Simple FET Preamp / Buffer

Newsgroups: alt.guitar
From: till@ (Don Tillman)
Subject: Re: Building your own (ONBOARD PREAMP!)
Organization: Lucid, Inc.
Date: 30 Sep 92 09:20:08



Well okay. Here's a nice design from the "op-amps are for weenies" school. Low distortion, low noise, low feedback, graceful overload, simple, elegant, inexpensive, etc. This baby sings. It has a transfer characteristic somewhat similiar to the first 12AX7 tube in a Fender amp.

The FET is a 2N5457. The voltage gain is subtle, 3dB or so. You can substitute another low-Vdss FET if you know what you're doing. Power drain is about 0.5 mA, so a 9v batter will last a good long time. It does start to sound a little grubby when the battery sinks below 7v.

It's also trivial to add a high-boost switch if you'd like; have it shunt the 2.2k resistor with a 0.05 uF (adjust to taste) cap. Or shunt the 2.2k resistor with a 10uF cap for more gain.

Obviously you want to wire it so it's powered off when the instrument isn't plugged in -- it's far superior to use a switched jack instead of the weenie approach of using the second connection of a stereo jack.

Also note that this design is easily phantom-powered, so you can even get by without installing a battery.

-- Don