[sdiy] SSM2164 Phaser - another way?

[David G Dixon]

> > The signal input of the 2164 is essentially a virtual 
> ground, so your
> > 30k+2164  becomes essentially a constantly grounded 30k.
> 
> That's the thing...  At high gain settings almost all of the 
> 2164's diff amp gain is being steered to the output and 
> almost none is feed back to the input, so there is not enough 
> loop gain to consider it a proper virtual ground.  The input 
> impedance of the 2164 will rise at higher gain settings.
> 
> (Whether or not this a good mechanism for a phase shifter is 
> another matter...)

The 2164 does develop a measureable voltage on its input terminals when the
gain is increased (I've measured it).  It is not large, but will create some
error if the design absolutely depends on that voltage being exactly zero.