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Zentroduction:
As an exploration into the potential performance of a very simple amplifier, the Zen amplifier has succeeded in creating notoriety and some controversy over the last 8 years. More importantly, it's novel and simple construction appears to have encouraged a large number of do-it-yourselfers to take up a soldering iron and jump in.
Having only a single gain device, the design's name is a pun on the Zen Koan, "What is the sound of one hand clapping?", but the point is quite serious. High quality sound can be obtained with simple and accessible circuits. Conversely, it is quite easy to design a complex circuit which sounds subjectively lifeless or even irritating.
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.
The Original Zen Amps:
Figure 1 is the simplified schematic of the original Zen Amp. Here we see a single gain device, a power MOSFET, operated in Common Source mode, where the input comes into the Gate of the device and the output which drives the loudspeaker comes out of the Drain. The MOSFET is biased by a constant current source from the positive supply, and a small network of resistors and capacitor set up the operating voltages and provide feedback. The full article can be found in Audio Amateur 2/94, with a revision in 3/94. It is also available on line at www.passdiy.com .
The Zen was followed up by Son of Zen (Audio Amateur 2/97) whose unsimplified circuit is seen in Figure 2. Here a single gain stage is formed by a differential pair of identical devices, allowing greater simplification and the removal of coupling capacitors and negative feedback.
Both these designs also inspired preamplifying circuits based on the same topologies, giving us Bride of Zen (Audio Amateur 4/94) and Bride of Son of Zen (Audio Electronics 5/97).
Let There Be Light *
Some of the DIYers who built the Zen Amp were put off by the complexity of having a constant current source comprised of two additional transistors and several resistors. However a constant current source can be replaced by a high power resistor in the Zen Amp, if you are willing to use a higher supply voltage and a resistor with high enough value to simulate a constant current source for practical purposes.
At the same time, others complained of the difficulty of obtaining the high power resistors required for the Son of Zen. I get a few such complaints, but they have been a source of inspiration.
And so the light came on and I had a bright idea: A light bulb is a power resistor which is conveniently obtained and which can dissipate large amounts of power without a heat sink. So what kind of light bulb might be appropriate?
I went out and bought all sorts of samples of light bulbs. A little measurement revealed that common household incandescent bulbs are quite useful for our purpose. Figure 3 shows the current versus voltage for a couple of 120 Volt lamps, one rated at 150 watts, and the other at 300 watts. The resistance (in ohms) of each of these at any point is simply the volts divided by the amps.
* ( Personally, I don't care for cute section titles, but if I don't put them in, somebody at Audio Express does. )
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