Grundig GDM311 part 2

What a great mic !


Another specimen was tested. There seems to be great sound variation from different specimens.


As natural sound as one could expect.


Some sibilant problems when used at very close distance.


If it sounds very bad it is likely there is something wrong or the polarity needs to be reversed.


With the correct polarity it sounds very clear, surprisingly so.


If it sounds thin on the 200Ω terminal, the cause is the following:


It already has an internal autotransformer of low primary inductance, ie a Pleiades filter. The low inductance together with the output impedance of the voice coil introduce a low cut so that the mic sounds correct to listener's brain.


When it is connected to an amplifier with an input trasformer of relatively low input inductance (ie 400mH) there are then two coils (inductances) in parallel. So inductance becomes even less and the the mic departs from natural and sounds thin to listener's brain.


If it is connected to a transformer of very high input inductance such as Sowter, Jensen etc (4H to 10H) the sound is great.


Alternatively one can use the high Z out of the internal autotransformer and connect to grid of electron tube (possibly through capacitor) or gate of JFET. The sound is then fantastic. The trasformer has an unusually high turns (step up) ratio. It may need termination for damping. The Pleiades V4 used does this automatically.


The sensitivity then is so high one can hear sounds from hundred of feet away.


And the mic still sounds full and natural when one speaks or sing at 1 inch distance.


Setup, signal path:


Grunding GDM311 - Sowter 3104x - Pleiades V4 at 3.4V - Realistic disco mixer mic mino input - HD580


Or


Grundig GDM 311 H connection - Pleiades V4, 3.4Volt - Realistic disco mixer at mono mic input - Sennheiser HD580


The capsule seems to be low impedance, 10-30Ω? .


It is stepped up by an autotransformer to 200Ω or 30KΩ.


The primary inductance of the autotransformer is very low, typicaly 10mH.


This is to remove the exagerated low frequency perception to listener's brain.


If more body is needed one could experiment and wind a Pleiades trasformer using a Magnetec Nanocrystalinne Nanoperm tape wound core. One could then increase the body of sound and tailor to what is needed at any impedance.


It seems to be The mic for ASMR as it sounds fantastic at 0in and 2 can make a great binaural pair if two are placed next to one' ears while recording.