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 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 stereo potentiometers. The potentiometer selected in previous revisions was the RK14K124 line from ALPS but this has now been upgraded to the Bourns PTV112 series which has more alternatives when it comes to shart lengths, taper and values. It's also the pot' used by the excellent Mutable Instruments.
If you wish you can recalculate the values of some resistors/capacitors to obtain different frequency ranges for each filter in the cascade. The formulas are in the schematic.
The Elektor project of 1986 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.
Regarding the power supply: a careful observer will notice that the circuit requires two symmetric power rails (+V and -V), and that those rails are only used to supply the op-amps. That means the audio circuit is only ground-referenced, and you should simply choose the supply voltage depending on which op-amp you selected. For the TL082 the power rails are typically +/- 15VDC. However given typical audio signal levels of +/- 1VAC the circuit should still run fine on +/- 5VDC (slew-rate caveats apply).
The latest revision of the schematic uses extra filtering caps on the power lines, thicker power lines and more ground vias.
The PCB was recently shortened (-1cm) and made slightly slimmer (-1mm) to allow easy integration of two units within a 4U Eurorack module (which just to be clear is quite different from a 19" rack). Note however that power and I/O connections are missing for such project.
A small template/prototype of front panel is also available in SVG format.
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 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 stereo potentiometers. The potentiometer selected in previous revisions was the RK14K124 line from ALPS but this has now been upgraded to the Bourns PTV112 series which has more alternatives when it comes to shart lengths, taper and values. It's also the pot' used by the excellent Mutable Instruments.
If you wish you can recalculate the values of some resistors/capacitors to obtain different frequency ranges for each filter in the cascade. The formulas are in the schematic.
The Elektor project of 1986 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.
Schematic
For those who want to do some simulations, Mike Stricker sent me the SPICE model of the circuit.
Regarding the power supply: a careful observer will notice that the circuit requires two symmetric power rails (+V and -V), and that those rails are only used to supply the op-amps. That means the audio circuit is only ground-referenced, and you should simply choose the supply voltage depending on which op-amp you selected. For the TL082 the power rails are typically +/- 15VDC. However given typical audio signal levels of +/- 1VAC the circuit should still run fine on +/- 5VDC (slew-rate caveats apply).
The latest revision of the schematic uses extra filtering caps on the power lines, thicker power lines and more ground vias.
PCB
As advertised before, the PCB is very compact: 19x61mm. It can fit comfortably in a 1U unit 19" rack (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 if you just want to start soldering.The PCB was recently shortened (-1cm) and made slightly slimmer (-1mm) to allow easy integration of two units within a 4U Eurorack module (which just to be clear is quite different from a 19" rack). Note however that power and I/O connections are missing for such project.
A small template/prototype of front panel is also available in SVG format.