gaussmarkov: diy fx

Wiring up the off-board components, particularly a 3PDT switch, can be confusing. There are many good explanations of how to do it (see for example geofex.com, tonepad.com, and generalguitargadgets.com) and this one adds to the pile by breaking down the logic of one of the more elegant layouts with a series of detailed figures. I walk through the case where the audio jacks are not insulated from a metal enclosure, there is an LED to show when the circuit is on, and when the circuit is off there is simple by-passing with the circuit input grounded.

It seems simplest to start with the wiring for so-called true by-pass, a straight connection from the input jack to the output jack. The input jack is on the left and the output jack is on the right. They will be reversed to the usual placement when the stompbox is closed up and turned over.

The input jack pictured here is a stereo Switchcraft jack called the 12B. The output jack is a mono Switchcraft called the 11. You can read a post about these jacks in 1/4″ Phone Jacks and Plugs. The switch is a 3PDT Taiwan Blue. The picture shows

1. a white wire from the tip lug of the stereo jack (on the right, for input) to the switch;
2. in one switch position, this will connect to the white wire that goes across the bottom of the face of the switch;
3. which will then connect to the white wire that runs from the switch to the tip lug of the mono jack (on the left, for output).

So we have a simple connection from one tip lug to the other: true by-pass.

In the other switch position, we want to connect the input lug to the input of the circuit and the output lug to the output of the circuit. These are added in the following image:

The blue wire should be connected to the input pad of the circuit board. The yellow wire should be connected to the output pad of the circuit board, or to the middle lug of a level pot if one completes the circuit. I am leaving the board out of these pictures for simplicity. Any unconnected wires in this tutorial are supposed to connect somewhere on a circuit board.

As it stands, we do not need the 3PDT switch because we are only using 2 poles, one for input switching and one for output switching. The third (middle) pole can do the switching for an LED that lights up when the circuit is engaged (or not by-passed). For that we will also need a power supply, which we will get from a DC voltage supply.

This image shows the LED wiring from another vantage point, with the wires for the guitar signal removed for clarity. The LED switching opens and closes the ground connection for the LED circuit. The ground for the circuit is the sleeve lug of the output jack.

• The DC jack has a direct wire to that lug: the green wire that runs all the way across the middle of the picture.
• The LED also connects to that grounded lug through a resistor (2.2K for limiting current) and two green wires, one that goes from the LED to the switch and another that goes from the switch to the lug. Note that these two wires are connected when the switch is in the “not by-passed” position.

Also note that in this particular setup, the sleeve lugs of both jacks are connected through the aluminum enclosure that holds them. The entire stompbox is grounded through the output cable. So the input sleeve lug is grounded by its connection to the output sleeve lug through the enclosure.

In setups with insulated jacks, one must make these connections with wiring. You should still ground the enclosure in those cases because this improves the ability of the enclosure to shield the circuit from outside radio frequency (RF) signals.

Here is a close-up view of the DC jack connections:

The top lug is the positive power supply connection and the angled lower lug is the ground connection. We will use the third middle lug later when we hook up a 9V battery as an alternative power supply.

The red wire is the positive power supply. This colour is consistent with the leads found on most 9V battery snaps: red is positive and black is negative (or ground). In these pictures, I am using green for ground because it shows up better.

I prefer to use the DC jacks that are fastened with a nut on the outside of the enclosure and that is what is pictured in these figures. DC jacks also come configured with the nut on the inside of the enclosure. I find this inconvenient because it requires me to install the jack in the enclosure before I solder the wires to it. As a result, if I want to remove the circuit from the enclosure then I must unsolder these wires. The input and output jacks, the 3PDT switch, and any pots all have their nuts on the outside. If the DC jack does also, then one can remove the nuts and the whole circuit lifts out of the enclosure completely connected.

Here is a close-up view of the output sleeve lug for ground connections:

At this point, two wires are supposed to soldered to this lug. One wire is coming from the DC jack (not shown) above. The other wire is “flying in” from the switch.

Here is an image of the switch wiring with all of the wires in place: input, output, and ground connections:

Note that there is an additional green wire. This is the short wire that connects the first (input) pole of the switch to the middle (ground) pole of the switch.

• When the switch is in the by-pass position, this short green wire connects the input of the PCB to ground.
• When the switch is in the engaged position, this short green wire does not connect to anything.

This additional wire feeds the stompbox circuit a quiet input signal when the circuit is by-passed. That is the trickiest part of the switch wiring, making a nice use of that otherwise unused lug on the switch.

Now we are ready for the circuit board. Besides the input and output connections, the board needs the positive and ground connections which come from the same places as for the LED circuit: the positive lug of the DC jack and the sleeve lug of the mono output jack.

Wires for those connections appear in the figure above. Generally, it is good to run your positive supply lines next to ground lines. This is true of PCB traces as well. So I am showing the positive board supply wire running next to the ground wire for the DC jack.

I am not quite finished. I still need to add the wiring for the battery. It will take a while to make a figure for that, but in the mean time it is easy to describe. The red battery snap wire connnects to the remaining free lug on the DC jack. The black battery snap wire connects to the ring lug of the (stereo) input jack. That’s the obvious one facing up to the right of the tip connector. This battery snap hookup accomplishes two things:

1. The battery negative terminal will be connected to ground only when there is a mono plug inserted into the input jack. In that event, the ring connector is in contact with the sleeve of the mono plug and a ground connection is made through the ground lug of the input jack.
2. The battery positive terminal will be disconnected to the LED and the board when there is DC plug inserted into the DC jack.

Because of these properties, the battery will supply power when there is no alternative DC power supply and there is an input for the stompbox. Otherwise, the battery is preserved.