More Experiments - Envelope Generators this time...
don at till.com
Thu Nov 23 01:51:24 CET 1995
Date: Wed, 22 Nov 1995 11:20:40 -0700 (PDT)
From: gstopp at fibermux.com
So here's where the brainstorm comes in. I was sitting around thinking
about the drawbacks of most designs. All designs use some kind of silicon
switch to charge and discharge the envelope cap throughout the various
phases of the envelope's cycle. These switches all have some finite
impedance which places restrictions on the time constant range - if you
want longer maximum times this means a bigger cap and/or higher resistance
pots. Pots become harder to get over 1 Meg. This means a bigger cap. This
means that your minimum times get longer too. As you get into the tens and
twenties of milliseconds your attack/decay/release times start to become
noticeably sluggish. Not that this is bad, but it should be adjustable and
not a design brickwall. Chip ADSR's use tranconductance amps internally.
Discrete designs use discrete NPN and PNP transistors. The EN designs
either use diodes or 4016/4066-type analog switches.
So I was thinking that the best kind of switch would be one with zero
on-resistance, which of course is a relay. Now don't get all huffy on me
and say that relays are "old technology" or "crummy"; the modern micropower
DIP relay has some pretty darn good characteristics, like logic-level
switching and the ultimate in signal-passing characteristics (it's like a
damn wire!). It wouldn't be appropriate to use one to reset the cap in a
VCO, but for slow-speed work (like keyboard keypresses) it is ideal. In
fact the Moog 951 Modular Keyboard uses a DIP relay as the sample/hold
switch for the keyboard CV.
Jeeze man, why not an automobile starter solenoid?
Seriously; there's something very wrong here. Before you reach for
that relay, stop and think for a moment exactly how low a resistance
you really need.
Those readily-available-dirt-cheap CMOS switches (4016 et al) have a
resistance of, from memory, something like 500 ohms or thereabouts.
As you said above, your maximum readily-available pot reistance value
is 1 Mohm. That gives you a 2000-to-1 ratio for your slope. If your
capacitance is about 20uF or so, that give you a range of 10 mSec to
20 Seconds, which is pretty much exactly what you want, right?
If you use regular boring TO-92 bipolar transistors you can get at
least an order of magnitude faster slope.
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