Thursday, June 25, 2009

How to wire up a Stomp Box / Effects Pedal

Today we're going to talk about how to wire up a stompbox.

Apart from the effects circuit itself, there are three things that have to be thought about when wiring up a stompbox, which are the stomp switch, the stereo input jack and the DC-in connector.

I’ll be referring to this image throughout the explanation (as with all images on here, click to enlarge):


1. The Switch




To try to make this easy to follow, consider the left-hand column of the switch to be the input column, the middle column to be the LED column, and the right-hand column to be the output column. If for any reason you only have a 6-pin switch, you can still use it, just without the LED column.


Off position

In the OFF position, pin 4 is connected to pin 7, pin 5 is connected to pin 8, and pin 6 is connected to pin 9 (in my diagram at least – you may have a switch with different numbers written on it, though often they have none at all).

Looking at just the left and right columns, the input comes in from the input jack via the yellow wire connected to pin 4, is directed downwards to pin 7, which is shorted to pin 9 via the jumper wire, then this is redirected up to pin 6 which is connected to the output jack. In other words, a bypass is in operation. As the effects circuit itself is completely disconnected, it cannot influence the signal, so this is true bypass.

As for the middle column, there is not much going on there. The ground/earth wire is connected to pin 5, but it is not redirected anywhere, so nothing happens, and the LED does not light up.

On position

In the ON position, pin 4 is connected to pin 1, pin 5 is connected to pin 2 and pin 6 is connected to pin 3.

Again, looking at just the left and right columns, the input again comes in from the input jack via the yellow wire connected to pin 4, is directed upwards to pin 1, which then goes to the input of the effects circuit. The output of the effects circuit goes to pin 3, and this is redirected down to pin 6 which is connected to the output jack.

As for the middle column, the ground/earth wire is connected to pin 5, which is redirected up to pin 2 and fed to the negative terminal of the LED, allowing it to light up.

OK, that’s the switch explained, but there’s more going on here.


2. The Stereo Input Jack


You may be wondering why a stereo input jack is required, when you are only using a mono signal. Well, that’s because the third connector (pin 3) on the input jack is used to control the power supply to the circuit.

Let’s assume for a moment that you are using a battery. Instead of the negative of the battery being connected straight to the negative of the effects circuit, which would mean the battery is in use ALWAYS (even when the circuit appears to be switched off), the negative of the battery is instead connected to the third connector (pin 3) of the stereo input jack. That way, when nothing is plugged into the stomp box, the third connector is not connected to anything and the battery is not being used. When you plug in a mono jack plug, it shorts the third connector to the shield/sleeve connector (pin 1) of the socket, which in turn connects it to earth/ground, powering the effects circuit.

You may wonder why you don’t just power up the battery at the same time as switching on the effects circuit. Without going into too many details, this is because the sudden surge of power leads to a loud popping noise, which no-one wants to hear.


3. The DC-in Connector


Lastly, we have the (5.5 x 2.1mm) DC-in connector. This also has three connectors and can sometimes cause confusion, since instead of having a positive inner (centre pin/tip), which would be the norm for most electronic items, it actually has a negative inner. I’ve numbered the pins in the diagram for clarity, as follows: pin 1 is the (negative) inner/pin/tip and is connected to the negative of the battery, pin 2 is the outer (sleeve) and is connected to the positive (+9v) of the effects circuit, and finally in a similar way to the stereo input jack, pin 3 is the 3rd, extra, pin and is connected to the positive terminal of the battery.

(The difference between this and the input jack is that in this case, pins 2 and 3 are normally shorted UNTIL you plug in a power supply, at which time pin 3 becomes disconnected.)

When nothing is plugged in, the positive terminal of the battery, which is connected to pin 3, is shorted to pin 2, and continues on its way to the positive of the effects circuit. When the power supply is plugged in, however, pin 3 (and therefore the positive of the battery) is disconnected, and the positive voltage from the power supply goes directly to the effects circuit via pin 2.

One more thing to note is that since the DC-in socket has a positive outer, it should be one with a plastic surround, so that the outer does not make electrical contact with the enclosure (which is grounded/earthed).

Something like this (below) is fine:

Whereas something like this (below) is NOT suitable:

 
 

Additional information


With thanks to CheopisIV (http://www.stuntmonkeypedals.com/) and bugg over at the mylespaul forums for clearing this up, it is a wise idea to ground the input of the circuit itself when not in use (in other words, while it is being bypassed). Without a grounded input, the input signal can float all over the place and by the time it gets amplified through the circuit, there can be quite a large uncontrolled signal in there. Now theoretically this wouldn't be connected to the output socket, since the circuit is being bypassed, but the noise can be picked up by other means, and so it is safer to just avoid it if possible. 

So we simply add one extra jumper wire from the leg of the switch that goes to the circuit input and the unused leg on the LED column. The resulting wiring would look like so:
While the circuit is bypassed, the circuit input is connected to the ground wire going into the centre pin (pin 5) of the switch. When the switch is engaged, this connection is broken and the circuit just works as previously described.

Finally here are a couple more circuits for people who want to take out either the battery or the DC-in jack:

10 comments:

Thomas said...

Thanks for this, it really clears a lot up!

Anonymous said...

Thank you so much! Now i know how to connect a led that is shining when the effect is activated.

Anonymous said...

Thank you very much. I have a metal surround DC power connector, which of course is running off of a negative tip connector. That is why my EQ effect I built was not working. After drilling the hole slightly bigger, fitting a rubber grommet in, I finally have it working.

tdale said...

In this schematic:
http://www.dirk-hendrik.com/swamp_thang.pdf

It seems like the center pin of the 9V connection, is not connected via the - side of the battery, but it's just grounded directely.

Does that mean that once the 9V plug is inserted, the circuit is working, even if the input jack is not inserted? Could that cause hum, even if the guitar is not connected?

stu said...

Hi tdale. Yes, you're right, the power supply's negative goes straight to earth, so it will always be using power, even when the input jack is not connected. However, apart from very small amount of power wastage, there won't really be any negative effects.

Justin Reed said...

Hi - not sure if this is the appropriate forum but i was wondering if it is possible to add a switchable trigger/gated pulse input that would close the stomp pedal switch?

my application is to send clock signals from synth/modular equipment that will activate the tap tempo of my stomp pedal, i think a momentary switch to allow the trigger pulse would be sweet as i could hold for 5-6 tap tempo and release to set.

Anonymous said...

Hi there , thanks for this post. One stupid question, how do you know what is the pin 1 of the stompswitch?
Thanks!

Unknown said...

No, not stupid. Make sure the lugs run horizontal as that is usually theway it goes. The switch has two states, on and off. Therefore which way up it is is not important (most people have the circuit engaged at the top of the circuit though as above). On; rows one and two are active. Off; rows two and three are active. Make sense? No....sorry about that

MrJohnny56789 said...

So why cant you just use a center positive jack or wire it for center positive? Im sitting on some jacks and cent+ wall warts right now that I've wired up for center positive and power circuits just fine. Why do people wire center negative?

MihaiM said...

@MrJohnny56789 : most of the guitar pedal supplies follow this "standard". Not the most fortunate, I know, just this is what stuck. Think of the pedalboards with central power source. They follow this rule, and inverted polarity would render your pedal incompatible with these supplies.