on pleiades filters

A useful property of Pleiades filters is that if 2 are connected one after the other the new cut off frequency is the sum of their cut off frequencies.


( Using the known relation for 2 inductors in parallel:  1/L=1/L1 + 1/L2 )


f = R / ( 2 x PI x L )   ==>


f = R / (2 x PI) x 1/L ==>


f = R / (2 x PI ) x (1/L1 + 1/L2) ==>


f = R  / (2 x PI x L1) + R / (2 x PI x L2) ==>


f = f1 + f2

pleiades preamplifier schematic

Please note tube is drawn upside down!


The grid needs  to be biased  positive hence the order of 4 Megohm resistor.


Input impedance is around 100KOhms.

Pleiades Microphone Preamplifier

Possibly the best way on our planet to amplify the tiniest signal with less noise is to use a battery powered electron tube.


This is done by using a high transconductance variable mu tube such as an EF183 at very low voltages.


The heater and plate can be supplied with 3.7V only and a few Megs resistor is needed between 3.7+ and control grid to free electrons and have an anode (plate) current of  >100 micro amperes.


This can make a nice portable single battery electron tube pre pre amplifier. Pleiades filters can be used as input and output transformers. It's a beauty to "hear" the absence of noise from battery clean dc supplies.


Could this be used to detect a most distant object in universe by being the front end of a radio telescope helping to unveil borders of the past?


Georg Neumann used lower than specified heater voltage and small plate voltage on the VF14 tube on the legendary U47 Telefunken Neumann microphone and changed our planet musically.

At 0Hz the Pleiades Filter acts as a short circuit

This damps the microphone membrane and dissipates pop noise without needing a pop screen.


It should be great for live vocals with dynamic mics protecting the first stage of the mixing console from the low frequency crowd PA and stage noise.


Great with ribbon microphones at any distance.

The most beautiful addiction is being under the sun

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The Pleiades Electra Amplifier can also be made with Junction Field Effect Transistors JFETS