ENS-76 VCO Option 2 question

[Bob Zimmer]

At 06:45 PM 3/24/97 GMT, you wrote:
>I must be missing something terribly important here, and if you have a
>little time...
>
>Can you briefly explain how this circuit converts triangle to sine?  I
>believe you that it should (and does with the old parts), but I am
>baffled.  It looks (to me) like the CA3080 is nothing more than a
>noninverting buffer (control pin tied to +rail thru constant
>resistance = constant control current) which feeds an inverting
>buffer...  Does the CA3080 have a real slow slew rate?  Or does it
>have to do with the fact that the second buffer (556) has a zero ohm
>input resistor?  (and in the latter case, I am *still* baffled).  I
>have never used the CA3080 myself, but I am looking for a simple
>triangle to sine converter that does not require filters or an AGC.
>
>- Scott Gravenhorst (Synthaholic)
>
>Programming: The Ultimate Computer Game. | Windows 95: The Ultimate
>Unfortunately, you never win.            |      Pain in the Butt
>********* Hell: Windows 95, 16 bit apps, and Banyan Vines. ********
>
>"I didn't do it."     -- Bart Simpson
>
I can't explain how it works as I lack the background, but it does!  Here is
the description as printed in Electronotes (EN#75-page 12, and in the PCC):

"The two opamps form a triangle-to-sine converter using the overdriven
differential amplifier principle."

This text points to a reference in the Musical Engineer's Handbook-Chapter
5b page 18) as follows:

"For a practical circuit, we can use the OTA since this has a differential
input structure (like all op-amps) and its total gain can be controlled
(unlike all op-amps).  A practical circuit that sets the peak triangle input
at about 80 mv is ...(see my previous post - Bob Zimmer). ... The OTA uses
the set of "Universal" diff amp curves, and the performance should be device
independent."

Hope that helps!

  >=== Bob Zimmer -- Philadelphia PA ===<
  >=== bzimmer at voicenet.com          ===<