[sdiy] Lowest distortion neede for VCA

[Harald]

I am just in the proces measuring THD while varying Id, I_abc and input. Vpp. 
Preliminary result for I_abc is that increasing it over 0.8mA have no impact 
in THD. I increased it up to 1.5mA and THD stayed constant.. (Id set to 1.4mA)

The LM13700 varys a lot. I checked 5 pieces and they varied between 0.1%THD to 
0.4%THD in the same seting. So some selection is needed to obtain best 
results.

Later i will check 3080 and 3280 for comparison .

I'll keep you posted.



 Am Montag, 16. September 2013, 01:23:55 schrieb Brian Castro:
> For synth VCA's used for attentuating audio signals I go right in the sweet
> spot as per the datasheet. For teh LM13700 I use the linearizing diodes and
> go for an Id and Ibias(max) of 0.8mA, and scale the input for 50mVp-p. This
> is pretty much the case with most synths I've documented or seen in service
> manuals.
> 
> Regarding linear vs log there may be some assumptions made here. If you're
> using the VCA as a volume control for a VCO etc, and the voltage is being
> controlled by a linear pot, then certaintly you can use a log control
> input. In this case you may consider the SSM/Cool Audio 2164. If it's to be
> controlled by a CV from a modulaton source (ADSR etc..) then yes usually a
> linear control input is used.
> 
> On Sun, Sep 15, 2013 at 9:19 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> > I'm with Tom.  I (and I woudl suspect most modular users) use linear VCAs
> > almost exclusively post envelope.  The envelope is already exponential.
> > 
> > > I disagree. Most of the "classic" synths use a linear VCA for
> > > the audio output too. The envelopes provide a log decay, but
> > > the VCA itself is linear.
> > > Whilst a log VCA might be more "logical" in some sense, it
> > > isn't actually what was commonly used. Check the schematics
> > > of your favourite classic synth to check, but my money's on linear.
> > > 
> > > 2 cents,
> > > Tom
> > > 
> > > On 15 Sep 2013, at 14:48, Harald <sdiy at haraldswerk.de> wrote:
> > > > Because lin are mostly used for VC processing and log for audio.
> > 
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy