Dual Modulator

This is going to be a shorter post, since this is a circuit I mostly copy/pasted from Kassutronics. He has a very detailed write up of the circuit on his site (link), so I’m only going to be writing about the changes I made.

I set out to build and envelope generator after getting frustrated with the available options. I had a specific image in my mind of what I was looking for, and couldn’t find it anywhere. I wanted just an attack knob, a decay knob, and a sustain switch. I found a lot of AD envelopes and ASR envelopes, but not many that could switch between the two, especially in the DIY realm. I also found a lot of big function generators with lots of features that could achieve this, but I wanted to keep it minimal. Just two knobs and a switch… and maybe a loop switch too. More of a utility modulation source than a big centerpiece module.

I stumbled upon Kassutronics ASR envelope, which turned out to be exactly what I was looking for. It’s a minimoog style envelope generator, where decay and release are both controlled by a single knob. I just needed to replace the sustain knob with a switch, and it would be good to go!

The Sustain/Loop Switch

It’s pretty simple to replace a pot with a switch - just make sure it includes a resistor in place of the pot.

Before

After

However, looking ahead at the Loop switch circuitry shows that this can be further simplified. Here is the full context:

When Loop mode is toggled on, the Sustain section is ignored. That means that these two switches only really have three positions between them. And when Sustain is off, it is the same as disconnecting the two switches from each other. That means these two switches can be replaced by a single on-off-on switch.

Pulse Output

While studying the circuitry, I found another change I wanted to make. There is a node called “STATE” that is high while the envelope is rising and low at all other times. I figured that when the module is looping, this could work as a pulse wave LFO. When not looping, it might be a handy fixed gate/trigger output.

However, this is op amp comparator, so the signal is either -12V or +12V at all times. I want a smaller range, maybe -5 to +5, and it would be nice if it could be bipolar while looping but unipolar otherwise.

The first change is easy, I simply used a voltage divider to reduce the signal to 50% amplitude. The second change was a little trickier. If you take the pulse output directly from STATE, it is always bipolar. If you take the pulse from after D5, it gives the desired polarity but stays high while the gate input is also high. Not what I want.

After messing around with it a bit, I realized I could use replace D5 with two diodes in series, and take the pulse output from between them.

In AD and ASR modes, the Pulse out passes through D5 so only the positive voltage is allowed to pass. However, in LFO mode, the negative voltage from state is passed through D6 from cathode to anode, which results in the Pulse output being bipolar while looping. Perfect!

Conclusion

Since I had the envelope down to just two knobs and one switch, I figured it would be easy to put two of these in one module. A dual modulation source, if you will. This was easily achieved, and the only remaining question was if they should interact at all. I decided to normal the second input to the first, with a jumper to determined whether the normal signal comes from the first input or the first Pulse out.

The last thing to do before selling this module was to reach out to Kassutronics and get his permission. I made a few tweaks to the inputs and outputs but the core of the module is exactly as he designed it. He replied that he didn’t mind if I sold them as long as I gave him credit in the documentation. Thanks, Kassutronics!

I might tweak this a little more in the future, but for now I am very happy with this module. The only change I might make would be to invert the pulse output so it is a gate delay. However, that would require more components and the PCB is packed to the brim already. If I were going to add that feature it would probably bump the module up to 8hp. For now though, I have two 2xMods in my rack and they are my most used modules, I use them in every patch.