Synth DIY: Stompbox Mixer

Recently I wanted to use two different effect pedals in parallel, but didn’t have anything handy that would easily allow me to split and then re-combine the signals. So I designed and built one! It’s a very simple device consisting of a passive multiple and a 3-into-1 audio mixer with input level pots and a single output.

Photo of DIY 9 volt Stompbox Mixer in use

The mixer circuit uses a single transistor and runs from 9V DC, so you can power it from the same supply you use for your pedals. It draws only a few milliamps. The multiple is entirely optional – it’s purely passive and is just 4 jacks tied together, but it’s a useful addition and you could fit both this and the mixer into the same enclosure.

The Circuit

DIY Stompbox Mixer SchematicHere’s the schematic. It can also be downloaded as a PDF here.

How it Works

This is a very simple single-transistor design that uses a generic NPN device. The circuit is a ‘common emitter’ type (a basic description can be found here). Signals are presented to the base, and the output is taken from the collector. In order for the output to be able to swing up and down (audio signals are AC, don’t forget), the collector needs to sit somewhere a little above half way up the supply when nothing is happening. Given that we’re running this from 9V, it’s only really suited to relatively low audio signals, but we still have enough headroom for a small number of mixed inputs. I had no trouble mixing three audio test signals.

I won’t go into much detail about all aspects of the design process here, but the core is the transistor Q1, the resistor R6 from collector to 9V, and the resistor R5 from base to collector. The gain (Hfe) of Q1 together with these other values sets the collector voltage around which point the signals are mixed. The first job is to pick a transistor.

Transistor Selection

A simple way to choose a transistor is to build the test circuit shown here, using just Q1, R5, and R6. Connect power and measure the voltage at the collector. The aim is to get a voltage here a little over half supply, but not too much higher. Something in the region of 5V is fine. I picked a BC108 with Hfe of around 220, which was my starting point for the other component values in the circuit. The 2N3904 is also a good choice, and easy to find. Hfe is not a precise value for any device but a ballpark of 200 will suit nicely.

Falstad transistor test simluation

If you find your transistors all giving collector voltages nearer to 4.5V or even lower, and sourcing alternative devices is not an option, try decreasing the 4k7 resistor value – for example, if your Q1 Hfe is nearer 300, a 3k3 resistor will suit better.

I should stress here that this design is absolutely a compromise for the sake of simplicity. ‘Close enough’ is fine. The risks are lower headroom and some distortion.

The Rest of the Mixer Circuit

The inputs are brought in via potentiometers and decoupled using small capacitors. Three input resistors mix the signals into the transistor base. Note that the capacitor and resistor in series on each input acts as a low-cut filter to reduce sub-audio content.

In parallel with the 1M resistor discussed before, we add another resistor and capacitor across the NPN’s base/collector (R4 & C4). This does several things, not the least of which is to set the gain of the inputs. The 200k resistor, in tandem with the 100k values at the input, would suggest a gain of 2 (200k/100k = 2) but the real value is somewhat lower. In practice, with these values I found unity gain around 80% of the way around the input level pots, so there’s a little bit of boost available to help balance levels if you need it. The pots, by the way, should be log (or audio) taper.

Finally, the output is decoupled so that the signal has no DC offset and moves around 0V.

Powering the Circuit

I used a standard 9V DC barrel jack of the kind seen on many effect pedals – in this case the positive tends to be on the outer sleeve, and ground on the centre pin, Boss-style. A diode across the input protects against reverse connection, the capacitor helps smooth the incoming supply, and a resistor/LED draws a few milliamps to indicate ‘power on’.

The Passive Multiple

This is simply four jack sockets with their ground/sleeve connectors tied together and their tip connectors tied together. Please note, they are intended to split one signal several ways, not to combine signals.


… And Finally

Here are some photos of the build. You can see there’s lots of room in that enclosure, but I wanted something that was stable on the desk with a few cables hooked in. It’s possible to make the final unit quite small as the circuit itself takes up very little room. Designing a layout is your only challenge. Of course you can use any kind of box, sockets, and knobs that you like. I hope you find this useful!

Stompbox Mixer design sketches
Stompbox Mixer stripboard build

Stompbox Mixer internals 2 of 2


Synth DIY: a White Noise generator (part 2 of 2)

In Part 1 I outlined how to make a very simple discrete white noise generator. In Part 2 I’m going to expand that into something a little more interesting.


To colour the noise, simple passive filtering is sufficient. Originally I was going to put a switch here with a couple of different RC networks in place for basic high- and low-pass, but instead opted for a potentiometer to fade between them. The filter circuit below is inspired by part of the classic EMS VCS3 synthesizer, though is not identical. The pot sweeps from high pass at one end, through to low pass at the other, with a balance in the middle.

Schematic diagram of the white noise colouration filter

Schematic diagram of the white noise colouration filter

R4/C4/R5 and R6/C5 form low- and high-pass passive filters respectively. The pot pans between them by proportionally grounding one, or the other, or somewhere in between. The two networks have the same cutoff frequency, and that remains the same as you sweep the pot. It’s really just a balance control, between high and low frequency content.


Now we just add a volume control and one more stage of amplification to buffer the output. The overall output from the whole circuit, with these components, is about 5v p-p maximum. This varies as you sweep the filter. Feel free to play with the gain – line level audio will hover around 1v p-p, modular signals nearer 10v p-p.

You can download the full schematic as a PDF here.

Please Note: the schematic below includes corrected R and C numbering in the first stage. The schematic posted in Part 1 accidentally starts the numbering at 2 instead of 1. The component placement and values, however, should be correct throughout.

Discrete white noise generator schematic

Discrete white noise generator schematic


The short answer is ‘yes’.

A single-sided DC supply is fine for the transistor circuits here, and 9v or more is sufficient to allow breakdown noise generation. If you use a dual supply, opamp stages would make for an efficient circuit with very easily controlled gain.


Pick out any NPN you have at hand, connect its base to 0v and its emitter through a resistor to your positive supply. Leave the collector free. Power up and use a scope to monitor the noise at the emitter. Choose a resistor between 100k and 1M. Lower values give greater noise level, to a point. Too high resistance, and you won’t see a suitable noise source. The results listed in Part 1 for different devices were based solely on some quick practical tests, and are to be taken as a guide only.

You should be able to use pretty much any common general-purpose NPN in this manner, though don’t forget to pay attention to the pin arrangement as this varies across different devices.


Here’s a photo of the circuit in development on a breadboard. The scope shows the white noise output, the meter shows current consumption. Note the 9v PP3 battery providing power.

Photo of the white noise circuit during testing

Photo of the white noise circuit during testing

Finally, here’s a couple of photos of the finished device, housed in a stompbox-sized enclosure, and with an LED added to show power on. The audio output is a 1/4″ mono jack, and the power input is a 3.5mm minijack. Not the most common style for 9v, but they do crop up, on old DOD pedals for example, and some EHX. It’s just what I had lying around.

The second photo of the pair shows the interior. You can see how the circuit fits easily enough into a small enclosure. The eagle-eyed reader may notice a component difference or two – that’s because the black box is a version I made some time ago, and I decided to recreate and update the design for this blog.


Photo of the finished white noise box

Photo of the finished white noise box

Photo of the finished white noise box - interior

Photo of the finished white noise box – interior


Synth DIY: a White Noise generator (part 1 of 2)


We’ve all heard white noise in synth patches – it sounds like the wind, adds breath to a pad, rattle to a snare. It’s also a useful source of randomness for modulation, either directly or via a sample and hold circuit.

Technically ‘white’ noise comprises all frequencies at all amplitudes. Despite this sounding complicated, we can generate white noise very simply. It happens naturally in transistors and all we have to do is amplify it.

Once we have our white noise, we can filter it. Different colours represent different frequency content. Many synthesizers only provide white noise, but some also offer pink, which has the high frequencies rolled off. Occasionally you’ll see noise labelled as blue, red, or brown.


I decided to make this circuit using discrete components only – no ICs for once! You could use opamps instead for the amplifier stages, but the transistor circuit is compact and runs from a single supply, in this case 9 volts. Battery power is more than adequate.


Please Note: the R and C numbering in this schematic accidentally begin at 2 rather than 1. This does not affect placement, values, or operation. These identification markers are corrected on the full schematic in Part 2.
Also note the pin arrangement for Q1 will vary depending on your choice of transistor.

Basic discrete white noise generator schematic

Basic discrete transistor white noise generator schematic


The noise itself comes from the first transistor, Q1. In most circuits, the voltage at the base of an NPN would be higher than that at the emitter, allowing current to flow between the collector and the emitter (transistor basics here if you need them, there’s no shortage of guides on the internet). However, for noise purposes we reverse that – we hold the emitter higher than the base. We also leave the collector unconnected. If the reverse voltage applied is sufficient, it produces noise that we can amplify and use.

Here I’m using a BC182L. This component will require some experiment on your part. Every transistor has a different breakdown voltage (ie., the reverse base-emitter voltage that produces the noise), and every transistor will give different noise quality. I had good results with the BC182L, but I recommend trying whatever NPN devices you have at hand. If you have an oscilloscope, testing each transistor along with just resistor R2 (here I’m using a 1M resistor) is enough to compare a few examples. My selected BC182L with 1M on 9V gave noise levels up to 100mV peak-to-peak. The output was measured at the emitter.

The following image shows a sample from my Rigol 1054z oscilloscope. Horizontal divisions are 1ms, vertical divisions are 20mV. The bright band is the momentary snapshot, the dark band behind it is the signal smoothed out over time. You can see the signal is around 100mV from its highest to lowest point. This is pretty much the strongest result I got from any of my transistor stock.

Oscilloscope display of breakdown noise in a BC182L transistor

Oscilloscope display of breakdown noise in a BC182L transistor

I also tried several other silicon NPN transistors – nothing special, just what I had handy. In order to get something in the region of 100mV p-p I had to change the resistor value for each of them. Here’s a quick list of my results:

BC107 — 200k
BC108 — 640k
BC182L — 1M
BC547 — 150k
BC549C — 270k

2N3904 — 200k

These values are a guide only. You should adjust up or down as required – lower value to get a higher output. Something between 100k and 1M should give you useable noise from a broad range of transistors, so don’t worry if what you have isn’t listed here.


The rest of the circuit around the second transistor Q2 is an amplifier. I won’t describe here how this works (feel free to research common emitter amplifiers), but with these parts the output was around 2V p-p. That should be loud enough for audio testing if you don’t have a scope. You could substitute an opamp stage here, which I won’t detail. Consider it homework ;).

Note the 10pF capacitor. This isn’t essential. In fact, the noise has a higher peak-to-peak level without it (see images below) but it will sound different. This small value capacitor rolls off the harsher top end frequencies, making the basic ‘white noise’ smoother. Adjust, or omit, to your taste.

I recommend prototyping this circuit hooked up to something you can listen with, as well as see the signal on a screen. The component values are not set in stone, and it’s worth experimenting.

White Noise figure with and without 10pF capacitor in feedback of transistor buffer stage

White Noise figure with and without 10pF capacitor in feedback of transistor buffer stage

Finally for this stage we can add a capacitor on the output. This will decouple the output from any DC bias when we hook it to something else. You can see the DC bias in effect on the next image. Consider that we are using a single-sided 9V DC supply. The noise has to happen between two positive voltages. Audio signals should be centred around 0V. Any difference between 0V and the centre of the audio signal is the DC offset, and this can cause various problems such as distortion or even speaker damage. The DC offset in the image below is around 4.5V (the dotted horizontal line at the centre of the grid is 0V, the major divisions are 2V).

White Noise output from first stage buffer showing DC offset

White Noise output from first stage buffer showing DC offset

The next image shows the same noise signal taken from after the capacitor but measured as an AC signal to remove the offset. See how it is bipolar around the centre point.

White Noise output from first stage buffer with DC offset removed

White Noise output from first stage buffer with DC offset removed

This is enough for a standalone white noise source, and if you choose your components well the output should be enough for audio. You may wish for more gain if you’re using this with a Eurorack modular or similar. Modular synth levels are around 10V p-p, and we’re not going to reach that with a 9V supply. Feel free to experiment with a 12V supply though. If you want to get a more substantial output, you can use a bipolar supply and an opamp gain stage instead of the second transistor. Alternatively, we’ll be adding an output stage later anyway.


Synthfest UK 2019: my DIY modular gets screen time!

For the last three years I’ve been working hard on a complete modular synth system (which is partly why this blog has been so quiet lately), and at last the graft is starting to pay off!

I took it to Synthfest UK 2019 last weekend to give people a chance to see it, hear it, and more importantly to play it. Happily, it went down very well, and to my surprise the crew from Sound on Sound magazine asked me for an interview! Of course I was excited to oblige, and here (unscripted, unprepared, and with terrible hair) is my 6 minutes of fame…

As outlined in the interview, the synth itself is currently two cabinets. The upper cabinet contains:

  • VCO
  • VCF (24dB low pass)
  • VCA (transistor design with lots of colour)
  • ADS(R) envelope (Decay and Release share a control)
  • Dual lag processor
  • White & pink noise
  • Dual LFO (with waveshaping)
  • Passive ring modulator
  • Passive filters (Low and High pass)
  • Passive attenuators
  • Passive multiples
  • PSU

The lower cabinet contains the following, at the time of writing:

  • Dual VCA
  • AR & ADSR envelopes
  • Envelope follower
  • Gate delay
  • Audio (log) mixer
  • Attenuverting linear mixer
  • CV sources and inverters
  • Dual Sample & Hold
  • Headphone output and extra gain control
  • Buffered mults & inverters
  • PSU

I was ill for a couple of weeks leading up to the show, so I didn’t get chance to complete and install the High Pass VCF (24dB) that I had working and half-built, so that’s going in next. That leaves one slot in the lower cabinet which will become a Dual VCO.

The synth as it was for Synthfest UK 2019:

Photo of my DIY modular at Synthfest UK 2019

Photo of my DIY modular at Synthfest UK 2019

There’s a long way still to go. Once the lower cabinet is complete, I’ll be making another cabinet of sequencing and control modules – clock converters, triggers, all that kind of thing. I’ve also got plans for various units that I think will become separate devices, and which are at various stages of development. I’m trying to keep the focus on finishing this pair of cabinets first, and taking things one step at a time.

My aim is to turn these into modules you can buy. That’s also in progress, but it’s not about to hit the streets just yet. Hopefully something will start to appear in 2020, but the most important thing is to get this right, so releases of any kind will happen when they’re ready. There’s a lot of work in turning an idea into a product!

Watch this space, anyway. Meanwhile, I’ll try to keep posting a few synth DIY circuits, and maybe some details about vintage synths, but I have had to focus my time and energies on the design and testing for now. This blog is definitely not done yet, I will keep it going as long as I possibly can, but posts will be more widely spaced than they were in, say 2014-16, when I was dissecting the Lambda and modding my old Werkstatt. I haven’t forgotten those people who message me here and ask questions, either! I still try to answer as many of those as possible, where it’s appropriate and I am able to find the time. Please do keep asking, I’m very happy that my little blog gets such an audience!

I wish you all the best, and hope to continue seeing you here for a long time to come!




Synth DIY: the LFO (an overview)

Photo of synth LFO controls

Some synth LFOs as they appear on the panel

What is an LFO?

LFO stands for Low Frequency Oscillator. It’s a general purpose modulation source that outputs a slowly cycling waveform. They are commonly used for effects such as vibrato, tremolo, PWM, and filter sweeps.

An LFO’s frequency is usually sub-audio, but in practice many LFOs venture into the low audio range (above 20Hz) or even higher. At the slow end, cycle time is a few seconds, sometimes a lot more – there’s no standard range for an LFO. Sometimes they’re just called ‘modulation oscillator’, which I guess is more accurate.

The output of an LFO will often be a sine or triangle wave, as they give good results for most purposes, but other waveforms are also common – square waves, ramps, and a ‘random’ or ‘sample and hold’ output are also popular (these are good for the ‘burbling’ effect often associated with synth sounds). Technically, a sample and hold circuit is another device, so I won’t cover that here.

Many older synths come with just one LFO, which means all your basic cyclic modulations (as opposed to one-shot modulations like envelopes, or stepped modulations like sequences) have to come from the same source. It would be better to have at least two LFOs, so that pitch modulation and PWM, for example, could use different waveforms at different speeds.

Happily, many new synths have multiple LFOs, and luckily for DIYers, making an LFO is a simple job. The parts are cheap and plentiful, the basic circuit is compact and easy to build, and one or two extra LFOs can enliven a basic synth patch considerably.

There are several ways to make such a circuit but I’ll limit myself here to two basic methods:

I’ll be adding links to designs on this page as I write up a few details, so bear with me!


Synth DIY: the Relaxation LFO

This circuit gets is name from the way it charges, then discharges, a capacitor. Think of it like breathing: inhale, exhale. The charge on the capacitor is the rhythm of the breath.

Relaxation LFO schematic

Relaxation LFO


Relaxation LFO simulation image

Relaxation LFO simulation

Watch this in action here: Relaxation LFO simulator

Download a PDF of the schematic here: Relaxation LFO schematic

This example uses two op-amp stages, run from dual supply rails. Let’s consider just the first op-amp for now.

It has something in common with a comparator – the output will flip high or low depending on which input is higher. Here we’re using feedback to control this operation.

Imagine the output of the op-amp is high. From this, feedback charges the capacitor. The capacitor takes time to charge up through the feedback resistor, with the charging being faster with less resistance (actually here we’re using one fixed resistor and one variable resistor so the user can change the charging rate). When the voltage present at the inverting input (-) goes higher than the voltage at the non-inverting input (+), the output flips low.

Now, the capacitor slowly ramps down because of the negative feedback from the low output. When the voltage at (-) drops below the voltage at (+), the output flips high and the cycle repeats.

With the values shown, the LFO ranges from about 0.3Hz to 30Hz, or 30 milliseconds to 3 seconds per cycle. This is a good starting point for experimentation.

Some Technical Detail

Looking at the simulation, we can see a square wave at the output of the first op-amp, and a triangle wave at the top of the capacitor. Ignore the second op-amp for now.

The output voltage of the op-amp will vary depending on your supply rails and which device you use. The popular TL07x devices, for example, will not swing fully to either rail.

If you take your ‘square’ LFO wave from this point, it will likely be hotter than you need. Using a TL07x on 15V rails, your LFO output would be in the region of 27V peak-to-peak, which is silly. We could add a few more parts to drop it down to something more practical.

The two resistors connecting the output of the op-amp to its (+) input act as a divider to determine the level at which the signal flips direction. If they are equal, the flip point will be half the op-amp’s output voltage, etc. You should adjust these values to get the right output level for your needs. A typical modular LFO might be +5V to -5V, for example. Useful resistor values might be in the range 10k to 100k or thereabouts, but absolute values are not critical. It’s the ratio that counts. For example, if your rails are +/-12V, and your op-amp thereby swings to +/-11V, you could use a 12k resistor from output to (+), and a 10k resistor from (+) to ground, which would give you a +/-5V LFO output (11V*10k/10k+12k = 11*10/22 = 5).

We can take the square LFO output from the junction of these two resistors, instead of directly from the op-amp output, and it will be the same peak voltage as the triangle. We just need to buffer it to prevent problems arising from connecting it to some other input somewhere.

The ‘triangle’ wave available at the capacitor is not really a true triangle, as the rise and fall are both slightly curved. This will be more pronounced the closer to the rails your flip point is. It probably won’t matter, and you might not even notice, if you’ve got something like a 5V flip point on 12V or 15V rails.

We can’t take the triangle wave directly from the cap; we need to use a buffer so as not to load it. This is the second op-amp.

We can use one buffer for both waveforms, and just add a switch to select them. You can then use the output of this buffer to feed multiple destinations without trouble. Doing it this way means we can have a useful LFO with just one dual op-amp chip and a handful of extra parts. It’s a small circuit that is cheap to build, and with only one knob, one switch, and one socket required, you can fit them into a small panel space. Of course, you could omit the switch and use separate buffers for each waveform, or just tap off one waveform if you don’t need the other. You should be easily able to adapt this circuit to your own needs.

Finally, how do we change the speed range of the LFO? Once we know the peak voltage level, we can change the values of the capacitor and the negative feedback resistor/s. We can use a variable resistor here, though we’ll still need a small fixed resistor at one end to give a limit to the maximum speed. A higher resistance means a slower LFO. A 10k resistor and a 1M pot give a 1:100 range; if you keep the 1M pot but make the fixed resistor only 1k, for example, your range will be 1:1000, with the slow time more or less the same – the smaller fixed resistor sets the fast limit. When you’ve decided on your speed range, pick a suitable capacitor to determine the absolute speeds. A higher capacitance means a slower rate. With the aforementioned resistor values, something in the low microfarad range is a good starting point.

The capacitor should be non-polarised, as it will be charged above and below 0V. You can get non-polarised electrolytics easily enough, and the little polyester film caps are fine too, though they are less common in the >1µF range. Make sure the voltage rating of the cap is more than the voltage swing you’ll be giving it.

Next time I’ll talk about integrating LFOs.

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: improving the VCO and VCF direct outs

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.

Schematic for improved VCO and VCF direct outs on the Moog Werkstatt

Schematic for improved VCO and VCF direct outs on the Moog Werkstatt

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.

Simulation of the improved Werkstatt output mod in Falstad

The improved Werkstatt output mod  demonstrated using Falstad’s online simulator

VCF out

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.

Werkstatt VCO and VCF direct out mod wiring

Werkstatt VCO and VCF direct out mod wiring


Werkstatt VCO and VCF direct output mod installed

Werkstatt VCO and VCF direct output mod installed


Werkstatt VCO and VCF direct output mod external appearance

Werkstatt VCO and VCF direct output mod external appearance


Before and after: VCO and VCF direct outs. Top image 1V scale, bottom image 5V scale

Before and after: VCO and VCF direct outs. Top image 1V scale, bottom image 5V scale


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

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!

Boss DR-55: replacement legends

There are two push-buttons on the DR-55: Start/Beat and Stop/Rest. They each comprise a switch body, a switch cap, a printed label, and a transparent hood that clips over the top. The labels on one of my DR-55s had faded long ago, and the hoods had yellowed slightly.

I decided to print some fresh labels to my own design. I never really cared for the musical notation symbols on the original, so I copied the design of the ‘hit’ and ‘rest’ enclosed circles which can be seen on the front panel and in the user manual.

Below is a jpeg containing two replacement legends which should be the correct size if you print at 100%.


Boss DR-55 replacement start/stop button legends

Boss DR-55 replacement start/stop button legends

And here’s how they look in situ:

Boss DR-55 replacement start/stop labels in situ

Boss DR-55 replacement start/stop labels in situ

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