The Rat Bypass Switch
For many years, true bypass switching systems utilized a DPDT (double-pole double throw) switch that left no provisions to toggle an indicator LED. Combining an LED with true bypass required the more expensive and less rugged 3PDT switch that was also less readily available. However, a clever system using a transistor switching mechanism with a DPDT switch appeared in the late 1990s first on one of the ProCo Rat models, then quickly adopted by the hobbyist community and gradually by commercial builders. The original Rat bypass switch used a darlington transistor but they later changed that to a jfet.
There has been much discussion and comment that this method is not true bypass but that is a falsehood and easily proven. Let us consider the internal workings of a DPDT switch. There are actually two separate switching systems inside the housing as shown in the graphic here. The dotted line illustrates how the poles of the switch sections are completely separate and the blue lines between poles represent the internal pole connections that are moved when the switch toggles. In Position 1, poles 1 and 2 are connected to each other and poles 4 and 5 are connected. When the switch is actuated, the internal connectors of both switch sections will move to the opposite position, shown here as Position 2. Now poles 2 and 3 connect, and totally separate, so do poles 5 and 6. At no time are there any internal connections between any of the poles on the left side of the switch (poles 1-2-3) and the poles on the right side of the switch (poles 4-5-6).
If we look at the common DPDT true bypass switching setup, it can be seen that when the switch is in Position 2, the signal goes from the input, through the switch contacts, through a jumper wire between poles 3 and 6 (shown in red), and to the output jack without any electrical contact with the effect circuitry. This is "true bypass". The audio signal is effectively passing through a "straight wire" in the effect box, therefore none of the circuit can have a detrimental effect on the sound. When the switch is toggled to Position 1, the effect circuit will be brought into the signal path and the input-to-output signal now goes through the circuit board. In Position 1, the jumper wire and poles 3 and 6 are not being used at all. Are we clear so far?
So to recap quickly, for the true bypass function the left and right sides of the switch do not connect to each other except through jumper wires while the input and output will be connected in a straight signal path without any connection to the circuit board.
The ProCo Rat switching system took advantage of the fact that by rearranging the configuration of the switch, one pole was left free, even in true bypass mode. This spare pole could be setup to (1) float when not in use, and (2) connect to the output when toggled thereby actuating the transistor that extinguishes the LED. Since the transistor has a resistor on its base (or gate) that connects to the voltage supply, the trickle of current through the resistor holds the transistor on when the input is floating. The basic schematic of the Rat bypass switch is shown below.
When the switch is in Position 1, the connection to Q1 via pole 6 of the switch is not connected to any part of the circuit. The 22M resistor on the gate of Q1 is keeping it saturated and the current can pass through it to light the indicator LED. If you trace the path of the input signal, the audio goes into pole 2 of the DPDT, through the internal contact to pole 1, out to the effect circuit, from the volume control back to the switch at pole 5 which is internally connected to pole 4 that is jumpered to pole 3 and thereby to the output jack. Whew! The circuit is modifying the signal and the LED is lit to indicate that it is working.
If the switch is toggled to Position 2, the audio goes into pole 2 of the DPDT as before, but the internal contacts now connect pole 2 with pole 3 and thereby to the output jack. Even though pole 3 is jumpered to pole 4, there is no internal contact with pole 4 so it has no effect on the signal. The gate of the jfet Q1 goes from pole 6 to pole 5 and to the output volume control. Since the resistance of the output volume pot is much lower than the 22M pull-up resistor on Q1, the gate is pulled down closer to ground and the transistor becomes high resistance so the LED is extinguished. Note that no point of the audio signal path is connected to the circuit board or the jfet, so obviously this is "true bypass".
It is interesting to note the two diodes on the source of Q1. They are used to make the source of Q1 at least 1.4v above the LED, or about 3.2v above ground, which makes it easier for the gate to be pulled down below the source voltage to actuate the transistor. This allows positive switching even with the wide variance in jfet characteristics that are often encountered with the same numbered parts or even in the same batch of jfets. The diodes allow just about any 2N5457 to be dropped in the circuit without having to measure the gate threshold of the jfet yet providing quick positive switching of the LED. Jfets are as tough and static-proof as bipolars and work quite well in this clever application.
With the Rat bypass switching system, a rugged DPDT switch can be used to have both true bypass and an indicator LED. Try it on your next project!
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