Synthesizer Extra's No. 01: SIMPLE AD/AR using the 7555

A fantastic little AD/AR envelope generator that is super easy to build and works very well.  Perfect little adition to the DIY synthesizer.

I was looking for a better AD/AR design than the one I had built and used upto now and I came across the Thomas Henry design. Here is the schematic.
This design uses the CMOS version of the NE555, the 7555 and it can be built on a piece of stripboard that is the same size as the LMNC version that I first used.

If you're new to all this; AD/AR stands for Attack Decay/Attack Release. It's a little Envelope Generator creating a control voltage that can open and close a Voltage Controlled Amplifier (or you can drive a filter with it. There are lots of options.)
In the picture below is the stripboard layout I made for it. I added all the features that Sam Battle has in his design, like the Arcade push-button with internal light and I added a little thing of my own, an option to double the output voltage of the Envelope from 0 to +5V to 0 to +10Vpp. I always find it handy to have a bit of extra charge on the envelope if I want to use it to control a filter for instance. You can of course connect a potmeter to the +10V output and so turn the output amplitude up or down from 0 to 10V. That way you can do away with the +5V output altogether. Since I already made and wired up the panel for this, I couldn't use the potmeter option. I simply exchanged the old stripboard for this new one and soldered all the wires back in place. You can also wire up the opamp in such a way that it inverts the envelope. That would be easy enough to do. As a final extra I put in buffer stages for the envelope output, for both the +5V and the +10V output. You could also wire one of those up to be an inverter. Lots of options here. One thing that is different from the LMNC version is that the Arcade Push Button won't work as long as a key is pressed down. The Gate signal has priority in this design.

The layout below is an early version and although it works fine, it is a bit messy. So I made a new layout which you can find a bit further down the article. I'm leaving the old one up in case someone who built it needs to reference it for troubleshooting or something.

SKIP THESE 2 LAYOUTS AND GO FURTHER DOWN TO THE UPDATED LAYOUTS TO BUILD THIS PROJECT.

Beware if you are using standard 24 x 56 holes stripboard, that the layout only goes from A to U not to X. So only 21 strips!

(Last revised: 1-March-2020: Changed attack and release pots from linear to logarithmic. 4-Oct.-2021: Cosmetic changes to layout.)

Here's a close-up of just the stripboard:

Bill of Materials:

Here's a look at how fast this little AD/AR is and it is super fast! It reacts to the Gate signal with practically no delay what so ever as you can see from these scope images. The risetime is about 12 µSec. The same as the risetime of the Gate signal (Gate = yellow, AD Out = blue). The gate signal has a bit of a skew in it half way up. That's due to some circuit specific stuff elsewhere in the synth but not really relevant because we are zoomed in so much it's practically instant. I mean, it's 12 millionth of a second in total:

UPDATED LAYOUT:

I made an updated version of the layouts above. I built it and changed the old one for this new one and everything works as it should so it's verified.  In the previous layout the output stages and 0 to 10V is a bit clumsy, although I guarantee that it works fine! The layout below is just a bit neater because I gained some knowledge over the past year and applied it here:

Beware if you are using standard 24 x 56 holes stripboard, that the layout only goes from A to U not to X. So only 21 strips instead of the usual 24!

SCHEMATIC:

Here is the schematic for this version. As you can see I added two buffers (which is a bit overkill but I wanted to use all the opamps available) and one opamp with a gain of x2 to get a 0 to +10V output. The buffers help to prevent this AD/AR from 'hanging' if you use it with inputs that have a bit of a low impedance (see text below). Both outputs are connected to a switch so you can choose between them. You could of course connect sockets to both outputs, instead of the switch and have two outputs permanently available, a 0 to +5V and a 0 to +10V. That's up to you.

The AD/AR works as follows: In AR or Gate mode, the Attack remains high for as long as you keep the key on the keyboard pressed down. After you let go the Release kicks in and the signal will fade out in the time you have set with the Release potmeter.
In AD or Trigger mode the Attack/Decay cycle still needs to have been completed before you can trigger it again but as soon as the Attack cycle has been completed the Decay kicks in, regardless of whether the key is still pressed down or not. For fast trigger sequences the Attack and Decay need to be set to short times because it won't trigger again until the cycle is completed, and that's perfectly normal.
So with the Attack a tiny bit open and Decay/Release fully closed you get a powerful envelope pulse of either 5V or 10V depending on how the switch is set.
In Gate mode you can have both Attack and Release fully closed to get fast short envelope pulses as the video below will demonstrate.
If you build this circuit with separate inputs for Trigger and Gate, and you feed it both at once, the Gate signal will take priority.

Here are some pictures of the stripboard using the new layout. I had made a mistake at first because I forgot this layout only had 21 strips instead of 24 so I made some cuts in the wrong place. That's why I placed the warnings with the layouts. And that's why there are some horizontal wirebridges in the lower ground strip (bottom black line).

12V vs 15V:

A little word on operating this from a dual 12V power supply. It will work but you'll need to change one resistor at the output. (R7 on the schematic). The 2K2 (R7) becomes a 3K3. This is necessary to give pin 6 on the 7555 the correct threshold voltage. I myself put in a 5K trimpot for R7 so I could experiment with the threshold voltage. It turned out that changing the resistance value of R7 mainly influenced the amplitude of the Envelope. In other words, you can set the initial envelope voltage with it. So after I learned this I took the trimmer potmeter back out and put in a 3K3 resistor.

The 'hanging' issue:
Because the resistor voltage divider at the original output influences how this AD/AR works I decided to add some extra buffer stages at the end, to stabilize the working of the circuit. I noticed that impedance differences, when connecting it to certain filters in my synth, can make the AD/AR hang sometimes. The release won't activate like it should, probably because the threshold voltage on pin 6 is disturbed somehow. I didn't want to rebuild the whole stripboard so I used a little piece of stripboard with just a single TL072 on it and buffered the +5V aswell as the +10V outputs. I stuck it onto the main board with hot-glue. It now works perfectly. No hanging or anything. I incorporated these buffers on the stripboard layout so they are now part of this design.

This design works a lot better for me than the LMNC one. This AD/AR reacts to trigger signals with an amplitude of +4 V and upwards and gate signals from +1.8 V and upwards with a maximum frequency of at least 60Hz. For triggering to work well, you need to open up the Attack a tiny little bit. The circuit is so fast that the envelope pulse shuts off before it has time to reach full potential. I tried different things to fix this little issue but I wasn't successful upto now. Anyway, it's nothing serious having to turn up the Attack a tiny little bit when using Trigger pulses. When you use Gate signals there's no problem.

I do strongly advise you use a logarithmic potmeter for the 1 M Attack potmeter. I used a normal linear one first but had trouble setting short attack times accurately. I've now put in a logarithmic one and it makes a world of difference. Works so much better. I really need to change the Release potmeter into a Logarithmic one too. That would make it much easier to dial in the Resonance or Cut-Off frequency when I use this to activate a filter. For the 4,7µF capacitor you can use a normal electrolythic capacitor. You don't need to use a Bi-polar capacitor in this circuit, unlike the LMNC one. You can put in extra electrolythic capacitors in parallel with the 4,7µF cap. to stretch the Attack time to the maximum length you want. I put in a 3,3µF and two 1µF caps for a total of 5,3µF which gives me almost 10 seconds maximum attack time. If you need longer Attack times just put in a 10­­µ­F cap.

Here are some technical specifications:
Minimum Attack time: 692 µSec
Minimum Decay time: 248 µSec
Maximum Attack time: 6 seconds with 4,7µF cap.  9 seconds with 5,3µF (which is what I installed)
Maximum Decay/Release time: ±30 sec.
Maximum input pulse frequency: ±60Hz

Here's a link to the Electro-Music Forum page that deals with this design:
http://electro-music.com/forum/topic-61297.html

Here's a little demo video of this AD/AR in action:

This second video shows one way of using the AD setting (trigger mode) of the AD/AR to control the cutt-off frequency of the ARP2600 filter. The Attack is fully closed so the instant a key is pressed the envelope voltage opens up the filter and then the Decay sets in and slowly closes the filter off as the envelope voltage fades down to zero. Watch the big blue light and listen to the effect on the sound.

The LED inside the Arcade push-button is connected to the +10V envelope output with a 4K7 resistor. It shines nice and bright. There's also a yellow LED on the panel between the input and the output. That one is connected to the output jack with a 1K resistor. It shines normally when you use +5 V out and extra bright when you use the +10 V output level. This is just a handy indication of how the output switch is set. It also reacts faster to pulses than the LED inside the push-button so it's a better indicator for that too. The LED was already built in so I thought I might aswel use it like this. :)
The Arcade push-button switch, which is the manual trigger, is fed with half the positive rail voltage (+7,5V) by means of the voltage devider formed by the two 68K resistors. I thought that was better than giving it the full whack of the +15V rail voltage. You can of course use other values for these as long as they are both the same. If you want to feed the switch with a different voltage then you can calculate that voltage as follows: Say R1 is the resistor coming from +15 V and R2 is the resistor going to ground. V = 15/(R1+R2)*R2
The arcade push button will not work as long as a Gate signal is present!!
Gate takes priority over manual trigger, just so you know that.

Okay, conclusion time: This design is a big improvement over the LMNC simple AD/AR and I can highly recommend using it. It works very well with patches where you feed it a fast trigger signal to control drum modules for instance. The switch which lets you choose between +5V or +10V output works perfectly fine but if you want more control just build it with the output controlled by a potmeter like I mentioned before. I do recommend you include the extra buffers at the end. They will insure that this AD/AR works perfectly under any condition. The only tiny little down point is that in Trigger mode the Attack needs to be a tiny bit opened to get a full envelope pulse. With Attack fully closed in Trigger mode, the pulse you get on the envelope output stops so fast that is doesn't have time to reach the full voltage potential. You could say it's too fast for its own good. You can see this happening on the oscilloscope. You get really fast pulses that don't reach the full voltage before they're cut off again. In Gate mode you won't have this issue and it works just perfectly. I really like this design and I highly recommend building it.

Here's a picture of how I added the buffer stages by glueing on a little print with a single TL072. This saved me from having to rebuild the whole thing.

Finally, for my own record keeping purposes, here's two pictures of how the finished synthesizer now looks, with two new VCO's and the Envelope Follower and the little oscilloscope of course:

Okay that's it for this article.
This article isn't really part of the synthesizer build itself so I named it 'Synthesizer Extra's'.  That's the header I will use for articles describing enhancements and changes to the original synthesizer that I build in the past 19 articles.
If you have any questions please leave them in the comments or post them on the special Facebook Group for this website. Okay, see you on the next one.

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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). 

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.)

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 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 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!

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!