Introduction: What Makes an Aleph an Aleph?

The Aleph family album includes the following commercially produced siblings and descendants: the Pass Labs Aleph 1.2, Aleph 2, Aleph 3, Aleph 4, and Aleph 5, the Volksamp Aleph 30 and Aleph 60, and the First Watt Aleph J. (There were also the Aleph 0 and Aleph 1, but they technically had three gain stages and so were a bit different from the others, more like ancestors.) 

So what unites all of the Aleph amplifiers? What makes an Aleph an Aleph? 

The short answer is that an Aleph amplifier is a pure Class A, single-ended, Aleph current source, solid-state amplifier with only two gain stages. 

Unless you're an amplifier designer, that's a lot to digest. Let's break it down. 

Class A: For a basic review of amplifier classes (A, B, AB, etc.), I can do no better than the top Google result at Electronics Tutorials. For our purposes here, it's enough to know that Class A amplifiers intrinsically offer excellent linearity, high gain, and low signal distortion levels. In the words of Nelson Pass:

"The primary virtue of class A lies in the smooth characteristics of its operating parameters. The gain transistors are operated in their linear region only, where the distortions are limited to smooth, simple forms, unlike the abrupt distortions created when the transistors in class B output stages switch on and off. In class A, the transistors are always on, eliminating the turn-on/turn-off delays which characterize the crossover of class B and even AB amplifiers. The distortion is inherently lower without the need for cleaning up via feedback, thus class A lends itself well to low distortion performance in a simple circuit with low open loop gain."

However, these advantages come with a significant drawback: inefficiency. Because the transistors in a Class A amp are always operating in a "fully on" condition, they convert a lot of energy into waste heat even when not amplifying any signal. This means that the Aleph J dissipates approximately 160-200 watts as heat through its heat sinks all the time, while offering only 25 watts of output power. 

This inefficiency explains the need for large heat sinks and a robust power supply. Those requirements make for an amplifier that, when measured on a per watt basis, is relatively large, heavy, and expensive. As a result, few commercial companies choose to make amplifiers that operate in "pure Class A." Nevertheless, Nelson Pass has written at length about the sonic advantages of Class A amplifiers (see here and here) . 

Single-ended: Class A amplifiers can either be single-ended, where a single gain device amplifies the entire signal, or push-pull, where two gain devices share the load, with one "plus" side supplying positive voltage and current to the loudspeaker, and the other "minus" side supplying negative voltage and current. Push-pull topologies have the advantage of being more efficient. However, according to Nelson Pass: 

"[P]ush-pull Class A circuits have two opposing gain devices producing the output signal, and though it is industrially effective and efficient, it is not the most delicate way to amplify a signal. Push-pull circuits give rise to odd ordered harmonics, where the phase alignment reflects compression at both positive and negative peaks and crossover nonlinearity near the zero point."

For more on the advantages of single-ended Class A, I recommend Nelson Pass's article on the subject, Single Ended Class A. (Note: Nelson Pass has also designed a well-regarded push-pull Class A amp, the First Watt F5.)

Aleph Constant Current Source: This active current source technique for biasing the gain devices in a Class A amplifier increases efficiency while reducing distortion. Nelson Pass received US Patent 5,710,522 for it in 1998 (now expired). The patent "detailed the operation of an active current source which was constant for DC characteristics but variable at AC frequencies and which could track an arbitrary percentage of output current to a load, relieving the active single-ended Class A gain device of some of the work." This approach effectively doubles the efficiency over previous constant current source biasing techniques and is described further by Nelson Pass in Zen Variations 2. 

Two Gain Stages: the Aleph amplifiers are built around only two gain stages (the Aleph 0 and Aleph 1 had three, but they are outliers from the rest of the Aleph family), reflecting Nelson Pass's view that the fewer gain stages, the better, all else being equal. In fact, there is a Nelson Pass amplifier family that relies on only a single gain stage -- the Zen amplifier. The Aleph is the next best thing, where the signal only encounters two gain stages between input and output. In a 2008 article, Nelson Pass laid it out like this: 

"An important thing about distortion – when you run a signal through a device which is even slightly non-linear, you have changed the signal forever. You can use various techniques to reduce distortion after the fact, but you can't go back. ...

We have seen that distortion complexity results when you pass a simple signal through a gain stage with high order nonlinearities, as in the example of the distortion spectrum of a Class A versus Class B output stage (Figure 5).

We have also seen that distortion complexity results when a complex signal is passed through a gain stage with relatively simple low order non-linearities (Figure 8).

And finally we have seen that distortion complexity is increased whenever you use negative feedback (Figure 11).

I can think of one more source of distortion complexity, that which results from passing a signal through successive gain stages. This is quite common because it is popular to use multiple stages in an amplifier in an effort to generate enough open loop gain so as to have plenty of feedback.

Paradoxically, you can visualize instances of feedback pyramid schemes, in which more gain stages are added to generate more feedback to partially correct for the distortions generated by the additional gain stage."

There is much more that can be said about the design philosophy of the Aleph amplifiers, but I hope this provides a basic introduction for those who are not trained in analog circuit design. For next steps, I recommend A Concise Guide to the Published Amplifier Circuits of Nelson Pass, which provides a topic-oriented index to all of the Nelson Pass articles at PassDIY. 

But for now, perhaps the most important thing for us here is what Mr. Pass had to say about the Aleph J: "I consider it the best of the Aleph series."