We may note in passing that in the evolution of music recording, various media and techniques were developed, each with its own problems, side-effects and equalization requirements.
The main problem with recording was the limitation and the skewedness of the frequency response.
An ideal frequency response would be similar to that claimed for a modern stereo amp:
However, the actual process of recording would imprint its own frequency response 'fingerprint' or bias. Thus a magnetic phono record response curve might look more like this:
That is, in the process of recording, the final product (a vinyl record and playback needle) has very loud treble, and very little bass.
As a result, the signal needs heavy correction or 'equalization' to restore the sound back to how it originally sounded. Thus the "equalizer" was originally a correction device in the playback chain to compensate for frequency response distortion in the recording and playback media chain.
The "Equalization" is imposed in the preamp stage, just after the signal is retrieved from the record through the magnetic cartridge. Like an adjustable tone control, this circuit imposes its own curve on the signal:
The two filtering processes (recording and playback) are supposed to cancel each other out, restoring the signal to its original balance, and achieving a more or less flat frequency response overall.
As a matter of history, early attempts at equalization had some variations, before some semblance of standardization sorted itself out in the marketplace, and in the interim, several hi-fi equipment makers offered alternate settings to more closely accommodate various recordings the methods used. The McIntosh AE-2 (pre)amplifier Equalizer control (1950) for instance offered both a 5-position switch and bass and treble adjustments:
Today, an RIAA equalization circuit is usually a simplified version (a compromise) of the various EQ curves and standards floating about in the 50s and 60s. It is assumed that most stereo systems will have some kind of 'fine tuning' tone-control adjustments, so that specialized RIAA circuits for each case are not really needed.
Those who are serious about playing back their vinyl records as they were really intended, and with the best fidelity however, will not be satisfied with such commercial compromises, and will want to have a selection of RIAA equalization circuits at hand for playing various records.
In terms of the variations, the following main cases are:
(1) the Columbia 78 rpm circuit
(2) the Columbia 33.3
(3) the RCA Victor 78 and 45 rpm,
(4) the RCA Victor 33.3, and Concert Hall 78 versions.
(4) the London (records) FFRR and Decca FFRR.
These are the most used and most popular versions.
If you have a swelling vinyl collection, it might be wise to do some sorting or labeling on the basis of the RIAA type or equalization needed.
The brand-names and dating will go a long way toward sorting out the issues.
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