How to easily calculate the inductance of a Pleiades microphone filter

Assuming a microphone impedance of 200 Ohms we use the number 200.


We divide this number by 2 then by 3.14 (π) and then by the desired cutoff frequency. This gives us the required inductance that must be connected to the microphone in parallel.


If instead of the cutoff frequency we divide by a given inductance we get the resultant cutoff frequency.


Example:


1. To design a Pleiades cutoff filter of 470Hz:

200 devided by 2 = 100 devided by 3.14 = 31.84... devided by 470 = 0.067H

so the required inductance to be connected parallel to the microphone is 67mH


2. Let's find the cutoff frequency of an inductance of 100mH in parallel with a microphone of 200 ohms output impedance


200 / 2=100  / 3.14 =31.84...  / 100 = 0.318 kHz

therefore the cutoff Pleiades filter proximity compensating frequency is 318Hz


Inductors can be very easy wound on the extraordinary Magnetec 073 Nanoperm core. It has extreme inductance index, small size, it's toroidal and therefore there is no hum even if it is unshielded. Everything can then be fitted inside an XLR inline male to female adaptor. Neutrik modules are strongly recommended.