This is the third post about switches. I already covered some basics about switches and what true bypass is. In this post I’m covering how to wire-up 3PDT switch – the most common stomp switch used in DIY effects.
As always – video first, if you want to have a closer look at the diagrams from the video just continue reading 🙂
3PDT – is it special?
Well, here’s how to wire-up 3PDT switch for true bypass:
The main difference from DPDT true bypass covered in the previous post is that we can now wire-up an LED indicator too.
Remembering from basics about switches – 3PDT is just a switch with 3 SPDT switches that share the activation action. This makes it equivalent to DPDT + SPDT … meaning we have an extra switch to play with, which makes wiring up LED super easy.
It has to be said that, besides having 3 switches, 3PDT is also bigger than DPDT switch making it a bit harder to fit in into smaller enclosure. But, 3PDT is also way more robust and should last way longer than DPDT switch (at least in theory, and highly dependent on the abuse it takes from the user).
How to wire it up?
Considering that this is a DPDT switch with an additional SPDT switch, we can use exactly the same wiring as used in previous post, we just add LED part, something like this:
As you can see in the diagram above, first two switches of the 3PDT are wired up exactly the same as the DPDT next to it.
Sometimes it is helpful to move around the wires – maybe it makes it easier to solder them if they were moved around. Remembering that the switches are electrially independent, we could move them around to get equivalent switching – consider this:
It is clear on the diagram above that wiring is slightly moved around, but the two wirings are equivalent. For DPDT I just swapped two switches, makes absolutely no difference. For the 3PDT switch I swapped 2nd and 3rd switch. Again, the wiring is absolutely equivalent – no difference at all.
The above diagrams were showing equivalent wiring, but there are few more options. Most of the modern pedals have pull down resistors to counter switch pops. See a ProCo modded schematic from one of my earlier builds:
In the above schematic, C1 coupling capacitor, not being a perfect capacitor, could develop some DC charge due to current leakage while the effect is disengaged. If this is to happen, when the effect is engaged by stomping on the switch, it could result in a loud pop. Resistor R1’s job is to bleed this leakage current to ground and help preventing the pop.
Have a look at this great article on sources of pops in pedals. Essentially, if the pull-down resistor is there, most of the time, simple wiring 3PDT arrangement is sufficent. However, sometimes that’s not the case. If you look at this diystompboxes thread, it covers alternative wiring options that might help in some cases.
The thread covers options:
- No Input or Output Grounding
- Effect Input Grounding
- Effect Output Grounding
- Both Input and Output Grounding
Essentially – all these more complex wiring alternatives are trying to do is to alleviate popping sound by connecting input, output or both input and output of the effect to the ground to try keeping the coupling capacitors at correct DC level.
The four above mentioned arrangements are pretty much all you can do with 3PDT switch. I mostly use the simplest wiring, it’s easiest, but I nearly as commonly use the following arrangement:
As you can see on the right hand side, top diagram shows the effect in bypass mode: the effect input is grounded and input jack is directly connected to the output jack. When the effect is ON – the connection is as you would expect: input jack is connected to effect input, effect output is connected to the output jack, and the LED is illuminated.
Of course, if you follow the wires, the wiring is slightly more meandering, but it works just fine. I picked this up from that diystompboxes thread – this particular arrangement is great because it makes soldering the wires easier beause all of them are on the outside of the switch.
There it is, wiring up 3PDT stomp switch, covered in full. I hope this makes sense and is useful.