R.G. KEEN
Diode Stew
This month, I felt like taking a little dive into
circuits for you solder-breathers out there.
to the output. Representative values for R4
and R5 are 10K fixed and 10K variable. It’s a
fun mod for the Distortion Plus.
There are two main ways that diode
clippers work, and there are subtle but
important differences in the two ways.
In the electronic sense, diodes are used
as one-way valves for current. The arrow
in the diode’s schematic symbol is the
anode, or positive
terminal, and the
bar is the cathode,
or negative terminal. A voltage that
is more positive on
the anode side than
the cathode side
lets current through
in the direction of
the arrow. When
the diode is reverse
biased (the cathode
is more positive
than the anode),
then the diode
blocks current from
flowing, as shown
in Figure 1.
signal level will be clipped off the top
and bottom. This kind of clipping is fairly
abrupt, and gives a ‘60s kind of fuzz all by
itself. The MXR Distortion Plus and the
Armstrong Blue Clipper, among others,
used this kind of clipping.
A slightly more sophisticated way to use
diode clippers is in the feedback loop of an
The final circuit in Figure 5 shows the feed-
back clipper of Figure 3 but with the signal
applied to the non-inverting (+) input of the
opamp. This forces the op-amp to add the
input signal to the feedback-multiplied sig-
nal. So the output con-
sists of a 50/50 mix
of the input and the
multiplied-and-diode
clipped signal, and the
clipping harmonics are
effectively cut in half.
You may already have
recognized it – this
is the circuit for the
clipping stage of the
Ibanez Tube Screamer.
In real diodes, you don’t get that one-way
valve stuff for free. The price is that there
must be some voltage lost across the
diode, even in the forward direction, before
the diode will begin to conduct. This forward voltage is different for different kinds
of diodes. For silicon diodes, it’s about 0.5-
0.7V, for germanium about 0.2-0.3V, and for
LEDs, it can be 1.5V–3V or more.
opamp, as shown in Figure 3. This works
differently from Figure 2. The opamp forces
a current equal to the current through input
resistor R2 to flow from its output to its
negative input through R3. The diodes
have the output voltage across them, but
only start conducting when the output
voltage of the op-amp gets more positive
or more negative than the diode voltages.
After that output voltage size, the diodes
conduct all the output current back to the
input. This is a more “delicate” way to run
the diodes in clipping, and this results in a
softer-sound.
The diode clipping
circuit in either the
resistor-diode or
feedback-diode forms
is highly flexible and
modifiable. If you experiment with these
circuits, play with both identical diodes and
non-identical diodes. Changing from, say,
1N4148 to 1N4007, or even one of each
may not make much difference. Using one
silicon 1N4148 and one or two germanium
1N34s will provide a subtle change in the
tone of the distortion. Using one silicon
diode and one LED makes a much more
obvious change in tone.
If I force a signal through a resistor, then
place a pair of diodes from the end of the
resistor to ground (as shown in Figure 2),
the diodes will do nothing to the signal
until it is more positive than D2’s forward
“penalty” voltage, and again nothing until
it is more negative than D1’s conduction
voltage. At voltages bigger than the diode
voltage, one or the other of the diodes
will conduct heavily, and any greater
You can modify the resistor-diode clipping
from Figure 1 as shown in Figure 4 by putting another resistor (R5) between the original resistor (R4) and clipping diodes. This
resistor causes some of the original signal
to be mixed with the clipped diode signal
and adds some soft-sounding roundness
Like making stew, making a good diode
clipper requires some thought, some good
ingredients, and some tasting as you go
along. If you like cooking your own clippers,
be sure to try some of these recipes.
R.G. Keen – Chief Engineer
Visual Sound
visualsound.net
56 PREMIERGUITAR NOVEMBER 2007
www.premierguitar.com
