Grampian DP4/L - Pleiades V6 - Sony TC-D5 Pro - Sennheiser HD580
The Pleiades V6 pre preamp is with an input transformer of 1:10 with 140mH primary inductance. Powered by 3 AA 1.2V batteries, 8Mohm from anode to grid to accelerate electrons. Anode voltage is 4V. Heater voltage even less, such that anode current is reduced to about 70 microamperes. Cathode connected to ground, see Pleiades V6 schematic.
For some strange reason even the treble detail is amazing. A sound preferred from a few very expensive tube mics and from other state of the art moving coil ones tested on previous posts. The Grampian ribbon is another world class mic (random corrugation reribboning after ribbons were damaged).
The sound reminds Castle in the Snow - Kadebostany, Stepping Stone - Duffy. Are they using a similar setup?
This mic, it's 25 ohm direct from the voice coil output impedance and the 140mH primary inductance complement each other miraculously. The very low voltage on the EF183 triode connected electron tube allow for quietest smooth operation as there should not be any secondary electron emission effects. Underheated cathode sounds so nice. The 100Hz cutoff -3dB point introduced by the interaction of anode internal resistance and output transformer compensates for bass heaviness and the whole sound is big, full bright, with excellent mid range, snap, punch and slight agression because of the aluminium diaphragm? In addition revealing any ambience at any distance with almost no hiss. What one would expect from the nest condenser, moving coil, ribbon, crystal mic combination?!
Another important factor is that at such low voltage conditions the EF183 is so low noise that it's equivalent noise input resistance at the control grid is very low, perhaps 2500 ohms?
The mic is stepped up by 1:10 so the impedance is increased by 100 times. So it's 25 ohms output impedance becomes or is transformed to 2500 ohms too. But the load of the tube at such operating conditions is 100Kohms ie 40 times higher. Or looking at the mic's terminals it's 25 ohms (considered in series in the Thevenin's equivalent circuit) is loaded by only 1Kohm. Since 100Kohm devided by 10 devided by 10 is 1Kohm. Again of course 40 times the mic's impedance.
Is this the reason for so much treble and HF detail?
By connecting the mic to the 1:10 transformer and then directly to Sony TC-D5 Pro the sound was big with somewat bass heaviness like radio adverts. Treble sound less prominant, a nice sound but not as treble detailed as with the V6 in circuit. One reason for the bass geaviness in that there is no rollof of 100Hz from the Sony. Other reason is that the mic is much more loaded now. The TC-D5 seems to have an input impedance of around 4Kohm. So 2500 ohms is loaded by 4000 ohms. Or the 25 ohm mic sees 4000/10/10 or 40ohms. So it is heavily loaded. This perhaps explains the fact that when Pleiades filters were connected to the mic (in parallel just before the 1:10 transformer) in order to reduce the bass heaviness, they were not as effective. An 82mH Pleiades filter had to be inserted to get the spectrum to balace. But the sound although very good big and somewhat dark, indicates that something was still not there.
Going back to the setup including the V6 the sound jumps out flying.
The interesting thing is that the mic arrived by the post in a couple of days after the Pleiades V6 test jig was ready. The MD211 200ohm and 140mH combination made for a bass light sound. The much heavier bass was greatly compensated on the MD21 HN but the treble was so excessive like the most razor sharp condenser and without any s or pop problems but then again treble was too excessive. When the DP4/L arrived it was as if the first role actor entered the stage.
The team that designed and built this mic, possibly by ear brain fine tuning, must be feeling very proud.
Reference:
Pleiades V6 Schematic
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