How a Record Works
Analog playback operates by continuous physical transduction. A mechanical stylus traces microscopic canyons cut into lacquer. Kinetic friction converts into electrical voltage, passes through equalization, and drives speaker voice coils.
Every stage in the signal path obeys Newtonian mechanics and electromagnetic induction.
The 45/45 Stereo Microgroove
A vinyl record is not flat. A continuous spiral trough runs from the outer lead-in rim inward to the center deadwax. This spiral measures approximately a third of a mile in total length.
The groove forms a ninety-degree V-shaped canyon. In the Westrex 45/45 stereo system, the two walls represent independent audio channels oriented forty-five degrees from vertical. The inner wall holds the left channel. The outer wall holds the right channel.
Mono signals cut lateral modulations side to side. Stereo signals cut both lateral and vertical vectors simultaneously.
The diamond tip rides suspended along both walls. It never touches the floor of the groove.
Electromechanical Transduction in the Cartridge
As the vinyl disc spins, wall undulations force the diamond tip into rapid physical vibration. The stylus moves thousands of times per second.
A rigid cantilever transfers this movement up into the cartridge body. At the rear of the cantilever rests a microscopic permanent magnet. This magnet vibrates within the electromagnetic field of two isolated copper wire coils.
By Faraday's law of induction, a moving magnetic field generates an electric charge across nearby copper coils.
Kinetic friction becomes electrical current. The output voltage measures between three and five millivolts.
The RIAA Equalization Curve and Preamplification
A vinyl record cannot physically hold flat audio frequencies. Low bass wavelengths require massive physical excursions that would throw the needle clear across adjacent tracks.
During master cutting, engineers apply the RIAA equalization filter. Bass frequencies are cut by nearly twenty decibels. High frequencies are boosted by twenty decibels to rise above background vinyl surface noise.
The phono preamplifier applies the exact inverse mirror curve during playback. Bass notes receive a twenty-decibel boost. High frequencies are attenuated back to natural balance.
Without an RIAA preamplifier, vinyl sounds thin, tinny, and nearly silent.
Power Amplification to Air Vibration
The phono preamplifier raises the tiny cartridge signal to standard one-volt line level. This voltage enters the power amplifier.
Power transistors amplify the electrical waveform, generating the current required to drive loudspeaker voice coils.
Inside the loudspeaker, current flows through a voice coil suspended in a magnetic gap. The alternating current drives the coil backward and forward according to Lorentz electromagnetic force.
The speaker cone pushes air molecules forward in compressive acoustic waves. The physical circuit is complete. Microscopic groove modulations become moving atmospheric sound waves.