"EMI’s Abbey Road Studio equipped itself with a TG12345 Mk I transistor recording console. The Beatles had recorded all their earlier albums on thermionic valve-based REDD desks. At first, Engineer Geoff Emerick didn’t like the solid-state transistor sound, according to Womack’s book. He thought it tamed aspects of the sound, like low-end distortion, which came through stronger with tube equipment. Emmerick came around, concluding the transistor softened the overall sound but brought out definition, clarity, and a deeper low-end."
Let’s not also forget that most of the Beatles output (and most everyone else’s during that era) was created via 4 track tape decks. This usually meant an increasing amount of ’bounce down’ record additional instruments and create a sonic effect Spector style. Brian Wilson was a big fan of this (anti audiophile) technique too.
It was the only way to create musical collages back then and some bands took it too far, eg The Mamas and Papas. On many of their records the background instrumentation is a mush that no system could untangle.
Perhaps another turning point was the switch to stereo. Most recordings up til 1965 would be primarily mono recordings. The Beatles themselves focussed on mono primarily up til The White album in 1968. It wasn’t just Phil Spector who preferred mono, John and George seemed to be fans too.
Until recently I usually preferred stereo recordings but I now have to admit that mono recordings can sometimes display a greater perceived sense of tonal density.
So the 3 major suspects when it comes to changes in sound quality might be the change from mono to stereo, the switch from tubes to transistors, and the increasing use of ’bouncing down’ tracks from tape to tape where each successive generation would mean a loss in fidelity.
Then we might also consider whether the change to 16/24 track machines was a plus or a minus. The musicians seemed to love the increase in versatility and more tracks equalled less bounce down, but they also equalled less tape width per track.
The introduction of digital recording in the late 1970s must be another factor.
We could argue forever whether these changes were a good thing or not, but surely we can agree that they all made a sonic difference.
There’s also little doubt that some of these simple recordings made pre-1965 can easily stand the test of time some 55 years later.
Depending upon your outlook that might be a slightly depressing thought or perhaps even an uplifting one.