[sdiy] Expo in one chip?

David G Dixon dixon at mail.ubc.ca
Wed Dec 27 00:29:06 CET 2017


That's an old version.  I've replaced the two transistors with another 2164
VCA, and you don't see the actual expo converting 2164 in that schematic,
but it takes the Expo VC output of the opamp and directly processes the VCO
core current.
 
The parts count really isn't that high.  Almost all of the parts on this
schematic are input resistors and pots for controlling the pitch, and these
are all going to be required no matter what.  The HFC circuit requires two
trimmers because my scheme adjusts both the slope and the intercept of the
correcting current, unlike most schemes which only control the slope.  This
is the key to the Dixie's (and Rubicon's and Atlantis's) excellent tracking.
Again, even an all-in-one expo converter chip would require external
correction trimmers, because every VCO core has tracking error.
 
I don't mean to presume anything about your design skills or level of
experience, but I think that if you really went carefully through a VCO
design, then you would realize that what you are asking for really wouldn't
save very much in terms of parts or labor, both in building and in
calibrating.


  _____  

From: Jacob Watters [mailto:jacobwatters at gmail.com] 
Sent: Tuesday, December 26, 2017 1:58 PM
To: David G Dixon
Cc: SDIY
Subject: RE: [sdiy] Expo in one chip?


Thanks David. Like this example posted on muffwiggler? 

https://www.muffwiggler.com/
<https://www.muffwiggler.com/forum/userpix/2077_dixie_cv_network_2.png>
forum/userpix/2077_dixie_cv_network_2.png


That sovles the resistor contact issue, but it still has a high part count.
Is there no such thing as a chip with linear in on one pin and expo out on
another?

On Dec 26, 2017 4:07 PM, "David G Dixon" <dixon at mail.ubc.ca> wrote:



Yes, it's called the V2164, and it is available from Coolaudio in bulk, or
from many retailers (Small Bear, Mammoth, etc).  It is a quad exponential
VCA chip.  One VCA is used to temperature compensate the expo, and another
is used as the actual exponential converter.  It requires an external
summing opamp.  A third VCA can be used as a precision high-frequency
compensation current source, leaving the fourth VCA for another use.  This
is how all Intellijel VCOs (which I designed) are controlled, and they have
a reputation in euro-land for very decent tracking and stability.


  _____  

From: Synth-diy [mailto:synth-diy-bounces@
<mailto:synth-diy-bounces at synth-diy.org> synth-diy.org] On Behalf Of Jacob
Watters
Sent: Tuesday, December 26, 2017 12:49 PM
To: SDIY
Subject: [sdiy] Expo in one chip?


Expo conversion is require in almost all VCO, VCF and VCA circuits. Matching
transistors and getting the right tempco resistor to be touching them so
they are the same temp is a pain. 

Has anyone ever made an expo conversion chip that had everything required in
one package? Is it still available?


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