Mechanical TV & Illusion Generators by James T. Hawes, AA9DT
FET Preamplifier, Part 2

FETs Deliver

You want that warm tube glow. You want that tube sustain. Plus you want portability and battery operation. It's a wonderful world. With a JFET, you can have all that and more . A JFET is the real deal. It delivers on the promise. And it's the only true answer.

FETs Versus Op Amps

If you've ever heard an op amp, you probably wonder why it sounds so pinched. The secret's in the circuit. A look inside reveals tens, hundreds or thousands of devices. You have feedback loops, compensation, current mirrors and a whole lot more.

The op amp sounds different because it is different. Go with a FET: You're back to simple construction and sweet sound. The price can be less than an op amp costs. Besides, a FET takes up less space than an op amp does. All you need fits onto a little piece of perfboard. Average power drain is about 2 mA. A 9-volt battery lasts a long time. (Not as long as with the J201 circuit. But long enough.)

FET Preamp Schematic

Schematic: High-impedance 
       preamplifier with MPF102 JFET. Easy to build. Battery-powered. Radio Shack stocks most parts.

Parts List

C1 Capacitor, 4.7 μF, electrolytic
C2 Capacitor, 10 μF, electrolytic
Q1 JFET, type MPF102, N-channel
R1 Resistor, 3.3M, 1/4-watt, 5%, metal film
R2 Resistor, 1.5K, 1/4-watt, 5%, metal film
R3 Resistor, 560Ω, 1/4-watt, 5%, metal film
R4 Resistor, 220K, 1/4-watt, 5%, metal film
S1 Switch, SPST, toggle
ZD1 Diode, 5.1 V, 1-watt zener (Requires 2)
NOTE: In some applications, the circuit works fine without R4 & ZD1.

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WARNING. The author assumes no responsibility for your success or failure in using methods on these pages. Further, the author neither makes nor implies any warranty or guarantee as to the accuracy or effectiveness of these methods. Proceed at your own risk.

Copyright © 2007 by James T. Hawes. All rights reserved.

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