A great advantage of 25 ohm microphones

Is that we have direct access to a low thermal noise low resistance voice coil.


State of the art electron tube (or JFET too?) pre preamplifiers and nanocrystalinne cores for input transformers permit a direct connection to the grid through just one low turn ratio transformer. For example a 1:10 ratio transferring 25ohms to 2500ohms. (The impedance transformation ratio of transformers is the turn ratio squared).


Whereas with 200ohm microphones (assuming they do not have an internal transformer to step up 200ohms) it is again almost impossible to connect directly to the grid. Again a transformer should be used although it can be of smaller ratio if the pre preamp has a very low equivalent input noise resistance.


Some microphones, some very successful ones, have an internal transformer to get from 25ohms? to 200ohms. So another external transformer may then be used making the count of transformers before the grid 2. This may even be a good thing. Signal transformers are miraculous devices. They may make the sound better to listener's brain. Is this because of gentle roll offs that make the frequency response flat from producer's brain to listener's brain?


Would it be better or worse if the voice coil of an SM57, Unidyne III etc assuming it is 25ohms is accessed directly? Then an external (one only) transformer can step up to 2500ohms. This 2500ohms output impedance can be connected to the 2500ohm equivalent input noise resistance of the grid of the EF183 battery operated of a Pleiades low noise pre preamplifier. The input impedance of the EF183 at the Pleiades V6 conditions is 100Kohms so the microphone will not be loaded for worse or better. The primary turns of the input transformer will adjust for proximity compensation. How would this setup sound like?


Of course the Unidyne III has an additional high Z internal transformer winding which can be connected directly to grid. It is a high ratio which may roll off some HF which may be a good thing. Bass compensation can be made by adjusting the coupling capacitor to grid or connecting an inductor (Pleiades filter) to the 200ohm winding or a much larger one to the hi Z winding.


References:


Flat frequency responce form actor's, 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