Variable separation?

[BillWay]

Can anyone help me figure out how to add a variable separation control to my Crack?

Thanks,
Bill

[Paul Joppa]

Here's one way:

http://www.meier-audio.homepage.t-online.de/passivefilter.htm

Meier Audio posted this a long time ago, but the page is still up. The algebra is a little confusing the way it is presented, but an example is provided. You could probably adapt it to the input of any headphone amp.

Note that it says "you're free to use this design for your own but please understand that commercial use is not allowed" - this is a design of theirs that is no longer in production but they retain the rights to the design - similar to the SEXy Speaker, for example.