15/02/2025
THE PHONO STAGE
The Phono stage is both an amplifier and equaliser.
The Vinyl Groove
A record spins at 33 1/3 RPM, at this speed it takes two seconds of music to travel approximately 36 inches around the outside of the record and 14.9 inches around the inside.
As the groove of a vinyl record spirals inward from the outside of the disc towards the label, the length of the groove per revolution on the inside of the disc is shorter than on the outside of the disc. Because of this, the reduced space on the disc near to the label restricts the spacing between the groove walls, the depth of the groove and subsequently affects the stylus ability to track the groove, consequently, the record suffers some hi frequency loss plus distortion halfway through the LP.
The Cutting Engineer
The cutting engineer compensates for this hi frequency loss plus distortion by cutting louder tracks on the outside of the disc and quieter tracks on the inside of the disc.
Louder tracks require more space for a deeper, wider groove depth and cut for volume and clarity.
The shorter in time the tracks are on an LP, the better, resulting in improved dynamics and volume.
The longer or more tracks that are cut, the quieter and less dynamic the record will be, plus there will be increased surface noise.
Study the grooves on a 12’’ remix record running at 45rpm and you will see that the groove is cut deeper and wider this is because there is only one track per side and the cutting engineer can use all the space available to cut a deeper and wider groove resulting in a louder and clearer vinyl cut.
This is why 12’’ 45rpm discs with one or two tracks always sound better than 12’’ 33 1/3 rpm with multiple tracks.
12’’ Vinyl Disc at 33 1/3 rpm offers an optimum of 12 to 14 mins and maximum of 22 to 24 mins cutting time
12’’ Vinyl Disc at 45rpm offers an optimum of 9 mins and maximum of 12 to 15 minutes cutting time.
The RIAA Pre-Emphasis Equalisation Curve
During the process of cutting the vinyl record the cutting engineer uses what’s called an RIAA Pre-Emphasis Equalisation Curve, this equalisation reduces frequencies below 1Khz by 6 dB per octave reaching -20dB at 20Hz
and boosts frequencies above 1Khz by 6 dB per octave reaching +20dB at 20Khz.
The reason for using this equalisation curve is to increase the cutting area on the disc.
By reducing the lower frequencies, the width of the groove is reduced, permitting longer track times.
The louder the bass, the wider the groove.
Boosting the high frequencies greatly reduces high-frequency surface noise during the cutting process.
The RIAA De-Emphasis Playback Equalisation Curve
During playback of the vinyl record the Phono Stage acts as both an equaliser and line amplifier plus uses a RIAA De-Emphasis Equalisation Curve, reversing the RIAA Pre-Emphasis Equalisation Curve used by the cutting engineer.
The RIAA De-Emphasis Equalisation Curve boosts the Low frequencies and reduces the Hi frequencies canceling the effect of the Pre-Emphasis curve applied during the cutting process.
For additional information about the phono stage click on the link below to visit The Listening Suite Hifiology Pages
https://www.thelisteningsuite.ie/the-phono-stage/
