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

[Nils Pipenbrinck]

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