How to choose the inductance of a Pleiades filter

This is done by experiment.


The judge is the listener's brain.


In general, the more:


Bass heavy a mic is, ie lower membrane reasonant frequency


Bass reverberant a room is


Soft is the voice to be captured by the mic


Loud the voice is listened to when played back


Close a directional mic is to source


High notes are sung (this needs high cutoff);


The more bass heavy the sound perception to the listener's brain will be.


So the Low cut turnover frequency will have to be higher, more than 1kHz in extreme cases.


So the less the inductance of the Pleiades filter or the primary inductance of the Pleiades input transformer will have to be.


This is good news as it makes the input transformer primary turns needed very few.


For example using a ribbon mic at a few inches while singing softly, less than 10 primary turns can be wound on a Magnetec M073 ring Nanocrystalinne core. If a step up ratio of 1:20 is desired, only 200 turns need to wound for the secondary. This clearly can be hand made almost everywhere.


The Pleiades transformer or the Pleiades filter is connected of course after the internal mic ribbon transformer. In the above example the 1:20 transformer will step a 200Ω impedance by 20x20=400 ie to 80KΩ. This can clearly drive the grid of an electron tube amplifier. If a Pleiades V4, V5, V6 battery powered electron tube prepreamplifier is used an amazing overall signal to noise ratio can be achieved even with a whispering voice and a low output ribbon microphone. Schematics are open source, public domain.


References:


Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Motion Picture Sound Engineers


Pleiades Filters - euroelectron


Pleiades schematics - euroelectron