AC coupling questions

[media at mail1.nai.net]

As you may know, I'm working on the capacitor FAQ.  (Since I have plans to
go away Saturday, I might stay in tomorrow night and finish it.)

Anyway, as it turns out I am also fixing some mods on a synth that has an
added audio input to the filter.  So for its own protection, a cap was
added to block direct current in the event a CV was accidently plugged into
it.

On this subject the FAQ says, "Use 0.47uf metallized polyester or
polyproplyene.  Values less than this will attenuate bass frequencies."
Mind you it makes no mention of impedances.  Regardless, a .47uF
polypropylene cap is the size of a bumblebee.

I've looked at a number of audio circuits and have found a wide variety of
capacitor values used for AC coupling.  While I'm fairly sure that a
capacitor of any value will block DC, we don't want to create a high pass
filter in the audio range.

Is there a capacitor value that is typically used for this in modulars??
Is there a simple formula that one could use to calculate the best value??
Further, is blocking DC always necessary??  I'm guessing typical coltrol
voltages wouldn't hurt modules with op-amp imputs.


PEACE OUT :)
MARK