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

Chris McDowell declareupdate at gmail.com
Fri Dec 4 06:49:27 CET 2020

haha yes, very good points. TL074 are $0.18 where I get them (lcsc.com <http://lcsc.com/>), and they take up much less than 0.7" in SMD. Component reduction is part of the art (and neurosis) for me. Your cavalier use of a 2164 in a control circuit is at once offensive and inspiring! :P 


> On Dec 3, 2020, at 11:37 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> Chris,
> 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 <mailto: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 <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! 
>> -dk
>> <New Scanner Schematic 3.png><New Scanner Schematic 1.png><New Scanner Schematic 2.png>_______________________________________________
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