I didn’t want to build a full ADSR circuit, I want that synth to be simple. Simple enough for a child to use it without questioning everything. It was pretty clear in my mind that a simple AD module will suffice and here’s it is :
The input section is a circuit that you can often see on 555 based attack generator. Yusynth uses it, schmitz also ect… I won’t give full details on it because there’s plenty of that on the internet and Yusynth did it wonderfully (and the links are below).
Another good thing to do when you’re looking for explanation is to look at old service manual. The moog micromoog for instance gives you the all description of the inside circuitry. I think Roland also did, their Service Manual are great, full of waveforms and voltages, piece of art ! Imagine that for you Ipad !!
the 555 timer is the classic. On a gate signal arrives, Reset (pin4) goes high, pulse at trigger (pin2) starts the timer. Out (pin 3) goes high, current charges up the cap (the speed depends on the value of the attack pot.). The cap is monitored by the 6th pin (TRSH), once it goes at 2/3VCC the chip triggers the transistor (inside the 555) linked to the 7th pin (discharge) and the current goes out trough the decay pot trough that transistor. It’s the classic 555 thing.
the traditional Icrap blurry picture.
One probe for the scope, one probe for the Waveform generator (to simulate gate signals).
Worked at the first power up !! (proud of myself I must say, actually the filter also did, is that meaning something ?)
the current draw is around 12.6mA (without the buffer op amp, the empty socket). It can goes from 1ms to 10sec if I did my math right. Once that board is completed I’ll make a complete schematic for it.
There’s an attenuation factor of 0.9 because of the voltage divider introduced by the 1K output resitor and the level knob. A bit annoying but the value of the level pot will certainly change.
A piece of advice if you want one (even if you don’t): Use code color for wires. On big circuit is helpul to debug. Mine is Red for V+ and green for ground. Don’t know for -V yet. Even though I don’t respect it strickly (I may use green other stuff I haven’t millions of wires) it’s a general habbit that I keep.
Before asking, no, there’s no layouts or documents to build it on stripboards like I did. Sorry, I do mines on the fly you should to !
thanks for reading.