Using the Grampian ribbon mic on various experiments it was best found to low cut with a Pleiades filter or transformer of about 2.3mH input inductance. This value gives a natural mid and treble production. Of course it can be fine tuned according to need
Further low cut if needed may best be done by other ways, in our experience nets it was done by varying the coupling capacitor from anode to mixer a series resistor of 10nF to the usual 1uF was typically used.
The Sennheiser MD -421 does not need a extra low cut the
Pleiades 19.5mH giving an excellent bass responce at a mic - mouth distance of less than 1 inch. This may sound contradictory with following entry with Canford input transformer of 419mH. At 19.5mH even cutting all treble, the bass still sounds unexagerated. More work needs to be done to decide. The MD421 inherent bass cut makes the experiment more difficult.
Electromagnetic compensation due to the inductor short circuiting the low frequencies is very effective as a pop and wind filter.
The extra treble due to close distance exaggeration was not reduced with a shunt anode capacitor as described below.
A clip can be heard on YouTube's Pleiades Radio channel 4th broadcast. No treble cut was used and if I remember well a Canford impedance converter (419mH measured primary inductance) was used as step up. This step up as documented is feeding a EF183 Pleuades 4.5vVbattery pre preamplifier with 3.9Mohms from anode to grid for positive (less negative) bias.
In fact treble cut must have been used as the cable capacitance makes a high cut filter with the Pleiades electron tube preamplifier output impedance.
Further low cut if needed may best be done by other ways, in our experience nets it was done by varying the coupling capacitor from anode to mixer a series resistor of 10nF to the usual 1uF was typically used.
The Sennheiser MD -421 does not need a extra low cut the
Pleiades 19.5mH giving an excellent bass responce at a mic - mouth distance of less than 1 inch. This may sound contradictory with following entry with Canford input transformer of 419mH. At 19.5mH even cutting all treble, the bass still sounds unexagerated. More work needs to be done to decide. The MD421 inherent bass cut makes the experiment more difficult.
Electromagnetic compensation due to the inductor short circuiting the low frequencies is very effective as a pop and wind filter.
The extra treble due to close distance exaggeration was not reduced with a shunt anode capacitor as described below.
A clip can be heard on YouTube's Pleiades Radio channel 4th broadcast. No treble cut was used and if I remember well a Canford impedance converter (419mH measured primary inductance) was used as step up. This step up as documented is feeding a EF183 Pleuades 4.5vVbattery pre preamplifier with 3.9Mohms from anode to grid for positive (less negative) bias.
In fact treble cut must have been used as the cable capacitance makes a high cut filter with the Pleiades electron tube preamplifier output impedance.
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