The masters are 15 IPS, full track mono or half-track stereo, Scotch 111 1/4-inch tape with acetate backing, on large reels with center hubs.
Listening to Nelson’s recordings, I can almost smell the tape and feel the tubes heat up.
The liner notes on several of Ricky Nelson’s albums described the technical details:
“Since high fidelity recording includes a true reproduction of all sounds without discrimination or distortion, the original recording was done on an Ampex 300-C Magnetic Tape Recorder at 15 IPS using Scotch tape.
“To effectively record the full frequency spectrum Telefunken U 47M microphones were used. Tape to disc transfer employed an automatic Scully lathe with heated stylus using a Grampion feedback cutter system.
Disc recording utilizes the standard RIAA recording curve.”
A large part of the sound was due to the legendary Telefunken (Neumann) U 47M microphones used on the sessions.
A blurb on http://www.telefunkenusa.com describes the U 47:
“Since its original development in 1946, the ubiquitous TELEFUNKEN U 47 has been the microphone of choice for generations of recording professionals. The first condenser microphone to offer selectable polar patterns, the U 47 is instantly recognizable (Figure 2).
Figure 2: The Telefunken U 47M. (click to enlarge)
From Frank Sinatra to the Beatles, the sonic performance of the U 47 is legendary as a vocal microphone.
Huge presence, with an extended frequency response that is extraordinary on any signal source, the U 47 has been a classic sonic benchmark for over 60 years.”
The U 47M used an M7 mic capsule, BV8 output transformer and VF14 vacuum tube.
The mic is famous for its clear sound due to a rise in the upper-midrange of its frequency response. When set to a cardioid pattern, its response is basically flat from 40 Hz to 15 kHz with a 4 dB shelf from 3 kHz to 12 kHz (Figure 3). The 7 kHz region is less boosted, which reduces sibilance.
This frequency response makes the U 47 especially suitable for capturing the sound of rock ‘n’ roll. Let’s explain.
Figure 3: U 47 frequency response and polar patterns. (click to enlarge)
Remember the Fletcher-Munson equal-loudness contours? The ear is less sensitive to lows and highs (around 4 kHz) at low SPLs than at high SPLs.
So when you record a loud rock band (say, at 110 dB SPL) and play it back at a normal listening level (say, 85 dB SPL), the sound is weak in the bass and highs—unless you compensate with EQ or with the mic’s frequency response.
Since the U 47 boosts the lows with proximity effect, and boosts the treble with its presence rise, its recordings simulate the fullness and edge of a loud rock band even at low volumes.