It is great that relatively thicker wire microphone voice coils exist with fewer turns such as Grampian DP4/L etc

They must be having a very low thermal noise.


On the November 1955 BBC Reseach Department report of their visit to Vienna, Hamburg, Cologne it is mentioned that the very fine wire for AKG 200ohm microphones was imported from Switzerland.


With 25 ohm voice coils being the British standard the signal voltage stepping up can be externally performed with a bigger and very efficient modern magnetic transformer core such as the Magnetec 070. For example the Pleiades V6 pre preamplifier uses for the primary (for 140mH inductance ) a wirewrap (which is think) wire coil of only 35 turns. The core is the Magnetec Nanoperm 070 and the step up ratio is 1:10. The sound leaves so far nothing to be desired when connecting a Grampian DP4/L.


Using this point of view it seems that the thin long wire of higher Z voice coils is a difficult and marvelous effort to get the step up ratio or impedance transformation on the voice coil itself.


200ohm microphones need an external transformer anyway.


The low noise achieved with the Grampian DP4 connected to the Pleiades V6 is impressive. So it should be with any other British, American etc microphones of 25 ohm impedance and good quality.


The overall frequency response is also amazing with top quality HF detail to listener's brain. The bass, mid, presence and liveliness is phenomenal too.


All there is between the 25 ohm voice coil and the grid of the front end electron tube is just one transformer (and a coupling capacitor from secondary to grid). The usual practice as when connecting voice coils that are already 200ohms.


This is thanks to the Pleiades V6 having a very low equivalent input noise resistance (not to be confused with input impedance).


Preliminary tests suggest an equivalent input noise resistance of the order of 2500ohms and an input impedance of the order of 100Kohms. The low input impedance is perhaps a fortunate byproduct of the 8Mohm electron accelerating resistor from anode to grid making the electron tube or electronic valve operate with normal transconuctance at only 4V at the anode. Heaters are underheated as in the Neumann U47 internal amplifying circuit. Reasons are attempted to be explained on other euroelecton posts.


The 25ohm voice coil when stepped up by 1:10 becomes 2500ohms which is a good match in terms of low noise to the 2500ohm equivalent input noise resistance. And the loading is minimal since an equivalent series output resistance of 2500ohms is loaded by 100Kohms.


Or calculating from right to left. A 100Kohm input impedance is transformed by the 10:1 transformer to 1kohm. And the equivalent input noise resistance of 2500ohms is transformed to 25ohms.


So the 25 ohm voice coil sees a 25ohm equivalent input noise resistance and is terminated by 1000ohms.


Other good news? The sound of this setup may be too bright for some microphones. But few primary turns may compensate for both bass and treble exaggeration. The voice coils of SM57, Unidyne III, 545 should be 25 ohms. Hmmm!


Signal path, Setup:


Males singing voice at 2in to 8in - Grampian DP4/L - Pleiades (Rag=8MΩ) - Sony TC-D5 Pro - Sennheiser HD580


References:


Pleiades V6 schematic


Flat frequency responce from actor's, singer's vocal chords to listener's brain, Motion Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Motion Pictures Sound Engineers


BBC Research Department Report No. A-041