• Marshall JMP1 footswitch

Maarshall JMP1 footswitch
The Marshall JMP1 preamp is MIDI controllable which I might have found more useful if I also had a MIDI effects processor. I borrowed a MIDI foot controller which worked fine for the channel switching but it turned out to be overkill. The controller allowed access to any patch in the JMP1 in banks of 5. I found I never used more than 5 patches in one night, and never switched banks. Remembering patch numbers was more than I was up for anyway.

The JMP1 also has a footswitch jack that allows access to 4 patches. The footswitch is hard-wired to patches 1, 2, 3, and 4, but it is simple to use the MIDI mapping and assign them to any other patch. Unfortunately, I couldn't find one of these footswitch units for less than about $130.

The switching done by the footswitch is all handled in the preamp. The footswitch plug is a 6 pin DIN with pin 6 supplying +5VDC, pin 2 being ground, and pins 1, 3, 4, and 5 being the patch selects. If you momentarily ground one of the patch selects, that patch becomes active. So making the switch is simple--4 SPST momentary pushbuttons, a DIN plug, case and cable. What is more difficult is making the LED indicators that show which channel has been selected.

Randall Aiken has plans for a 3 channel switch that has the digital logic to run displays for the 3 channels. He's taken it off his site, but given me permission to repost it. It would be possible to use a CD4073 3 input AND gate to get 4 channels. This chip packages 3 gates in a chip, so 2 chips are required to get the necessary 4 gates with 2 gates being unused. I approached it differently with the same outcome using 2 CD4081 chips. This chip puts 4 dual input AND gates in one package, and I needed 8 gates, so 2 chips filled the need. I don't know enough to talk about the benefits of doing it either way.

footswitch schematic

I modified Aiken's circuit as shown. Note that only part of the total circuit is shown here. The top 4 gates constitute one chip, the bottom 4 the other chip. The pin numbering for the lower chip is similar to the numbering for the upper one. Pin 14 is +V and pin 7 is ground.

The other modification is that the 47K pull-up resistors in AIken's design aren't needed for the JMP1. The JMP1 has 4K7 pull resistors internally. The 47K resistors should be omitted as the voltage sags below 5VDC if they are used, and while this didn't seem to be a problem, it might arise if the line voltage varied--I don't know.

Total cost for parts was about $25 plus shipping. I used a scrounged multi-wire length of phone wire that had 8 conductors--only 6 are needed.

Subpages (1): Aiken switch schematic