Clipping Stage
The clipping stage is a fairly ordinary variable gain opamp stage with a few tricks to shape the amount of clipping and the frequency at which it occurs. The signal from the input buffer stage feeds the (+) input of the opamp section, so the output is in phase with the input. In all members of the TS family except the TS10, this is a direct connection from the coupling capacitor to the emitter of the input buffer. In the TS10 there is a 220 ohm resistor in series with the (+) input. In all models the input is biased to the 4.5V bias source with a single moderate value resistor, usually 10K; this seems not to make any difference in the sound. The gain of a noninverting opamp stage set up like this is [1+ Zf/Zi] where Zf is the equivalent impedance of the feedback network from the output pin to the (-) input, and Zi is the equivalent impedance from the (-) input to AC ground.
Zi is formed by the series resistor and capacitor from the (-) input to ground. This combination is frequency selective, in that the capacitor impedance is inversely proportional to frequency. At DC, the capacitor is an open circuit; it's impedance goes down as frequency goes up. At very high frequencies, the capacitor is effectively a short circuit, and the resistor is the sole determining factor in the gain, the capacitor impedance being negliglible compared to the resistor. At the point where the capacitor impedance equals the resistor, the gain of the entire circuit begins to fall off toward a gain of one. With the stock 4.7K resistor and 0.047uF capacitor, this frequency is 720Hz. Only notes and harmonics above this point get the full gain of the distortion stage, and everything below it gets pregressively less gain - and distortion. This fact probably accounts for the lack of "muddiness" of the distortion in the TS series as bass notes are clipped least.
Zf is the parallel combination of the clipping diodes, a 51pf capacitor, and the series combination of a 51K resistor and the 500K "Drive" control. Ignoring the diodes and capacitor for a moment, and assuming that the signal frequency is above the 720 Hz rolloff point of the Zi network, the gain of the clipping stage is (51K + drive control)/4.7K. This means the gain of the whole stage can be varied by changing the drive control setting between 1 + (551K/4.7K)= 107 (about 44db) and 1+ (51K/4.7K), about 12.
Going back to the clipping diodes now, the diodes have no effect until the signal at the output is greater than the threshold voltage of the diode. In the stock TS series, these are silicon signal diodes, with a turn on voltage of about 0.5 to 0.6v. As a diode turns on, the equivalent resistance of the diode goes down as the diode turns on harder. Effectively, there is a small range from about 0.4V to 0.7V (the exact voltages depending on the type of diode, the package, the doping, etc, etc.) where the diode resistance goes from an open circuit to a very low value, perhaps a few ohms for signal diodes. So as the diode turns on, the gain of the opamp stage changes, going down to just over 1 if the diode can be considered a short circuit compared to the 4.7 K resistor on the (-) input. Even if the Drive control is set for a gain of 100, the diodes conducting cause the gain to drop to 1 for only those portions of the signal where the product of the input signal and the gain exceeds the diode threshold. This means that the signal is "clipped" at the forward voltage of the diode; because there are two opposing diodes, this happens for both signal polarities.
A "typical" guitar signal if there is sch a thing, can be expected to be in the range of 30 to 100mV for most pickups when the string is picked, trailing off as the note decays. For such signals, and especially in the lower notes, with the drive control turned down the sound out of the TS is fairly clean as the signal hardly breaks over the clipping diodes. With the drive control turned up, even a 30mv signal would be boosted to 3V if not limited by the clipping diodes, so there is enough gain here to give some distortion at least on the pedal's own to any reasonable guitar signal.
The small 51pf capacitor across the diodes acts to soften the "corners" of the clipped waveform somewhat. This softens the distortion somewhat. The action of the 51pf is most noticeable when the drive control is maxxed out, so it softens the distortion most when the gain (and distortion) is highest. Put another way, the capacitor's impedance goes down with increasng frequencies, so it starts cutting the gain of the stage when the impedance of the capacitor is equal to the resistance of the 51K resistor plus the setting of the drive control. This rolloff frequency is lowest in the audio range and most noticeable when the resistance of the drive control is highest.
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