It depends on the type of singing, playback sound intensity (Fketcher-Munson curves) singing voice effort (voice effort frequency spectrum production curves), etc.
Preliminary tests show a terminating primary inductance of the order of 470mH giving flat frequency response from singer's brain to listener's brain.
More experiments needed.
The Pleiades V6 head amp was used. See next euroelectron post.
(Next day addition)
2 Sowter 9126g input transformers were tried too. These were kindly supplied by Brian Sowter near 2000. They are iiside easy to open octal bases and laminations can be freely rearranged to obtain a suitable primary inductance or bass response. Currently they are programmed with metal laminations as follows. 7 Es on top of each at 0 degrees other then 7 Es facing opposite direction ie 180 degrees then 7 Es again back to 0 degrees etc.
The primary inductance of the first transformer is 220mH and of the other transformer 260mH. These were measured at a low frequency. Each transformer has a turns ratio of 1 to 10.
The latter transformer gave very nice results suggesting a suitable terminating inductance for the MD211 for a not loud singing voice to be around 260 millihenries.
It is being considered adding other winding to the transformer currently used on the Pleiades V6 (140mH) so that both windings in series give 260mH. The 140mH original winding can be used as usual with the Grampian PD4/L and the smaller winding with the least inductance with a Grampian GR1/L or GR2/L ribbon microphone to compensate for the proximity effect. Possibly 3 XLR Neutric female sockets can be provided in the octagon orange Hammond box so that connections are ready without the need of switches. In this way the Pleiades V6 booster mic head amplifier can have options for 25Ω or 200Ω mic impedance and provision for some low cut options towards flat frequency response from music producer's brain to listener's brain.
But there is at least something else that needs to be tried. How would the MD211 sound terminated by an inductor in series with a variable resistor so that the low cut compensation slope can be adjustable? What would be a best in practice inductance, resistance range.
(Later addition)
The Sowter input trasformers were connected with about 1 ft crocodile clips. Only live, return was connected and not a third cable for screen. This gave an interesting sound with a slight increased high frequency giving a slight edge to the sound. By connecting a third cable from mic housing to V6 housing the euphonic edge disappeared. Some RF? induced hiss decreased too. Or is this a capacitive effect of adding the screen surface to the circuit?
This may make the sound appear a bit more dull or bass heavy. So perhaps a slightly lower inductance should be used.
On the Harry Olson books it can be read how important it is that if high frequency response is exagerated, a corresponding lower frequency response should take place so that the result sounds spectraly balanced. Bandwidth should be decreased or increased on both spectrum sides.
(After a few days addition):
An inductor was connected in series to a resistor and the total in parallel or shunt to the MD211. The sound is excellent. No bass heaviness but sound with full body or big sound, great mid, and HF. See a newer post:
http://euroelectron.blogspot.gr/2018/05/a-resistor-in-series-with-inductor.html
references:
Flat frequency response, Sound pictures recording and reproducing characteristics - Loye, Morgan - Journal of the Motion Pictures sound engineers
Music, Physics and Engibeers - Harry F. Olson
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