Referring to the Pleiades V6 schematic:
Cc is 22nF. On this post the electron tube used is the Nuvistor 2DV4. Heater voltage supply is 1.2V.
Signal path, setup:
Male singing voice at 2-4in - Sennheiser MD411 U3 - Pleiaves V6 mic booster amp - Sony TC-D5 Pro at XLR mic in - Sennheiser HD580
As the Sennheiser MD441 U3 does not have a low cut adjustment the sound is very bass heavy at a close (2-5in) voice distance due to proximity effect, Fletcher-Munson curves, voice effort curves.
Connecting an inductor in parallel (Pleiades filter) improves the sound but the sound quality leaves much to be desired for this particular mic and various Pleiades filters combinations.
Connecting an inductor in series with a resistor ie a Pleiades (R,L) filter in shunt with the mic greatly improves sound as the slope seems much more gentle. See previous posts including the use of gentle slopes on world class omnidirectional mics such as MD211.
This is about another way to get a great vocal sound using the MD441 U3 world class mic at close distance.
Since yesterday when the first Nuvistor 2DV4 experiments begun, a great sound is heard using the MD441 U3 directly connected to a world class input transformer, the Altec Peerless 4722.
Normally the sound would be very bass heavy but the high pass, low cut effect was left to be made by the electron tube and its interaction with the output transformer. No overload or mid peak artifacts can be heard as in the case of a standard Pleiades filter (inductor) which might magnetically saturate in an unwanted way.
Using the 2DV4 electron tube in the following to be described conditions the sound is big, very clear and treble detailed with world class attack. Strongly reminding the world class reference of When I Fall in Love sung by Nat King Cole on one of the best ever made vocal recordings.
It happened almost by chance.
The 2DV4 experiments are about using this fabulous electron tube with just one AA 1.2V battery powering both heaters and anode. Pleiades pull up bias is used to reduce negative bias by a 1.9MΩ resistor from anode to control grid.
By connecting the pull up resistor the electron tube immediately starts to operate and give an uncanny real voice signature.
At such conditions the anode current is very low, possibly of the order or 1μA.
It is precisely this low current that helps achieve this better than usual and more natural quality. As anode current is reduced so plate impedance is increased. (This is the electron tube output impedance).
The electron tube anode impedance forms an LR low cut filter with the output transformer primary inductance.
Such a low anode current must produce a high plate impedance and therefore a high, high pass or low cut cutoff frequency.
This frequency may of the order of more than 700Hz.
It so happens that the above objective and subjective bass heaviness effects are compensated and the impression is a flat frequency response from singer's brain to listener's brain.
Increasing the anode potential from 1.2V to 2.4V while making the pull up resistor 6MΩ made the anode current increase to perhaps 2-4μA but the sound quality become less natural as bass heaviness was coming into play.
For some reason not understood yet letting the electron tube take care of low cut gives an impression of a gentle slope and a smooth bright sound. A similar world class sound was produced by supplying anode with 9V and leaving the grid of 2DV4 floating or at space potential. The anode current again was of the order of 1μA.
As a rule of thumb using omnidirectional microphones which do not exhibit the objective effect of proximity by exhibit the subjective effects a higher anode current is needed. Possibly of the order of 20-40μA. See previous posts on Sennhsiser MD211.
Further reading:
(Flat frequency response from producers brain to listener's brain), Sound Picture Recording and Reproducing Characteristcs - D. P. Lowe, K. F. Morgan - Journal of the Society of Motion Picture Engineers
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