[sdiy] VCA response - continuously variable between linear and exponential

houshu at muj.biglobe.ne.jp houshu at muj.biglobe.ne.jp
Sun Mar 10 02:49:03 CET 2013

Hello Neil and List,

I designed variable current sources for LM13700
with bipolar power supply.


As usual, I tested it only on LTspice.
Quite good exponential curves were obtained thanks to 
low impedance design.  If you don't need precise linear response,
An audio-curve 2k-ohm pot seems to be good for continuous control.  
If available, reverse-log curve pot with switch would be the best
to have precise linear response.

Temperature dependency was not so bad.  The response shrunk just 10%
in horizontal axis even when the temperature changed from 5 deg.
(in C) to 35 deg.

Saitama, JAPAN

----- Original Message -----
From: houshu at muj.biglobe.ne.jp
To: "neil harper" <metadata at gmx.com>
Cc: "Synth-Diy" <synth-diy at dropmix.xs4all.nl>, houshu at muj.biglobe.ne.jp
Sent: Sunday, March 3, 2013 01:53:37
Subject: Re: [sdiy] VCA response - continuously variable between linear and	exponential

Hello Neil and List,

I've designed continuous curve VCA.
Precise exponential can not be obtained.
But, fractional power curves were available.

Final VCA design (Built for battery operation, and worked as in simulations.).

Current source designs:

This is basically a function generator combining log and exponential, 
which may be analyzed by theory of translinear circuit.

Continuous curve control is possible, but the required
resistor range is too large to obtain linear-like response.  
So I chose 3 curves.

Here is comparison of the 3 curves and linear and exponential: 

If you can wait for 1 month, I'll design a current source circuit 
for LM13700, though I myself would not test it on a breadboard.

Saitama, JAPAN

----- "neil harper" <metadata at gmx.com>
hey all,
i'm looking for some advice/pointers on how to get a VCA response curve
to be continuously variable between linear and exponential. i can do
switchable between the two, but is there any trick to getting everything

if it helps, my VCA is lm13700 based but i don't mind starting with
something else.

//neil harper
every wave is new until it breaks

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