Mu-follower circuits have been well discussed, ... so I will leave the basic circuit to one side for now, and discuss RIAA.
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
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
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
The brand-names and dating will go a long way toward sorting out the issues.