Front End Amplifier for Moving Coil Microphones

This is on connecting a low noise front end pre preamplifier between the microphone and a normal or ordinary mic input.


A well designed front end amplifier for a moving coil dynamic microphone would not necessarily reduce the dynamic range but on the contrary increase the subjective dynamic range.


It may do instantaneous peak limiting of very large peaks so that the signal is more suitable to feed ordinary op amp microphone inputs. Op amps in fact were not made in mind for the extremely demanding music signals. (Op)erational (amp)lifiers were used for exactly what the word says; for addition, subtraction and other mathematical operations of signals in analog computing. Nevertheless operational amplifiers have done a great thing for humanity. They have kept the need of electron tubes and other class A operation (electrons flowing at all the times) designs such as Neve transistor preamps which can cope with the demanding music signal and condition it so that then ordinary designs can cope. In a few words the advantage of the class A amplifier is that it is extremely linear as the signal is small (near the horizontal axis ie crossover point) and progressively non linear as the signal starts getting too big. A similar principle to looking at the earth, flat for small travels, curved for large ones. Similar to our ear brain mechanism, linear for small signals and nonlinear for large ones so that the massive dynamic range around us is perceived smooth and not painful.


Music signals are possibly the most demanding signals on the planet. 120dB of dynamic range of the acoustic world means multiplication by a million million times between faintest and loudest sound intensity. And this dynamic range must artfully fit to just 10 to 20 dBs for commercial recordings for known reasons of keeping music listenable at most real world environments. Artfully may mean doing this while keeping the subjective dynamic range, impact, edge, smoothness and loudness of the musical performance in a recording medium. So that we can then do almost anything. Listening through ordinary amplifiers, do digital archiving etc.


In fact the same present to humanity came by the introduction to music of the digital technology. Everything analog became immediately important. For example analog recording, signal transformers, FM pre post emphasis, disc recording. Most of these are instatenious peak limiting signal conditioning processes. Disc recording for example contains treble boost in recording. Treble cut is used at playback which cleverly rounds any peak clipped or limited. A similar technique is used by Mike Oldfied making his guitar sound. Firstly clipping the waveform through the input stage of a tape recorder then frequency emphasis then overdriven amplifier and then de emphasis to round the waveform as described in one of his interviews.


And the interest in analogue technology keeps increasing, the synergy between analog and digital becoming increasingly important. The interplay between the real world, the analog world, and the download upload or binary digit or BIT information world or digital world is important.


Thankfully operational amplifiers and digital technology have kept active perhaps the most beautiful and fascinating part of analog electronics.


Now a very susccsful example of a top flight amplifier between microphone capsule and the following normal mic preamplifier is the amplifier inside a Neumann U47 microphone.


The high impedance capsule drives just very few components in a simple electron tube artful amplification stage. The tube used is the VF14 at underheated cathode heaters. The tube drives an output transformer as is done on single ended class A operation 300B power amplifiers to bring the impedance low to 200Ω so that the signal is ready to drive long cables and a normal microphone input.


At least two advantages are achieved. The signal out of this arrangement is quite hot at a few millivolts per Pascal. The noise is low as the VF14 at underheated heaters and relatively low plate voltage is top flight in quietness terms. So the signal is high and low noise, therefore ridding above the noise of an ordinary preamplier.


The second advantage is that the signal is conditioned in the sense that a very high peak causes diode rectification as on the side chain of gain compressors. This makes the grid more negative reducing the electron flow hence transconductance hence the gain of the front end. So it acts like our eyes adjusting to very intense light or our ears adjusting to very intense sounds. And it is done instantaneously. Our eyes do this using the iris. Our ears do this by adjusting the 3 tiny conducting bones between ear timpani and inner ear to impedance match. Much like an automobile gearbox taking all infinite? values of gears.


So the extreme high dynamic range of an acoustic signal (120dB means multiplying by a billion?) is made more suitable for the ordinary consumer and ordinary pro electronics.


And this type of compression is not conceived as compression as an electron type is capable of producing nice sounding overtone harmonics keeping the edge of a loud subjectively sound. A well known secret of the music industry as is explored in the Russel O. Hamm paper.


So why not doing the same for moving coil microphone capsules. They have great reputation in the studio for great sound. For example Shure unidyne III for Brian Wilson at Pet sounds. Shure SM7 for Thriller - Michael Jackson. MD421 or MD441 for Stevie Nicks. Electro-Vooce RE15 for Elvis Presley or Dolly Parton.


And there are excellent examples of omnidirectional moving coil microphones. Such as MD21, Electro-Voice 635a, Beyer M55.


So what would be the simplest and purer signal path in the same reasoning that Georg Neumann used for the U47 using just one tube and as few as possible components?


A suitable implementation is for example the Pleiades V6. All Pleiades circuits, concepts are open source.


The voice coil, ie the mic, is connected to an input transformer so that the signal and the impedance is increased passively with no increase in noise. A great advantage of moving coil mics is that they are low impedance and balanced so that the cable between the moving coil capsule and front end can be of practically any length.


The high impedance secondary of the input transformer is connected through a capacitor to the grid of an EF183 triode connected electron tube.


The tube may have directly connected at its anode the output transformer for getting us back to low impedance.


So again we have all the advantages as mentioned in the VF14 circuit of the Neumann U47 microphone.


We do not need a polarizing high voltage. And in fact a tube such as the variable transconductance and high transconductance EF183 can operate beautifully with just a few volts at the anode.


This is done by neutralizing the self assumed negative grid voltage from the cathode electron cloud by a 4MΩ resistor from anode to the control grid.


So the tube can operate normally at only 3.4 volts as is done on the Pleiades V4 preamplifier.


The same battery can supply the heaters too.


It happens that underheating (lower heater voltage than specified) reduces almost all effects of too much boiling electron traffic which induce negative potential to grid. In addition the very low anode voltage reduces almost all of secondary emission effects and noise is further down to a new level.


So the tube becomes low noise in an unheard way, making the analogue to a very sensitive electron magnifying lens as on electron microscopy.


The amplifier can fit in very small space which includes a li-ion battery similar to the ones for electronic cigarettes. This insures the purest low noise DC supply that lasts for many hours. Or it can fit inside an XLR, female to male adaptor if a K117 JFET is used as for example on the Pleiades K117 preamplifier. Power supply can then be automatically given through the mic connector of the recording device as when using electret mics. Phantom power, USB or Apple lighting power is possible too and possibly feasible on the electron tube versions too.


The output transformer may be omitted if driving a higher impedance. Would it be possible to drive an A to D converter directly? If may microphones such as M55 are physically electrically connected in series parallel combinations the signal would be large. Would this be the simplest or purest signal path on the planet from the analog world to the digital world while close micking instruments to direct mono or direct to stereo?


References:


Tubes vs Transistors - Russel O. Hamm


Operating features of the Audion - Edwin Armstrong


Other euroelectron posts, for example:
On preserving transconductance of electron tubes at very small potentials
Pleiades modular microphone


Robert Orban papers on FM peak limiting


FM peak limiting explained on www.261.gr


Neuman U47 schematic


Pleiades V4 schematic, V5 description or V6 specification on euroelectron blogspot


Acknowledgements:


To Lefteris Logaridis who insisted years ago that a stand alone VF14 front end amplifier be built and for making a U47 available for study.


To Panayiotis Diamantopoulos for insisting years ago that the simplest K117 JFET one stage front end be made and provided schematic.


To Hliana Stalika for insisting that the grid should not be negatively biased long before I came by chance across the paper of Edwin Armstrong. In fact the grid is already biased negative by an internal process. Why should we bias it twice making the tube inoperative at low potentials? Hliana gave the way to free electrons.