Pleiades input transformer to compensate a Grampian ribbon mic for proximity effect

A Pleiades transformers was made using Magnetec's 388 Nanoperm core. Primary turns were 6 and secondary turns were 110. Primary inductance was 0.6mH which was previously found to be a good value to compensate the Grampian ribbon for proximity of about 1 inch. Assuming a mic output impedance of 50 Ohms this gives a low cut turnover of 20KHz! Pop behaviour was very good due to the electromagnetic damping of the ribbon from the short circuit offered by the Pleiades filter at close to 0Hz.


The secondary was capacitor coupled to a Pleiades input amplifier made of a EF183 electron tube triode connected and 4MOhms from plate to grid!, and a bit more than 3V at plate, Vb being 4,5V!


Voice was extremely natural with excellent transparent detailed and soft sibilants, possibly due to the resonance of the ribbon being at sub low frequency giving very low displacements at HF and the excellent transient response of the very lightweight ribbon. Some apparent hiss may be improved if the step up ratio is increased from 1:18 to 1:30. I am pleased after less than 20 years of experiments to hear such a nice and natural voice reproduction from the excellent ribbon mic. The Grampian mic had its 25-50Ohms output connected to the 0.6mH Pleiades primary winding.

Positive bias on grid can make an electron tube to operate fantasticaly at extremely low plate voltage

One of the best presents of my student was how she got disapointed when I told her that the control grid is usually biased negatively. She said, no it should be positive to free electrons.


So it was tried and a 4MOhms resistor from plate or Vb to grid made an EF183 operate as an excellent low noise, small signal microphone amplifier at Vb of only 4.5V. Heaters were also supplied with 4,5V, (less than specs rating), Georg Neumann style.


It sounds amazing.


Preliminary tests show that the input impedance of this configuration is around 100KOhms.

Pleiades Input Transformer for MD-421 at less than 1/4 in mouth distance

After extensive trial and errors, 20mH primary inductance was found to give a good male voice balance for the Sennheiser MD421 used at distances of less than 1/4in.


A Pleiaades transformer of 1:7 was connected to the grid of an EF183 through a capacitor. Plate, g3, and g2 were connected together. 4MOhms were connected from plate to grid ! And Plate supply voltage was 4.5V!
A load resistor of 15K was used to feed a transistor mic input stage through a coupling capacitor.


Sound was very clear, low noise (even when used in a noisy environment), every voice nuisance can be heard but somehow sibilants or naturalness leave something to be desired. Prior experiments with a Grampian ribbon mic terminated with a Pleiades 0.6mH and feeding an Altec 4722 feeding the EF183 stage gave very promising results of naturalness that you can only listen at BBC voice announcements. Hiss left something to be desired. No free lunch in  nature.