Why does the Grampian GR1/L ribbon mic needs so low terminating inductance to sound correct at small distance?

The inductance needed in parallel with this brilliant mic is ridiculously low, typically 3mH to 14mH.


The mic sounds correct without any EQ needed when recording at large distances, eg greater than 1-2 meters.


When we sing close to them the sounds gets extremely bass heavy and we need to compensate for the proximity and some psychoacoustic effects.


The fact that a so small inductance is needed makes input transformer design and construction a paradise. Especially if a Magnetec Nanoperm ring core is used the turns are so low the primary winding may consist of just say 7 turns.


Why such a low inductance needs to be connected in parallel?


It is best to find the right inductance exoerimentaly by what sounds correct to listener's brain. Increasing or subtracting turns until the sweet spot is reached for the particular voice, distance, song etc.


Here are some reasons of the very low inductance needed.


They are 25ohms so they need an inductance of 10 times less compared to a 250ohm mic for the same effect.


They are ribbon. So the bass increases the more we come close. At a few inches even the mid range would have increased.


The more softly or intimately we sing close to a mic the more bass or treble is produced by our vocal apparatus. This is described by the voice effort curves.


The more loud the acoustic reproduction is, the more our brain becomes sensitive to bass and treble. This is described by the Fletcher, Munson or equal loudness curves.


A proposed signal path for excellent signal to noise ratio:


Grampian GR1/L - Pleiades V6 head amplifier with a low inductance input transformer - a mic preamplifier


Since at very low frequencies the impedance of an inductor is very low, mich current is flowing through the ribbon. So there is a magnetic force to it as on ribbon loudspeakers. This force opposes it's cause which is our voice. If it did not oppose we would have energy from nothing which is against the law of conservation of energy. This is called Lenz's law. So the ribbon is damped and the pops are greatly reduced. Nevertheless you act at your own risk and always protect those precious excellent sounding mics.


References:


Microphones - Borwick


Flat frequency response form singer's vocal chords to listener's brain, Sound picture recording and reproducing characteristics - Loye, Morgan - Journal of the motion pictures sound engineers


Pleiades V6 schematic


Conceptual Physics - Hewitt


The Feynman Lectures on Physics - Feynman, Leighton, Sands