Oakley 3U User Manual - page 10
Circuit Description
This module is probably one of the simplest we make, and indeed, if you have already looked
at other overdrive circuits you may well recognise some of the features used in this particular
design.
The module is powered in the conventional way from a split rail of +/-15V. This comes in to
the module via the 0.156” MTA connector PWR. L1 and L2 in conjunction with C16 and
C10 provide high frequency filtering and decoupling. They essentially act to keep the power
supply as free from noise as possible. The main overdrive and distortion circuitry is actually
run from a lower +/-5V supply. This is generated in the usual, if somewhat overkill, fashion
of using two three terminal regulators, U3 and U4. These generate a very stable quiet low
noise split 5V rail which the more sensitive parts of the overdrive circuitry can run. R15 and
C20, and their negative equivalent, provide additional filtering and isolation from main 15V
rails.
D5 to D8 provide discharge paths during power up and down cycles which could potentially
harm the ICs on the circuit.
The input signal is firstly attenuated by an inverting amplifier block based around U2A (pins
2, 3 and 1). This circuit reduces the input signal to around a quarter and provides the
following sections with a constant source impedance. Without this part of the circuit, the
overdrive core and the balance pot may affect the source signal's integrity and cause
unwanted distortion if you were using the Overdrive module in parallel with another module.
U2A's output is then split, one side going to the balance pot which will provide the 'dry'
signal, and the other side going into the overdrive's core circuitry.
The core of the overdrive module is based around the same circuit as used in many guitar
overdrive foot pedals. To my knowledge this type of design first appeared in the Ibanez
Tubescreamer TS-808 pedal, but it also appears in other commercial pedals including the
Boss OD-1 and SD-1, and other later Ibanez pedals. The TS-808 pedal has become something
like the equivalent of the TB-303 in the guitar world and the original green units sell for a
great deal of money. However, the basic circuit is actually quite simple and it is one that I
have used in various home made pedals for some time.
There are great deal of DIY TS-808 clones out there and many of them will talk about the
huge differences in using different diodes and types op-amps. I found that, for synthesiser use
at least, the actual sonic differences due to the actual devices used are not quite as obvious as
internet lore has decreed. However, I will leave it to the builder to experiment here and I am
sure that the Oakley Sound Forum would be a great place to discuss your findings.
The key in the basic TS overdrive circuit is the usage of a standard non-inverting high gain
op-amp stage, U3A, with two diodes in reverse parallel with the resistative feedback path.
These diodes act to limit the voltage across the feedback resistor. These are shown in the
schematic as D1 and D2. They can be switched in and out of the circuit with SW2, the
overdrive and distortion selection switch. Because they act upon the feedback voltage and not
the output signal directly they do not behave as a traditional clipping circuit. In the latter any
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