"Bypass" operations
"Bypass" is a misnomer for the TS series. They all use JFET electronic switching, and all of them have at least the two signal buffers and a JFET switch (plus the additional emitter follower in the TS10) in series with the signal even when "bypassed".I have deliberately super simplified the footswitching arrangement in the schematic, as this is the thing least likely to need modification. In addition I show a conceptual drawing for the switching, replacing the FETs with idealized switches.
The electronic switching is done by two JFETs, 2SK30A in early pedals and 2SK118 in later ones. These FETs are set up with both source and drain tied to the 4.5V bias source through high value resistors. Since they are N-channel devices, they are "on" and look like about a 100 ohm resistor when the voltage between gate and source is 0, and "off", looking like a multimegohm resistor, when the voltage from gate to source is negative. The gate of each fet is tied through a diode to the electronic switch control flipflop so the gate can be pulled to ground, which is -4.5V compared to the source. When the gate is allowed to float by the voltage on the other side of the gate diode being higher than the 4.5V on the source, leakage effects let the gate drift up to the 4.5v of the source over a period of a few milliseconds. This allow the switch to turn on softly with no audible click. An RC network on the outboard side of the gate diode does the same for the turn off.
The two JFETs are connected to out-of-phase outputs of a simple discrete flipflop made from two more NPN transistors. The flipflop is set up so it changes state once each time the "bypass" footswitch is pressed, and so it is "biased" to come up in the "bypassed" state. The actual footswitch is a sealed dome momentary switch, a fugitive from computer keyboard technology. The switches are available on the surplus market at very low prices. The two out-of-phase outputs (when one is high, the other is low, and vice-versa) are connected to the two gate control networks for the JFETs so that only one JFET is on at a time.
One JFET is connected to the signal at the emitter of the input buffer transistor, and the other is connected to the output of the volume control. The other side of both JFETs connect to the base of the output buffer transistor. Because of the flipflop action, one and only one JFET is conducting signal at any time, so the output buffer gets only the buffered input signal or the processed signal through the clipping stage and tone controls, but not both. The buffered input stage signal is the sound you hear when the effect is "bypassed".
ไม่มีความคิดเห็น:
แสดงความคิดเห็น