Sound Waves, Phonograph, Stylus, Piezoelectric & Magnetic Cartridges, Thermionic Emission, Vacuum Tube, PN-Junction, Transistor and Amplifier

Posted by PITHOCRATES - May 2nd, 2012

Technology 101

The First Phonographs used a Stylus attached to a Diaphragm to Vibrate the Air and a Horn for Amplification 

Sound is vibration.  Sound waves we hear are vibrations in the air.  A plucked guitar string vibrates.  It transfers that vibration to the soundboard on the guitar body.  The vibration of the soundboard vibrates the air inside the guitar body.  Amplifying it.  And shaping it.  Giving it a rich and resonant sound.  Creating music.  And we can reverse this process.  Taking these vibrations from the air.  And putting them into a piece of wax.  Via a vibrating needle.  Or stylus.  Cutting wavy grooves into wax.  And then we can even reverse this process.  By dragging a stylus through those same wavy groves.  Causing the stylus to vibrate.  And if we transfer those vibrations to the air we can hear those sound waves.  And listen to the music they make.

The first phonographs could reproduce sound.  But they didn’t sound very good.  The first phonographs were purely mechanical.  A stylus vibrated a diaphragm.  The diaphragm vibrated the air.  And a horn attached to that diaphragm was the only amplification.  Sort of like cupping your hands around your mouth when shouting.  Which reinforced and concentrated the sound waves.  Making them louder in the direction you were facing.  Which is how these early phonographs worked.  But the quality of the sound was terrible.  And played at only one volume.  Low.

Electric circuits changed the way we listen to music.  Because we could amplify those low volumes.  By changing the vibrations created from those wavy grooves into an electrical signal.  The first phonographs used a piezoelectric cartridge.  Which the stylus attached to.  The piezoelectric cartridge converted a mechanical pressure (the needle vibrating in the wavy groove) into electricity.  Later phonographs used a magnetic cartridge.  Which did the same thing only using a varying magnetic field.  The vibration of the needle moved a magnet or a coil through a magnetic field.  Thus inducing a current in a coil.  Then all you needed was an amplifier and a loudspeaker to make sweet music.

Small Changes in the Control Grid Voltage of a Vacuum Tube make Larger Changes in the Plate Voltage

The first amplifiers used vacuum tubes.  Things that once filled our televisions and stereo systems.  Back in the old days.  Up until about the Seventies.  A vacuum tube operated on the principle of thermionic emission.  Which basically means if you heat a metal filament it will ‘boil off’ electrons.  The basic vacuum tube used for amplification consisted of a cathode and an anode.  Or filament and plate.  And a control grid in between.  Sealed in, of course, a vacuum.  Creating the triode.  The cathode (filament) and anode (plate) created an electric field when connected to a large power source.  The cathode is negative.  And the anode is positive.  When negatively charged electrons are ‘boiled off’ of the cathode the positive anode attracts them.  The greater the heat the greater the thermionic emission.  And the greater the current flow from cathode to anode.  Unless we change the electric field to inhibit the flow of current.  Which is the purpose of the control grid.

Small changes in the control grid voltage will make changes in the large current flowing from cathode to anode.  That is, the larger current replicates the smaller signal applied to the control grid.  This allows the triode to take the low voltage from a phonograph cartridge and amplify it to a higher voltage with enough power to drive a loudspeaker.  Which is similar to diaphragm and horn on the first phonographs.  Only the amplified electric signal moves a lot more air.  And better materials and construction create a better quality sound.  Amplifiers with vacuum tubes make beautiful music.  High-end audio equipment still uses them to this day.  Including almost all electric guitar amps.  So if they have the highest quality why don’t we use them elsewhere?  Because of thermionic emission.  And the heat required to ‘boil off’ those electrons.

Vacuum tubes worked well when plugged into line power.  Such as a radio in a house.  But they don’t work well on batteries.  Because it takes a lot of electric power to heat those filaments.  And you need pretty big batteries to get that kind of electric power.  Like a car battery.  But even a car battery didn’t let you listen to music for long when parked with the engine off.  Because those tubes drained that battery pretty fast.  So there were limitations in using vacuum tubes.  They draw a lot of power.  Produce a lot of heat.  And tend to be pieces of furniture in your house because of their physical size.

Small Changes in the Base Current of a Transistor is Replicated in the Larger Collector-Emitter Current

The transistor changed that.  Making music more portable.  Thanks to semiconductors.  Material with special electric properties.  Based on the amount of electrons in the atoms making up this material.  Atoms with extra electrons make material with a negative charge (N-material).  Atoms missing some electrons make material with a positive charge (P-material).  When you put these materials together the N and the P attract each other.  Electrons cross the junction and fill in the holes that were missing electrons.  And the ‘holes’ cross the junction and fill in the spaces where there were excess electrons.  (When an electron moved, say, from right to left it made a hole and filled a hole.  It made a hole where it once was.  And it filled a hole where it now is.  So it looks like the hole moved from left to right when the electron moved from right to left.)  Neutralizing the N-material and the P-material.  But creating a charged region around the junction.  And it’s this electron flow and hole flow that make these PN junctions work.  When you add a third material you get a transistor.  Made up of three parts (NPN or PNP).  Emitter, base, and collector.

To get the electrons and holes flowing you start applying voltages across the junctions.  A large current will flow from the collector to the emitter.  Similar to the current flow in a tube from cathode to anode.  And a small base current will change that current flow.  Just like the control grid in a vacuum tube.  Small changes in the base current will make similar changes in the larger collector-emitter current.  Just like in a vacuum tube, the larger current replicates the smaller signal applied to the ‘control’.  Or base.  This allows the transistor to take the low-level signal from a phonograph cartridge and amplify it to a higher level.  Just like a vacuum tube.  Only with a fraction of the electric power.  Because there are no filaments to heat. 

Low power consumption and the small physical size allowed much smaller amplifiers.  And amplifiers that everyday batteries could power.  Creating new ways to listen to music.  From the pocket-size transistor radio.  To the bigger stereo boombox.  To the iPod.  Where the basic principle of how we listen to music hasn’t changed.  Just how we vibrate the air that makes that music has.


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