Understanding the 3PDT Switch and Building a True Bypass Double Looper

Most people who own a true bypass boutique pedal are familiar with what the little blue 3PDT switch looks like, but very few are actually aware of what is going on when they step on it aside from the fact that it turns the pedal on or off. Let’s take a look at how the switch does what it does and then go a step further and actually build a simple true bypass looper with 2 loops.

3PDT stands for “triple pole double throw”. The 3PDT switch that is a standard in most pedals is also what is called an “ON-ON” switch. This means that the poles are always connected to one of their throws. In this case there are 3 poles and each pole has 2 throws that it can switch between. The number of throws each pole has will tell you how many positions a switch has. So the 3PDT will only have 2 positions. In the diagram below, the red arrows indicate where a connection is made. You can see there is no “OFF”.

Here we have an example of the 3PDT wired for true bypass with an LED status light. The pole on the left is connected to the signal from your guitar, the pole in the center is connected to the signal going to your amp, and the pole on the right is connected to the -9v power supply (more commonly referred to as “ground”). In the “engaged” position, signal travels from your guitar to the pole on the left. The pole on the left is now connected to the top left throw. The signal now travels to the circuit…let’s say a fuzz circuit. The fuzz does it’s thing and now it’s sending the newly distorted signal to the top center throw. The top center throw is now connected to the center throw, which is connected to your amp. The right pole is now connected to the top right throw which send negative current to the LED. Since the LED’s positive terminal is already connected to +9v, the circuit is now complete and it lights up.

Here we have an example of the 3PDT wired for true bypass in the “bypass” position. The poles are still connected to the same things as before, but now the throws have changed so the guitar signal does not go to the fuzz circuit anymore and there is no longer any negative current going to the LED. Since there is a jumper from the bottom left throw to the bottom center throw, the signal from your guitar is passed directly to your amp with out any alterations made to your original tone. Since the bottom right throw is connected to nothing, nothing is what happens in the bypass position and the LED status light turns off.

Ok. So now that we understand how the 3PDT footswitch works. Let’s build a true bypass double looper. Here’s the layout. You’ll need:

1 enclosure (125b or larger)

5 ¼” mono jacks

1 ¼” stereo jack

3 LED’s with bezels

1 battery snap

3 4.7k resistors

2 3PDT footswitches

First connect all the Ground connections. These are the black wires in the diagram below. Connect the black battery wire to the “ring” of the in jack. This will create a master power on/off switch when you plug in your instrument cable. Connect 4.7k resistors to the anodes (positive ends) of each of the LEDs. You can use a pair of needle nose pliers to bend the leads into circles to give you something to solder to. Then bend the other end of the 4.7k resistors into circles and connect them to the red battery wire.

Now connect the negative ends of the LEDs to the lugs of the footswitch as shown in the diagram below. Now you can see how the LED circuits will be completed to or broken from ground when you step on the switches.

Now we begin the tricky part - routing the instrument signal. The green wire shows the direct “bypassed” route that goes straight from the tip of the in jack to the tip of the out jack. When the on/bypass switch is turned on, the signal is sent down the brown wire to the A/B switch where it will be routed to one of the “send” jacks. When the signal comes back from one of the “return” jacks it will be routed to the purple wire which goes back to the green wire that runs to the out jack.

Now when we wire the send/return jacks to the A/B switch, we can see that when the switch is in the “A” position, the signal will loop through the brown wires and when the switch is in the “B” position, the signal will loop through the purple wires.

There ya go. Take your time. If you need to trouble shoot, use the “continuity” setting on your meter and trace out the signal paths. Thanks for reading.

Keith Vonderhulls

keith@buildyourownclone.com