Comparing Microphones

It is not as easy as it seems.


Each microphone like to be terminated by or likes to see a particular input tranformer primary winding inductance so that it approximates the ideal of flat frequency response from producer's brain to listener's brain.


The primary inductance seems to have a big influence on the end result as it controls whether the sound will be bass heavy or bass light according to what is the acoustic intensity when recording and what is the acoustic intensity when reproducing. So a specific value of inductance is a guideline.


For example the Grampian DP4/L (25Ω) sounds so good terminated by 140mH that it may be thought it sounds slightly more detailed than real life. More detail is possibly because we usually reproduce a voice at a higher acoustic intensity level than the original voice acoustic level. Compared to all mics tested so far the Grampian is the most natural in every respect but it has to be terminated with 140mH and preamplified by a ultra low noise booster amplifier such as the Pleiades V6.


The Sennheiser MD211 prefers to see an inductance of the order of 260mH in order to approach the ideal of flat frequency response from producer's brain the listener's brain. [Lowe, Morgan]


The same model of a mic but with a double output impedance should like to see a double optimum inductance.


Directional mics should like to see a lower inductance for compensation of the proximity effect. (Pleiades wave filters). But there is danger of overloading the core and perhaps an inductor in series with a resistor (improved Pleiades wave filter) connected shunt (in parallel) to the voice coil should be used as is done on the AKG D1000.


The inductance values suggested are rough guilines. Another reason for this is that there is already some low cut effect due to the intersection of the V6 booster amp electron tube anode resistance with the output transformer primary inductance.


Signal path, setup:


Male voice singing When I Fall in Love - microphone - Pleiades V6 front end booster amp jig with octal base input transformer - Sony TC-D5 Pro used as followup preamplifier - Sennheiser HD-580


Reference:


Brain to brain flat frequency response curve, Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Society of Motion Pictures Sound Engineers