[sdiy] CEM3340 question -- interpolating scanners (again)

David G Dixon dixon at mail.ubc.ca
Fri Dec 4 06:37:29 CET 2020

1) Buffers are awesome.
2) TL074 opamps are $0.50 each.
3) it takes up 0.7" of PCB space.
I don't really see a downside, and it just works without fuss or compromise.
Embrace the buffer!


From: Chris McDowell [mailto:declareupdate at gmail.com] 
Sent: Thursday, December 03, 2020 9:27 PM
To: David G Dixon
Cc: Dakota Melin; Synth-diy at synth-diy.org
Subject: Re: [sdiy] CEM3340 question -- interpolating scanners (again)

[CAUTION: Non-UBC Email]too many replies, sorry!  

I goofed, the spread is more like 300mV. if you barely tweak the bottom
resistor, you can narrow it to ~120mV. If you tweak your expectations, you
can use whatever lol 

On Dec 3, 2020, at 6:45 PM, David G Dixon <dixon at mail.ubc.ca> wrote:

Here's my schematics (attached as PNG files).  The trapezoid generator
(picture 2) looks remarkably similar to Chris McDowell's.  These schematics
were generated in Multisim.
On the Control schematic (picture 1), the pot on the left is the Width
control, and the pot in the centre is the Fader.  Both are subject to
voltage control (as indicated by the little function generator graphics).
The 1.87k resistor on the Width pot is to limit the upper voltage to about
10V, and the weird buffer circuit on the bottom of the Fader pot is to set
the bottom fader voltage to -12mV.  Originally I had this grounded, but the
voices wouldn't shut off completely when the fader was all the way down if
the Width control voltage was low.  Putting this tiny buffered voltage there
fixed it, and I had a spare opamp on the board, so no worries.
The opamp with the 9.09k resistors is there to apply a linear transformation
to the Width voltage.  This linear equation is V_2 = (V_1 + 2)*5/11.  This
voltage becomes the upper voltage on the Fader pot, and that is what
guarantees that the trapezoids fit nicely into the Fader window at all Width
settings.  This voltage also drives a linear VCA which scales the incoming
Fader control voltage so that this CV has the same effect on channel
scanning regardless of the Width setting.  


From: Dakota Melin [mailto:dksynth at gmail.com] 
Sent: Thursday, December 03, 2020 10:38 AM
To: David G Dixon
Subject: Re: [sdiy] CEM3340 question -- interpolating scanners (again)

[CAUTION: Non-UBC Email] 
Would love to see these new scanner schematics! 


<New Scanner Schematic 3.png><New Scanner Schematic 1.png><New Scanner
Schematic 2.png>_______________________________________________
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