From: META000 <META@UNB.CA>
Subject: MXR Distortion +
Organization: The University of New Brunswick
Date: Fri, 2 Oct 1992 22:03:34 GMT

Ok, folks... I found my schematic, and since, for some zany reason,
lots of people are interested, here is the schematic for what I
gather is the original MXR Distortion + (circa 1978).  This
schematic was traced out from one I borrowed.  Given the simplicity
of the circuit, I am pretty confident of its accuracy.

Unfortunately, the system here tends to screw up the spacing of
characters, adding and deleting spaces here and there, so I'll have
to verbally describe the circuit, step by step.

1) The circuit employs a 741-opamp, 4 x 1meg resistors, 2 x 10k
resistors, 1 x 4.7k resistor, 2 x 1.0uf electrolytic caps, 1 x .01uf
cap, 1 x .05uf cap, 1 x .001 uf cap, 2 x 1n270 diodes, a 500k pot,
and a 10k pot.

2) The input goes to a .01 cap then to a 10k res, then to pin 3 on
the opamp (the noninverting or + input).  A .001uf cap also goes
from the input (i.e. before the .01 cap) to ground (to shunt off
high end so you don't end up fuzzing high frequencies).

3) Pin 3 also connects to a 1meg res and 1.0uf cap (in series, with
the + end of the cap connected to the resistor), with the - end of
the cap going to ground.

4) Pin 2 (the - or inverting input) on the op amp connects to a
series of components, first a .05ufcap, then a 4.7k resistor, then
to a 500k pot, used as a variable resistor going to ground (i.e.,
the 4.7k res connects to the center terminal on the pot, and one of
the side lugs goes to ground).

5) a 1meg resistor joins pins 2 and 6 on the opamp

6) Pin 4 on the opamp goes to ground.

7) Pin 7 is connected to two 1meg resistors in series, then to
ground.

8) The point where the two 1meg resistors are joined is connected to
the point where the 1meg resistor coming from pin 3 meets the + side
of the 1uf cap.

9) Pin 6 of the opamp connects to the + side of a 1.0uf cap.  The -
sideof the cap connect to a 10k res, then to a 10k pot wired as a
standard volume control, then finally to the output jack/switch.

10) Two 1N270 diodes, wired in parallel, one the reverse orientation
of the other (i.e., the stripe is on the other end), are connected
between the input to the volume control and ground.

11) This is the entire original circuit.  As those of you more
technically inclined can tell, its a standard non-inverting amp,
with a variable gain of 3 to about 200 or so, using standard
clipping diodes at the output.
   You can substitute a quieter op amp forthe 741, use metal film
resistors, and plastic caps (for all those below 1.0uf) for a "mil
spec" box.  I reduced the noise on a friends' using a .001uf cap in
parallel with the diodes to shunt high frequencies to ground.  My guess
is that the 1N270 diodes are critical,  however, you can probably use
a numberof substitutes in your circuit for some of the other parts.
The 1.0uf cap going to ground can probably be replaced with a higher
value one such as 2.2uf or 10uf, perhaps even improving the sound.
If you have a very "hot" signal going into it, you might consider
replacing the 4.7k res with, say, a 22k or 47k, and a 1meg pot.
This will reduce the maximum gain, and allow you to achieve gains <
3.  This will let you dial in softer sounds.  If you want, you can
stick a cap in parallel with the feedback resistor (the 1meg joining
pins 2 and 6) to reduce the high end and shape the tone.  You
probably don't want to go much higher than 220pf on this.  If you
want, you can stick in a 3-position toggle switch and alternate
between no cap, 47pf and 220pf for different voicings.  There you
go. I realize this is a fairly low-level description, but  I'm
hoping some of those entry-level do-it-yourself-ers may be inspired
to take up soldering pencil and create.  It's amazing how many
classic sounds were created by very simple means.

While I remember, the .05 cap can probably be replaced with
something higher, like a 0.1uf, without any noticeable change in the
sound, perhaps a wee bit more bass.  Indeed, bass players may find
this circuit sounds better with the .05cap increased and the
capacitor in the feedback path.
Finally, I highly recommend Hammond 1590B or BB boxes.  These may be
sold as Eddystone radio boxes in the US.  They are cast aluminum,
and the original MXR chassis.  They are very easy to machine, and
extremely rugged.  Expect to pay about $10 from an electronic supply
house.

Mark Hammer