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.
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.
CAN I USE A DIFFERENT VOLTAGE SUPPLY, eg. 12v or 15v?
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.
HOW DO I SELECT A TRANSISTOR?
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.
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.
WHAT IS WHITE NOISE?
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.
BASIC WHITE NOISE GENERATOR
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.
HOW IT WORKS
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.
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:
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.
BUFFERING THE NOISE
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.
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).
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.
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.
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:
- 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
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
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:
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!
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.
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.
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:
A very simple Attack-Release envelope generator can be built with a dual op amp and just a handful of extra components. The input stage is basically the same as my op amp gate buffer, with only its output resistor changed; the rest is a simple low-pass resistor/capacitor setup with an output buffer. Here’s how it works:
The input acts as a comparator. When the gate input goes high, the comparator output goes high, and the capacitor is charged up via D2 and the Attack pot RV1; when the gate goes low, the comparator goes low, and the capacitor discharges through the Release pot RV2 and D3. The diodes directionalise this process, so the attack time is governed only by the Attack control, etc. The output is a very simple unity-gain follower.
With the values shown, attack and release times range from just a couple of milliseconds to around 5 seconds. Larger values for the pots and/or cap will extend the times proportionally, smaller ones reduce them. The 560 Ohm resistor sets the minimum time against a given capacitance.
With an op amp such as the LM358, the output will swing between 0V and approximately 1.5V below the positive rail. If a lower output level is desired, add a potential divider of resistors in the low-mid single Ks after the output buffer amplifier, taking the overall output from their junction.
Supply voltage is not critical, but as mentioned above, the LM358 op amp will swing to around 1.5V below supply at maximum. It does, however, swing to ground too, which when operated on a single supply is necessary in obtaining a correct ‘gate low’ output. If you cannot find a 358, use another op amp which will swing rail to rail, or ground to near-positive.
A circuit like this makes a nice addition to synths with only one envelope, such as the Moog Werkstatt and Mother 32, or Arturia Microbrute. It will run from a 9V battery and is small enough to build into the Werkstatt itself, or indeed any small external box of your choice. You could easily build one for a Eurorack modular system too, and it will run happily on +12V or +15V.
For details of how to modify the Werkstatt, take a look at my Werkstatt page.
Op Amp AR, parts list:
U1: LM358 or similar
D1-3: 1N4148 or equivalent
C1: 1µ poly non-polarized
R1,2: 100k 1/4W resistor (I use 1% Metal Film types, but 5% Carbon are also fine)
R3: 82k —”—
R4: 18k —”—
R5: 560Ω —”—
R6: 1k —”—
RV1,2: 1M linear pot
Input and output connectors as desired.
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.
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.
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.
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.
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.
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.
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.
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.
43k 1/4W 1% MF resistor
1/8″ panel mount jack socket