Synthesizer Build part-63: FASTEST ADSR IN THE WEST by Rene Schmitz.

The fastest ADSR in the West. A simple to build and fully featured Envelope Generator. I thought it was about time for another ADSR project for the website and this one worked out great and it has no trimmers to set. I even added a few extra's to make it even better.

This is an other 7555 based envelope generator like the YuSynth one from project 24. Some people seem to have problems with that one so that's why I choose to do this project now.

This ADSR will work on both +/-15V and +/-12V. At +/-12V the maximum envelope amplitude is just under 8 Volt. If you run it on +/-15V the peak envelope value will be 10 Volt. It's a very fast Envelope Generator. The minimum risetime of the signal is 1mSec or 1/1000th of a second.

Now this is roughly the same setup as in the previous 7555 ADSR from project 24 but it does work a lot better, especially when using Schottky diodes. The whole problem with the DC offset voltage left on the ADSR output comes down to the forward voltage drop of the diodes. That's why Schottky's are so useful because they have only a third of the voltage drop of silicone diodes. Now what would be even better is to have diodes with no voltage drop. To achieve that we have to put the diodes inside the feedback loop of an opamp. That's what happens in the Kassutronics Precision ADSR which might be the subject of a future article.

SCHEMATIC:

Here is the schematic for this circuit. I've redrawn it from the sketch posted on Rene Schmitz website.

The opamps are numbered in the order they are used on the stripboard. All diodes have been replaced with Schottky diodes which work much better in this circuit.

Click here for a FALSTAD SIMMULATION of this circuit.

I've added some extra's to this design. The original design only uses two opamps but I wanted to include a LED and also an inverted output so I decided to use a quad opamp, the TL074, and include an attenuverted output where you can have in inverted envelope signal with the potmeter turned fully counter clockwise, attenuation with zero signal when the potmeter is at the 12 o'clock position and a normal envelope when the potmeter is turned fully clockwise. I took the design from the AD/AR attenuverter mod from Ole Stavnshoej design (project 44). This is a great option to have when you use the ADSR with a filter. Turning the attenuverter will give the filter some very cool resonance response.

I decided to adapt the design to more run of the mill parts, like for instance I used 1M potmeters instead of the 2M2 ones in the original schematic. I changed the 220Ohm resistors to 100Ohm types and the capacitor from 2µ2 to 4µ7 to keep the original timebase intact. This is all explained in the text underneath the original schematic on the Schmitzbits website.

The circuit is relatively simple so I was able to build it up on a very eurorack friendly sized piece of stripboard. It's only 21 strips by 33 holes. Although I left the V,W and X strips on the board, they are not populated. You can use them to connect L brackets to mount the board to a panel. The three transistors on the Gate input represent the same setup as we've seen before in the Yusynth 7555 ADSR only there he had no resistors at the base of transistors 2 and 3. It works as follows: the first two transistors make up a schmitt trigger which turns any input signal into a sharp gate signal. That signal now goes through a capacitor that turns any long gate signal into a short pulse. That pulse is inverted in the third transistor stage to make it acceptable for the 7555 chip, going in at pin 2. 

Once the ADSR has been triggered the sustain level for that cycle is frozen. You can not add sustain while the ADSR is in its cycle, unlike the Digisound ADSR which can do this. Not that that's important. it's just something I noticed while testing the circuit.

The 1M resistor, in red on the schematic, can be added to provide for some input hysteresis. This will improve triggering on slowly changing waveforms. In the layout below, the purple 1M resistor on the left indicates where it should go if you want to include it. I left it out. Only include it if you really think you're going to need it. If in doubt, Leave it out.

LAYOUTS:

Here are the layouts I made for this project. As always they are verified. I used them to build my module. It's important to use logarithmic 1M potmeters for Attack, Decay and Release. The time based parameters. Otherwise it will be much more difficult to set these parameters accurately. It will work with linear types but get logarithmic pots for these. Sustain is a level control so that can and should be a 10K linear type potmeter.

I used Schottky diodes throughout this design because with 1N4148 diodes there's a DC offset voltage present on the output. Using Schottky diodes helps to prevent that.

Wiring diagram:

Stripboard only:

Cuts and wirebridges seen from the component side. You know the drill by now; mark the cuts on the component side with a Sharpie or Edding pen and then stick a pin through the marked holes and mark them again on the copper side. Then cut at the marked places with a sharp, hand held, 6- or 7mm drill bit.

Here is the Bill of Materials. I altered the diode types to Schottky diodes because they will work much better in this design. I put in the BAT4* series (like: BAT41, BAT42, BAT43 etc) because they work really well and have good availability in webshops. Any Schottky diode will do though.

OSCILLOSCOPE IMAGES:

Here are some screen shots from my oscilloscope to give you an impression of what the signal looks like. All testing was done with a +/-12V powersupply:

Here's the normal envelope output. The envelope signal does have a small positive offset voltage of 400mV I noticed. But this won't cause any VCA to stay open so it's of no consequence. 

However I changed the diodes for Attack and Release into Schottky diodes and that reduced the offset to just 16mV (16 thousandth of a volt) which is the same as 0V to me. The offset voltage is the result of the fact that the 4.7µF cap has to discharge through a diode and a diode has a voltage drop over it of about 0.6V (with silicon diodes). So the lower that voltage drop the better. With Schottky diodes the voltage drop is only about 0.2V which allows the cap to discharge as good as fully.

Here's the normal output in yellow and the inverted in blue coming from the attenuverter mod I added on myself. It works like a charm.

Here you can see, in the blue trace, the attenuverter in action. I'm turning the potmeter as the trace goes from left to right.

This is the signal at a pretty high rate at almost 3Hz. No problem for this ADSR.

In yellow you can see the pulse as it comes out of the third transistor and into pin 2 of the 7555. It's a inverted pulse, triggered by the gate signal, that starts the ADSR.

This time the yellow trace is the gate signal at the input. This was measured after the 10K input resistor. The gate signal was a +/-5V pulse wave from an LFO.

PICTURES:

Here are some pictures from the build proces.

Stripboard with cuts and wirebridges done.

Finished stripboard ready for wiring up.

All wired up ready for testing

I decided to use this ADSR for my DIY Kosmo synth and not for Eurorack so I took the YuSynth ADSR and replaced the stripboard with this one. I had to widen one hole to fit the attenuverter potmeter to which I added a bi-coloured LED to fill up another hole where a switch had been. I used a 4K7 resistor to connect it to the attenuverted output socket. I also re-used the manual trigger button that was already present in the panel. I took two 47K resistors and made a voltage divider so when I press the manual trigger it sends 7.5V to the gate input. (My DIY synth runs on +/-15V mostly).

Backview of the panel:

Here's what it now looks like mounted into the synth. My ADSR module has two Gate inputs each with a Schottky diode in series with the socket (soldered straight to the socket). This is to prevent +7.5V entering the Gate socket when I push the manual trigger button.

Luckily I could re-use the potmeters, which were 1M logarithmic types with a 10K linear pot for the sustain, the same as in this project.

You can see the blue LED underneath the attenuverter potmeter. The hole I had to fill up was 6mm and this LED is only 3mm so I used hotglue and made a sort of white blob that lights up red or blue. Worked out pretty well :)

I kept the dual gate inputs from my previous ADSR because I think it's handy to have. The gate inputs have Schottky diodes on them so that when I push the manual trigger button I don't get 7,5 Volt pushed into the gate patch cable(s). It's a safety feature I advise you to copy if you are going to include a manual trigger button.

Troubleshooting tip: If your Decay and Sustain are not working then the most likely cause will be a broken Sustain potmeter. It happend to me when I built it into the panel I used for the YuSynth 7555 ADSR and it turned out it had a broken Sustain potmeter all the time.

DEMO:

Here's a video I found on YouTube of someone demonstrating this ADSR in action. He's using it on the cutoff of a lowpass filter. Sounds pretty sweet. If he had the version with my attenuverter mod it would have sounded even better LOL ;) 

So that's another one done. I thought it was about time for a new ADSR project on this website, especially since some people seem to have problems getting the YuSynth 7555 ADSR of project 24 to work right. That's weird though because I always rated that one as near perfect but I think this will make an excellent alternative especially with the extra's I added. I'm really chuffed that it worked so well. Okay, I hope you will enjoy building this one. 

If you have any questions or comments about this project then please post them in the comments below of on the special Facebook group for this website.

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