How ribbon mics work

We know from nature or physics that stationary electric charges create electric fields. And moving electric charges create magnetic fields.


The ribbon microphone has a strong magnet. Where are the moving charges then?


It is the electrons inside the magnet that have constant spins and create this stationary magnetic field which is guided to the lightly suspended aluminium ribbon which is the membrane.


What next?


When we speak, sing or a musical instrument plays, molecule vibrations (sound) set the lightweight sensitive ribbon to vibration too by collision. (In fact collision is an electrical force, the electrons of the air atoms repel the electrons of the aluminum molecules etc, they never touch)(same as when we swim in the sea, billion billion electrons from the water repel billion billion electrons of our body skin)


So what then?


The ribbon is made of aluminum which is conductive which means has billion billion free electrons in it or elementary charges.


As the ribbon vibrates they vibrate too and BINGO, each one now creates a magnetic field which interacts with the stationary magnetic field from the mic magnet body.


Interaction means force and those tiny electrons are forced to move back and forth inside the ribbon. So we have an emf or voltage across the ribbon which is the music electrical signal.


But what really happens is that those electrons dancing (by magnetic interaction) to the music signal make (by repulsion) those electrons at the end of the edges of the ribbon to repel those in the wire that connects the ribbon to its stepup transformer. Similar magnetic effects are happening inside the transformer.


Those electrons dancing at the primary of the transformer are fewer than those at the secondary winding because the secondary has much more turns.


In fact this may not be correct. Those at the primary may be many too because the primary is a thick wire.


What we gain by the thin wire is pressure or voltage. Like closing a garden water hose with our finger? In fact this may not correct too. It is conservation of energy. The power input to the transformer must be equal to the power output from the secondary plus losses. Power is voltage x current. There are many turns effectively connected in series at the secondary. So we have much voltage but less current. This is impedance being transformed or increased. Impedance is defined as voltage/current, all being complex or 2 dimensional numbers because there is phase involved too.


Anyway.


The electrons of the secondary can repel their neighbours and the music vibration finally arrives to the input transformer of the preamplifier. The voltage is further steped up. Impedance is much more increased. And then electrons again by repulsion make those near to the grid of an electron tube move back and forth. An electron tube has a high input impedance so there is a match and energy is not reflected back to the microphone, only a little. This little reflection may help damp the membrane. See how electrons are interacting all the way though and is it not a shame there is no World Electron Day yet!


Back to the electron tube. Electrons dance to the music signal in and out of the grid wire, inside the grid. This causes a larger current called anode correct which is supplied by an external high voltage (or low battery voltage if it is a Pleiades electron tube preamplifier) to be modulated. Again by electric force.


So again we have amplification, active this time as we supply power by the battery, solar cell etc.


And then a singer like Amy Winehouse can sing to the ribbon microphone Back to Black. And the amplified movement or music dance of electrons arriving though similar electric or magnetic field processes at the end of the chain to the analog to digital converter and then to YouTube:


Follows the video showing Amy singing to the RCA 77DX ribbon mic

https://m.youtube.com/watch?v=2NVvzRt8fTs


Summary: Stationary charges create electric fields. Moving charges magnetic fields. Accelerating or vibrating charges electromagnetic waves (for example when we look to ourselves in the mirror, jealous electrons inside the metal part of the mirror vibrate in the same way to those of our body and we can see our moving image at the mirror!


References:


The Feynman lectures in Physics


Richard Feynman on YouTube


Applied Electronics - T.S. Gray - MIT


http://www.coutant.org/rca77dx/index.html


Elements of Sound Recording - Frayne, Wolfe