Synthesizer Build part-54: JOYSTICK CV Controller (Eurorack).

 An easy to build joystick module that outputs 2 CV voltages to control anything from pitch to filter cutoff and anything else that can be changed with a control voltage.

The finished Joystick module installed in a Nifty Case.

Before I started building my modular synthesizer I had a brief try at flying FPV drones. I bought all the gear and some cool drones but the damn things were way too fast for me to control. This was before the stabilized DJI FPV drones were on the market. Anyway.... the gear landed in the coupboard for a few years. I have now successfully taken the hobby up again and now I can fly FPV drones but my transmitter/controller was now outdated so I used one of its gimbals for this project. I found a good schematic on the Mod Wiggler forum.

Close-up of the circuit:

The image below shows the circuitry for one axis. You need two of these circuits to work both axis of the joystick, left and right [X-axis] and up and down [Y-axis].

HOW THE CIRCUIT WORKS:

It's a very simple circuit. Each of the 2 axis of the joystick is assigned two opamps. The voltage coming of the joystick potmeter goes into the inverting input of an opamp and added to that is voltage from the Zero Point trimmer to make sure the voltage is at zero when the joystick is in the rest position (middle). The gain of the opamp is adjustable with the 1M potmeter marked Range. This determines the maximum voltage you get when you push the joystick fully to one position. This goes from 0 to 10V max. when used with a 12V powersupply.

The CV voltage then goes into a second opamp which has an offset potmeter so we can turn the signal into a unipolar one if we want (all positive or all negative voltage) or just give it some offset or even just to make sure the voltage is zero when the joystick is in the middle position. 

This module is meant for Eurorack (dual 12V powersupply) but it will run just as well on a dual 15V powersupply and if you build it for a Kosmo or 5U synthesizer you have more space on the faceplate to accomodate some extra features.

The joystick I used came out of a Taranis QX7 RC controller/transmitter and it has the following resistance values:

When in the middle position (rest) the resistance is 1,31kΩ. Fully right is 2,15 kΩ and fully left is 550 Ω. Same for the up-down potmeter.

The circuit will take a wide range of joystick resistance values so practically any joystick can be used.

I left the springs installed so the joystick always returns to the middle position when let loose.

Ideas for extra features:

The circuit is very bare bones but you can extend it with, for instance, a momentary switch that cuts the CV voltage if you push it, or one that makes contact if you push it and so outputs an extra gate signal.

An other idea that was suggested to me is to have two input sockets with the voltage connected to the socket switches (normalized) but then you can input an audio signal that cuts the voltage and then the joystick controls the amplitude of the audio thus creating a Manually Controlled Amplifier (MCA).

I'll leave all that up to your imaginations. I didn't have room for extra functions on my panel so I left it as presented here.

I did put in two bi-coloured LEDs to give a visual representation of the voltages on the outputs. It glows red for positive and blue for negative voltages. I connected them straight to the output sockets but with a big 10K current limiting resistor so they only glow at their brightest with the full voltage applied and don't pull down the CV outputs. Also to keep the number of components to a minimum. It works like a charm and looks very cool. Their brightness is a good indicator for the amount of voltage present at the output sockets.  They start glowing at around 2V and then get brighter with higher voltages. I mounted the LEDs above the joystick so they are in full view.

You can use any type of quad opamp for this circuit. I used one of my fake LM324 chips from China and because there are no high frequencies involved it works just fine. You can use a TL074, TL084 etc. They all work fine as long as the pin-outs are the same. It's a good idea to use miniature potmeters for the offset and range controls to save some space on the faceplate. The offset potmeters don't have to be 10K, I used 100K potmeters myself. The range potmeters do need to be 1M otherwise the range of the range will be different ^___^

The trimmers can also be different values. I used 200K trimmers. Afterall they are just voltage dividers in this circuit, so the value is not that important.

CALIBRATING:

The zero point is the point at with the joystick is at rest, right in the middle and in this position the CV outputs must be at zero Volts. You set the zero point with the two multiturn-trimmers.  

The best way to set the zero points for both axis is to have both the Offset and the Range potmeters at the 12 o'clock positions and then connect the CV output to an oscilloscope or volt meter and turn until the voltage is zero. 

Then set the scope or meter to a more sensitive setting and again correct until it reads zero Volts. Try to get it as accurate as you can. After you're done calibrating both channels you don't have to touch the trimmers again. 

Here is the schematic I used for the layouts:

I made a Falstad simulation of the circuit which you can see by clicking here.

Here's one observation I made about this circuit. The voltages from the wipers of the joystick potmeters go through a 51K resistor into an opamp, the gain of which is determined by the 1M potmeter (Range). I noticed that the Range potmeter reaches its maximum at about 1/3rd before the full clockwise position is reached. I think this is due to the 51K resistor. I think it will be better to put in a 91K or even a 100K to get the gain in step with the throw of the potmeter. 

The gain of this stage is determined by the formula: Av = (-Rfeedback/Rin) = (-1M/51K) = -19,6 (the minus simply means the output is inverted). This is too much and that's why the potmeter reaches full gain way before it's turned fully clockwise. With a 100K the gain would be -10 and that would result in the full throw of the potmeter being used. To play it save and make sure you get all the gain you can before you reach the fully clockwise position of the Range potmeter I would suggest using a 91K resistor instead of the 51K on the layout. I've changed the Bill of Materials to include two 91K resistors. However I have not made this change in my own module because I can't access those resistors easily anymore, so I can not guarantee it will fully solve the potmeter throw issue but I can't see why it wouldn't work because the mathematics says it will.

The Falstad simulation doesn't really show this discrepancy so do not rely on it for component values. 

LAYOUTS:

Here is the layout I made for this circuit. It is verified, I used it to build my project. It is small enough to fit flat behind a 14hp Eurorack panel. Beware there are two copper strips underneath the IC that are not cut. They connect the grounded pins together. Pins 3 and 12 and pins 5 and 10. 

There are three 100nF caps visible in the layout but I also put a 100nF cap over pins 6 and 7 of the IC. This is to suppress any voltage spikes or noise. This cap is not visible on the layout and because I had no room for it on the component side I soldered it straight to the pins on the copper side. So there are 4 caps in the Bill of Materials. (I didn't use any bypass caps myself but they are in the layout and B.O.M.).

Here is the stripboard only view. 

Here is the layout for just the cuts and wirebridges. 

As ever mark the cuts at the component side and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut them with a sharp hand held 6 or 7mm dril bit.

And finally here's the bill of materials. It's quite a cheap project if you already have a joystick in stock and anyway, joysticks aren't that expensive if you know where to look. The resistance value of the joystick potmeters isn't that critical. The circuit just uses them as voltage dividers so any value will work. They usually don't go down to zero Ohms. The one I used goes from 550 Ω to 1K3 to 2K15 in the lowest, middle and highest positions.

You can find joysticks on AliExpress for under $20,- for a pair. Just Google: "Radio Rocker Joystick 5K." Those should work just fine.

How to determin which wire is for up and which for down, left or right with a joystick.

Connect an Ohm meter to the middle wire and one of the outer wires of one of the potmeters on the joystick. Say for instance we're looking at the potmeter for the Y-axis (up and down). Now we measure the resistance while moving the joystick up. If the resistance goes down you have the correct wire for the up position. If the resistance goes up that wire should go to the down position on the stripboard, for the Y axis. So if you have the correct wire for a specific direction the resistance between the middle wire and that wire should go down when moving the joystick in that direction, because the wiper of the potmeter moves closer to it. I hope that makes sense.

Here's a screenshot from my oscilloscope. Yellow = X-axis, Blue = Y-axis. In this picture I moved the stick to the outer most positions and you can see both voltages land on exactly 10V maximum with Range turned fully clockwise and no offset applied.

PICTURES:

Here are some pictures I took during the building process:

This is the faceplate I made. Notice the two square holes. I tried fitting two push switches for extra Gate outputs but I came back on that idea because I didn't have enough room to accomodate that.

I made the big round hole with a hand held jig saw.

The finished face-plate with everything installed but without the stripboard. As you can see the knobs are very close together which isn't ideal so when you design your own faceplate for this module take some time to find out the best places to put these potmeters. If you use miniature potmeters you have more room to move them about to find the best placement.

Below is the stripboard with all components mounted except the power connector. The bottom two strips I later cut away go have some more space for the gimbal to move because when I tried to mount the board behind the panel I needed a bit more space. The bottom two copper strips are not used so I could just cut them off.

Here's how I mounted the stripboard behind the panel. I used some plastic tube as a stand-off. If you do the same, drill a few small holes in the sides very near both ends so the glue can run into those and provide a good grip. Then I hot glued that to the back of the panel, making sure the glue flowed around some of the mounting screws for the joystick, for extra grip. Then I hot-glued the stripboard to that stand-off after the wiring up was all done. I had to be careful not to disrupt the movement of the joystick gimbal, keep that in mind when mounting the stripboard behind the panel. There's almost no place to drill a hole through the stripboard for a normal M3 threaded stand-off so this seemed like the best solution. Works fine.

And here's the finished product. Front view:

Back side. The depth of the module is just under 4 centimeters. It's 14hp wide (7CM):

Finally a little demo video of the module in action in my 'Nifty Case'. This is just a simple patch I put together in 5 minutes. The X-axis CV is controlling the cutoff of the filter in the Doepfer A-111-6 synthesizer voice and the Y-axis CV is controlling the reverb amount from the FX-Aid.

Okay, that's it for this one. Quite a simple build. The only thing I did wrong was that I forgot that the wipers of the offset potmeters connect to the inverting inputs of the opamps so I had the offset potmeters wired the wrong way around. An easy fix. This is a very easy to build module and, I think, a very useful one especially for live performing. It's in fact the equivalent of a synthesizers modulation- and pitch-bend wheels all in one.

If you have any questions or remarks please put them in the comments below or in the special Facebook group for this website.

Synthesizer Build part-36: DUAL VOLTAGE PROCESSOR.

This is the Fonik Buchla Style Dual Voltage Processor. A very useful module for altering Control Voltages with five different functions!  Offset, Attenuation, Inverter, Glide or Lag control and, if you follow the tip below the schematic image, it can also be a CV splitter. 

Now also with Eurorack compatible layout. 

I wanted a Voltage Processor module in my synth for a long time and I was thinking of copying the ARP2600 VP, but that one is fairly limited in its options and more specialized specifically for the ARP2600 so when I saw this design I thought it would fit much better in my system. This module lets you alter the offset of a control voltage by 0 to +5V or -5 to +5V. It lets you attenuate and invert a control voltage by means of an Attenuverter and it has a Lag control that is a direct copy of the Lag control from the ARP2600, with a 1 MOhm potmeter and a 470nF capacitor (The ARP used a 100nF cap). This alters the slew rate of, for instance, a Squarewave and rounds off the corners turning it into a Sharkfin Wave. In fact it adds a 90° phase shift to the signal. 

If you want that control to behave more like a Glide control to smoothly go between different notes with a 1V/Octave signal then use a 1µF electrolithic capacitor. Try it and experiment. Maybe use a 100K potmeter instead of a 1M one.

Besides control voltages this module can also handle audio signals.

This module will work fine on either a dual 12V or a dual 15V powersupply so no problem for you Eurorack fanatics =).  In fact, there's a Eurorack friendly layout further down the article. One thing though, with a dual 12V supply normally the maximum offset would be 4 Volt instead of 5 Volt but I addressed that issue and fixed it by changing some resistor values.

I guess you could say that the Eurorack equivalent of this module would be something like the TipTop Audio MISO (Mix, Invert, Scale, Offset) which costs a 110 Euro's. 

The circuit is primarily meant for control voltages but it can handle audio signals just as well. Even at very high frequencies it won't distort the signal. With audio you can use the Lag control to turn a Triangle wave into a Sinewave although with less amplitude. It won't be perfect but it's possible. It can also turn a 0V to +10Vpp signal into a +5/-5V signal by adding a -5V DC Offset voltage to it. The other way around works too of course, turning +5/-5V into 0V to +10Vpp signal. Very useful.

The circuit consists mostly of 47K resistors but you can actually alter the value of those and use for instance all 91K resistors. I actually did this as a test with the second part of this dual module and it didn't change the working of the circuit in any way. Just make sure you use the same value for all 7 resistors. Don't make them lower than 47K though. You can also use other quad-opamp chips instead of the TL084. You can use TL074, LM324 or any other, as long as it's a low noise opamp (good for use with audio, which most of them are these days) and they have the same pin-out as the TL084 (and most quad opamps also have that these days).

This circuit was designed by Chris MacDonald and modified by Peter Grenader and then further improved by Matthias Herrmann who added the Lag (Glide) control function. The only thing I did was adding the Offset switch, changing the potmeter values from 50K to 100K, changing the value of the Lag Capacitor from 1µF to 470nF and adding the 470 Ohm resistor before the Lag potmeter to eliminate noise issues, based on practical testing.

The original schematic and a PCB design can be found in this original PDF and I made a new drawing from that schematic which is posted below. Like I just mentioned, they use 50K panel potmeters in the schematic but I didn't have those so I used 100K potmeters. Again, this made no difference what so ever. You must however use a 1 MegaOhm potmeter for the Lag control because this, together with the capacitor, forms a simple lowpass filter and these values are important to get the correct frequency response. The original schematic uses a 1µF capacitor for the Lag control but with testing I found out that this is way too much. So I changed it for a 150nF in the layout but that turned out to be not quite enough. (The original ARP2600 Lag control uses a 100nF capacitor.) In my own build I experimented with different values and I ended up using a 270nF and a 180nF in parallel to make a total of 450nF and that works fine. So I set the capacitor value on the layout to 470nF. I found that this gives the best Lag control response in my case. Of course, if you don't have a cap of that value available, you can use an other one with a value close by. Anything between 300nF and 700nF will work fine and you can put two (or more) in parallel to create the value you want but test it and check to see it works like you want it to. Use an oscilloscope set to DC mode for measuring the output.

If you don't want a Lag control but a 'Glide' control you can use a 1µF electrolithic cap. My advise again is to experiment and use whatever suits your needs.

Setting the trimmer (T1):

The trim potmeters are for setting the attenuverter mid point, but they don't have too much of an impact so you don't have to use multiturn potmeters for those. The normal ones will do fine.  I added a switch to the offset control so you now have a choise to offset a control voltage from 0 to +5V or from -5 to +5V. 

Here's the procedure for setting the T1 trimmer:

- Turn all potmeters to the fully counter-clockwise position.

- Turn the attenuverter potmeter to the 12 o'clock position (mid point).

- set the offset switch to 0 to +5V position.

Connect an oscilloscope or multimeter to the output and turn the trimmer until the output reads exactly zero volts when the attenuverter is at the 12 o'clock position.

A little quirck I found, at least in my build, is that there can be a lot of noise on the output if the Lag potmeters are set fully closed (counter clockwise). Because this was the case with both sides of the Dual Processor I figured this was a fault in the circuit design so I added a 470 Ohm resistor between the Lag potmeter and R6. The value is low enough not to influence the Lag filter and it gets rid of all noise issues that I had.

The schematic drawing doesn't include any de-coupling capacitors but they are included in the layout. Just four 100nF ceramic caps on the power rails as close to the chips as possible. If you experience hum on the audio output you could even put some 10µF to 47µF electrolytic capacitors on the power rails. There's room enough left for that. Make sure they are rated 25V or higher and put one on the +15V to ground (negative pole to ground) and one on the ground to -15V (negative pole to -15V) rails. I leave that up to you but for my module it wasn't necessary to include them. (The electrolytic capacitors are not included in the layout, only the de-coupling caps.)

Here's the schematic drawing which I re-made from the original, from the above linked PDF file. The Dual Voltage Processor consists of two of these circuits side by side with only the Ground as a common link:

I made a Falstad simulation of this circuit. Some of the component values have been alterred a little to make the simulation act more like the real thing. The Lag potmeter value changed from 1M to 100K, the cap from 470nF to 100nF and the 470 Ohm resistor has been taken out.

--- CLICK HERE ---

LAYOUT

Here is the verified stripboard layout I made for it. It's the same layout once repeated and mirrored to make it a dual module. This layout was made for the Kosmo format modules but now there's also a more compact layout below for Eurorack size modules.

TIP: Solder a wire from the input-socket of stage one to the socket switch of the input socket of stage two. That way the signal on input one will be present on both inputs and can be processed by both stages and split in two. If you connect a patch cable to the input of stage two, that first connection will be broken by the socket switch and it's back to normal. (Normally we call this normalling a connection ;) Very useful me thinks! 

Of course you need input sockets with built in switches for this but most types have that as standard.

Stripboard only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide. 

Bill of Materials:

EURORACK LAYOUTS:

As of December 2021 there is now also a new layout for the Eurorack format. The stripboard is 24 by 41 holes. Just like in the Kosmo format layout above, the right part of the dual voltage processor is a mirror image of the left part so I could place all the connections to the potmeters on the edge of the stripboard on both sides. The TL084 has 4 identical opamps in it so it doesn't matter which opamp is used for which part of the circuit. As I mentioned earlier, you can use other quad opamps like the TL074 or LM324 for this without problem.

Make sure you connect the three ground strips at the top together by putting some extra solder on the eurorack power connector. Otherwise put in some wirebridges to connect the three ground pins together.

Here is the wiring diagram:

Stripboard only view:

I built this version on Dec 10th 2021 and everything worked fine except that I had to use a lower value capacitor for the Lag control. The layout has a 470nF cap in it and that works fine in my Kosmo format panel but for this one I had to use a 10nF cap. Not sure why this one is different, maybe it's the fact that this runs on +/-12V instead of +/-15V or it's the potmeters I used I don't know, it's a bit of a mystery. Anyway, it's not important because if you find, when testing, that the Lag potmeter doesn't work over the full throw then you need to lower the capacitance. Just a matter of experimenting. The cap can easily be de-soldered and changed for another one.

The trimpotmeters are for setting the Attenuverter midpoint and as with the previous layout they don't have much influence but you need to set the Attenuverter so that at the midpoint, when the waveform is a flat line, that line is at the zero volt mark. Measure this with a scope and make sure all the other potmeters are turned fully counter clockwise and the offset switch is set to 0/+5V.

Btw, because this module is running on +/-12V the actual offset voltage is plus or minus 4 Volts, not 5 Volts but I only discovered that after I made the panel so I kept the labeling as is.

EDIT: To get a higher offset voltage in this Eurorack version you must change the resistor R6 from a 47K to a 68K. This will actually give you +/-6V offset voltage. R6 on the lefthand side of the stripboard is the 47K between pins 13 and 14 of IC-1 (positions I and J-16). On the righthand side it is the 47K between pins 1 and 2 of IC-2 (positions I and J-27). Change those for 68K's and your offset voltage will be upto +/-6V. If you need +/-5V then use a 56K with a 1K2 in series. You may have to do some tweaking if you want the voltage to be just right for you. You could also use a 100K trimpot to dial it in accurately. 

The 470 Ohm resistor(s) are not in the original circuit schematic. I put those in myself when I built the first version because the Lag control produced some noise when the potmeter was fully counter-clockwise. This resistor sort of prevents that the Lag pot can be fully closed. It has no negative influence on the amplitude or sharpness of the signal so it works fine.

Here's a picture of the finished Eurorack module:

VIDEO DEMO:

Here's a video with a quick overview of the different functions. 

I watched a demonstration video about the ARP Odyssey and in it they showed the effect that the Odyssey's Lag control had on the filter cut-off control voltage. It made the filter make these 'Wah' sounds. And I'm very chuffed to see that the Lag control in this module has the precise same effect on a filter.

Here are some pictures from the build process:

In the picture of the panel (below) the 'Lag' control is still called 'Glide'. That's what it's called on the schematic but I chose to use the same term that ARP uses in the 2600.  I think it's a more accurate description because it actually creates a phase shift of about 90 degrees (see also the article about the ARP Envelope Follower). So that makes the signal lag behind the original in a small way. 

The picture below shows one side of the dual module wired up and the other side has not yet been wired up. The LEDs of that side are still mounted on the print (which was necessary for testing) instead of in the panel.

When the module is in 'rest' position so to speak, all potmeters should be set fully counter clockwise and the switch set to 0/+5V. That way, any signal you put in will come out unchanged. You can then alter it by turning the controls.

Okay that's an other one done. If you have any questions please put them in the comments below or on the EddyBergman Facebook group. Please read the whole article before asking questions.

DISCLAIMER: The author of this article does not accept any responsability for the correct functioning of this, and any other, module/project on this website. What you build, you build at your own risk. All project layouts are thoroughly tested before publication, it's up to you to replicate them and the author can not be held responsable for any mistakes made.

Synthesizer Build part-20: ARP2600 ENVELOPE FOLLOWER with pre-amp.

An external input module derived from the famous ARP2600 synth. It produces a Control Voltage, Gate and Trigger pulses from an audio signal and it has a clean audio output at synthesizer level for further treatment. In fact it offers you a third modulation option besides the LFO and the Envelope Generator (or ADSR).  You can also use the envelope or trigger output as a sync pulse to sync different modules together.

Although I pretty much finished the first stage of my synthesizer build when I wrote this article, I got inspired to try and add one more module to the case after watching this documentary about the ARP2600. I always wanted some sort of external input module in my synthesizer and in this documentary they talk about the opening of a famous song by The Who called 'Who Are You'. Pete Townshend plugged his guitar into the ARP's Pre-amplifier and through the Envelope Follower to get the effect you can hear in that song. So I started to look into Envelope Followers and asked on Facebook for schematics. It turns out these schematics are all variations on the same theme and look and perform very much the same. That's easy to understand as they all need to perform the same task.

NOTE Feb. 2025: AS THIS WAS ONE OF MY EARLY PROJECTS THE DESIGN OF THE INSTRUMENT PRE-AMP CIRCUIT WASN'T REALLY THAT GOOD SO I HAVE NOW UPDATED THIS ARTICLE WITH NEW SCHEMATICS AND NEW LAYOUTS.

WHAT IS AN ENVELOPE FOLLOWER?
Now what is an Envelope Follower I hear you ask and to be honest, I didn't know myself until a week before starting this build. An Envelope Follower (or EF) creates a Control Voltage who's amplitude follows the amplitude of the input signal. So the control voltage sort of follows the contours of the volume of the input signal. This is nicely illustrated by the oscilloscope pictures below. And as an extra it also produces Gate and Trigger signals if the input volume (or amplitude) passes over a certain threshold, so this can also be used as a Gate Extractor of some sort.  So in other words, you can input external audio signals and get control voltages, gates and triggers from them plus a clean amplified audio output. Just what I wanted.
I did some research and it turns out that Alan R. Pearlman (founder of ARP Instruments Inc.) won a prize for designing a tube based Envelope Follower in 1948 and he wrote a thesis about it for his senior year at Worcester Polytechnic. I dug around and found the ARP2600 service manual in which I found the schematic for the Envelope Follower with pre amplifier. The chip they use for the preamp is the 1339-01 which is long obsolete I believe (I couldn't find it) so I decided to make the pre-amp with the venerable LM386 at first. But I later found out that these chips shouldn't be used for pre-amps because they have a low impedance output meant to power loudspeakers or headphones. Not ideal, so I based the instrument amplifier of my updated version on the pre-amp that Ray Holmes used in his Envelope Follower module. That in turn is a Ken Stone design. For the electret microphone pre-amp I stuck with my previous 1 transistor design because it works so well and it's such a simple design. I really like using it.

HOW THE CIRCUIT WORKS:

Here's how this circuit works (quoted from the ARP 2600 service manual):
A1, CR2, CR1 and A2 comprise a full wave rectifier for the audio signal. The positive portion of the wave, on pin 6 of A1, goes through CR2 and into the non-inverting input of A2 (pin 3). The negative portion of the wave passes through CR1 into the inverting input of A2 (pin 2) so that the output of A2 is always positive. The rectified signal is then filtered by R12-15 and C7-10 and then amplified and buffered by A3. 

R12 to 15 and C7 to 10 form a 24dB/Octave low pass filter. This is straight from the original ARP schematic and it works very well. The filter's cut-off frequency is 53Hz. This filter makes sure the high frequency audio part of the input signal, which is rectified by the two diodes, is filtered out and we are left with a low frequency voltage that follows the amplitude of the audio input signal. The signal is attenuated quite a bit by this filter but is then boosted again by the almost 10x gain of opamp 3 with the 10Meg feedback resistor (R17). Ray Holmes lowered that value to 4,7Meg to run this circuit on +/-12V so I followed in that and it works very well.

Here is the original schematic from the ARP2600. (The microphone pre-amplifier it uses is the standard datasheet circuit for the 1339-01 chip):

As you can see it doesn't have a 'Gate out' or a 'Trigger out' so I took those functions from the PAiA schematic and I came up with the schematic below which I used for my build. The component numbering follows the numbering on the original ARP schematic, as far as possible.

With the circuit below the gate output will be around +8V. I changed the value or R24 (4K7) and R25 (51K) in the layouts below to produce gate and trigger pulses of exactly 10V. The circuit was designed to work on +/-15V so these alterations had to be made to make it work on +/-12V.

(Last revised: 20-Feb-2025 Made completely new schematic with new instrument pre-amp based on Ken Stone design.)

Here's the KiCad schematic. I made the changes necessary to have this work on +/-12V

THE INPUTS EXPLAINED:

In the schematic I drew above, I put in all the different points at which we can input signals of a different level or amplitude. 

The Envelope Follower has three inputs that are normalled together.

The first input is the most sensitive, this is the microphone input. It uses a transistor amplification stage that goes into the instrument amplifier via the socket switch (normalled). The on-board microphone on the panel is also connected to this stage but that connection will be broken if you insert a cable into the Mic input.

The second is an instrument amplifier. If you want to use an instrument like a guitar you can plug it in there and the connection with the microphone preamp will be broken.

The instrument amplifier is normalled to the direct input of the envelope follower. You can input a signal directly into the E.F. if that signal is at the synthesizer level (+/-5V ot 10Vpp).

I've also added a LED to the Gate output to get a visual indication of the working of this circuit which is very useful to have, especially to see if the input is clipping.

So one more time for clarity: the three different inputs are there to accommodate different input LEVELS! 

- The input for the electret microphone can handle tiny signals in the 10 to 100 milliVolt range which then get amplified by the transistor pre-amp to around 2Vpp and then by the instrument pre-amp to boost it up to 20 Volt peak-to-peak max. before they go into the envelope follower. The electret input has voltage on it! (upto +12V) The next two inputs do not!

- The second input can handle input levels from the 100 milliVolts upto 2 volt range, for use with guitars or dynamic microphones for instance, and this gets amplified by just the instrument pre-amp to boost it to synthesizer levels for input into the envelope follower. It has a gain potmeter to adjust the levels.

- Finally, the third input does not have any pre-amplification so this input can only be used for signals that are already in the +/-5 to +/-10 Volt range (10Vpp to 20Vpp) like synthesizer or drum machine signals.

At first the idea behind the 3 different inputs was to serve as a substitute for the x10, x100, x1000 preamp range switch that was on the original ARP2600 Envelope Follower. In the original ARP2600 the range switch was a 3 way switch that changed the feedback resistor over the pre-amp opamp with a choice of 10K, 1K and 100Ω. 

Now, with this new version of the Envelope Follower, with the new instrument pre-amp design, it has a 10K gain potmeter over the instrument pre-amp opamp and that can also be seen as a substitute for the gain switch in the ARP2600 but having the 3 different inputs makes this module much more versatile. 

All inputs are normalled together so when nothing is connected to the inputs, the envelope follower gets a signal from the electret microphone mounted on the panel. That connection is broken when you insert a microphone into the mic pre-amp. The output of the mic pre-amp goes through the instrument pre-amp to the envelope follower input. That connection in turn is broken if you plug something into the instrument input and that connection gets broken if you connect something directly to the envelope follower input. So the envelope follower input always gets the right amplitude range from whatever you want to use as input source. On top of that it has its own level control so you always get the correct levels.

Here's where the socket switch is located on the 3,5mm mono sockets I always use for all my projects.

 

Leading the envelope signal into a VCO doesn't sound very good, at least not when the envelope is produced from the human voice. It's better to use it for a VCA controlling volume. After considerable testing I added one feature. An envelope smoothener. It's just a 47µF cap over the output jack which can be switched on and off. It is effectively forming an extra lowpass filter with a cut-off frequency of 3.4Hz, filtering out the higher frequency spikes and pulses. This is in fact the same as the ARP2600 'LAG' control. More about this at the bottom of this article.

LAYOUTS:

This is a new and verified layout design which I made in Februari 2025. If you need the old ones, contact me on Facebook and I'll send them to you. I kept the microphone preamp from the previous version because it works so well. I tried the one used by Analog Output in his E.F. module but I couldn't get it working.

Wiring:

The resistors R20 and R21 (33K and 47K) determin the voltage threshold of the Gate and pulse outputs. They form a voltage divider that gives off +5V to pin 13 of the TL074 which is set up as a comparator. Any envelope signal higher than +5V will produce a gate and trigger signal. If you want to change that threshold you can change R20 for an other value which you'll have to calculate. (These resistors are located at the top left of the stripboard) however there's no reason to do that. You can create more or less gate and trigger pulses by varying the input level and gain.

The voltage amplitude of the actual gate signals is determined by resistors R24 and R25. Using the values in the schematic the gate and trigger pulses will be around the 8 Volt. I changed the values of these resistors in the layouts to 4K7 and 51K which produces pulses of exactly 10 Volt. The previous version had them at 10 V too. (these changes are also in the Bill of Materials)
Stripboard only: 

Cuts and the wirebridges. This is seen from the component side.

As ever, mark the cuts on the component side with a permanent marker like a Sharpie or Edding 3000 and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut the copper strips at the marked places with a sharp hand held 6 or 7mm drill bit. With this method you have the least chance of making mistakes.

Bill of Materials following the numbering of the schematic.  There are some components with duplicate numbers but don't worry about that. The right amounts are in the bill of material.

TEST RESULTS / SCOPE IMAGES:

And finally some test results in the form of screenshots from my oscilloscope. 
The trigger pulse takes about 100 mSec to die out completely but if you want that time to be shorter just put in a smaller capacitor for C12, the 3n3 that is at the Gate to Trigger junction in the schematic drawing. The Gate and Trigger outputs are about 10Vpp. 

All scope screenshots are from the new version. The yellow line is the envelope output, the light blue is the microphone preamp output, the dark blue is the Gate or Trigger output and the purple is the instrument preamp output.

You can see that all traces are set to 5V/Division except the light blue which is 1V/Div.

In the picture below you can see the gate signal at a nice +10V like it was with the old one. All I did was change R24 from a 15K to a 4K7 resistor to up that voltage.

Here's an example of the function of the 'smooth' switch. One side is smoothed and the other is normal.

In the screenshots below dark blue is the trigger output, purple is gate, yellow is envelope out and cyaan is audio output.

Notice the lag or phase shift that occurs if you engage the smooth option. That's why the original control on the ARP2600 was called 'Lag'. It introduces a 90° phase shift.

Here's a close-up of the picture above showing the lag a bit clearer. Compare the peak of the cyaan (light blue) coloured waveform with the yellow and you'll see a slight delay in the yellow peak.

The different level controls work very well and I can get Gate, Trigger and useful Envelope voltages from this circuit while wispering in the microphone or, giving it more attenuation, I could be shouting in the microphone, makes no difference. The LED will indicate when it clips by being on continuously so you simply attenuate more and that's it. With all these different inputs and level controls this circuit can take an enormous range of input signal voltages.

One thing to remember, the Gate and Trigger signals need to go into high impedance inputs like opamps (and that's usually the case anyway, so no problem). If you pull any current from them their voltages will drop.

Here's how to make a simple hand held electret microphone with a 3,5mm mono socket and a patch cable:

Just solder the mic to the socket. Electret microphones can be bought on eBay for around $ 5,- for 20 pieces. They're really cheap. Get the ones with two legs. You'll see that one leg is electrically connected to the case. That's the minus or ground pole.

Here's a link to an eBay listing: https://tinyurl.com/5n6bfhsy

Pictures from the build proces:

Wirebridges put in.

All components put in. Don't mind the wires, they were for testing.

Here's the panel I made for the Eurorack sized module. It's 14hp wide (7CM) which is a size I almost always use because it means I can mount the stripboard flat behind it, making the module less deep than if the board is mounted at a 90° angle.

Finished module. I built an electret microphone into the panel itself which is switched off when an external microphone is connected to the socket. Above the gate and trigger outputs there's a little 3mm blue LED. (blue was the only color I had left.) It lights up when a gate pulse is created and it also makes for a great clipping indicator because if it stays on all the time you know you will need to lower the level or gain. Very useful actually. If you patch the audio output into the input of a module like Mutable Instruments Rings, you can get some very cool sounding string plucking sounds.

The normal/smooth switch connects a 47µF capacitor to the envelope output to smooth out radical changes in voltage. It acts as a lowpass filter with a cutoff of 3Hz.

Backview. The module is just 3,5CM deep. The stripboard is held in place by one M3 stand off and the rest of the stability is provided by the wiring itself.

Here's a link to Ray Holmes (Analog Output) article about his envelope follower module:  --CLICK HERE --

Okay, that's an other one done!
I hope you enjoyed this article and if you have any questions please put them in the comments below or post them in the special Facebook Group for this website. You can follow this blog to keep up to date with the latest posts.
See you on the next one!