The value of phantom blocking cap depends on the input impedance of the preamp. On a V672, the input impedance is equal to the sum of the values of the input resistors, remembering that the V672 is essentially a "virtual earth" input.
The corner frequency of the RC high pass filter formed by the blocking caps and the input resistors is calculated using the normal formula e.g. the -3db frequency is found where the input resistance is equal to Xc of the blocking caps.
If you use the typical 750R + 750R input resistors seen when using V7672 as a micpre, and desire a -3db frequency of say 2 Hz, solve for Xc = 1500 ohms at 2Hz.
Xc = 1/ (2 * Pi * f * C) so setting Xc = 1500 ohms and f = 2 Hz C needs to be 1/(12.56 * 1500) or about 53 uF. Wow! That's either an electrolytic or a big film cap. I use a cap on each side, but I guess that not strictly necessary.
For other input impedance you could use the same process.
(In my V672 "PreBuddy" front end board, I have 2* 47uF which gives about 4Hz with the 2*750R input, since the 2 caps are essentially in series)
Cheers!
Michael
