vc adsr

[Arthur Harrison]

Don't discount the choice of the LM13600 for your OTA!  
This part has the advantage of built-in linearizing diodes 
which can be used to improve dynamic range, and thus, 
signal-to-noise.  Also, the '3080 is a single OTA, where the 
'13600 is a dual.  The fact that two matched OTAs are 
available on one chip is a great advantage for all kinds of 
applications.  (Too bad National Semiconductor doesn't
show examples of these in their data sheet.)  When using
the '13600, I suggest using an external op amp as an  I to V
converter, and not the transistor pair on the chip.

LED illuminated LDRs offer some advantages such as
low noise and low (virtually no) distortion.  However, LDRs
are a little slow, and you may have to do a considerable
amount of offsetting and trimming to achieve the desired
attenuation range.  Also, the tolerances of both LED output
power and LDR resistance may make duplication a problem.

Lastly, the OTA can probably offer better dynamic range
(at least 55dB) whereas you may have trouble getting a single
LDR to do that. 

-Art

-----Original Message-----
From: Beszedes Arpad <beszedes at cc.u-szeged.hu>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Date: Wednesday, November 11, 1998 2:52 AM
Subject: vc adsr


>Hi!
>Thanks for everyone who replied about vc-adsr. I am collecting various
>possibilities and will buid some test-circuits. If I decide which to
>chose, I will share my experiences with other diy-s!
>
>Using LDRs with LEDs sounds cheap, but how much is it linear and
>wide-range?
>I like the idea of using OTAs as current sources (maybe CA3080, beacuse
>it's cheaper than the LM13600), the problem is that to use it as a
>general vc-resistor (floating) it needs 2 opamps, so the circuit could
>be too complex (and expensive).
>
>Regards, Arpad.
>
>-- 
>***********************************************
>*  Arpad Beszedes                             *
>*                                             *
>*  Research Group on Artificial Intelligence  *
>*  Hungarian Academy of Sciences              *
>*  Attila Jozsef University, Szeged, Hungary  *
>*  e-mail: beszedes at cc.u-szeged.hu            *
>*  tel.: (+36) 62/45-41-45                    *
>***********************************************