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.
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.
Here’s the Korg MS-04 schematic as PDF download.
And here’s a quick video of it working:
There are three ways to get audio from your Werkstatt: the VCO direct out, the VCF direct out (both on the pin header), and the main audio out (the 1/4″ jack on the rear panel).
The VCO Out signal is a sawtooth or pulse, depending which wave the VCO is switched to, at 0-5V. This is pure, dry VCO with no further processing, though of course it will be pitch- and/or pulse-width-modulated, depending on your modulation routings. The VCF Out is taken directly from the output of the filter, bypassing the VCA, and is nominally -2V to +2V. The main audio out is at typical line level (a couple of volts peak to peak), and comes through the filter and VCA.
If you want to use the Werkstatt as an extra oscillator for a modular, for example, you’ll probably want to use the VCO direct out. If you’re running the filtered sound into an external VCA for more varied amplitude modulation or to use with a high-pass filter maybe, you will probably want to take the Werkstatt’s audio directly from the VCF out. If you’re using the Werkstatt as a standalone expander, the main audio out will do just fine.
If signal levels were the same all the way along, none of this would be a problem. However, as with other aspects of the Werkstatt’s design, it needs some tweaking to integrate perfectly. Here’s how.
VCO direct out
Let’s say you’re using the VCO direct out. Eurorack has typical VCO signals of 10 volts peak-to-peak (see Doepfer’s Signals in the A-100 section, for example), centred around 0V (that is, -5V to +5V). To get the closest match sonically we want the Werkstatt’s output to match the other oscillators you’re using. Not all modules with mixers on board will boost as well as cut their inputs, so we can add a small circuit to give a true -5V to +5V VCO Out on the Werkstatt. The schematic below shows both the VCO and VCF mods. More on the VCF shortly.
How It Works
The VCO out mod is a basic non-inverting amplifier with an offset to make the positive-only signal bipolar. The gain is set by the two 20k resistors (1+20k/20k = 1+1 = 2) and the unity-gain reference point is at 5V. That is, 5V in gives 5V out. 0V in would be a difference of -5V from this reference point, so this is multiplied by the gain of 2 to give a difference of -10V, which taken from the +5V reference gives -5V out. In this way, the 0-5V input becomes -5V to +5V out. You can see it in action at this link. Below is a screenshot of the simulation.
Likewise, if you want to run the Werkstatt’s VCF output into an external module, boosting the signal to match requires just a small circuit, almost identical to the first. The signal level drops as resonance is increased, but to keep our circuit simple we won’t worry about that. The schematic is on the same sheet as the VCO output, posted earlier on this page.
How It Works
This is also a non-inverting amplifier, but this time with no offset as the VCF signal is already bipolar – all the amplification happens around a 0V centre point. Positive signals get more positive, negative signals get more negative. The VCF direct out is normally about 5V peak to peak at maximum, so we just double that to get the more useful 10V range. The gain is set the same way as the previous circuit, and we get an output of -5 to +5V maximum.
Installing the mod
I built both these circuits onto a small piece of stripboard mounted onto the panel with one of the minijacks. There’s just enough room, as can be seen in the photos. This allows the use of both halves of a dual op-amp so nothing goes to waste. There’s also plenty of room on the experiment pads at the top of the Werkstatt’s PCB, though you may find it a bit cramped if you’ve already got a couple of mods in there like I have…
The photos show the locations on the PCB of the various supply rails you’ll be wiring up to: -9V and +9V to power the op-amp, +5V for the VCO amplifier reference, and GND. These are all labelled on the top side of the PCB anyway so it’s easy to find them. I shared the ground that my existing mods were already using, which is connected to the nearby screw post via a solder tag. See my CV mod for details.
U1: TL072 or equivalent
R1, R5: 10k (1/4W 1% Metal Film used here, but it’s not critical)
R2, 3, 6, 7: 20k
R4, 8: 1k
jacks, wire, stripboard: as per your own choice
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.
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:
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!
As a companion to my simple op-amp AR envelope circuit, here’s a discrete version. It has the same basic functionality – gated input, variable attack and release times – but is made with transistors instead of integrated circuits. Power consumption is very low (just a handful of mA), and it runs from a positive supply of your own choosing. Like its op-amp cousin, it could be powered with a battery, or in a Eurorack system, or you could add it into an existing synth like the Moog Werkstatt as a mod.
The main difference between this and the op-amp circuit, aside from it being discrete, is that I have included a very simple way to set the level of the envelope output (see below for details).
RV1 is Release, RV2 is Attack. The Gate input can be anything over a couple of volts. Negative-going inputs (eg., from a bipolar LFO) will be removed by D1. The output goes to nominally 0V when fully off (closer than the op-amp version, in fact).
R1, R2, R6, R7, R10: 100k
R3, R4: 47k
R5, R8: 560 Ohm
RV1, RV2: 1M linear pot
C1: 1µ non-polarized
D1, D2, D3: 1N4148 or equivalent
Q1, Q4, Q5: BC549C or equivalent
Q2, Q3: BC559C or equivalent
How it works
Compare the first pair of transistors with my discrete gate buffer circuit. A positive voltage on the input turns on Q1, taking the base of Q2 low. This turns on Q2, taking its collector high. This is how we drive our envelope.
Now compare the diode and potentiometer arrangement with my op-amp AR. Once you’re past the transistors, it works in basically the same way.
Q3 inverts the output of Q2, so when Q2 is on, Q3 is off, and vice versa. When the collector of Q2 is high, the capacitor charges through diode D3 and pot RV2 (Attack). When the gate input goes low, the transistors Q1-3 switch off, off, and on, respectively. In this state, the capacitor discharges through RV1 (Release) and D2.
Note the two 560 ohm resistors: one on the emitter of Q2, the other on the collector of Q3. When the gate input is high and the capacitor is charging, current flows through Q2’s emitter resistor; when the gate is off and the capacitor is discharging, current flows through Q3’s collector resistor. These two resistors put a lid on the current flow and limit the fastest times for Attack and Release. The value is a trade-off between current and snappiness. With the values shown, maximum current through these resistors is around 16mA and the fastest rise and fall times of the envelope are around 2ms.
The final two transistors in the circuit after the capacitor are the output buffer; notice the two resistors between them, forming a potential divider. With the values shown, if you run this circuit on 12V, the envelope output will be around 8V max.
There are better ways to set the peak level of an envelope, but my aim here is to keep things simple as a base for experiment.
The most obvious things to tweak are the envelope times and the output level.
The values of the two potentiometers affect the attack and release times, but the envelope can be substantially stretched by using a larger capacitor. It would be easy to add a switch that connected, say, a 4.7µF or 10µF capacitor in parallel with the existing one, which would multiply the envelope’s times substantially (use perhaps a 25V electrolytic, with its -ve terminal to ground).
The two resistors between the output buffer transistors can be adjusted to suit your requirements. If you want full-scale output (ie., envelope peak closer to the supply voltage), remove R9 and R10, and connect the emitter of Q4 directly to the base of Q5. In fact, this circuit will also work with just a single NPN as a buffer (miss out Q4 and the divider resistors, connect the cap to the base of Q5), but amongst other things the ‘zero’ value is less close to actual zero; if you want to experiment with a single transistor here, setting the level of the output can be done by replacing the 10k resistor on its emitter with a pair of resistors as a potential divider, or even a 10k trimmer with the output taken from the wiper.
Feel free to experiment with the circuit in Falstad’s handy online simulator.
By default, the Boss DR-55 does not receive any kind of incoming clock. The ‘FS’ footswitch input takes a latching footswitch that starts and stops the existing clock, but that’s it. Although you can clock other equipment from the DR-55, it would be nice to be able to use an external clock to sync the Boss to, which would allow the Boss to trigger yet more devices with its CSQ and DBS outputs (active on Accented steps only and every step, respectively).
My mod as detailed here does exactly that. By replacing the existing FS jack socket, adding a small circuit, and replacing a jumper, we can safely trigger the DR-55 from an external trigger.
A quick internet search will turn up an existing clock input mod which is simpler to do and requires no extra parts; however, it puts the RAM at risk of damage from high triggers, and it does not sync the Boss’ own DBS output. It also requires ‘arming’ by hitting start before external triggering.
My own mod, though more complex, overcomes all these issues: the trigger input is protected, both the Boss’ trigger outputs maintain their correct functions, and triggering occurs without ‘arming’. The only two functional disadvantages of my mod are that you must set the Boss’ tempo to Fast, and to reset the pattern when stopped mid-way you need to remove the trigger plug.
I’m going to blog another small mod which will overcome the latter inconvenience [EDIT: No I’m not! I sold both my 55s, thereby halting this particular project].
The Clock Modification in detail
Below is a diagram which shows everything you need to know about building this mod. Below that is a parts list. Key to this is the replacement FS jack socket; it needs to be TRS (ie. a stereo jack), with single pole changeover switches on the tip and ring contacts. I used a Lumberg KLBPSS3 (datasheet here, Farnell UK stock page here).
The additional circuit can be made very small indeed (3 rows * 8 holes on stripboard), and there is plenty of room for it inside the DR-55, particularly towards the right-hand end. The photos below illustrate my own placement.
There is one jumper to be removed, the one immediately to the right of the Variation switch. The replacement connections for the upper and lower point of this removed jumper are shown in the diagram, and you can see in the photos how I wired this up.
In brief: remove that jumper, solder the two points to two jack pins; build the extra circuit, and solder that to the jack and to the main PCB; replicate two of the pre-existing connections from the jack to the PCB. That’s it. I also stuck a small folded piece of card to the PCB to stop the extra circuit from shorting against components.
The image above is a jpeg; click here for a PDF: Boss DR-55 clock input mod revised
1 * TRS 2-pole changeover jack socket – eg. Lumberg KLBPSS3
2 * 47k resistors – I used 1/8W for their smallness
1 * 10nF capacitor – I used a ceramic, again for smallness, but polyester film etc. would be usual
1 * 1N4148 signal diode or equivalent
1 * BC549C transistor or similar standard NPN
Here’s the modified DR-55 (also incorporating my DC supply mod):
And here’s a close-up of the clock mod:
Here are two angles to show the extra circuit more closely:
How to use your new trigger input
The new trigger input will accept any positive pulse over a couple of volts. It’s edge triggered, so the pulse can be any length over a couple of milliseconds. The operating principle is to use the DR-55’s existing clock, but to gate it on for a very short duration; normally when the clock is gated off again, the pattern resets, but the new jack socket enables us to disable that by breaking the reset connection when a jack is inserted.
As I mentioned earlier, the Tempo must be set to Fast (ie. all the way clockwise) for correct function. This is because the DR-55’s clock, once triggered, finishes its pulse cycle. If this is longer than the incoming trigger cycle, it will ignore the new trigger; if we set the speed dial to its fastest, we can clock the DR-55 at any rate up to its natural maximum.
The pattern will cycle round as usual, but if you stop mid-pattern, new triggers will continue where they left off. To reset the pattern at this stage, you need to unplug the trigger jack and hit Stop. This is not ideal, I know
, and I will be making an amendment to correct this later [EDIT: project halted, see above. I have no current plans to do any further work on the DR-55].
For now though, this mod works fine, as shown in the (slightly rubbish) video below:
One of the drawbacks of the DR-55 as it comes unmodded is the power supply. In its original form, the DR-55 takes only batteries, and though this might be good for reducing cable clutter and having to find yet another wall-wart, it does mean you need to keep a regular stock of fresh AAs, and can guarantee that just when you want to use it, your DR-55’s batteries are too drained for the unit to function correctly.
Luckily, it is a relatively simple process to modify the DR-55 so that it takes a commonly-found 9V DC supply instead. I provide instructions for this below. It’s not the only way to do the job, but this is how I did it, and it works just fine. Modding the DR-55 in this way means it no longer accepts batteries, which means two things: 1) you will need access to a 9V adapter, and 2) pattern data will not be retained on power-off. Given that filling the memory of this humble machine can be done in less than five minutes, and I never use this outside my own home studio, I never found memory retention to be an issue. It would be possible to design a DC input that also catered for memory backup via battery, but I’m not going there.
There are two basic stages to this modification:
- Making a 9V DC input: the basic voltage supply circuit
- Installing the Mod: adapt some wire links on the output jack and PCB
Making a 9V DC input
Because the DR-55’s RAM can be killed by voltages higher than around 7V, we take a 9V input and regulate it down to between 5V and 6V. I chose to use a 5V regulator propped up with a diode to give around 5.6V, but you could also use a 6V regulator and omit D2. The input jack I used is a 3.5mm mono minijack of the kind often used for audio and CV interconnects, mainly because I had lots of them and the holes are easier to drill than the larger ones needed for a plastic-bodied insulated barrel connector. Use whatever type you prefer, but note the polarity of your incoming DC, and don’t connect the +ve to the case… with a tip-positive 3.5mm jack, the sleeve of the input jack is connected to the shell of the socket, so it makes sense for that to be the ground. Some barrel connectors do likewise.
Here’s the schematic:
Here’s the final circuit built onto stripboard. It will be panel-mounted using the socket:
Installing the Mod
Now we have a simple DC input, we could just solder the +ve and Gnd outputs to the corresponding solder points on the main board – that is, where the battery clip attaches. Black is ground, red is positive. This works, but you still need to insert an audio cable to turn the DR-55 on. I chose to remove that ‘feature’, as there are no longer any batteries to protect from accidental drain. It’s a simple mod that just means a couple of wiring changes.
The diagram below shows the required re-wiring. The audio output socket is wired by default to both ground and audio signal, as well as having two pins wired to act as a switch when a jack is insterted. We want to retain the audio and ground connections, but not the switch. We remove those wires and instead bridge the corresponding points on the PCB.
Here’s a photograph of the full mod (note the wiring):
I damaged a track while desoldering the battery wires, which is why the red wire goes to the un-numbered hole next to point 9. They’re directly connected, happily.
Below are some photos of the hole I drilled for mounting the new DC input, and the final appearance when mounted and labelled with cheap Dymo (should have gone with black… oh well):
So there you have it. My humble DR-55 now works from a regular 9V DC wall-wart supply, and switches on whether or not its audio is connected. The hardest part is putting the DR-55 back together again…