A Low Cost, Special Purpose AR Generator
By Harry Woodell, Springfield, VA
Being a poor student, I have added several "homebrew" circuits to my 2720 in an attempt to derive as much versatility as possible for minimum cost. One of my most used units is a stripped down version of the 2720-4 Envelope Generator. The 2720 User's manual showed how to generate an ADSR envelope using two 2720-4's. However, since one is triggered by the keyboard step trigger output, the input flip-flop and UJT reset circuitry are not needed. Thus the circuit shown will replace the standard 2720-4 in this application. This circuit can also be used wherever a step or gate trigger is available and only a simple AR envelope is required: sequencers, envelope followers, or even the repeating trigger source shown on page 6 of issue #1/76 of Polyphony.
The circuit, as shown below, is quite straightforward and uses common parts. All transistors are 2N2712, all diodes are 1N914 or similar. C2 should be a mylar capacitor, and C1 will be electrolytic.
Feature by Harry Woodell
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