A Brief History of Equalization



The equalizers of today evolved from the need to minimize transmission losses on long distance telephone lines. In about 1901, as the telephone industry was expanding, it was realized that the loss in level and reduced frequency response of the long line telephone circuits could be improved by the insertion of series inductance, at critical intervals, into the long toll lines. These “loading” coils essentially raised the operating impedance and improved performance by offsetting the effects of the line distributed capacitance and inductance. By 1915, loaded lines were common and, with the introduction of vacuum tube repeater amplifiers, transcontinental telephone service became a reality. Researchers at the Western Electric laboratories were in the forefront in bringing about these developments.


By the late 1920’s, the new- fangled radio craze was in full bloom and improvements in amplifier design made it possible to improve both radio transmission and receiver audio quality. At the same time it became necessary to transmit radio programs by land line in support of the expanding radio networks which were being set up. In order to provide the radio interests with quality nation-wide program distribution circuits, the Western Electric gang began developing “program line filters” to “equalize” the otherwise poor frequency response given by the loaded toll lines. The equalizers were all passive networks using RCL components to shape the frequency response of the line and get it as “flat” as possible. Amplifiers were used to “make up” for the inherent insertion loss of the filter networks. The overall result was quite impressive, by the mid 1930’s it was possible to have a radio circuit from New York to Los Angeles with a frequency response of 50 to 8000 Hz with a signal to noise ratio better than 50 dB.


Improvements in the phonograph record industry, beginning with the transition from acoustic to electrical recording techniques in 1925, created a need for “louder” records in order to offset the noisy shellac compound being used to press the records. By reducing the low frequencies  and boosting the highs during mastering, and then using a complementary curve during playback, it was possible to get greatly improved noise figures and dynamic range. This was the beginning of all the “curve” nonsense during the 1950’s Hi Fi craze that created a dozen or more “curves” finally culminating in the RIAA curve still in use for vinyl mastering. The advent of magnetic tape recording in 1947 also evolved a series of tape “eq. curves” defining the pre-emphasis and de-emphasis needed to get flat response and decent signal to noise ratios from the tape medium. We also needed equalization for optically recorded movie sound tracks.


The broadcast and recording industry soon found use for the many passive filter circuits being developed by the telephone researchers. By the mid 1940’s there were equalizers being offered as program filters to be used for sound effects and other program production work. Most recording studio mixing consoles were either custom built with built-in passive eq circuits, mostly found at the major labels, or were modified broadcast mix boards with outboard equalization or kluged in circuits, found at the smaller studios. Very few equalizers back in the day were active with frequency shaping in the amplifier feedback loop. This approach had to wait for the advent of solid state circuits. One of the most popular equalizers, the Pultec series, arrived in the mid 1950’s using Western Electric patented circuits.


As stereo and multi-track recording entered the field, the use of equalization to perk up the tracks and mix bus became common, often to the detriment of the recorded material. Sometimes too much of a good thing can spoil the soup and many a good cut was ruined by over-equalizing. The “lets fix it in the mix” concept has its limits.


So the humble equalizer we use today to tweak up our tracks and mixes is the result of over a hundred years of fussing around,  trying to get the sound we put into one end of the pipe to come out the other end sounding good. Now we can do it with parametric circuits and digital plug-ins, all of which are technological progress. But to some folks who are more attuned to the true sound, nothing equalizes like a passive network followed up with a good make-up amplifier. That was how the original Pultec was built and how Psidex makes the PEQ-1, as a robust passive network followed by a gutsy make-up amp. And that’s the story.