Friday 7 February 2020

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 and Gate and Trigger pulses from an audio signal. In fact it offers you a third modulation option besides the LFO and the Envelope Generator (or ADSR). It also outputs an amplified Audio signal from whatever you put on the input (microphone, guitar etc.) which you can feed back into the synthesizer for further processing.

Although I pretty much finished the first stage of my synthesizer build, 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.

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.

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. Just before the output opamp we find a 24dB/Octave low pass filter. I copied this 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.

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 this as the schematic which I used for my build. The component numbering follows the numbering on the original ARP schematic, as far as possible:

(Last revised: 17-Feb.-2020. New drawing. Added smoothness switch + 8V wiring. 31-Jan.-2022: Changed duplicate R26 into R22.)

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 most sensitive input is the electret microphone input. Electret microphones are in fact capacitors with one movable plate, they are not able to generate a signal themselves. They can only manipulate an existing voltage to carry the audio signal so the input is connected to +8V through a 27K resistor to deliver the voltage to the microphone to enable it to work. So this input has an 8 Volt voltage on it!! Keep that in mind and only use it for electret microphones. You can choose an other type of circuit if you wish but you'll have to adapt the schematic and layout accordingly. That's up to you.
-- The next input is the one at switch S1 which bypasses the transistor microphone pre-amp and inputs straight into the LM386 pre-amplifier chip. This can be used for a guitar for instance. I also tested it with a run of the mill dynamic microphone (600 Ohm) and that works very well too but the volume knobs all had to be fully opened up. 
-- The last input, which can take voltages of a Control Voltage level, upto +10Vpp, bypasses the whole pre-amplifier circuit altogether. It can be used to input a signal at modular synthesizer levels such as straight from an oscillator, filter or even a HiFi audio amplifier's speaker output for instance. Because the input of the Envelope Follower needs such high signal levels, the circuit includes the LM386 amplifier to provide those high levels.
Switch S2 let's you choose to input a signal from the on-board amplifier or from an external amplifier into the Envelope Follower. I've incorporated all the inputs in the layout below. I don't like trusting the switches built into the jack sockets so I always install separate switches instead but you can use jacks with built in switches of course. 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 preamp and the LM386 to boost it up to 8 Volt max. before they go into the envelope follower. The electret input has voltage on it! (upto 8V) The next two inputs do not!
- The second input can handle input levels from the 100 milliVolts upto 1 volt range, for use with guitars or dynamic microphones for instance, and this gets amplified by just the LM386 preamp to boost it to 8 Volt max. for input into the envelope follower.
- 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 like synthesizer or drum machine signals
The 3 different inputs can be seen as a substitute for the x10, x100, x1000 preamp range switch that was on the original ARP2600 Envelope Follower. In fact you could make such a switch by using a 3 pole rotary switch and connecting one input-socket to the wiper and have each of the poles go to each of the inputs. The ranges wouldn't be exactly x10, x100 and x1000 but it'll have the same function.

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.

Here is the stripboard layout that I made and used for my build. As always, the layout is verified and has been successfully used by many people already. The components are quite spread-out but I had to keep the overview to be able to troubleshoot it, should that be necessary. You could easily make a more compact layout yourself, but this is what I used and this is how I wired it up. The only things not on the layout are the extra two 6½ mm (or ¼ inch) input jacks you can see in the pictures below but they just go parallel over the 3½mm Pre-Amp and E.F. input jacks. The ones connected to the switches. All potmeters viewed from the front. Layout is verified.

(Last revised: 10-March-2020.  Corrected a missing cut in copper strip at output LM386. 11-May-2021: Cosmetic changes to layout. Removed resistor colour codes and indicated wirebridges to ground with green colour.)

Close-up of just the stripboard. The layout is verified and has been successfully used by others:

Here's an overview of the cuts and the wirebridges. This is all seen from the component side.

And here's the layout with just the cuts. Seen from the component side. 
As ever, mark the cuts on the component side with a waterproof Sharpie 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. (If there are discrepancies between the BOM and the layout just go with what you see on the layout.)

And finally some test results in the form of screenshots from my oscilloscope. (I only had 3 channels available on my scope 'coz I blew up channel 1, aaargh! :p  ) (it's fixed now).
In this first picture the purple line is the trigger output which is 10 Volt, the light blue is the envelope and the dark blue is the output from the pre-amp. 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. If you want that value to be lower you can exchange R24 for a 50K trimmer. R24 and R25 form a resistor voltage devider that determins the output voltage of the Gate pulses. So by making R24 higher in resistance you bring down the Gate voltage.

In these 4 pictures below, the purple line represents the Gate output. The rest is as described above:

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.
The pre-amp works very well with this Envelope Follower. It's a good combination. Btw, this preamp design is the same as the one I used for the 100 LED oscilloscope project, only that one didn't have adjustable gain.

Some pictures of the stripboard at its testing stage:

Here's a look at the finished module:

Here it is built into the synth in the only place I had left:

The 6,5mm or quarter inch jacks are for instrument inputs like a guitar.
There are four outputs which are all 3,5mm jacks. Gate, Trigger, Envelope and pre-amp audio out, with a LED to indicate Gate activity.

EXTRA ADDITION / Lag control:
Okay, as I write this it is 9 days since I first published this article and I have done some more experimenting and I thought the envelope looked a bit rough sometimes, especially with speech input so I did some experiments to see if I could smoothen it out a bit. I put a 47µF capacitor over the Envelope Output with a tiny little switch so I can turn it on or off.  Together with the 1K output resistor this forms a simple lowpass filter which works very effectively. The cut-off frequency of this filter is 3.4 Hz so it suppresses all fast voltage fluctuations. You can easily adapt this filter to your own needs. Click here to find a handy calculator for finding the right capacitance for a given frequency. This filter doesn't affect the Gate and Trigger generating circuit because that is fed straight from the output of the opamp before the signal goes into the filter.
I have incorporated this smoothness control in the schematic drawing and also in the stripboard layout. This is in fact the same approach as the ARP2600's 'Lag' control. The ARP has a Lag control which you can switch in series with the Envelope output. This Lag control is nothing more then a 100nF cap and a 1Meg potmeter forming a lowpass filter that gives a phase shift of 90°. Some people also call it a Slew Rate Limiter because it delays the time that sudden voltage changes can take place. In other words it would turn a squarewave into a Sharkfin-wave. I only found this out after studying the ARP2600 service manual and finding the schematic for the Lag control on page 29. 
I couldn't incorporate the full Lag control. Indeed I had almost no room left on the panel to add even a single switch so I moved the Gate-LED up and used that hole for a micro on/off switch that pushes in or out. I glued it in place with hot-glue. This works fantastically. Here are some of the test results from the oscilloscope. The lightblue waveform on the left is the envelope without smoothning and then I push the switch on half way in and you can see the wave smooths out to the right. Dark blue is the audio waveform. You can see in the bottom scope picture that the lowpass filter introduces a small lag of about 25 milliseconds which corresponds to about a 90° phase shift around 3Hz.

Here's a picture of the panel with the new switch. It's the little white square underneath the LED, I labeled it LPF 3Hz (Low Pass Filter 3Hz).

Okay, that's an other one done!
I must say I am particularly proud of this build because it is the first project that I designed myself and where I didn't use pre-made plans from other people. Of course I didn't design the circuit but I did convert it from 1970's components to 2020's components and made the layout. And the fact that it worked so well right from the first testing stage makes it a very satisfying project for me to look back on.
Anyway, 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!

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  1. Hi, will this work with +-12V Eurorack-Style?

    1. Yes this will work on 12V.

    2. hey, if you are interested, i am currently building this. i added a HPF to the input, you can check my progress here

    3. 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!

  2. D1/D2 uses 1N914 while D3/D4 uses 1N4148.
    Can I safely use 1N4148 for D1/D2?

    1. Yes you can. It doesn't matter which you use where.

  3. Hello 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??

    1. Hi 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!

    2. I've made a new Bill of Materials. It's posted above.

  4. hi Eddy, thank you for the schematics and descriptions!

    the 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 !:))

    1. 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.

  5. Hallo Eddy,

    da ist ein polarisierter 1uf Kondensator ! Kann ich hier auch eine bi-polare Folie nehmen ?
    Danke im voraus,viele Grüße

    1. Yes, that one is part of the lowpass filter. I don't think using a bi-polar cap would be a problem.

  6. Hello 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?

    1. 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.


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