<p>That's really easy to do. Look at the schematic here:<br />
http://casperelectronics.com/images/finishedpieces/droner/V2/DroneSchematic.jpg<br />
Before each tremolo switch there is an RC network that does just what you are looking for but the attack and decay time is very short. We installed this to soften the tremolo a little bit and eliminate a clicking that appeared when using a hard square wave.<br />
Replace C44, 45 , 46 , &amp; 47 with larger value caps and you will get the more gradual attack and decay you are looking for.
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