In ZV8 we dipped our toes into the waters of power JFET transistors using the new Lovoltech LU1014D in a simple circuit.  The focus of the project was on the JFET itself, and except for a cascode transistor the rest of the amplifier used only passive components.  Here in four installments we will increase the complexity of the circuitry around the JFET with an eye toward distortion performance surpassing any of the Zen projects to date.......

First, we will put in an active supply filter to reduce power supply noise and turn-on thump.  Then we will replace the light bulbs with a 2 amp constant current source.  It will not glow warmly on cold winter evenings, but it will draw less current, deliver about 50% more power, and has lower distortion and noise......

There are few other changes to the circuit of Figure 1, but you will note the addition of an audiophile-approved bypass capacitor on the output, and variability in the value of R3.  Later we will see that that this value can be used to optimize the performance around the characteristics of individual JFETs......
 

ZV9.pdf (209k)

Why power JFETs you ask?  Don’t we have enough transistors already?  We do have lots of choices in devices, but a vertical JFET brings some particular characteristics to the table that we don’t get elsewhere.  When we look at characteristic curves for devices we see what I would call the “triode character” and the “pentode character”, as illustrated in Figure 2....... 
 

 ZV8 power jfet (530k)

During the past few years a number of excellent designs have been published for domestic audio amplifiers. However, some of these designs are now rendered obsolescent by changes in the availability of components, and others are intended to provide levels of power output which are in excess of the requirements of a normal living room. Also, most designs have tended to be rather complex.

In the circumstances it seemed worth while to consider just how simple a design could be made which would give adequate output power together with a standard of performance which was beyond reproach, and this study has resulted in the present design.

He then described a Class A power amplifier using three gain stages of Bipolar transistors in a topology which continues to be admired for its elegant simplicity and sound quality.......

My own approach is to make the signal path as simple as possible, work to lower the distortion of that basic circuit before feedback is applied, and then apply minimal (or no) feedback, largely in agreement with the comments in Linsley-Hood’s original article.  The result is not always the best objective measurements, but the sound is often interesting. 

The 3-stage topology of the JLH amplifier routinely uses simple Class A operation and about 33 DB negative feedback to achieve this performance, and this lured me to consider what kind of amplifier I could achieve with an even simpler circuit and less feedback.  The output stage and the intermediate phase splitter cannot be dispensed with and still resemble a JLH, but you can certainly remove the input transistor.

By thumbnail calculation, the input transistor of the JLH contributes about 27 dB of voltage gain.  Remove it and the open loop gain of the amplifier drops into the region of about 28 dB.  If we scale back the gain of the amplifier from 22 dB to 18 dB, we end up with about 10 dB of feedback – a very minimal amount.  Unfortunately only 10 dB of feedback means that the original amplifier will likely deliver something more like 1.5% distortion at 10 watts.  Since such a figure is better than that seen on many single-ended Class A tube amplifiers (SET), this might be an acceptable amplifier.  Actually, since the input transistor is no longer contributing to the distortion figure, we would expect the performance to be better than that, and maybe that’s the point. 

PLH_amplifier.pdf (350k)

In the second part of this article, we will additionally raise the efficiency to over 16% by driving coupled inductors instead of resistors. This will about quadruple the efficiency of the original Son of Zen, but with much better performance in all areas except for the input impedance.

Of course, it is also a slightly more complex circuit, and will keep in mind that the Zen and its brethren are all explorations in achieving good amplifier performance as simply as possible. What constitutes "good" is observed from both an objective (measurement) and a subjective (listening) viewpoint. It is my personal opinion that the simpler the circuit, the more similarity there tends to be between these two, but not always....

zen-v7.pdf (200k)

U.S. Patent # 5,376,899 describes an amplifying circuit topology that takes advantage of the character of matched balanced amplifiers that are cross-coupled to provide cancellation of distortion and noise. The result provides high performance with very simple linear circuits and has been dubbed Super-Symmetry, an homage to particle physics, and is also known popularly as the X circuit. Super-Symmetry works by exploiting the complementary characteristics of matched balanced circuits to differentially reject distortion and noise, and applies a small amount of feedback to extend this symmetry, making the distortion and noise even more identical on each half of a balanced amplifying circuit. In this article we are going to apply this technique to a previous project, the Son of Zen. 

zen-v6.pdf (100k)

Previously we have limited ourselves to single-ended Class A amplifiers, but it’s time to try a complementary (push-pull) design. Since this is supposed to be a tutorial series, we need to ask the primary question, "What is meant by complementary operation and why do I want it?" read on

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The Penultimate Zen is the sum of several incremental improvements to the original Zen amplifier of 1994. Eight years just flies by, doesn’t it? These improvements are contained in parts 2 through 4 of the Zen Variations, and is likely the last version of this amp, although by no means the end of the variations on the theme of single stage amplification. read on

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zen-v4.zip 25k

In Part 2 we developed a new active current source for the Zen amplifier. In this part, we create a power supply regulator suitable for the Zen amplifier projects. read on

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This is Part 1 of the Zen Variations, and each part will illustrate one of the many ways to build a single stage audio amplifier. There are a lot of possibilities here; I recently counted out several hundred permutations. After considerable meditation, I winnowed these down to approximately 30 interesting and non-trivial examples, and it is these we will explore one at a time in no particular order.... read on..

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Legacy projects:

These projects are available as downloads only, due their age no PCBs or special parts will be offered. See part substitutions or information on possible component crosses and or substitutes. Adobe Acrobat 4.0 is required to view these files and can be downloaded for free from Adobe. ..............continue..