Terminating a microphone with a much higher impedance than its output impedance

Practice shows it sounds very nice.


Pleiades examples:


Grampian GR2/L (25-50 Ω) - Sowter 1:20 - Ferrograph electron tube tape recorder with 1 MΩ input impedance
(Nice and bright smooth sound)


Grampian DP4/L (25Ω) - Pleiades 1:10 with 140mH primary inductance  in a Pleiades V6 mic booster configuration
(Nice and bright smooth sound)
Mic sees 100KΩ:10:10 ie 1KΩ since the electron tube at the Pleiades V6 schematic has an input impedance of 100kΩ.
1KΩ is 40 times higher that 25Ω, the mic's output impedance.
The Pleiades V6 configuration is very low noise, ie of low equivalent input noise resistance so such connection still gives very low noise.


So should for example an AKG D190 be just stepped up by 1:4 in a Pleiades V6 mic booster front end amplifier?
So the mic's impedance would be stepped up to 200Ωx4x4 which is much lower than 100KΩ.
This gives a 1:30 margin ie mic is loaded by a 30 times higher impedance.


But if the above arguments are correct why does a Sony TC-D5 Pro with an input transformer sound better to Sony TC-D5M when an AKG D190 is connected. The input impedance of the Pro model is lower.



Later addition 20082309:


When the secondary of a transformer sees a higher impedance broad reasonance is less damped so there is a high frequency rise. So the above examples are not mic's behavior but input transformer's behaviour or both. See also Coil design and construction manual - Babani, Rupert Neve interviews on how a transformer can make subtly a brighter sound, Radio Engineering - Terman