[sdiy] help with clicks on VCA

[David G. Dixon]

Dan,

I had a similar problem recently with gates and a 2164-based VCA, but the
problem was solved by decreasing the gate voltage and/or running the gate
through an ADSR first.  I can't imagine how a subaudio triangle wave could
be causing clicks in a VCA.

There are several potential problems with OTA-based VCAs.  One is that the
I_abc pin rides two diode drops above the negative rail, so there can be
significant problems if the modulating signal isn't biased downward by
several volts.  Another is that the OTA seriously distorts the input signal
unless that signal is limited to a peak-to-peak amplitude of just a few
millivolts (this is partly alleviated by using the linearizing diodes).  A
third is that using the Darlington buffer to amplify the output can be
tricky unless it is properly biased.  Most VCA designs with LM13700 avoid
both the diodes and the buffer, and there is not usually a problem.

(Building VCAs from linearized 2164 is much more straightforward.  I'll
never go back to OTAs for this!)

Can you send along the schematic of the VCA you're using?


> I am trying to make a tremolo.
> 
> I have tried with an OTA vca and with an SA571 vca
> 
> the SA is better as I dont have to worry as much about the peak to peak
> values coming in and the CV rejection is better
> 
> BUT
> 
> neither are ideal
> 
> when driving with a squre or TRI wave i get all kinds of CLICKS imposed on
> the audio (i am not talking about
> when there is no audio coming in. i am talking about clicks imposed ON the
> audio)
> 
> 
> anybody have any ideas on how to remove these clicks?
> 
> someone recommended slewing the Control waveform.
> 
> i was quite surprised that an OTA was so prone to this
> 
> 
> thanks
> --------------------------------------------
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