Trigger Cable for Moog Synthesizer

This week I got a modular sequencer, which is normally part of a modular synthesizer. I have a MiniMoog Voyager synthesizer that contains many of the components a modular synth would normally include, so I wanted to connect the two of them up.

Analog synthesizers need two connections to be played by a sequencer: a voltage that specifies the pitch of the note to be played, and a trigger/gate that tells it when a key is pressed and released. On the trigger/gate, Moog uses a different system from most other synthesizers.

The others use 0 volts for key released and 5 volts for key pressed down, which is called V-Trig (voltage trigger.) Moog's designers wanted people to be able to plug in a simple foot switch to trigger sounds, so instead of incoming voltages, they expect the two contacts to be shorted for key pressed, and not shorted for key released. This is called S-Trig (shorting trigger.)

This article tells you how to create a cable that will allow a non-Moog synth or sequencer to trigger a Moog synthesizer, or even a waveform from a Moog. It uses the 1/4" phone plug used by all modern Moog synths, including the MiniMoog Voyager, the Little Phatty, the Sub Phatty, the Sub 37 and their new modular synths. (Classic Moog synths use a different kind of plug.)

The schematic above shows the circuit recommended by Moog. And you can get the parts you need from Radio Shack. (Yes, Radio Shack is still in business and still selling these parts. They just have fewer stores.)

In addition to normal audio cable, a 1/8 watt 10k resistor and a standard 1/4" phone plug, I recommend you get these items from Radio Shack. The oversize plug from RS is better for this than other ones I've seen, and they have the exact transistor and diode recommended by Moog.

P/N Description
276-1617 NPN Transistors
276-1122 Switching diode
274-1539 Oversize 1/4" phone plug

If you need the audio cable, resistor and the standard 1/4" phone plug, you can get them at Radio Shack too. These are the part numbers:

P/N Description
278-0513 Audio cable
271-006 1/8 watt 10k ohm resistors
274-1536 Standard 1/4" phone plug

I recommend you also get a short piece of heat shrink tubing with an opening just big enough to fit over the 1/8 watt resistor. Once you have all the parts, you are ready to begin the assembly

Cut the cable to the length you want. It is not critical to make the cable short. I used some green guitar cable I had because the color will help me to remember it's not a normal cable.

Strip one end of the audio cable and solder the shield of the cable to the ring of the plug. Solder the insulated inner wire of the cable to the tip of the plug. Screw on the cover of the plug. It is a good practice at this point to test with a meter to ensure the tip and ring are not shorted together. This plug will connect to the non-Moog trigger output.

Select a 2N3904 from the transistors in the package from Radio Shack. There are 3 different kinds of transistors in the package, so ensure you have the right kind. A magnifying glass may help to see what is written on the parts.

Place the transistor between the two solder paddles, with the pins pointing away from the plug. Pass the transistor's "collector" lead (on the left as you look at the writing) through the hole in the plug's tip paddle. Pass the transistor's "emitter" lead (on the right as you look at the writing) through the hole in the plug's ring paddle.

Strip the free end of the cable. Make sure to pass the plug cover onto the cable, so it can be screwed on later. Run the shield braid from the cable through the hole in the plug's ring paddle. Get one of the 1N4148 diodes. Run the end without a black stripe through the plug's ring paddle.

Bend the middle lead of the transistor up. Make sure none of the leads from the transistor are touching any of the other transistor leads. Solder the leads going through the plug's paddles to the paddles.

Solder a 1/8 watt 10k resistor to the cable's insulated inner wire. Either end of the resistor can be used.

Slip the short piece of heat shrink tubing (clear in the photo) over the resistor and its connection to the cable.

Twist together the middle lead of the transistor, the unsoldered end of the resistor and the unsoldered end of the diode, making sure not let the leads touch either of the plug's paddles. Solder the twisted leads together. Clip off the excess and bend them to allow the plug's cover to be screwed onto the plug, but be careful not to cause the twisted leads to touch the paddles or other leads.

You can test that the cable works by connecting the normal 1/4" plug's tip to +5 volts and the ring to ground. When connected this way, a meter's diode/short tester should beep when the black lead is connected to the oversize 1/4" plug's ring and the red lead is connected to the oversize plug's tip. When +5 volts is disconnected, the meter should not beep.

I found that, sometimes, when I wiggled my trigger cable, it would be intermittent. So I took the cover off of the trigger end, and filled the space between the paddles with hot glue, to prevent the components from moving. This has made the cable very reliable.

Your trigger cable is ready to use! Remember that the oversize 1/4" plug connects to the Moog trigger in, and the normal 1/4" plug connects to the voltage trigger output.

You can see it here in my setup. I'm using an LFO output to trigger the synth.