This is the kind of embarrassing "light-bulb moment" I suspected might come 
out of this posting.  How I could've overlooked the need for bias is beyond 
me.  I'll try it, but the suggestion is already gonging with the ring of 
truth.

Sheepish thanks,
RR

>
> What's keeping the non-inverting input (of the TL072) from drifting off
> into a rail?  I didn't hear anything in your description that sounded
> like a DC path.
>
> Dave
>


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