This project is based on the parametric equalizer proposed by Elektor in the late 80s or early 90s and later published in the book "Creations electroniques" in 1993 (Publisher: Publitronic). Their design involved three stereo potentiometers per channel, which means a lot of cables from the front panel to the circuit board. It's quite tiedous to build and IMHO prone to noise from within the enclosure. To solve these problems and make the unit more compact I have put everything on a single board, potentiometers included. No more cables!
A graphic equaliser has only one control per band: the gain. A parametric equaliser has 3 controls: gain, frequency and width. While a graphic equalizer requires a lot of bands to correct the sound, a parametric equaliser is more acurate and usually only one to three filters are used in series.
Several modifications were made to allow these improvements. First, it's an all-SMD board: easier and cheaper to build. This is necessary because clearance is limited with the front pannel. More importantly, it is important to realize that several PCBs will be stacked next to each other for the different filtering stages (usually 3, as Elektor says) and therefor we have no space to extend the PCB without spacing the potentiometers too. The only other option would be to use DIL circuits and several stacked PCBs: not very practical or cost effective.
