From: till@acid-rain.lucid.com (Don Tillman)
Subject: Re: Building your own (ONBOARD PREAMP!)
Reply-To: till@lucid.com
Organization: Lucid, Inc.
Date: 30 Sep 92 09:20:08

   Date: Tue, 29 Sep 1992 14:38:56 GMT
   From: rstern@col.hp.com (Richard Stern)
   Organization: HP Colorado Springs Division

   Try this op-amp based pre-amp using the following circuit. 

(Umm, proper engineering practice requires you to decouple the 9v supply
and to bypass the lower 1Mohm resistor with a .1 uF (or so) cap,
especially if you're driving a load.)

The design you present is nearly identical to the "Guitar Handbook"
circuit that started this conversation, and suffers from the same
problems -- high feedback, bad feedback topology, ungraceful overload
and recovery, noise, and the signal has to go through a dozen non-linear
devices inside the chip before it gets out.  Exactly the sort of thing
that gives solid-state amps a bad reputation!

   Date: Mon, 28 Sep 1992 13:15:11 -0500
   From: Kurt Taylor <kurt@istwok.ods.com>

   Please post a schematic of your fet pre-amp. (I have tried many op-amp based 
   circuits with little success, unless you like BAD pre-amps :'))

   Date:  Tue, 29 Sep 92 10:34:54 -0400
   From: tjs@godzilla.eecs.umich.edu (tim stanley)

   You mention your preference for FET based pre-amps.  Can you give me
   any pointers to where I can lift a schematic of such a critter?

   Date: Tue, 29 Sep 1992 07:55:32 GMT
   From: chuck@spectrum.cs.unsw.oz.au (Mark Andrew Hawling)

   Can you post one of your FET designs that you think is good.

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.


                 +------------------------+--- +9v
                 |                        |
                 <                        |
                 < 6.8k                   |
                 <                        |
                 |     5uF                |
                 +----||----+--- out      |
                 |          |             |
               |-+          |             |
  in ---+----->|            |             |
        |      |-+          |             |
        |        |          |             |
        < 10M    < 2.2k     < 51k        ---
        <        <          <            ---  10uF
        <        <          <             |
        |        |          |             |
        +--------+----------+-------------+--- gnd


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
Xref: sparky alt.guitar:8478 rec.music.makers:9927 comp.music:4585 comp.dsp:2262 sci.electronics:16348