My hunch is a buffer will end up being the system preamp.
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Boozhound Labs JFET Buffer kit
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- 2sk170 JFET transistors - Toshiba - matched quad - 1mA match - GR - 2SK170GR
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Bias and gain setting of jBoZ
Since the question has come up and not answered, the situation with the gain, bias, and resistor values is as follows.
The device is self biased, and we want the highest value of bias possible for lowest distortion which makes us want to use a low value Source resistor to ground.
At the same time, it would be nice to not have too much gain by virtue of that Source resistor value. The apparent resistance of the JFET is about 50 ohms, so the gain into a 10 K ohm load is about 1000 / 60 which is about 24 dB.
We can get lower gain, but the distortion will increase - it’s all a trade-off. I didn’t make any effort to exactly optimize these values - they were just thumbnail calculations.
I measured the circuit this afternoon, and it came out mostly as expected, but the best distortion numbers came from a 16V supply value.
Original MosFET Bride of Zen
Due to its extreme simplicity, the Zen amplifier’s interfaced needs to be indulged a bit, both at the output and also at the input. With respect to its output, it is designed for an 8 ohm load with an efficiency of 94 dB or greater. I have driven the Zen into Thiel 3.6 loudspeakers, 88 dB efficient at 2.3 ohms, but the result is not pretty. With a pair of Klipsch speakers it will throw off its mild mannered disguise and behave like a party animal.
The Zen amp input makes a preamp do a little more work than usual. Depending on the specific input values, its input impedance is typically 4.75 KOhm, and it needs about 2.5 volts input to drive it to full output. Its gain is about 15 dB, which is 10 dB less than most power amps and it requires more signal from the preamp for equivalent levels.
The point of a preamp circuit here is to function as a gain circuit with a high input impedance, a low output impedance, and a volume control. This preamp has a 50 KOhm input impedance, variable gain to about 15 dB, and will deliver the 2.5 volts at .1% distortion or less.
JFET Bride of Zen by Nelson Pass
We will place the volume control at the output of the circuit, giving us a higher output impedance than is typical. Depending on the position of the volume control, the output impedance will range from 0 to 1.5 KOhms.
Normally you see the volume control potentiometer at the input to the preamp circuit, and in this way you can insure that the input will not be overloaded. Placing the potentiometer at the output has two advantages, however, the first being that all circuit noise will be attenuated as the volume setting is reduced. Placing the volume control at the output allows the input of the gain stage to look directly at the source. This improves the bandwidth and distortion of the circuit by providing a lower RC figure for the input capacitance of the MOSFET. If you desire to place a potentiometer at the input of the circuit, feel free to do so before the input coupling capacitor in place of R107.
Because the input to the Zen amplifier is a resistor operating into a virtual ground summing junction, it is insensitive to a resistive source impedance. The gain of the amplifier will alter by the addition of the source resistance, but the distortion and bandwidth performance will not change appreciably. A circuit driving the Zen amplifier need not have a particularly low output impedance, as long as it is resistive in character.
Is a maximum 1.5 KOhm source impedance a problem? Not in my experience, as long as it is passive in character, as in the case of this circuit which has no feedback. You could conceivably see some rolloff at the top end, but even 1000 pF loading will only take the bandwidth down to 100 KHz.
The Bride of Zen, designed by Nelson Pass and described in The Audio Amateur, 1994, is a very simple line-level preamplifier which contains just one medium-power MOSFET per channel. It is meant to be used together with the Zen (or Son of Zen) single-ended MOSFET power amp. A complete description of the BOZ can be found at http://www.passlabs.com.Mr.Pass has given a very lucid account of the design philosophy, construction and alignment (the only alignment consists of adjusting the quiescent current which should be 40 mA per channel at a supply voltage of +60V). The lay-out for a printed circuit board can be downloaded from Passlabs.
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