Useful inductance values for microphone input transformers

The input inductance can be used for making the microphone sound flat from singer's vocal chords to the listener's brain. (See nearby euroelecton post). It shunts the microphone in a way that varies with frequency. The low the frequency the more it shunts the microphone. So because of the series usually 200Ω output impedance of the mic a voltage drop is developed right at the microphone's terminal. The curves is a gentle 6 dB per octave low cut, and the 3dB point frequency is dictated by the mic's impedance and shunt or parallel inductance of the transformer or Pleiades filter connected. The Pleiades filter is just an inductor connected in parallel.


The input inductance is adjusted by the number of primary turns.


These are just guidelines of values used in the Pleiades filters and transformers.


The best judge is you and your listeners, you may tailor to your needs.


Whereas great transformers exist on the market, an often found primary winding inductance is 10H. This may be great for recoding a symphony orchestra many feet away form the microphone but not nessasarily for close mic vocals.


So very roughly.


SM57, 58, Unidyne III, D112, RE15, RE16, MD441, (C12a, C24 non omni, they may need treble reduction too), etc.
From 47mH to 100?mH, it depends on how close the mic is and the singing register. Higher registers of the same singer may need more low cut.


MD21, 635a, AKG D130, (C12a, C24 at omni, they may need treble reduction too), etc
From 82mH to 300mH, they do not suffer from proximity effect hence the higher inductance. It depends on low soft or loud the singer wants to sing (Voice effort curves) and how loud the producer wants the end result to be (Fletcher Munson curves).


Beyer M55
As this is a mic that sounds directly very closely flat in frequency responce to listener's brain, no bass or treble compensation or reduction is needed.
300mH or more (it too may sound bass heavy at inductances of the order of Henries). Male singing voices at higher registers (Aquarious-5th dimension) may need an inductance of less than 140mH.


Sony F-98 Low
On male singing voice at 1in it sounds quite bass heavy to the listener's brain. A Pleiades filter of only 53.5mH inductance made it sing and shine at it should at the high frequencies too. L can be higher or less depending on singer or song. An input transformer of such a low primary inductance would be a joy to make as it needs only a few primary turns.


Ribbon microphones such as Grampian 25-50Ω
A typical value may be 3mH (it depends greatly on distance 3mH for 2in?)
It sounds with its well deserved high end detail.


The Grampian DP4/L, ie the 25Ω version sounds amazing terminated with a 140mH input inductance. See Pleiades V6 microphine booster preamolifier.


The Sennheiser MD211 sounds thin with 140mH, it should need 200mH to 400mH.


References:


Pleiades filters - Euroelectron


Sound Picture recording and reproducing characteristics - Loye, Morgan - Journal of the Society of Motion Picture Engineers