Daniel N. Flickinger introduced the first parametric equalizer in early 1971. His design leveraged the high performance op-amp of his own design, the 535 series (USPTO #3727896) to achieve filtering circuits that were before impossible. Flickinger's patent (USPTO #3752928) from early in 1971 showed the circuit topology that would come to dominate audio equalization until the present day, as well as the theoretical underpinnings of the elegant circuit. Instead of slide potentiometers working on individual bands of frequency, or rotary switches, Flickinger's circuit allowed completely arbitrary selection of frequency and cut/boost level in three overlapping bands over the entire audio spectrum. Six knobs on his early EQ's would control these sweepable filters. Up to six switches were incorporated to select shelving on the high and low bands, and bypassing for any unused band for the purest signal path. His original model boasts specifications that are seldom met today. [ citation needed ]
It's probably true to say that the phase changes introduced by many classic equaliser designs make an important contribution to their sound, so it doesn't always make sense to choose the technically more precise linear–phase option. You should also bear in mind that linear–phase equalisers introduce quite a lot of delay, which means that they'll increase the latency of your DAW by a significant amount. I like to think of traditional minimu-phase EQ as 'art' and linear-phase EQ as 'science'. Put another way, analogue or minimu-phase EQs tend to be better for creative tonal shaping where you're looking for a musically pleasing sound, whereas linear–phase designs tend to be better for corrective, 'surgical' jobs, such as notch filtering.