A Pleiades filter is an inductor, (reactor), (coil) connected directly across the microphone output (shunt, in parallel) in order to cut bass frequency content.
This is to compensate for the proximity effect, Fletcher-Munson curves, voice effort curves.
The microphone used for Aquarious - 5th Dimension was an RCA 77DX at the V1 setting which connects the inductor across the secondary of the ribbon input transformer. As producer Bones Howe comments this not only gave the correct sounding low cut but also eliminates mouth and tongue artifacts leaving the melody to shine through. This may be explained by the electromagnetic damping effect of the ribbon by the shunt inductor according to Lens's law or conservation of energy. (If the ribbon was not damped there would be a very high current flowing through the low impedance at low frequencies of the shunt inductor giving more energy output than is input by singing which is impossible).
Pleiades filters can be built inside an XLR female to male adaptor. A suitable small magnetic core is the Magnetec 073 Nanocrystalinne tape wound ring core. A wire is wound inside this core and its ends are connected to signal pins ie 2 and 3. Pleiades filters are open source and public domain.
References:
Classic Tracks - 5th Dimension - Sound on Sound
https://www.soundonsound.com/people/classic-tracks-fifth-dimension-aquariuslet-sunshine
Flat reproduction from singer's vocal chords to listener's brain -
Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Motion Pictures Sound Engineers
Instructions - RCA 77DX
http://www.coutant.org/rca77dx/77dxinst.pdf
Pleiades Filters - Euroelectron
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