In Search of the Miracle

It seems funny the weak link in the 50's was not microphones but possible lack of low noise electron tube preamplifiers.


Coming back to today or a few days ago. The Pleiades V6 battery operated electron tube pre preamplifier is ready at its experimental Pleiades jig, and this time with both Nanoperm magnetic core input and output transformers. These Pleiades transformers are hand wound on the Magnetec excellent nanocrystalinne cores.


Almost exactly by the time V6 is ready again, a Grampian DP4/L microphones apears for the first time by the post! This particular version (L) is only 25 ohms. It is a vintage pressure moving coil microphone of 25 ohms coil impedance, made in England. The sound is breathtaking.


Signal path, setup:


ambient sounds together with close male singing voice - Grampian DP4/L - Pleiades V6 - Sony TC-D5 Pro - Sennheiser HD580


Big sound, no noise, so low hiss, extended treble, great natural bass, great midrange. And the most important of all correct sounding to the listener's brain.


By comparison when it is just connected to Sony TC-D5 Pro, the sound is somewaht bass heavy, with much hiss, the sound somewhat burried in noise.


With the V6 both the quality of the Grampian PD4/L mic and the Sony TC-D5 recorder shine!


The DP4/L is just 25 ohms.


The EF183 on the Pleiades V6 has such a low equivalent input noise resistance  that a high step up transformer ratio is not needed. On this example it is just 1:10 therefore increasing the 25 ohm impedance to 2500 ohms, by the square of the turns ratio.


Modern nano crystalinne materials for signal transformer magnetic cores permit a given primary inductance with just a few turns. This insures still lower noise are winding wires can be thicker and shorter, so less resistance so less thermal noise. Additionally the Pleiades V6 has such a winding directly connected to the anode, a technique used even before the 30's and also much used on esoteric single ended 300B power amplifiers still made and used by the most extravagant audiophiles. A wire at the anode means further less thermal (electron agitation) noise, very important for a front end stage that will dictate the signal to noise ratio of a whole system from the record producer's brai, air, musicians, microphone,... up to youtube and the listener's brain.


The Pleiades V6 is no ordinary amplifier. It operates with just 4 volts at the anode and heater circuit. Temperatures are very low. There should be no secondary electron emission effects and could well be the state of the art in low noise amplification. It is based on the Neumann U47 microphone internal amplifier schematic. The most highly regarded microphone on the planet, used by the best,. Madonna, Sinatra, Cohen, A=Ha....The heaters are deliberately underheated on bot preamps for better noise performance and better sound quality. It seems electrons do not like to be crowded. They need some space as we humans do.Operation by just 4 volts at the anode is made possible by the 8 Mohms anode to grid electron accelerating resistor which also reduces the impedance of the tube to about 100Kohm.


With an electron tube circuit of so low noise a high input transformer step up ratio is not needed.


So the small step up ratio, the few primary turns (because of the mic's low Z) result in a breath taking sound quality. The sound is big, natural with no hiss and the high frequencies are extended.


Back in the 50's where battery operated electron tube circuits were not readily available such as the Pleiades V6 the tiny voltage from a 25 ohm mic had to be stepped up by transformers of x30 or morein order to cover the noise of the front end electron yune of electronic valve as it is called in England. Hundreds of meters of thin wire resulted in thermal noise hiss and reduced the response at HF y stray capacitance and leaage inductance, etc. A similar situation was for the 200ohm voice coil as the wire on it must had been a thinner wire to start with.


To put it simply when the following stages are up and no mic is connected at the input a lot of hiss or electron thermal agitation noise is heard. When the input of the Pleiades V6 is short circuited the hiss almost disappears.


So when the Grampian DP4/L is connected its 25ohms is close to a short circuit and almost no hiss is heard. But the sound and ambience sound is so big.


Lefteris was right 20 years ago. It is not the capsule on the U47, he said, that does the trick. It is the electron tube amplifier inside it.


Vasilis, a great man, helped. His shop at Monastiraki in Athens, ust below Akropolis. For some strange reason he let us search his shop's electron tube warehouse. Hundreds of electron tubes, until we found a Telefunken VF14! We could not believe. He knew and he almost gave it away to us.


The electron tube was almost dead and with a lot of hiss. A higher heater voltage was applied by a tube tester for some seconds to restore cathode emission. It recovered beautifully.



It was found that Georg Neumann under heated the VF14 electron tube inside the U47. Just by looking at the U47 schematic and comparing to the VF14 data in an old edition of the excellent french book Radio Tubes.


Some years after, after a lot of Pleiades experiments the same cathode underheating is being done on the Pleiades V6 using the EF183 electron tube triode connected.


When the DP4/L is directly connected to TC-D5, the sound is thick, bassy, with no treble detail. The sound is quite buried in hiss. But the Sony TC-D5 Pro is one of the best recordsrs of mankind has has an excellent sound capability.


When the Pleiades V6 is inserted between the Grampian mic and the Sony recorder preamplifier everything jumps out at you.As a big sound as can be imagined with soft speaking voice and no hiss.At a closer distance to the mic it starts getting unbelievable.


Perhaps one reason condenser microphones found favor was their high quality and simple front end preamplifier. Tthe front end or first stage noise performance dictates the signal to noise ratio in our case up to the listener's brain.


Dynamic mics can have at least as much detail and low noise as a condenser. And do not have the resonant frequency of the membrane tuned to a high frequency.


The week link was possibly not the mic. Was it the transformer - electron tube preamplifier interface technology?


With only 4 volts at the anode, underheating the cathode, electron acceleration by a few megohms from anode to grid, battery operation, an electron tube such as EF183 can be state of the art low noise. The Pleiades V6 is a proud example.


Hliana was right insisting that the grid should not be externaly biased negatively. Possibly influenced by a Montalk article of esoteric philosophy. So just 8 Megoms from anode to grid did the mircle of electron tube operation with normal transconductance, amplification factor, and anode resistance.


And most important of all low noise.


That's magical. No hiss, big but natural sound with every treble detail from a 50's, 60's mic. The information was there right at its output. Cream mid range, unexagerated bass. All the spectrum smooth sounding and with flat frequency response from vocal chords of singer to listeners brain


40 ago when being 7 years old an electronic construction game's task was building a miracle amplifier. The name of the game is Polycyclomatic 501. It was bought from Radio Katouma in Athens.


Build a miracle amplifier was the title of one of the experimental constructions.


It was thought that since it is a miracle it must be something really special.


It was a 2 stage germanium transistor single ended class a amplifier. All the capacitors and resistors around the 2 transistors were connected up by the stationary springs. The tiny output transformer wire connection set. Speaker connected. All set. After double checking the circuit it was time to connect the 9V battery cable clip.


The heart was beating fast.


The battery was connected but nothing happened. No miracle. Just a click sound on the small speaker. The click was repeated when the battery was disconnected.


Double checking the circuit again.


But no miracle.


It was not known that you have to connect something to the amplifier's input so that it can be amplified.


So many amplifiers had been built since that day in search of the miracle.




 
References:


Pleiades V6 schematic


Neumann U47 schematic


Flat frequency response from actor's, singer's vocal chords to listener's brain, Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Motion Pictures Sound Engineers