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.
(Last revised: 20-Feb-2025 Made completely new schematic with new instrument pre-amp based on Ken Stone design.)
THE INPUTS EXPLAINED:
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.
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.
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.
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 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.
Here's an example of the function of the 'smooth' switch. One side is smoothed and the other is normal.
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:
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.
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:
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.
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.
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!
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.
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 --
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!
If you find these projects helpful and would like to support the website and its upkeep then you can buy me a Coffee. There's a button for that underneath the menu if you're on a PC or Mac. Or you can use this PayPal.Me link to donate directly. All donations go towards new projects and the upkeep of this website. Thank you!
Hi, will this work with +-12V Eurorack-Style?
ReplyDeleteCheers
Yes this will work on 12V.
Deletehey, if you are interested, i am currently building this. i added a HPF to the input, you can check my progress here
Deletehttps://lookmumnocomputer.discourse.group/t/envelope-follower-with-high-pass-filter/1182
cheers
Hey that's awesome! Interesting idea to let the drums create a melody through the envelope follower. I've bookmarked the link and I'm interested to see your progress with this project. Thanks for letting me know!
DeleteD1/D2 uses 1N914 while D3/D4 uses 1N4148.
ReplyDeleteCan I safely use 1N4148 for D1/D2?
Yes you can. It doesn't matter which you use where.
DeleteHello Eddy, first of all thank you so much for all the schematics you are sharing, it really is amazing. I am now trying to make the ARP2600 envelope follower you've shared. Only I ran into some trouble with the components. I found out that the BOM doesn't match the values that are in your schematics. C2 for example is 20pF in the schematic and 10uF in the BOM. Which one should I follow??
ReplyDeleteHi Jula, sorry for the confusion. I hadn't noticed the discrepancies in the BOM. For C-2 you must use a 20pF. If in doubt go by the schematic. I will look over the bill of materials and correct any mistakes. Thanks for pointing it out!
DeleteI've made a new Bill of Materials. It's posted above.
Deletehi Eddy, thank you for the schematics and descriptions!
ReplyDeletethe bom says to use 2 x 68k but i can only find one on the stripboard layout, but one with the value 51k. is there only one 68k and a 51k?. also the stripboard has a 120k and 1,5k that there is not in the bom:)
the best regards, Albert
ps. I'm looking much forward hearing this module !:))
Hi Albert. Yes the 51K goes to the base of the transistor that feeds the LED. It doesn't really matter if it's 51K or 68K. The other 68K is part of a voltage divider for the Gate generating section. That needs to be a 68K. In general, if you find discrepancies just go with what you see on the layout. The BOM was made some time after, so it might be off a little. When I started posting these projects I didn't post BOM's with them, that came later on request. Good luck with the build. It's a bit of a outsider this module but really cool to have an external input for the synth and something that creates an envelope voltage according to the volume of the input signal. You can do some weird things with that.
Deleteamazing, thanks a lot!:))
ReplyDeleteHallo Eddy,
ReplyDeleteda ist ein polarisierter 1uf Kondensator ! Kann ich hier auch eine bi-polare Folie nehmen ?
Danke im voraus,viele Grüße
Yes, that one is part of the lowpass filter. I don't think using a bi-polar cap would be a problem.
DeleteHello Eddy! I'd like to build an Envelope Follower but I do not want to include the pre amp and mic parts. So I can take all those components related out of the project, but there's a 8V current you use to light up the LED (correct?) and I also do not need to reduce 15V to 8V in my case. I'm using 12V btw, so I was thinking to go directly with the +12V rail to the LED part using the 2.2K resistor. Do you think this will work?
ReplyDeletethanx!
Francesco
Yes you can cut out the whole pre-amplifier section and connect the LED to 12V. That's no problem. 2K2 is enough resistance for a LED on 12V.
Delete