[sdiy] My scheme and some questions on a A-SR envelope shaper

[Scott Gravenhorst]

I have a schematic for an ADSR that uses one 4069UB for the logic portion.  

http://home1.gte.net/res0658s/fatman/ADSR.html

This is a true ADSR.  Without the opamps, the circuit will peak at about 1/2
V+ with an "off" level of one diode voltage.  Input gate voltage needs to be
 bit more the 1/2 V+ (up to V+) so a TTL compatible gate signal will work. 
The threshhold for peak is detected by 2 inverter gates which resets the
flipflop.  The output buffer (opamp, especially the first one) subtracts the
one diode voltage and the second opamp scales the ADSR output waveform.  The
flipflop is obvious in the schematic and is built from 2 inverters plus some
diodes to create the 2 cross coupled NOR gates.  If the opamps are not used,
the negative supply is not required and the circuit will work from a single
+5 volt supply (the schematic calls out 8 volts, but it should work at 5). 
In this case, you could use a FET to buffer the output, perhaps a source
follower, diode voltage subtractor and then a gain stage (NPN) if desired.

Maybe you can get some ideas from this.

"Ishaan Dalal" <izx at xizx.net> wrote:
>For my barebones synth project, I hit upon an idea to make a rudimentary
>ADSR (without the decay), using the following scheme: Use the base square
>wave of the note to re/trigger one half of a 4538 dual monostable, which
>charges a capacitor through a diode and appropriate resistor. The control
>voltage from the R-C junction goes to my VCA. After the attack, the tone is
>sustained for as long as the key is pressed. The moment it is released, the
>cap discharges through another diode-R combo back into the monostable
>output, giving the release.
>
>What I would like to do is to add a sustain "time-out", where even if the
>key was kept pressed longer than n seconds, the A-SR would automatically
>begin the release. The easiest way would be to use the other monostable on
>the 4538 to time the attack+sustain, however, this would not release if the
>key was pressed only momentarily but go on for the full duration. I could
>probably do it with some flip-flops and using the resets of the monostables,
>but I'm strapped for space, and if possible would like to just use the other
>monostable on the 4538, along with simple logic (read: constructed with
>diodes/resistors and/or transistors), that releases the note after n
>seconds, even if the key is still pressed.
>
>Any ideas?
>
>Cheers,
>Ishaan
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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