 kOhm, so it’s important to mate it with a preamp whose source impedance is under 1 kOhm. The PrimaLuna EVO 400, Joule Electra LA-150 MkII, Don Sachs Model 2, and Wavebourn Labs Edelweiss-3 tube preamps worked well and resulted in astonishing textural liquidity. I don’t say this lightly: it proved superior in this respect to all other solid-state amps that I’ve auditioned over the years, and that includes all of the First Watt designs. The sound was pure and sweet with excellent transient clarity. Tonal colors were spot on without any gratuitous brightness. The soundstage depth perspective was nicely fleshed out while image outlines were tightly focused. Bass lines were detailed with sufficient punch to satisfy my needs with classical music. In all, it was capable of a very natural, realistic presentation, and a foot-tapping boogie factor. It has been said that the Aleph 5 is a solid-state version of a tube amp. I mostly agree, but with a couple of caveats; tube colorations are missing in action and bass reproduction is actually more extended and detailed.

Because of its design simplicity DIY clones of the Aleph 5 have been popular over the years. Here is what Nelson Pass had to say on the diyAudio forum (with an added touch of humor), dated 3/27/2003:

“My list of the most effective techniques for getting the best sound out of an Aleph:

1) measure the linearity of each output transistor in addition to matching and use the most linear

2) bias the hell out of it, with overkill heat sinking

3) substitute in combinations of diff pair devices, picking the best performing ones in terms of distortion, noise, and sound

4) a good pi filter on the supply

5) power transformer two feet away

6) filter cap on reference Zener

7) a glass of fine Cabernet

8) better caps

9) better connectors and wire

10) better resistors”

There are even Chinese copies available for purchase on Aliexpress.com, a testimonial to what should be considered a classic product, meaning one that has withstood the test of time.