[sdiy] Lowest distortion neede for VCA - linearizing the LM13600/13700 & other dual VCAs

Robin Whittle rw at firstpr.com.au
Tue Sep 17 07:04:37 CEST 2013

Hi Ian,

The purpose of this circuit:


is to linearize the audio signal AKA to reduce audio distortion at high
signal levels.  The control current for these particular VCAs - LM13600s
~= LM13700 - is linear, in that doubling the current to the control
input doubles the current of the output signal.

A VCA with an exponentially responding control voltage or current
control input would have a response such as increasing the control input
by X volts or mA doubles the current of the audio output signal (and
therefore its voltage in the current to voltage converter which follows)
and adding another amount X would further double the output.

Building a circuit to make a log/exponential (however it is referred to)
control voltage or current responding VCA into a linear responding one
would involve producing the log of the control signal to feed into the
original VCA's control input, which exponentiates it to produce its
internal current.

  - Robin

On 2013-09-17 2:04 PM, Ian Fritz wrote:
> At 07:58 PM 9/16/2013, Robin Whittle wrote:
>> I have not tried this technique, but it looks good to me:
>>    Low-cost audio VCA has high performance
>>    Mike Sims, Lectrosonics Inc, Rio Rancho, NM
>>    EDN 1995-01-19
> Several folks have built this with good results.  Makes more sense than
> linearizing an expo chip, seems to me, and less expensive.
> Ian

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