The Pleiades V6 pre preamp is ultra low noise.
How ultra?
The Grampian PD4 25ohm version sounds great on the Pleiades V6. Full body, great detail, high frequency extended, no noise, almost no hiss.
Bit how about removing the 1:10 Pleiades input transformer of 140mH primary winding inductance and going straight to the control grid?
It was decided to connect the 25 ohm mic directly to the grid through the usual 22nF capacitor.
Signal Path, setup:
Grampian DP4/L with crocodile clip cables - grid of EF183 of Pleiades V6 - SOny Tc-D5 Pro - Sennheiser HD580
The result was not very good.
There was noise and hiss, the signal buried in noise.
A systematic error may be the crocodile clips from the 25 ohm mic.
Also the the V6 is still in breadboard state with no hardwired soldered connections.
The from mic was unbalanced!
The experiment may be repeated later on.
It is interesting that a radio program could be heard on the headphones! This strongly proves how mighty electron tubes have a frequency response to Megahertz and this without the need of any feedback. It is the surrounding components that restrict bandwidth, [Neil Storey].The small crocodile cables was the antenna. RF oscillating and amplitude modulated by music electrons were in and out of the grid. In fact those near the grid. It is the wave energy or electromagnetic wave that travels not the particles themselves, like sea waves. The cathode, grid diode rectification effect was demodulating the rf wave or creating a side chain signal . This low frequency wave or the side chain signal here is the music itself! If we had an extremely fast compressor neddle what we could see if it was connected to the anode circuit (as a microammeter) would be the music signal itself!. Of course we can see this if we connect an electron ray tube or cathode ray tube oscilloscope. A Neumann U47 V14 amplifier compresses at very strong signals by this rectification effect. The rectification effect changes the bias, and the gain of the electron tube is reduced. It is amazing how these were known more than a 100 years ago, see the seminal article on electron tube by Edwin Armstrong inventor of FM radio. Another reason why radio was heard is the non linearity of the triode at large signals, nothing is a straight line in nature. It is amazing how technology goes so fast that it keeps forgetting itself.
Back to transformers.
But when the 1:10 low inductance (140mH primary inductance) input transformer is inserted in the circuit everything is big, impressive and low noise. Even distant birds through double glassed windows can be clearly and with detail heard. Almost no hiss.
The 25 ohm output impedance of the mic is stepped up to 25x10x10 by the 1:10 input trasformer. So the driving impedance to the grid becomes 2500ohms.
The Pleiades V6 seems very happy with this and it is a very low value anyway.
The mic is almost not loaded as the input impedance of the EF183 electron tube is about 100Kohms at the operating conditions.
So the 25ohm mic is loaded by 100Kohms/10/10 or by 1Kohm.
Reference:
Pleiades V6 Schematic
Electromagnetism - Grant, Philips (a nice chapter on transformers)
Electronics, a Systems Approach - Neil Storey - (one of the best books on the planet, it was a tremendous joy watching him lecturing his analog electronics course at Warwick University. At the end of the 15 lectures all students stood up and applauded. The notes of the course must had been a starting point for the book. Neil Storey is a hi fi equipment experimenter since young)
Operating Features of the Audion - E.H.Armstrong - (possibly the best paper on electron tubes by the master himself, inventor of FM)
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