As you can see from the images on the right - (you can click on each image to see a larger version) - the intial prototype EQ modules were constructed at the lowest possible cost... That way it wouldn't be too painful financially, if the project didn't work out!
The individual active filter blocks were constructed onto stripboard, and mounted into very low cost enclosures -- 70x50x35mm potting boxes.
Each module would also require a stereo TRS 1/4" (6.35mm) jack socket, a panel mounted 10k (log) potentiometer and 2 x 2.1mm power plug sockets...
Total cost - less than £10 per module.
The schematic below shows a 'standard' multi-feedback bandpass filter, together with control and mixing electronics built around a TL074 opamp. This was the type used for the initial experiments....
• Just click on the image above for a PDF of the schematic and connection details •
These can be useful 'building blocks' to start with, with a wide range of options over the filter parameters. Alternative centre frequencies - different filter slopes with varying 'Q' possibilties - variation in control ranges, and so on.
There's useful filter design information to be found in
THIS ONLINE CALCULATOR
The components and parameters selected for some typical band pass filters are listed here...
Here are 1/6th octave plots of 5.5KHz and 8KHz 'cut' band pass filters, built using the component values suggested in the above table...
There are of course many other options - with low and high pass filters, as well as band pass - and it should be possible to build them with the same construction format:
• Configurable 'line level' filters
• Powered from 24V DC
• Fitted with a 1/4" (6.53mm) 'TRS' stereo jack socket to allow connection to/from the standard 1/4" line level 'insert jack' found on most audio interface and mic channel strips.
• Each filter block has a high impedance input (nominally 10kΩ -- although any value can be selected by simply changing two resistors).
• The output impedance is a nominal 600Ω.
• Each block has nominal unity gain (+/- whatever the filter adds or subtacts)
• Bandpass filters are fitted with a low noise TL074 op-amp.