The objective gain of Pleiades V4 preamplifier

The Pleiades V4 pre preamplifier (booster), consists of just an EF183 electron tube triode connected. Cleanest power supply is by 4 AAA batteries for both heater and plate.

The grid is neutralized from the self negative bias induced to it by the cathode electron emission. This is done by a 3.9ΜΩ resistor from Anode to control grid.

The anode load resistor is 10KΩ.

The cathode is connected to ground further minimizing components.

Input and output is DC isolated by coupling capacitors.


Since it has high input and output impedance it can be directly connected to musical instruments.


The following measurement nevertheless refer to its use as a studio low input and output impedance pre preamplifier.

At real worst case conditions the objective gain was measured to be only 6dB. If a 7:1 output transformer is used the gain should theoretically double and the output impedance increasing from 50Ω to 200Ω. The gain can further increase by using an input transformer of higher stepup ratio taking into account that the input impedance is aproximately 100KΩ. Gain can be further optimized by the right Rag and heater temperature reduction. Gain can further increase by direct transformer output coupling, eliminating the load resistor. Anode voltage will increase for a given battery voltage as the only voltage drop will be the transformer primary winding resistance, see Pleiades V6.

Setup for measurement:

440Hz with harmonics acoustic wave Seiko metronome - SM58 converting sound to electricity - Canford Neutrik low to high impedance step up transformer (1:15) - Pleiades V4 - Canford Neutrik high to low impedance step down transformer (14.5:1) - transformer mic input of  SONY TCD5 PRO II - level is read at TCD5's VU meter (the right channel was used for the test)

These few dBs of gain can be thought of as dBs of noise reduction if noisy preamplifiers are fed. (By reducing the gain of the driven preamp its self noise goes down).

More gain is not nessasarily a good thing as the driven preamp has the risk of overloading more easily. (Headroom is reduced). The point of using V4 is as very good front end "lens" to get the most detail out of a sensitive voice for example without introducing hissing thermal noise.

A positive outcome is that a dynamic microphone having V4 with one tube as its preamplifier now gives a higher level as condensers do and therefore higher sensitivity with least noise at least
comparable to condensers if a world class dynamic microphone is used.

If V4 is used to drive a higher impedance load such as the 10KΩ mic input impedance of a Marantz CP-430 or the 10-50KΩ input impedance of Tascam porta 1, 2, 7 etc then the 2 transformers at the output interface are eliminated making gain much higher as well as further simplification of the signal path. So one is left with only the input transformer which is the first and best magnifying lens on the planet. The possibility of using low voltage batteries or battery makes the use of the input transformer shine. There is no power transformer induced field, no rf, no heater hum, so the input transformer shines noiseless at its best even without any shield if a Magnetec Nanocrystalinne core is used (Pleiades V5).

Objective measured gain is not related to subjective gain. Class A transistor and even more so, tube amplifiers sound louder than op amps as they perform instantaneous peak limiting increasing the average to peak level. This done while fooling the brain that a very loud sound exists due to the trumpet like harmonic distortion that gradually gets into play.

So the objective signal is low while the subjective high which is exactly what we want before sending the signal to an analog to digital converter.


(The Pleiades V5 has a much higher gain as it uses 2 EF183 electron tubes, the output being attenuated, to get more tube sound ie juice from music. The Pleiades V6 has one stage but output is transformer coupled as inside early Western Electric condenser microphones. So the anode voltage drop is lower due to the elimination of the anode resistor. It's gain is higher than V4. Pleiades preamplifiers have cathodes underheated as is done on the Neumann U47 V14 tube to maximize information extraction.

Reference:

Tubes vs Transistors is there an audible difference? - Russel O Hamm - Journal of Audio Engineering Society