Aphoristic as it may sound it worked as a Pleiades (R,L) filter.
A specially made mic Sowter 9127g transformer with option of removing magnetic core was used for this test. This was ordered from Brain Sowter possibly before 2000. It had been waiting for so long.
The core was removed.
Inductance and resistance of primary and secondary windings was measured.
A secondary combination of about 315Ω,37mH could be found.
It was connected across pins 2,3 of mic under test.
Signal path:
Male singing voice - various mics - Sowter transformer secondary used as Pleiades (R,L) filter - Sony TC-D5 Pro - Sennheiser HD 580 - singer's ears-brain
Hum and noise could be heard which disappeared when the bobbin was inserted into the mumetal octal base can. If a toroid coil is used then possibly a mumetal can might not be needed.
Alligator cable clips were used to insert the coil in circuit.
When the coil is connected some bass and mid heaviness disapears.
It worked well on Vivanco DM200, Shure VP64, not as well on Sony ECM-250. Each mic needs a different resistance, inductance vector. Different (R,L) coordinates. For example Sony ECM-250 is fine with 40mH but needs 140Ω or even less according to how close the mic is used (proximity effect compensation).
How about an audio signal transformer with no magnetic core?
This might have not been tried yet.
Very low primary inductance may be sufficient. See yesterday's posts. For example 8mH can be fine in certain applications. This creates a high high pass turnover frequency. But bass is not lost if primary winding has a high resistance, typically 50- 80 ohms. This is because resistance on the Pleiades (R,L) filter created changes the low cut or high pass slope to a very gentle one. Less or much less than 6dB per octave.
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