Tag Archive | CV

Korg MS-04 Modulation Pedal

The Korg MS-04 accessory was contemporary with the MS-series analogue monosynths in the late ’70s and early ’80s. It looks like a standard volume pedal, and weighs in around 1kg owing to its sturdy metal construction. Essentially it’s a bender pedal that provides a variable voltage to control your synthesizer. It also provides an LFO with triangle and positive-only square waveforms, and a random output that is sampled at the LFO rate. The range of the LFO goes from around 1s/cycle at the slow end to 70ms/cycle at the fast end (around 1Hz to 14Hz). It adds a glissando feature, which puts the bender pedal through the sample-and-hold instead. It has two outputs, which can be switched on and off, and which can output either the LFO alone, the pedal voltage alone, or a mix of both. One output runs at ±1.2V peak, the other at about ±5V. An LED indicates the LFO rate. It is powered by two 9V PP3-style batteries.

Inside, there is a small PCB with a few dozen components on, and a whole lot of wiring connecting the panel-mounted pots, switches, and jacks. Some components are soldered directly to the panel parts, and there are multiple wires of the same colour that don’t always run to the same points, so trouble-shooting is slightly messy. Luckily, it’s a simple enough circuit, and the spaghetti wiring is the only thing that need cause any headaches here.

Inside the Korg MS-04 modulation pedal. So many wires!

Inside the Korg MS-04 modulation pedal. So many wires!

The only schematic I could find online was blurry and hard to read, and lacks component numbering. I did my best to clear it up, and added the missing information. Unfortunately, half way through, my software crashed and my only saved file was of lower quality than I would have liked – hence the soft text over about half the image. I’d already put plenty of time in by this point, so it’ll have to do!

My additions are self-explanatory, I think. Parts that are not numbered are soldered directly onto panel hardware. The only missing values are the diodes – D2 to D5 are plain old signal diodes, D1 is a zener that I failed to make note of during repair. Sorry.

There seems to have been a change in some component values during production. These are labelled with ‘1’ and ‘2’ in black squares. The unit I took my details from had the lower value resistors and larger capacitor at points ‘2’.

There is no protection against only one battery being installed. The unit is switched on by the insertion of a cable into either output jack, so it is advised to remove the connections before fitting/replacing the batteries.

Korg MS-04 schematic, enhanced with component designations

Korg MS-04 schematic, enhanced with component designations (JPG)

Here’s the Korg MS-04 schematic as PDF download.

And here’s a quick video of it working:

Moog Werkstatt: adding a VCA CV input jack

I’ve already blogged about the reasons you might want to mod your Werkstatt, and have posted a list of mods here, so to complete the VCO/VCF/VCA trio, here’s the VCA input at last!

How it Works

This is a very simple mod, and just replicates the existing patch header input. All you need is a 10k resistor, some wire, and a jack socket.

Werkstatt VCA CV input mod schematic

Werkstatt VCA CV input mod schematic

The easiest point to solder to on the board is JMP62. This is located just above the Decay pot. This point is where incoming VCA CV signals are passively mixed. It’s probably best to solder the new parts to this jumper on the underside of the PCB, which you can see on the photos of the wiring:

Werkstatt VCA input JMP62 location

Werkstatt VCA input: JMP62 location

Werkstatt VCA input JMP62 on underside of PCB

Werkstatt VCA input: JMP62 on underside of PCB

Werkstatt VCA input soldering

Werkstatt VCA input: soldering

Werkstatt VCA input panel

Werkstatt VCA input: side panel


10k resistor
Jack socket

Further thoughts

As the Werkstatt already has a pin for VCA modulation on its patch header, you might think this mod seems less immediately useful than the VCO or VCF CV inputs. The main issue is that there is no way to simply switch off the Werkstatt’s own VCA – you can select EG shaping, or ‘on’ for drones, but you can’t bypass it for use with an external envelope. What does this mean in practice, and why bother adding a CV input for it?

If you’re using the Werkstsatt with a modular, you’ll have more interesting envelopes than the attack-decay type on the Werkstatt. You might well also have something that will give an offset. It should be possible to use a negative offset to counteract the Werkstatt’s ‘on’ VCA CV, and mix in a more interesting envelope signal; setting the Werkstatt’s VCA to ‘on’ and feeding this CV input with that mix will then allow you to contour the Werkstatt’s VCA as you like.

It would also allow you to use the Werkstatt’s own patchbay to feed the VCA with its LFO, and simultaneously blend another LFO in with this CV mod. Mixing different LFOs gives a lot of movement to a sound, and can be very enjoyable to play with. You can also sequence the VCA level this way, while retaining other modulation via the pin header.

As always, my mods are not definitive – the best thing to do is experiment, and adapt, and find out what works for you and this excellent little synthesizer. Enjoy!

Synth DIY: Gate Buffer

One of the simplest DIY utility circuits you can build is a gate buffer: you put a gate signal into one end, and get a gate signal out of the other.

Although this might sound unnecessary, there are several reasons you might want a gate buffer:

  • compatibility problems between gate/trigger inputs and outputs on different equipment: see my page on the Arturia Beatstep, for example
  • the need to trigger multiple devices from one source: passive splitter cables or mults sometimes result in signal loss and therefore unreliable triggering
  • tightening up the edges of gates/triggers: for various technical reasons, some trigger outputs are relatively slow to rise and/or fall; in a worst-case scenario, this can skew the timing of down-line devices. A buffer with multiple outputs can deliver a set of tight, sharp pulses simultaneously.

I offer two simple designs here, one using discrete components, the other using an op amp. Both require just a handful of parts, both will run off a wide range of DC supply, including a 9V battery, and both can be made very compact if you ever want to include them inside another piece of equipment as part of a build or mod.


Discrete (transistor) Buffer

The transistor buffer is a two-stage circuit, with each stage inverting the incoming signal.

Think of a gate signal as a logic on, or a logic off. When there is no gate present, the first transistor is held off by its base resistor. The base of the second transistor is therefore tied to +V by the two 47k resistors; as it is a PNP type, it is therefore off, and the output is held low.

Conversely, when the input is high, the first transistor is switched on, and the base of the second transistor is taken low. This pushes the second transistor into conduction, and the output is taken high.

Precise voltage levels depend upon the level of the gate signal going in, and the positive supply rail. The circuit will operate on a wide range of positive DC supply: in a 5V logic circuit, from a 9V battery, a 12V or 15V rail in a Eurorack system, etc. The input resistors and diode provide input protection; so, for example, you can send a bipolar square LFO into it with no ill effects, or use it to make a reliable 9V gate from a 15V one without the impedance issues of a simple passive potential divider. It will also allow you to increase a low gate to a high one, so you could (for example) run a 5V signal into this, powered on an existing 15V rail, and get a 15V gate out. With a standard signal diode and two normal low-power transistors, you can trigger this circuit with just a couple of volts.

Gate Buffer using Transistors

Gate Buffer: Transistor version


Op Amp Buffer

The op amp version of this gate buffer circuit consists of a single op amp stage set up as a comparator: one voltage is compared to another, and the output goes high or low depending which input is the higher.

The potential divider at the inverting input provides our reference voltage. The non-inverting input takes the external gate signal we want to buffer.

When there is no gate signal, or it is low, the inverting input is higher, and the output is therefore low. When the gate signal is high, the non-inverting input is higher, and the output is high.

Gate buffer: op amp version

Gate buffer: op amp version

The circuit is designed to run from a single-sided supply, ie. ground and positive. For this purpose, an op amp such as the LM158/358/324 (single, dual, and quad versions respectively) is suitable as the low output state goes to the 0V rail. Their high output state is around 1.5V below positive supply.

The voltage reference provided by the potential divider at the inverting input should be adjusted for purpose: using a 9V supply, the values given will trigger the comparator at around 1.6V; even with a low battery, this circuit should trigger around 1.2V. With a 12V or 15V supply, replace the 18k resistor with something in the region of 10k-15k. This would keep the trigger level around 2V or a little lower, which is high enough to be a clear ‘on’ signal, but not so low as to be confused with a slightly high ‘off’ signal (the Arturia Beatstep ‘off’ gate signal hovers around 0.6V, for example).

It would be possible to use a dual-rail op amp just as well, which would require the addition of a diode on the output to clip the negative-going signal.

I have used an op amp here rather than a dedicated comparator; devices such as the 311 cannot be directly substituted in this circuit.

Moog Werkstatt: adding a proper Filter CV input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

After the VCO Frequency CV and Gate inputs, perhaps the next most useful control we can modify is the VCF cutoff frequency. The Werkstatt already has switches to select either LFO or EG filter modulation in positive or negative amounts. Many synthesizers also have a Keyboard Tracking control which routes the CV generated by the pitch control source to the filter cutoff, allowing the filter to open up as higher notes are played. The amount of this modulation is often governed by a pot — giving continuous variable control — but is also often implemented with a switch — giving either preset amounts of modulation, or at its most basic just on/off (that is, 100% or nothing). At 100% Keyboard Tracking, a self-resonant filter can be used as a sine wave oscillator, the pitch of which will follow the keyboard.

Various filter tracking controls

Various filter tracking controls


The Werkstatt’s filter has a CV input on the header, which is fine for simple self-patching, but two problems show themselves when you want to control this parameter from an external source: firstly, the necessity of hacking a cable together as described previously; secondly, the accuracy of tracking. The Werkstatt’s existing filter CV input point does not, in my experience, give accurate 100% tracking from an external V/oct CV, which spoils sounds that require the resonance to boost harmonics that are locked to note pitch.

The mod below overcomes these problems by giving the Werkstatt a separate, tunable Filter CV Input that can be trimmed to give suitably accurate pitch tracking.

How it Works

As with the Pitch CV Input mod, we’re going to simply duplicate the existing control input and make a slight alteration. The existing header input for cutoff control mixes its CV via a 47.5k resistor. In order to be able to give tunable tracking, this mod is going to use a 43k resistor and a 10k variable trimmer in series. 100% tracking should be somewhere towards one end of the trimmer’s range.

Werkstatt filter CV mod schematic

Werkstatt filter CV mod schematic


Solder the two extra components to the board, take the third leg of the trimmer to TP17 (purple wire in the photos below), and take the outer leg of the resistor to the input jack, which is mounted and grounded exactly as for the CV and Gate jacks.


Werkstatt filter CV input mod PCB top

Werkstatt filter CV input mod PCB top


Werkstatt filter CV input mod PCB rear

Werkstatt filter CV input mod PCB rear


Werkstatt filter CV input mod rear wiring

Werkstatt filter CV input mod rear wiring


Werkstatt filter CV input mod jacks

Werkstatt filter CV input mod jacks


Fine Tuning

Tuning the tracking is similar to tuning the pitch (a process described in the manual) — with the Werkstatt open, connect the external CV and play as normal, using full resonance on the filter, with the cutoff tuned so you can hear the pitch of its self-oscillation. Adjust the trimmer so the filter’s resonant frequency scales up the keyboard at the same rate as the note pitch — that is, two notes played an octave apart should give a resonant filter peak an octave apart.

Because the Werkstatt’s VCO cannot be silenced without modification, it might be easier to disconnect the pitch CV control while tuning the filter; alternatively, if you have a way of multing the pitch CV, connect it to both pitch and filter. You might try setting the filter to resonate at an obvious harmonic such as a 5th above the pitch, as any deviation in the tracking will result in some noticeable sonic artefacts.

More Ideas

Of course, you might not want a simple fixed 100% tracking filter. It would be possible to add a pot to allow the user to vary the tracking amount; you could install a switch to select between different resistors to give preset fixed trackings; you could route the pitch CV to a break-contact on the filter CV input jack so that it tracks by default unless a jack in insterted to over-ride it. My own mod is simple and quick and functional, and hopefully will provide a point of departure for your own experimentation.


Werkstatt filter CV input mod in use

Werkstatt filter CV input mod in use


Parts used:

43k 1/4W 1% MF resistor
10k trimmer
1/8″ panel mount jack socket

Moog Werkstatt: adding a proper Gate input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

In its original form, the Werkstatt’s own keyboard generates the Gate signal to trigger the envelope, and there is no obvious ‘Gate Input’ on the header. The existing Gate Out can be (ab?)used as a Gate In, but it’s not ideal, because as with most of these header points, anything coming in here isn’t buffered from the internal signal.

Adding a proper Gate In to the Werkstatt is straightforward enough, though a little more involved than the CV input; my approach doesn’t require the cutting of any traces, the only hack-work being the hole in the enclosure for a jack socket. It does require the end of one wire to be soldered to rather small SMT (surface-mount) components though, so you’ll need a suitably fine tip for your iron and a steady hand.

Werkstatt gate mod schematic

Werkstatt Gate Input mod schematic


How it Works

The Werkstatt’s keyboard scanner outputs a logic high at U19 pin 3 when it detects a key press. As well as stopping the scan and loading the current key value into a latch (which feeds the VCO CV), this signal is buffered to provide a Gate, and differentiated to provide a Trigger. The Key On signal is buffered inversely by the Schmitt trigger of U14-F before being flipped back positive by U14-D. In order to add our external gate without affecting any other part of the keyboard circuit, we only need to bring the input of U14-D low. In this way, we can use both the Werkstatt’s own keyboard and an external Gate without having to switch between control sources.

The solution is to use a simple NPN in saturation to take U14 pin 9 to ground when its base is taken high. In other words, a positive external Gate will take the gate inverter input low, just as does the keyboard gate detector. Because there is a diode in the way (D14), our added transistor is isolated from the keyboard scanner clock and data-bus, so there won’t be any accidental mis-readings of the keyboard CV.

Another advantage of this solution is that the Werkstatt’s own envelope retains its Gate/Trigger operating modes, as our external Gate also gets differentiated; we are activating the Werkstatt’s envelope, not over-riding it.

The modification takes just four components and a socket, and fits easily on the PCB. The hardest part is soldering the wire from the collector of the transistor to the appropriate point on the Werkstatt’s circuit – I chose to solder it across the connection between R89 and C64, as the two solder points make a convenient place to lay a thin wire and give it a firmer purchase.

I presume you’ll be doing both CV and Gate input mods; the socket ground can be wired to the CV In socket ground, which I wired to a solder tag around the nearby PCB mounting screw (see also the CV Input page).

Werkstatt gate input mod smt solder point

Werkstatt Gate Input mod SMT solder point


Werkstatt gate mod extra components highlighted

Werkstatt Gate Input mod extra components highlighted (PCB top)


Werkstatt gate mod PCB rear highlighted

Werkstatt Gate Input mod extra components highlighted (PCB rear)


Parts Used:

33k 1/4W 1% MF resistor
100k 1/4W 1% MF resistor
1N4148 signal diode
BC549C NPN transistor
1/8” panel mount socket

These parts are what I had handy. Pretty much any NPN with reasonable gain can be used here, and the signal diode is a generic one.

Moog Werkstatt: adding a proper CV input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

The existing header on the Werkstatt allows for a VCO pitch CV to be patched in. Although the pitch can already be modulated by either the LFO or the EG (selected using a panel switch), the patch header input means you can use both modulation sources simultaneously – or an external CV, if you can cable it up.

When you start wanting to connect control sources to the Werkstatt, one problem is pretty obvious: the patch pin header provides a signal path, but there’s no ground. The user manual suggests hacking cables together, taking a ground from the cable to a screw on the case (or the ground on the audio output jack), but this isn’t a very neat solution. Better to add a proper CV input jack so you can directly and simply hook up your external CV source using standard cables.

Moog themselves (at the time of writing) do sell an add-on jack board, which provides both a row of minijacks and a signal ground, but I decided against buying it for two reasons: 1) it still doesn’t offer a true Gate input, which I felt necessary; 2) the jack board replaces the patch pin header – adding mods like mine means you can use them and the patch pins simultaneously, giving more possibilities.

How it Works

The circuit is very simple. Looking at p.2 of the official schematic, we can see the existing header CV input is mixed in via a resistor R46 and trimmer VR5. This trimmer can be carefully adjusted to give a 1V/octave response for your external CV.

It would be super-easy to simply wire a jack to the CV point on the header, but this has the disadvantage that inputs are not isolated from each other. Better (and still easy) is to replicate the two passive components and route them to the same mix point.

Here are my additions to the circuit:

Werkstatt CV modification schematic

Werkstatt CV modification schematic

Here is the mod in situ:

Werkstatt CV input mod (top)

Werkstatt CV input mod (top)

Werkstatt CV input mod, rear

Werkstatt CV input mod (rear)

The handiest solder points for connecting the extra components to the existing circuit are TP14 and TP10. Either will do:

Werkstatt CV input mod routing

Werkstatt CV input mod routing

The jack is wired to be brought to the side panel beneath the header. In this photo the Gate mod jack is also in place. The jack grounds are wired together, and then to a solder tag that connects to the nearby screw post. The existing screw is long enough to accommodate a washer or two:

Werkstatt mod ground point

Werkstatt mod ground point

Drilling the hole in the case is simple and quick, and a label finishes the job:

Werkstatt with CV and Gate mods

Werkstatt with CV and Gate mods

The accompanying Gate Input mod is also detailed on this site.


Parts Used

68k 1/4W 1% MF resistor
100k trimmer
1/8” panel mount socket
3mm solder tag
3mm washers (x2)

Moog Rogue: adding a Filter CV input

The Rogue in its original form includes keyboard CV and Gate inputs and outputs. It’s a slightly odd arrangement of TRS (stereo) jacks – one for the CV, with the input on one terminal and the output on another, plus signal ground, while the Gate connector has an even more awkward arrangement of short-to-ground on one terminal and positive trigger on another, either of which can act as input or output. It’s not great. But it does work, providing you have the right cables.

Rather than split these into separate jacks, which would be a handy mod, I just use custom made cables for my interconnects between CV devices. However, the Rogue lacks a CV input for the filter, which I thought might make a useful addition. It’s a simple enough job.

I decided to use a 3.5mm minijack for the sake of convenience. Pick a spot on the rear panel and drill a suitable hole:

photo showing a newly drilled hole for the Moog Rogue filter CV input modification

Freshly drilled hole for the Filter CV input jack


photo showing a newly drilled hole for the Moog Rogue filter CV input modification (inner)

Inner shot of the Filter CV input jack hole

I had a switched 3.5mm mono socket to hand, so used that:

photo showing the Moog Rogue Filter CV input jack (inner)

New Filter CV input jack (inner)


photo showing the Moog Rogue Filter CV input jack (outer)

The new Filter CV input jack from outside

Solder a ground wire from the appropriate jack terminal to the Rogue’s jack PCB. Its large solder plane is suitable and easy to work with:

photo showing the ground wire of the new Moog Rogue Filter CV input jack

Grounding the Filter CV input jack

The filter CV input requires a 45k3 resistor to give the same scaling as the keyboard. As presented here, there is no onboard scaling, so if you want to run the input as anything other than 100% follow, use an external amplifier/attentuator. It would also be possible to build such a circuit into the Rogue, but I chose not to for simplicity.

photo showing a resistor soldered to the Moog Rogue Filter CV input jack

Solder a 45k3 resistor to the Filter CV input jack

The other end of that blue wire goes to the filter CV node on the main PCB. Rather than outline it on the schematic, I’ll just show you where that is on the board. This is the point where various filter CV inputs are summed. It’s an easy job to solder a wire at the top of this area:

photo showing the Moog Rogue Filter CV summing node

Filter CV summing node

Here’s a wider shot. Note the longer black wire is a previous bodge and not related to these mods and repairs:

photo showing the Moog Rogue Filter CV summing node and input mod wiring

Filter CV summing node and input jack wiring

I used a Dymo labeller to add the finishing touch to the back panel:

photo showing Moog Rogue Filter CV input jack mod

Filter CV input jack mod, finished!

Hey presto! A filter CV input scaled at the same degree as the keyboard. I use a Kenton MIDI-CV interface, which has an auxiliary output for filter CVs, amongst other things, so this little mod should come in quite handy.

Moog Rogue: open wide

Opening the Rogue is easy enough. There are five screws to undo on the base and back, and two on the front. It doesn’t matter what order you work in, as long as care is taken while moving the Rogue when it is only partially screwed together, so as to avoid spraining or cracking the screw points. Four screws are located at the corners of the base plate, as outlined in red on the photo below. One screw is located at the bottom of the rear panel, also outlined in red on the photo. Two screws are located on the front, one at each end of the keyboard. The second photo shows one of these partially undone.

photo of the screw points on the base of a Moog rogue

Moog Rogue screw points on baseplate

photo of screw points on front of Moog Rogue

Moog Rogue screw points on front of keyboard

Turn the Rogue upright and lift the lid. The third photo shows my previously-modified Rogue open like this; note the wiring at the left that connects the keyboard to the upper part. This is present in an unmodified Rogue. Be careful not to strain this.

photo of Moog Rogue open for work

Moog Rogue gut shot

To remove the main PCB from the top part, firstly remove all the knobs from the rotary controls and sliders. They should just pull off with little difficulty. The main PCB is attached to the top section by mean of three screws at the front, and a clip at the rear, which is itself attached to the body via three more screws. It is a simple matter to unscrew these in whichever order you feel works best. The clip simply slides off the PCB. The photos below highlight the locations of these screws from inside and out. Note that in the photos I have also removed the jack PCB. This is attached to the rear panel by the nuts around the jack sockets. Simply unscrew them and the PCB pulls away.

photo of Moog Rogue internal screw points

Moog Rogue internal screw points

photo of the location of Moog Rogue screw points for the main PCB, external

Moog Rogue screw points, main PCB, external

The final photo shows the Rogue open with the main PCB and jack PCB removed from the panel.

photo of a Moog Rogue open with PCBs removed for work

Moog Rogue open, with PCBs removed for work

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.


Schematic for a V-Trig to Yamaha CS-Trig converter

Schematic for a V-Trig to Yamaha CS-Trig converter

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.

V-Trig to CS-Trig convertor installed in Kenton Pro-2

V-Trig to CS-Trig convertor installed in Kenton Pro-2

V-Trig to CS-Trig convertor installed in Kenton Pro-2, detail

V-Trig to CS-Trig convertor installed in Kenton Pro-2, detail


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