signal is simply limited to a set maximum or minimum voltage. The TS circuit is more subtle
than that.
The gain of the op-amp non-inverting amplifier is set by the resistances within the feedback
loop, but because of the way it works this is always more than or equal to one, ie.0dB. That
is, the output level is never less than the input. The exception to this is at very high
frequencies when the op-amp reaches its operating limits. Ignoring the diodes for now, U1A
has its passband gain set to a minimum of 11, via R2 and R1. The gain pot allows this gain to
increase still further by increasing the feedback resistance. With the gain pot turned up full
the maximum passband gain of the op-amp without the diodes is 111.
A diode will behave in such a way as to limit the positive voltage across it to no more than
0.6V. It does this by effectively changing its resistance depending on the voltage it has across
it. Two diodes connected in reverse parallel, head to tail, tail to head, will seek to limit the
voltage across them to +/-0.6V. D1 and D2 are connected to act like this.
So as the output level of U1A rises, either by turning up the gain pot or by applying a bigger
input signal, so does the feedback voltage. Once the feedback voltage gets to 0.6V
or below -0.6V, the diodes start to conduct and the rise in output level is tempered. However,
the gain of U1A cannot fall to below unity, so the diodes only appear to act on the amplified
signal. Thus what we get at the output is a clipped amplified signal
plus
the original signal
superimposed on it. This is the TS sound and it has great tonal characteristics.
I ought to add that a great part of the overdrive sound is also due to frequency shaping. C2
rolls off the gain at lower frequencies. C5 and the slew rate of the op-amp act to curtail high
frequency components. It is these that set the width of passband, that area in which the op-
amp acts as an amplifier. In addition C9 will effectively block any DC signals to the
amplifier.
The Oakley Overdrive has a major difference that the original TS doesn't have. We can
switch out the diodes completely and allow U1A to become an unlimited high gain amplifier.
But of course, everything has limits and this time the output is limited by the supply voltage
to U1A. This is set by the +/-5V power supply and this causes the output to clip at around
+4V for positive excursions and -4V for the negative. The op-amp cannot produce any output
signal higher than this.
However, when SW2 switches out the two diodes in the feedback path, it also switches in, via
SW1, two more reverse parallel diodes, D3 and D4. In conjunction with current limiter R4,
these two diodes act as a traditional diode clipper and truncate the voltage across them to
/-0.6V. This is the 'distortion' mode and two different types of clipping are now
possible in this mode. The diode clipping is apparent with high input signals with low gain, or
low input signals with high gain. However, at high gains with medium to high input levels,
both the op-amp output limiting and diode clipping work together to give you a more
pronounced effect.
Immediately after the diode clipping circuit there is some additional high frequency filtering,
based around the actions of R4 and C14. This low pass filter reduces the harshness and gives
a rounded tone to the sound. Coupled with the bass end roll-off of C2, the overall frequency
response over the distortion sections is focussed on the mid-range part of the audio spectrum.
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