Hawes Mechanical Television Archive by James T. Hawes, AA9DT
FET Preamplifier, Part 1

Now, with Easy to Find Parts! Some time ago, J. Donald Tillman designed an elegant JFET guitar preamplifier. He posted the circuit on the Web. Overnight, this circuit became very popular. And it's still a winner today. The reasons why are easy to see. This amp is easy to build. The circuit works like a champ. And the JFET's tube-like warmth sounds great. Plus, the high-impedance input won't load your guitar. At the other end, the medium output impedance combats high-frequency losses due to loading and long cables. And by the way, this FET preamp is great for microphones, too!

Bad News...and Good. Unfortunately, Tillman's amplifier requires a J201 FET. The J201 is a very special device and an excellent choice. Yet it's uncommon to hobbyists. But my site solves the mystery. I've designed a similar preamplifier with an everyday Radio Shack JFET.

Buy It Today. Use It Tonight. I've read the forums. Hobbyists want to buy now and play tonight. They stop at Radio Shack. But Radio Shack only sells one FET: The famous MPF102. Hobbyists try to use an MPF102 in Tillman's original design. Yet Tillman optimized his design for the J201. The specs for an MPF102 are very different. Ignore these differences, and your amplifier might not work. For example...

  • The J201 requires only 0.2mA to 1 mA of drain current.

  • The MPF102 operates on 2 to 20 mA. Run it on less, and you may get an attenuator instead of an amplifier! I can prove that statement. See... Proof

Pinout for an MPF102 JFET

Summary of the Differences

On the table, notice that the MPF102 requires more current than does the J201. The current requirement is a major reason why you can't just plug an MPF102 into Tillman's circuit. This is a very poor substitution. It will disappoint you. (You might as well strum piccolo music on a bass guitar. You'll have more luck.)

Device IDSS
(Drain current, mA) 		VGS GFS (Gain) RDS Typical
Vdd
J201 0.2 - 1 -40 0.5mS + 750Ω 10
MPF102 2 - 20 -25 2 - 6.5mS 150Ω 15

Achieve results that you can be proud of! When you build a circuit, you go to a lot of work. Make the effort pay off! My page offers a new preamp design, starting with the MPF102's specs. Now you have the right design for an MPF102 JFET preamplifier. Build carefully. You'll wind up with a satisfying project that you'll appreciate for years.




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♦ WARNING. This is your project. Your achievement is entirely yours. I assume no responsibility for your success in using methods on these pages. If you fail, the same is true. I neither make nor imply any warranty. I don't guarantee the accuracy or effectiveness of these methods. Parts, skill and assembly methods vary. So will your results. Proceed at your own risk.

♦ WARNING. Electronic projects can pose hazards. Soldering irons can burn you. Chassis paint and solder are poisons. Even with battery projects, wiring mistakes can start fires. If the schematic and description on this page baffle you, this project is too advanced. Try something else. Again, damages, injuries and errors are your responsibility. — The Webmaster

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

•URL: http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm •Webmaster: James T. Hawes
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