last time i promise was: [sdiy] 5v ADV Bass / AD EG Question.

[Bill Berzinskas]

now that i think of it..   i'm using the VCO from the lmX58 family.. which 
uses a virtual ground as well. . would this have an effect on the VCA?

----- Original Message ----- 
From: "Nils Pipenbrinck" <np at inverse-entertainment.de>
To: <WBerzinskas at nc.rr.com>; "Synth-DIY" <synth-diy at dropmix.xs4all.nl>
Sent: Friday, March 18, 2005 8:24 AM
Subject: Re: last time i promise was: [sdiy] 5v ADV Bass / AD EG Question.


> WBerzinskas at nc.rr.com wrote:
>
>>Hello Jay..  Thanks for the info... schematic - 
>>http://midwest-analog.com/bass.html
>>
>>
> I just had a short view at the VCA, and spotted some things that look 
> wrong:
>
> First off you reference the signal at the inputs to GND. Don't connect the 
> 680ohm resistors to ground but to the virtual ground instread (2.5V in 
> your case.).
>
> Then you should adjust the voltage divider (100k / 680ohm into pin2). The 
> input of the 3080 OTA likes to see a signal difference of something 
> between 75 and 150 mV at it's input terminals. (it's a matter of taste how 
> much distortion you want, the higher the input, the more distortion you'll 
> get). Also out of the blue, the impendance of the voltage divider looks 
> higher than nessesary. I would choose a value of 22k for R21 and pick R1 
> and R2 that you'll see a 100mV difference at the inputs.
>
> Last thing, and most probably the cause of all troubles: The output of the 
> OTA is a current output. You won't see a usable voltage at the output 
> until the current can flow, and I'm not sure if the opamp at the output 
> will do this job. You need a resistance between the output and the virtual 
> ground to turn the current into a voltage.
>
> Nils
>
>
>
>