Oakley Sound Overdrive 3U User Manual

Oakley Sound Systems

3U Oakley Modular Series

Overdrive

Soft and hard clipping module

User Manual and Builder's Guide

V1.1.3

Tony Allgood B.Eng
Oakley Sound Systems
CARLISLE
United Kingdom

Introduction

This is the User Manual and Builder's Guide for the issue 1 Overdrive 3U module from
Oakley Sound.

This document contains a basic summary of its operation, a how it works section, a full parts
list for the components needed to populate the board and some basic testing methods.

For general information regarding where to get parts and suggested part numbers please see
our useful Parts Guide at the project webpage or http://www.oakleysound.com/parts.pdf.

For general information on how to build our modules, including circuit board population,
mounting front panel components and making up board interconnects please see our
Construction Guide at the project webpage or http://www.oakleysound.com/construct.pdf.

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The 3U Overdrive Module

This is a simple but very effective waveform modifier module. It has two basic modes
selected by a switch. Hard clipping is a hard edged distortion type effect useful for grunging
up your signal. Soft clipping is a more subtle effect and produces a clean sounding overdriven
amp sound.

Three front panel pots control the sound. 'Gain' controls the overall gain of the 'pre-amplifier'
stage and essentially determines the level of overdrive or distortion. The 'timbre' or tone
control is an effective EQ control that affects the frequency response of the effected output.
The 'mix out' or balance pot is essentially a wet/dry mix control that allows you to add only as
much of the effect as you want.

Voltage control is determined not within this unit, but as part of the signal chain placed
before this module. Using a VCA to control the signal level that is fed into this unit will
determine the strength of the overdrive or distortion. Using a VCF to alter the timbre of the
signal fed to this module will control the overall harmonic level far more than using a filter
alone. In fact, hard sync type sounds can be easily obtained by simply sweeping the filter's
cut-off frequency.

The Oakley Overdrive module for the Eurorack format. This module has been built by Krisp1.

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Using the Oakley Overdrive – A Personal Perspective.

Since this module has been available I have read many comments from users regarding its
usefulness. Probably more than any other Oakley module it has produced the largest split of
people who either loved, and still love it, and those that were profoundly disappointed with it.
Truthfully, I can't say I particularly enjoy receiving negative comments but in some ways it is
those negative comments that make one work harder to perfect any design. The problem is
that with some things it simply boils down to personal taste. I think the Oakley Overdrive is
one such thing – it either produces a sound that you like or it doesn't. Furthermore, since it
does very little other than overdrive the input signal it is a bit of a one trick pony. If you like
the trick then all is well, if not, it's time to move on.

Nevertheless, it may be that the module is somewhat misunderstood. After all, the most
common criticism with the module is that it doesn't actually do much. In this perspective I
wish to argue the case for its defence. It actually does do a lot but it all depends on what sort
of signal you are putting into it.

The Oakley Overdrive, in its overdrive mode, is quite simply a derivative of a famous
overdrive guitar pedal. This highly popular pedal has been around for many years and it is
widely copied. Several years ago I made up my own stomp box version of it. Initially, driven
from a 9V battery, then later from a 9V wallwart, I pretty much slavishly copied the original's
circuit. Later still I modified the main gain stage and then added a hard clipping circuit (the
distortion mode). I used this for many years with my SH-101 and SH-1 and it's pretty much
the plaintive solo sound on my own music albums.

The Oakley Overdrive module uses very similar circuitry to this prototype pedal. The basic
circuit topology remained the same but I changed the design to allow it to run from a split
power supply. I was careful to recreate the same voltage limits as in the 9V design as I was
keen to replicate the op-amp's saturated output characteristics. I also tailored the gain stages
to be more responsive to the levels seen in a modular synthesiser.

The key thing about using this module is to not use any signal that has a high harmonic
content. Indeed, inserting a raw square wave into it will do nothing. Overdriving a square
wave simply produces another square wave. The tone control may have an effect but it
probably won't lead to an increase in distortion. Inserting a sawtooth will also have little
effect. In fact it is more likely in this case that the sound is dulled – the sawtooth waveform
would be turned into something more square wave like and therefore less harmonics would
result. None of these things are perhaps what we would look for in a distortion unit.

Therefore to get the best out of the unit, when playing just single notes at one time, it is
important that the input signal should have a relatively low number of harmonics.

Where the Overdrive shines is when you use the module after a filter - especially one set to
relatively high resonance. The filter will select certain harmonics and the overdrive module
will accentuate these still further. So much so that the harmonic's frequency will dominate
over the fundamental. The effect is similar to the classic Moog Prodigy sync sound when
sweeping the frequency of a slaved VCO. And overdriving a simple one VCO - one VCF
arrangement is used frequently with the TB303 to give those rasping squealing acid lines.

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However, it can be also used more subtly on more mellow filter sweeps with the overdrive
adding gentle overtones to a solo synthesiser lead.

The actual input level is very important too. The Overdrive module will produce much more
strident results when the input level approaches 5V peak. If the input signal is too weak then
the resultant overdriven effect will be much less. This means it is possible to get voltage
controlled overdrive effects if you use a VCA in front of the module.

It is traditional to have the distortion after the final VCA. In a fixed architecture synthesiser
this is generally the only way you can do it. But I find that using it between the VCF and the
final VCA gives me right sounds for solo work. The VCF will naturally vary the signal level
as the cut-off changes and having the VCA last will ensure that any additional hiss produced
by the high gain amplifier in the overdrive module will be only heard along with the signal
and therefore drowned out.

More complex sounds, such as chords, drum loops and so on, will be far more effected than
simple notes too. The more varying frequencies that are present in the input signal the more
distorted the output will appear. Generally speaking if you are after subtle overdriven effects
it is best to keep the input signal to just a few harmonically related notes.

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