Yamaha CS Trigger Input Modification
Problem: Yamaha CS not triggering from an external Gate
Solution: small converter circuit
I had a Yamaha CS5 for some time, a neat little monophonic synth with one oscillator, one envelope, switchable HP/BP/LP filter, a simple LFO, white noise, and a single VCA. It has Control Voltage and Trigger input jacks round the back for interfacing with other devices.
The CS series uses a Hz/V (Hertz per Volt) CV, and the better modern MIDI-CV interfaces can handle this with no problem. The Trigger levels are comparatively awkward though, with ‘off’ being nominally +3 to +15V, and ‘on’ being nominally 0 to -10V. I say ‘nominally’, because the outputs of these CS synths are stated as +3V for off, -7V for on.
Why is this awkward? Well, there are two other common systems – Positive Gate (aka V-Trig), and Short to Ground (aka S-Trig), which I shall not discuss here – and whereas the other systems have been employed by several manufacturers, Yamaha was, and is, on its own with theirs. Though many CV interfaces are stated as being compatible with Yamaha CS synths, I have found this not to be reliably the case.
The problem comes when a Short to Ground signal will not trigger a Yamaha Gate. For whatever reason, some units just don’t provide a good enough trigger output to correctly pull down the inputs of some Yamaha CS triggers. I suspect a number of things, but won’t speculate here as I found an easy and practical solution.
I owned both a CS5 and CS15, which use very similar, but not identical, trigger input circuits. My Kenton Pro-2 MIDI-CV interface would trigger the 15, but not the 5.
The Pro-2 is an older model, and has been long superceded by better units, but at the time I wanted to get the Kenton and the CS5 working correctly. My solution was to build a small buffer board and install it in the Kenton, adding a separate Trigger Out jack on the Kenton specifically designed for Yamaha’s system.
It works very simply. The Kenton provides a +15 Positive Gate by default. Its own subsequent conversion to S-Trig being insufficient, I added to the V-Trig output a single op-amp with a few resistors to provide both offset and scaling of the signal, transforming it into the ‘correct’ +3/-7V, and routed the new Trigger output to its own ‘CS-Trig’ jack socket. The schematic can be found below in both JPEG and PDF formats.
The circuit can be built onto a small piece of stripboard; I used a TL072 as it’s what I had to hand, but almost any op-amp will do. Mine was powered from the dual +15/-15 supply rails in the Kenton, but you could equally well install it within your CS synth if desired – just pay attention to where in the circuit you install it. Perhaps add a second jack for this input if you wish to leave the original in place (for example, if you wish to run your badly-triggering CS from another CS). Another option would be to install a switch to select the type of Gate input being used. That’s up to you; I present only the basic circuit that converts one gate to another.
NB: actual output values are 3.74V for ‘off’ and -6.45V for ‘on’, but they are within tolerance and much closer to Yamaha spec than the regular S-Trig.
PDF version: CS Trig schematic
Here are a couple of photographs of the extra board in situ in the Kenton Pro-2. Note the angled PCB at the bottom left is Kenton’s own optional Hz/V CV board (from the factory the Pro-2 only provided V/Oct CV). My extra circuit is mounted on the small piece of stripboard at top left. It takes power from the Kenton’s 15V rails, and takes its trigger input from the Kenton’s V-Trig +15V Gate, and it outputs a near-Yamaha-spec +3/-7V off/on gate signal to a dedicated jack socket which I added myself. The unused half of the dual op-amp is not connected to anything other than 0V and itself, as indicated on the schematic. If you use a single or even quad op-amp in this circuit, re-arranging the pin-out is up to you.