Today I had a go at breadboarding the envelop generator as described here: STUFF - LOOK MUM NO COMPUTER
I have found a potential issue with the circuit when using a long attack. There seems to be a delay between the start of the gate signal and the rising voltage of the envelope.
Here is a slightly different version of the schema for you to use as reference: lmnc - adar - EasyEDA
So the first opamp (U1.2) is configured as a comparator, so it will output either 12 or -12 volts which is used to charge/discharge C1. When the waveform comes out of the second op-amp, it still swings from -8 to 8 volts (on my scope at least) and the diode D3 at the end basically cuts away the bottom part of this waveform.
The consequence is that when using a long attack time (say 2 seconds), it will take half that time (1 sec) for the envelope to start rising because the first portion is negative voltages which are cut away by the diode. Can this be fixed easily?
My idea was to connect the opamps to ground rather than -12V. I really don’t understand why we need to use 12V anyway in this circuit. Would that be a sensible idea?
I would appreciate your input as I’m learning electronics with a the nice side-effect of building a moduler synthesizer at the same time Thank you in advance.
Welcome,
I actually noticed that also a few days ago, but since I tried it without sound, I supposed it was the LED forward voltage that took some time to be reached and that there was nothing wrong.
I just tried it and hooked up both opamps to ground rather than -12V and it causes new problems apparently.
I get a constant output voltage of 1.2V while no gate is triggered. This is probably caused by the opamps not going all the way down to the negative rail.
I’m not that good at reading datasheets yet, but perhaps I can resolve this by swapping the TL072 for something like a LM358. I believe its output can go all the way to ground. I don’t think its a problem that the output cant go all the way up to 12V as long as it can get to say ~10V.
Would love to hear what the experts have to say about this
Yeah, if you wanted to power it with +12 V and ground you’d need a rail to rail op amp. It also makes me nervous that the full scale output of the first op amp is the inputs to the second op amp. Again you’d need a rail to rail op amp to handle that without distorting.
But I think another way to fix it would be to put a voltage divider on the output of the first op amp, to take it down to say ±5 V, and then add a 5 V offset to that before going to the diodes and pots. Then leave out the output diode.
Thank you in advance.