The MD211, understandably one the best microphones on the planet sounds bass thin when terminated with a 140mH primary inductance input transformer as currently used in the Pleiades V6 pre pre head amp.
(140mH is perfect for the Grampian DP4/L (also for the reason that it is 25Ω) for male singing voice).
So the Pleiades (1:10 140mH primary) was removed and the TM513 step up transformer was connected to the MD211. It's primary inductance is (2H at LF, it is much less at HF due to the material used for the laminations) more than 10 times higher so it would go down 10 times lower in frequency (LR filter with R=200Ω, the output impedance of the mic).
There was phase inversion. So an Phase reversal XLR adaptor was removed from the Sony TC-D5 pro XLR mic input. Insisting the high inductance step up transformer the mic immediately sounded fuller but bass heavy. This is possibly due to Fletcher Munson and voice effort frequency spectrum curves.
So it was decided to connect instead of the usual 22nF coupling capacitor to the EF183 control grid a variable Heatkit capacitor box. The Heathkit Condencer substitution box CS-1. This provides a quick parametric low cut filter. Small length alligator cable clips were used.
While a male voice singing together with Sade singing at a soon further away (reproduction through Pleiadss 2N3053 - PHILIPS AD5046N) the best value for correct sounding voice to listener's brain was with a 3.3nF series capacitor. (The grid of the Pleiades V6 is an approximate load of 100KΩ due to the 6MΩ electron accelerating resistor connected between anode and grid. From this an approximate 6dB per octave cutoff frequency 3dB point can be calculated).
Note the Pleiades V6 was operating with 3 AA 1.2V batteries near the end of the charged life so anode current was 40μA. At these conditions the output transformer of the V6 low cuts more than 100Hz as the output impedance of the EF183 is increased. (RL low cut filter between electron tube output impedance and output transformer primary inductance).
So with charged batteries an even less than 3.3nF coupling capacitor may be needed to avoid bass heaviness.
The sound was excellent.
What would be the best input transformer primary inductance so that the MD211 sounds most natural or flat from singers vocal chords to listener's brain?
Signal path, setup:
Male singing voice at a few inch - MD211 - TM513 - CS1 at 3.3nF - V6 grid of EF183 - TC-D5 pro - HD580
(Later addition)
Next day ie today the 3 AA batteries are fully charged and inserted to the battery holder on the Pleiades jig configured for the V6 head amp. The anode current is now 80μA. (Note there is always a series resistor to the heaters for underheating). The sound as expected is more low frequency heavy.
By dialing on the capacitor box again the nearest value, that is, 2.2nF was found to give best vocal results. Ambience sounds nice too and not bass heavy. There is some hiss noise due to the box structure of the capacitor box picking up possibly rf.
A fixed small capacitor of 2.7nF is plugged to the breadboard and the C box disconnected. Noise is must less. Sound is great. Possibly at the threshold of becoming bass heavy. Ambience including Sade singing from the Pleiades Labs kitchen where the Pleiades 2N3053 - Philips AD5046N single single ended, single speaker setup is, sounds great to the headphones picked up by such a world class mic.
Next is to find a suitable mic stepup transformer of suitable primary inductance so that no low cut is need performed by a small capacitor.
The German inline XLR to 1/4in GSM1 is connected with crocodile clips. It has a primary inductance of 470mH at low frequencies (232mH at HF). The 22nF capacitor to grid is resumed. The male voice singing sound is great with a lot of body. At the verge of being bass heavy.
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References:
Pleiades V6 schematic
Flat frequency response from producer's brain to listener's brain, Sound Picture Recording Reproducing Characteristics - Loye, Morgan - Journal of Motion Pictures Sound Engineers
On preserving transconductance of electron tubes at very low anode voltage - euroelectron
Bibliography:
Radio Engineering - Terman
Communication Engineering - Everitt
Electronics, a systems approach - Neil Storey
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