MD211 N microphone BBC (1:2) transformer TCD5 impedance matching

Here is a relatively simple experiment to show how important is impedance matching.


1st setup:


Sennheiser MD211 N - Sony TC-D5 PRO - Sennheiser HD580


It does not sound loud and it sounds bass heavy or unfocused to listener's brain. Possible hiss.


2nd setup, signal path;


MD211 N - BBC step up transformer, 1:2, 115mH:382mH - TC-D5 PRO - HD580


It sounds fantastic. Loud, not bass heavy, natural, world class, correct to listener's brain. Great signal to noise ratio as the Sony REC level potentiometer does not have to be at 10 for full modulation but at say 7.


Looking at the whole system form singer's vocal chords to listener's brain it is worth noting how louder it sounds.


Not as loud or sensitive as when a Pleiades V6 active battery electron tube prepreamp is connected between mic and Sony recorder but the sound is there. Much simpler, passive ie no power consumption. No electron tube. Less weight A 1:2 transformer can be made with a small ring core such as Magnetec 073, fit inside a Neutrik barrel XLR module. One can have a world class sound with a passive prepreamp booster transformer that fits inside the pocket.


Some measurements;


The Sony TC-D5 PRO input impedance is measured 4.5Kohm with an escort ELC-120 LCR meter.


When the BBC transformer is connected, the new input impedance reads 360Ohm.


When the impedance of the BBC transformer is measured (transformer unconnected), primary is 220Ohm, secondary is 800Ohm.


its winding inductance measurements are, primary to secondary:
115mH to 382mH


so by division and then taking the square root this should be a 1 to 1.8 transformer, or the impedance of the MD211 should be increased from 200ohms to 650 ohms. This is found by multiplying 200 times 1,8 times 1,8


The low primary inductance ie 115mH cuts some bass thereby reducing bass heaviness due to Fletcher- Manson, Voice Effort curves etc and creates flat frequency response from singer's vocal chords to listener's brain.


Only problem was that this particular BBC transformer was probably not made for microphones. It is not encapsulated or mu metal shielded etc and there was buzz pickup from environmental RF or magnetic field noise.


If one uses a mumetal or nanocrystalinne tape wound core with so relatively few turns needed to get those small inductances  it could be made very easily.


One could immediately have a world class, state of the art sound that should beat almost anything.


The BBC transformer used is the LL/210A and was bought long time ago from Canford UK.


reference:


Flat frequency response from singer's vocal chords to listener's brain, Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Motion Picture sound engineers