Dual ADSR / VCA Combo - Part 1
Dual ADSR / VCA Combo! (Part 1)
Introduction
Hey Kittehs! I’ve been knee-deep in home improvement tasks, but today, we’re starting another analog circuit for eurorack.
My system is missing some very important pieces. Can you guess? Yup, I’m missing an ADSR and you know what they say you can’t ever have enough VCAs. And since the VCAs I have are mostly passive I figured our module will be a combination of a VCA and ADSR with additional inputs and outputs to use them independently. Ok so lets go over the module circuit.
Module Overview
For this module I’ve chosen to alter and combine two rather popular circuit designs I found online. For the ADSR option I will base the design off René Schmitz’s “Fastest Envelope in the west” The main reason I’m choosing this is for the low part numbers and the fact that I still have a bunch of 556 timers so I can easily get two from one chip. We’ll go into details in a bit.
For the VCA I figuered that it was time to do an active circuit and I really didn’t want to do a differential classic design which was transistor based. The main reason being that I really don’t want to deal with having to match components. So instead we’ll build one around the LM13600/LM13700 OTA IC. And while we could use start off one of the circuits from the datasheet, I found a blog post from TMW on the Electric Druid website with a slightly different design that seems to be a little better performing so we’ll go with that. The VCA uses one OTA and the IC comes with two so this is perfect as we will get two VCAs out of a single chip.
Let’s talk features! Normaled ADSRs to the VCA, flexible CV inputs, a retrigger function, I/O for days, LEDs for feedback, and a switch for short/log ADSR times. Exciting, right?
Circuit Explanation
I’m not going to go too much into detail on the original circuits themselves, but if you want to learn more the links to the reference material is on the bottome of this post. However, I will go over the high level details and emphasize on my alterations and reasoning for them.
Note that ADSR is outputing 0-10v CV but the VCA expects 0-5v internally so we’ll need to account for that in the schematic. This is the first major modification I had to do but it’s actually a rather simple one with the use of a voltage divider.
The inverted envleope was an interesting one, but also easy since we’d dont this same thing for punky’s new brew. However this time I’m adding a switch that allows me to switch between 10v - 0v and 0v - -10v. To accomplish this one lug of the switch goes to a 5v reference and the other to ground. Sending either path to the non-inverting input of an inverting amplifier VCA will do the trick.
One issue I ran into with the ADSR’s non-inverted output is that the ADSR wasn’t closing all the way. This is likely due to the diodes used to charge and discharge the capacitor. To account for this I simply added a negative ofset via a 5k trim at the output. This allows me to ensure that regardless of what components I use I will always have a way to get the closed position at 0v.
The trigger is just going to use a 5-ish volt reference and then triggered via switch.
The retrigger was accomplished by a mod that I borrowed from Rich Holmes off his rework of a very similar design by Kassutronics Precision ADSR schematic. Note that while similar, the precicion ADSR cicuit is not the same circuit as the one we are working with today. They are very close tho.
For the VCA there isn’t much to be said other than I changed a few of the part values to account for what I experience when I breadboarded this and observed the performance with my oscilloscope. I added an LED off the output signal so I had to account for the impedance changes and adjust the feedback resistor on the output buffer stage. Other thant that it’s just mechanical thinggies and attenuation to make sure that signals mixed in are within the spec.
Note that plugging a 10v cv signal to the 5v jack won’t harm the circuit but it will distort the output and cause output signals to be higher than the eurorack standard. So, we’ll make sure input jacks are properly labeled when we get to the PCB desing work in the next part of this project.
Build Progress
So far I’ve tested the circuit on the breadboard and discovered that the OTA does distort the signal some, especially at the lower frequency ranges (about less than 60Hz). The distortion isn’t too bad tho and if anything it just gives the sound some warmth so, bonus?!?
Challenges Faced
The journey wasn’t without its twists. Tweaking ADSR for smaller pots, a nod to Kassutronics for a diode mod, and navigating the quirks of the OTA. It’s the real deal, folks.
While doing this design I ran into a few issues that forced me to make some desitions. First I tried to add the modification described on Kassutronics for the diode issues causing the envelope to not return to 0v but was unable to reproduce the desired outcome. I think this was due to two reasons, first that mode was for a similar but not equal circuit by Jonathan Jacky; see links to the original post bellow. Second, I modified the values of the ADSR so I could use much smaller potentiometers to set the stages. The circuit called for 1M or 2M2 pots and I have 50K slider pots on the way. In the end I accounted by that 0v issue by adding an offset. The outcome was acceptable to me so went with it instead.
The other big challenge had to do with the nuances of this OTA. The “Control” input isn’t expecting a voltage so the IC itself isn’t a VCA intead it expects a current, so is that a ICA? heh… At fisrt I tried a few circuits of my own on the simulator and I got pretty close to what I wanted. That design was based out of the circuits found on the datasheet btw. However while trying to learn about voltage/current converters I came accround the design by TMW and by following along I was able to get a much better understanding of how this IC operates. This allowed me to modify his circuit to fit my needs better, and ultimately turned out to be a way better design than what I could have come up with so I went with that instead.
Cliffhanger
Parts are en route, and PCBWay is on board to sponsor, yay! However, holiday delays mean we’ll have to hit the soldering iron in the a future episode. But hey, they say good things come to those who wait.
BTW, PCBWay is having a holiday sale right now. So check link below so you can take advantage of this holiday sale!
Closing thoughts
Once the module is complete I’ll be putting the files on GitHub so if you want to be in the know be sure to subscribe to the YouTube channel. If you like what I’m doing here be sure to hit the like button in the video as well as it encourages me to keep making more videos!
I’m always looking for constructive feedback so if you have comments or questions be sure to post them.
Microcontrollers are peeking over the horizon. Development environments, LFO modules, and more. Stick around!
With that we’ve come to the end this post. Thank you everyone for your wonderful support and encouragement. Until next time, stay creative and ride the positive rail!
Links and References
Fastest Envelope in the West: https://www.schmitzbits.de/adsr.html Linear “Vintage” LM13700 VCA: https://electricdruid.net/design-a-eurorack-vintage-vca-with-the-lm13700/ Kassutronics Precision ADSR (Similar but different circuit design): https://kassu2000.blogspot.com/2015/05/precision-adsr.html Precision ADSR with mods: https://www.youtube.com/watch?v=HcEP0gQBN4U LM13700 datasheet: https://www.ti.com/lit/ds/symlink/lm13700.pdf My blog: https://www.scruffycatstudios.com Our Sponsor: https://www.pcbway.com