Modifying a Russian EX Big Muff π with a Switching Power Input Socket

I was  recently asked by a friend to modify their old-school Russian Electro-Harmonix Big Muff π, with a standard 2.1mm, switching, DC power input socket (often known as a ”Boss style” power jack), for use with a 9v battery. It was a fairly straight forward job, but I thought I would do a short blog post about it, as knowing how to add a DC input socket is super useful for a whole range of projects. These DC sockets let you switch automatically between battery and adaptor use, while conserving the battery when the adaptor, is plugged in.

Before we go through the mod, let me say something about safety

This project and modification are designed only to be used with the standard 2.1mm ”Boss Style” switching DC input socket that takes a 9v, centre negative DC power adaptor. 

Power from the mains AC power supply (the electricity from the plug in the wall) WILL, AND WANTS, TO KILL YOU.  Never mess with anything that plugs into a wall socket unless you are 100% confident with the power adaptor, that you know what you are doing, and the circuit you are working on.  Absolutely NEVER mess with a device WHEN IT’S PLUGGED INTO the wall.  Please be careful, and always put your and others’ safety first.

 

Now… let’s take a look at the layout of the 2.1mm DC input socket:

Pinout for the 2.1mm DC power socket showing the lug numbering.
Pinout for the 2.1mm switching DC power (”Boss Style”) input socket, showing the lug numbering.
Top view of the 2.1mm DC power socket showing pinout
Top view photo of the 2.1mm switching DC power (”Boss Style) input socket, showing pinout
Side view of the 2.1mm DC power socket showing pinout
Side view photo of the 2.1mm switching DC power (”Boss Style) input socket, showing pinout

 

Lug 1 – Ground (All ground connections)

Lug 2 – Power (From battery)

Lug 3 – Power (To board)

 

BE VERY CAREFUL- Check, double check, and triple check  your soldering and the pinout of the socket before connecting a power adaptor, or you risk frying your board. 

When running on battery, lugs 2 & 3 are connected. When a power adaptor jack is plugged in, the connection between the lugs is broken, saving the battery, and power is only drawn from the adaptor via lug 3.

 

So here is how the mod went:

1. I opened up the Russian Big Muff:

Russian Pots!
Russian Pots!

 

2. I drilled a 12mm hole for the DC socket. The metal is steel and quite tough but I managed to get through it:

Fresh hole for the socket
Fresh hole for the socket

 

3. The old 9v battery snap was broken so I’m gonna use a new one:

Old battery snap not looking very healthy.
Old battery snap not looking very healthy.

 

4. I inserted the DC Socket:

DC Socket in place.
DC Socket in place.

 

5. I then soldered the black wire from the new battery snap and the black wire from the old battery snap (that was still connected to the board) to lug 1 on the socket:

N.B. if there wasn’t already a wire soldered to the board (from the old battery snap), I would have had to add one from lug 1 to a ground point on the board).

Using the old wire coming from the board meant I didn't have to touch the board (always a good thing!)
Using the old wire coming from the board meant I didn’t have to touch the board (always a good thing!)

 

6. Then I soldered the red wire from the new battery snap to lug 2 on the socket:

The red wire from the new battery snap.
The red wire from the new battery snap.

 

7. The red wire from the old battery snap was still connected to the board so I soldered it directly to lug 3 on the socket (I also added a small piece of insulating mounting tape to prevent the lugs accidentally touching the board):

This red wire in lug 3 will take power from the adaptor to the board
This red wire in lug 3 will take power from the adaptor to the board.

 

8. Finally I remounted the board, and added a fresh battery:

Pretty!
Pretty!

 

The enclosure has been screwed back together and a fresh battery added.
The enclosure has been screwed back together and a fresh battery added.

 

9. And here she is working away:

It's alive!
It’s alive!

 

– Thom Conaty

thom@maker.ie