[sdiy] FW: Later MS-20 filter with 2164

[David G. Dixon]

This is what it was supposed to look like:

> 
>                                                33k
>                                           |---/\/\/--- GND
>                                           |
>                                           /
>                                           \
> opamp neg --- 1N4728 >| --- |< 1N4728 --->/ 100k pot
>                                           \
>                                           /
>                                           |
>                                           |
>                                        opamp out
>