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… More...

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.

In part 2 we developed an improved active current source load for the single gain device which is at the heart of the amplifier. Originally designed for the Aleph amplifier series, this current source doubles the output current and significantly lowers the distortion of the circuit over the original constant current source. As the original Zen amplifier is limited in both power and fidelity, this is a welcome improvement.

The original Zen amp is also limited in its rejection of power supply noise, and benefits from having a quiet, stable power supply. In part 3 we discussed some possibilities for power supply regulation for this and other Zen amplifiers.

The final shortcoming we will address in the Penultimate Zen is the low input impedance. Depending on the version and the desired gain and distortion figures, the original Zen amp has an

There is another, more clever way to accomplish this follower, as seen in Figure 2. Here we use a P channel Mosfet with the Drain connected to ground and the Source biased up by a current source, in this case just a resistor. There are several advantages to this approach:

First, the input can be direct coupled, as it operates at ground potential, eliminating the input capacitor and bias circuit.

Second, it is self-adjusting in the sense that the Source will operate at +Vgs where Vgs is the Gate to Source voltage for the Mosfet, or about 3.5 volts. The amount of current supplied by the resistor is not critical, and can be quite high if desired, as the Mosfet will only dissipate that current times the Vgs value. For the ZVP3310 we will be using, you could run it as high as 50 mA, which is a lot.

Third, the P channel non-linearity will tend to operate in opposition to the N channel distortion of the gain Mosfet, giving some distortion reduction due to cancellation. You can adjust this cancellation a bit by adjusting the bias current and loading of the P channel device.

Fourth, because the input system is at virtual ground, we can swing all the drive current we want without being concerned about the voltages across the input device, since it will be held very close to the constant Vgs of the P channel input. No Miller effect, hardly any gain or capacitance modulation, better performance.

Talk amongst yourselves. I’ll give you a topic: Is this still considered a single gain stage circuit? If a Differential pair or Darlington or a Cascode is considered single-stage would this be also?

input impedance that varies from 600 ohms to a couple thousand ohms. For many audio sources, this input impedance is simply too low to give optimal performance. The ideal input impedance would be something up around 47 K ohms.

We will address this fault here and now, and then go on to present a final circuit with a nice finished printed circuit board design.

Upping the Input Impedance

The issue of increasing the input impedance of the Zen amp is a delicate one. There is no way around the need to add circuitry to accomplish this, but in the spirit of the Zen amp, we wish to do it minimally.

The input junction of the gain Mosfet of the amplifier is operated at virtual ground, so that previous Zen amps have an input impedance which is the value of the input resistor between the input connector and the Gate of the Mosfet. You could simply increase the values of the input and feedback resistor, but then you bump into the non-ideal nature of the Mosfet.

The Gate of the Mosfet has essentially an infinite input impedance at DC, but offers capacitance between the Gate and Source and the Gate and Drain pins. The amount of the capacitance is not so great and by itself does not impose serious limits on the resistor values. The problem is that this capacitance varies under differing voltage and current conditions, and is thus non-linear in character, creating distortion at higher frequencies. You can see this effect clearly on the distortion versus frequency curves of the previous Zen amplifiers, where somewhere above the midrange the harmonic distortion begins increasing with frequency. The higher the resistance at the input, the earlier this effect starts.

To have a high input impedance while maintaining low distortion at high frequencies, the amp needs a buffer. The most obvious approach is to use an input follower as seen in Figure 1. Here a small N channel Mosfet is biased up to served as a Source follower, presenting a very high input impedance, and a fairly low output impedance while following the input voltage. Since we use a small Mosfet for this purpose, the input capacitance is tiny enough to not have a significant effect at audio frequencies.

The Circuit

Figure 3 shows the circuit of Figure 2 incorporated into the Zen Amp with the regulated power supply. Q4 is the added buffer transistor, and it is biased by R13. R9 and C7 filter the voltage provided to R13. The feedback loop of R2 and R3 which previously connected to the Gate of Q1 are now connected to the Gate of Q4 and have values two orders of magnitude higher, giving the amp an input impedance of 47 K ohms.

This circuit was designed to idle at 2 amps, which is more than it actually needs to achieve the 25 watt into 8 ohm figure desired. R0 and R1 together form a .33 ohm resistor, and if you leave R1 out, the bias figure drops to 1.3 amps. With both R0 and R1 and a 50 Volt rail, the amplifier will dissipate 100 watts, and with R1 removed, the circuit dissipates 67 watts at idle.

Depending on your heat sinking and power needs, you can include R1 or not. If you are planning on driving 4 ohms, you will definitely want it. If you leave R1 off, you also should change the value of R16 from 1.5 K ohm to 1.0 K ohm.

As with all the Zen amps, very few of the component values are critical, in fact none of them are, so if you have resistors and capacitors which come close to these values, feel free to use them.

As discussed previously, many substitutions will work for the transistors. Q2 and Q5 can be chosen from a wide array of IRF type N channel devices as long as they have the power and current rating to do the job, which would be around 150 watts and 10+ amps. They do not have very much influence on the sound compared to Q1 and Q4.

In the case of Q1, the IRFP044 is the preferred part, but you can get almost as good performance from the IRFP140 and 240 parts, and pretty much anything else that is similar in character.

In the case of Q4, the Zetex ZVP3310 performed the best among the parts tried. The IRF9510, 9610, and similar parts worked, but have higher capacitance, and thus higher distortion at the top end with these input impedances. This can be improved by lowering the values of R2 and R3. If you can accept a 10K input impedance, you could consider using these alternate P channel Mosfets.

While we’re talking about the input, note that the input of this amplifier is not protected against high voltage transients. Input voltages in excess of 20 volts will have some chance of damaging the input Mosfet Q4. If this occurs, it should not create damage to

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