A nice test for low noise electron tube pre preamplifiers...

is to connect to them a 25ohm microphone and listen to what they can do.


The Pleiades V6 was connected to the Grampian DP4/L. the 25 ohm moving coil version and the sound was phenomenal. Ambient sounds could be heard from far away with almost no hiss.


Signal Path, Setup:


Far ambient sounds or male singing voice at a few inch - Grampian DP4/L - Pleiades V6 - Sony TC-D5 Pro - Sennheiser HD580


Compered to feeding direct to Sony TC-D5 there was much less hiss noise. One has to bear in mind that the Sony must be more suitable for mics from 200 ohm to 800 ohms?


The Pleiades V6 at this experimental stage has an input transformer of 1:10 with 140mH primary inductance.


So the impedance of the mic is stepped up by 10x10=100 times, that is to 2500 ohms.


The sound is very big and low noise at least on the same league as a good condenser.


And the treble is amazing possibly because the mic is not loaded.


The input impedance at the grid of the EF183 at such low voltage conditions is about 100Kohm. So the 10:1 input transformer will bring it down to 100Kohms/10/10 or to 1Kohm.


So a 25 ohm mic sees 1kohm. An impedance 40 times larger. The Pleiades V6 is so low noise that no hiss is introduced. Is this 1 to 40 low loading relation a reason for the great treble?


But the question or a good test is what would happen if this mic is connected directly to the grid through the 22nF capacitor, ie without the input transformer.


That should be a really good test on how low noise a pre preamp is.


So far what was done is short circuiting from the 22nF input terminal to ground.


While before connecting anything to the grid hiss is very loud as soon as a short circuit crocodile clip cable link is connected the hiss almost stops in a dramatic silent way.


The Pleiades V6 seems to have a very low equivalent noise input resistance.


How low?


References:


Pleiades V6 Schematic


Thermal or Johnson noise