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

Chris McDowell declareupdate at gmail.com
Fri Dec 4 05:24:37 CET 2020

This is great, thanks.

> The trapezoid generator (picture 2) looks remarkably similar to Chris McDowell's.

It sure does, and I definitely don't feel as bad about my part count now seeing this! ;) though mine doesn't do any of your lovely width controI. I have a few questions:

1: I don't quite understand how you get trapezoids out of this vs just triangles! I'm missing something fundamental here about how you manipulate the input before the main control channels. 

2: Why the 100k to ground in the control channels, in the "subtractor but invert at a threshold with zener" stage, and similarly why the 49.9k input there? I ask because it -seems- like the 100k input and no resistor to ground in my circuit achieve the same thing, but maybe I'm missing something in yours. 

Thanks for sharing, this is a very thorough approach. I am spooked by the variability and softness of zener turn on voltages at low currents, which I generally tend toward, so I'm not going to jump ship from the programmable threshold approach just yet. 

It is funny, seeing this approach and working through an alternative described to me by another list member. Sort of however you swing it, you need 3 op amps for a combination of a buffered resistor string, a "single sided wavefolder" (how I think of it), and some subraction or offset. You can save the buffer op amps by using small resistors and accepting some resulting error, but I'm averse to using current in that way. My employer has had me chasing down stray 10s of mW for the past decade in designs, guess it has rubbed off. 

Your design is similar to the one described to me in that you offset the incoming ramps (more reasonable than my stab I think!) vs offsetting the resulting peaks, but interesting that it results in a very similar circuit. 

Fun stuff. I suppose I could actually build one sometime! 


> 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! 
> -dk
> <New Scanner Schematic 3.png><New Scanner Schematic 1.png><New Scanner Schematic 2.png>_______________________________________________
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