[sdiy] Interpreting four-pole-w-feedback circuits

Donald Tillman don at till.com
Thu Mar 6 19:07:27 CET 2008

   > From: Aaron Lanterman <lanterma at ece.gatech.edu>
   > Date: Thu, 6 Mar 2008 11:53:01 -0500
   > Sender: synth-diy-bounces at dropmix.xs4all.nl
   > BUT... I'm bewildered by the four-pole arp patent circuit. The stages  
   > look inverting (since the opamp-cap-in-feedback is an inverting  
   > configuration), the four of them in casecade should give total  
   > noninverting behavior, that's OK... but... it looks to be like the  
   > feedback loop is POSITIVE. I'd expect the input to either go through  
   > an inverting opamp before being fed back, or for it to go to the  
   > negative terminal of the first OTA (so that first stage would be  
   > noninverting, but the other three inverting, so it would all go  
   > through with a minus sign).
   > What am I missing? Where is my thinking going wrong?

Hey Aaron,

Good eye!  You're right, the ARP patent drawing is in error.  

If you check the production schematic, you'll see the inversion.  See
Joachim Verghese's ARPTech site for schematics of the 4072 and 4075
modules with their diff inputs and how they connect up the resonance

(The ARPTech pages are long gone, but I've kept an archive version
here: http://till.com/arptech)

  -- Don

Don Tillman
Palo Alto, California
don at till.com

More information about the Synth-diy mailing list