Parametric Audio Equalizer
This project is based on the parametric equalizer proposed by Elektor in the 1980s and later published in the book "Creations electroniques" in 1986 (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 bandwidth. While a graphic equalizer requires a lot of bands, a parametric equaliser is more acurate and usually only two or three filters are enough.
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 and therefore 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...
Other modifications were necessary for the values of several components. This was prompted by the decision to use off-the-shelf ALPS stereo potentiometers. The line RK14K124 (IIRC) has a limited number of resitances available so that the gain control must be re-designed to accomodate a pot value of 10K instead of 22K.
You can recalculate the values of some resistors/capacitors if you wish to have different frequency ranges for each filter in the cascade. The formulas are in the schematic.
The project of Elektor also included a 'Baxandall' tone adjustment (bass/treble with adjustable cut frequency) which I also built in the early 1990s but I haven't taken the time to make a new PCB for it yet.
For those who want to do some simulations, Mike Stricker sent me the SPICE model of the circuit.
As advertised before, the PCB is very compact: 20x72mm. It can fit comfortably in a 1U unit (horizontally) or in a 2U unit (vertically). PCB interconnection is straightforward: the connections for power and signal are aligned when you place the filters next to each other, hence very short wires or jumpers will suffice. Gerber files are available for those who just want the damn thing ASAP :-)
Last update: 20150228, new schematic and PCB. Updated text. Added Gerber files.
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