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Wiring up Switches – True Bypass

In my previous post I covered some basics about switches, in the follow-up post, I’ll cover wiring up 3PDT switches. In this installment, I cover what does a true bypass means. Actually, what options do we have to wire-up our DIY effects?

I cover “always on” effect … ha! Then I cover “hardwire bypass“, to finally cover “true bypass” and how to wire-up effects for A/B testing.

Here’s the video if you don’t want to read, but you can always come back to look up the diagrams:

True Bypass

For this demo I’m using a very simple effect:

Schematic of a very simple booster used for demoing the switching options
Simple Booster for Demonstration

This is a very simple booster based on LPB effect. It is slightly simplified – I got rid of the volume pot because the effect is there just for demonstrating switching. Accordingly, I scaled R3 and R4 resistors to give the booster about 13dB gain. Just enough that we can hear the difference.

I deliberately did not draw any connections to the battery, or to the ground, or for that matter to input or output jack. If we imagine that this is a PCB board with the components soldered on, we’d have 4 connection points (holes) to solder wires to the rest of the hardware. Actually, most of the time, I don’t even bother including lots of the hardware on the effect diagram. Look at this example:

MXR Distortion Plus diagram illustrating the point that most of the time I don't show all hardware on the effect diagram
Example: MXR Distortion Plus

The circuit above is quite more complex – but it has only 4 connection points to the rest of the pedal: IN, OUT, +9V and GND. This is just to illustrate the point, you can wire it up whichever way you want it. You want it powered by battery only, that’s fine. You want both DC jack and the battery – fine. You want it with true bypass with a DPDT switch, fine, hardwire, fine … you get the point.

That’s why I normally just include the rest in a separate diagram, because the rest of the components don’t really affect the sound:

MXR Distortion Plus - the rest of the hardware in a separate schematic
Example: MXR Distortion Plus – the rest of the hardware

Always On

I’m wondering, should we even have a switch? 🙂 I suppose, we should always ask ourselves that question. If we don’t, always on is an option:

Diagram showing how to wire-up our effect so it is always on
Always On Effect – not really a bypass 🙂

As you can see from the above diagram – the connections are as I described them when I described the effect. There’s +9V connection – I used battery here, there’s GND and Effect_IN and Effect_OUT are connected to input and output jacks respectively.

Even if you plan on to have the effect always on, it is real pain to actually bypass it when it is on the breadboard. When you’re testing it out, you might want to still use a switch to engage it or disengage it. But having it always on is a legitimate option.

Hardwire Bypass

If you only have a SPDT switch – you might get away with hardwire bypass:

Diagram showing how to bypass the effect with an SPDT and hardwire scheme for bypass
Hardwire Bypass with SPDT

This is very simple – input jack is always connected to both effect input and to the output SPDT switch. SPDT switch either bypasses the effect by directly connecting input jack to output jack, or it connects effect output to output jack.

This is absolutely viable bypass strategy – if you have MXR’s Micro Amp boost, you probably have it in your effect chain! But, this is not a good option for all effects. The effect I’m using here is particularily bad.

If you watched the video you could hear how much the effect affects the bypassed signal. The tone suck is huge because input impedance of the effect is very low and for this bypass scheme, the effect input is always connected to the signal input. This creates a voltage divider and always attenuates the sound.

In order for this effect to be negligible, input impedance of the effect has to be very high. That’s why this works for MXR Micro Amp – input impedance of the effect is very high. In my demo effect – that’s not the case and the impact is very clearly audible.

True Bypass

Minimal switch requirement for proper true bypass is DPDT switch. From the previous section – it is clear that SPDT will not cut it in all cases. We need to switch both input and output signal, not just output:

The simplest true bypass wiring with DPDT switch
Simplest True Bypass schematic

The above is the simplest true bypass wiring. It does not have LED indicator, but that is fine – lots of pedals did not have them originally.

This is true bypass – in one position – input is directly connected to the output. In that position effect is completely bypassed, meaning both effect input and effect output are disconnected.

In the other position, input jack is connected to effect input, and output jack is connected to effect output, thus, the effect is engaged. It really is very simple wiring up scheme. I use this most often when I do my breadboarding.

There are clever ways to wire-up the LED despite not having another switch available. Original RAT did not have LED, then it used DPDT switch with a JFET and some clever circuitry to drive LED indicator, before finally moving on to 3PDT. Possibly better – and more compliated way to do this is millennium bypass.

I used the millennium bypass version several times and it works great, but with 3PDT switches getting cheaper and being more robust than DPDTs – there’s little point in not using 3PDT switches for your actual build.

A/B Testing

Where I find DPDTs even more useful is when I’m testing various options for my effects. I breadboard two versions of the effect and then I make changes to the one. I can use DPDT to flip between the two versions of the effect and test what my changes did to the sound:

How to wire up two versions of the effect for A/B testing
A/B testing

If I want to test two versions of the same effect, or two totally different effects, I can use the diagram above. The wire connecting directly input and output – thus making it true bypass – is replaced, and the 2nd effect is wired up instead. So flipping the switch engages one effect or the other. Handy!

That’s it for this post, in the next one I’ll be talking about wiring up 3PDT switches – which is probably the most common way to build your pedal.

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