AW: Re: AW: Re: AW: Re: PCB for MAT-04 VCA

[gstopp at fibermux.com]

     Yeah after I wrote that I went back and looked at it, and realized 
     that I had a zero ohm connection from the PNP collector to pin 5 of 
     the 3080. I added different resistors until I found one that imposed 
     the current limit just before clipping - it turned out to be 100K in 
     this case. What a nice round number! Now there is a very friendly max 
     amplitude limit point at just below the supply rails.
     
     I seem to be coming up with the same solutions as you suggest, right 
     before I receive your replies!
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: AW: Re: AW: Re: AW: Re: PCB for MAT-04 VCA
Author:  Haible Juergen <Juergen.Haible at nbgm.siemens.de> at ccrelayout
Date:    3/3/97 9:41 AM


>      I was able to make a "real" exponential VCA by re-creating the
>      3080-type tri-square VCO exponential converter, and just using the 
>      3080 part as a VCA rather than as an integrator. This worked great,
>      although when the gain was unity at CV = 5V, adding to this CV just a 
>      little causes the VCA gain to increase very rapidly (exponentially,
>      even!) into distortion. Maybe a zener to limit the CV?
     
... or simply using a current-limiting resistor from your transistor to 
the
Iabc input of the OTA. (Zeners are noisy, and don't really cut sharp at 
low
currents)