Live Electronic Musical Instruments:
choosing diodes for fuzzing

Dave Krooshof

Wat ledje?

This guide in fuzzing was contributed by several members of the SDIY email list.
Thanks for all the answers. Especially for Jurgen Haible's and Harry Bissell's.

Discussed here is the use of diodes in the feedback loop
of an opamp with the goal to gain a nice distortion/clipping.
The idea is that diodes start to conduct more the higher the
over them is. Typically, their resistance drops dramatically
once this voltage exceeds a certain threshold.
Since the opamp amplification ratio depends solely (sort of)
on the ratio between resistors in the circuit, a resistor
that bails out on a certain voltage, limits the amplification
and thus clips and fuzzes the signal.
For the standard silicon diode, this voltage is around 0.7 volts.
For germanium it's lower, for LEDs it's about twice as much.
Unlike Zener diodes, normal diodes and LEDs do not come with
spec-sheet with a curve showing the relation between
voltage and resistance. Let alone that their behaviour in time
is described so clearly that you can decide what you want
to use to clip against. But it seems it's these issues that do
matter a lot when you build a fuzz.
Here's a little guide built on experience.

My fuzz has patchable LEDs, so if you find something nice
drop the part number in my mailbox and I'll check it out.
Your own experience is welcome here, too.
Here's the typical fuzz schematic, it's the basic situation
discussed here on this page:

Choose your clipping style: with or without opamp saturation.
It matters at what voltage you'll clip:
close or well below +-VCC. If you choose to clip at a voltage
close to VCC, the saturation and thus the quality of the chosen
opamp will matter.

Control how much edge you want
by using a pot (1M or 500K log) in parallel.

Decide on what kind of edge you want.
Here's a list of tastes:

Low saturation:

Below 1.5 volt? Then LEDs are out.
- smooth, soft distortion with a fixed maximum signal
level ("hard ceiling") : 4148 with 100k resistor parallel
- Mellow: 1N4148 as long as they do not clip fully.
- shottkey diodes like 1N5817...
these start to clip at such a low level... its possible
to put an inverse-parallel pair in your guitar with a
toggle switch... and have a passive fuzzbox. ;^)
- Try a germanium diode bridge rectifier configuration
with the 'AC' input across the signal (or in the feedback
path) and a variable load across the 'DC' output part
of the bridge rectifier.

With saturation or high voltage:
- Wild: red LEDs.   Both the Fender M-80 and a Danelectro stompbox
  use red LEDs. They light up the most of all LEDs too!
- Clean, High voltage (3.6 volt clipping): white LEDs.
  They look and sound different on the scope
- Cold, clean clipping: Blue LEDs
- The bipolar red-red LED... this way its just one component
  in stead of two anti parallel LEDs.
- Try the LEDs in a bridge rectifier configuration...
- Green: not yet tested
- IR: yet unknown

Uneven clipping (negative an positive tops clipped differently) :
Unevenly clipping sounds a bit voicelike. The
human voice's soundwaves can also look softclipped
on one side (if the phases are still correct),
but I assume also the uneven time-behaviours of the different
LEDs matter, as this seup tends to get a lot wilder then
using mirrored diodes. This wildness then also depends on the setting of the shunt pot.

Low voltage:
germanium and silicon diodes each pointing the
opposite direction as the other.
High voltage:
try one white going one way and two green in
series going the other. Substitute red, blue etc.

With this guide, you should be able to choose your fuzzing components well.
Fuzzing is an important stage in synthesizers as well as in guitar-stompboxes
arrays, as they are often the last stage in the patch. But also if it's patched
earlier in line, say before a low pass filter, your fuzzing edge will be a
distinct colour to your instrument.

                      Bright leds, big city: The Hague, 14-10-'02
    futher reading:
    A Musical Distortion Primer
    tube screamer anatomy


Dave <email Dave Krooshof>   See Also: 'An Electric Tuba'
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