40 Band (String) Filter. www.haraldswerk.de

Motivation

This is my adaption of the Juergen Haible string filter. This filter has a long history, beginning with Ian Fritz "Filter Bank Design", Electronotes EN#107", Bernie Hutchins, "A 39-Channel Variable Q Filter Bank", EN#115 and Arthur Benade, "Fundamentals of Musical Acoustics" 2nd edition 1990. Albeit, this implementation uses another filter topology as his predecessors. I quote Juergen here: "Also very different from the classic Electronotes single-opamp filters (and as far as I can tell, also from Moog's string filter), I'm now going for a two-opamp biquad topology. This generally not so well-known filter uses two integrators like the ubiquitous 3-opamp state variable and biquad filters, but unlike these, one of the integrators is non-inverting: The so-called Deboo-Integrator." I have made a few changes though. I moved it to +/-12V Eurorack format. The PSU is left out. Added volume pots for every filter band. Made a complete different new PCB design. Added a VCA for overall volume control with control voltage. Made Q voltage controlled

Specs and features

40 band filters with volume control
Overall volume control with VCA
Q voltage controlled
Left and right output
Spread control
On/off switch for partial filter banks
Power consumption around 200mA pos. rail and 110mA neg. rail

Implementation

Schematic

40 Band (String) Filter block diagram — 40 Band (String) Filter: Block diagram

40 Band (String) Filter frequency sweep — 40 Band (String) Filter: Frequency sweep

40 Band (String) Filter schematic control board 01 — 40 Band (String) Filter: Schematic control board 01

40 Band (String) Filter schematic control board 02 — 40 Band (String) Filter: Schematic control board 02

40 Band (String) Filter schematic main board 01 — 40 Band (String) Filter: Schematic main board 01

40 Band (String) Filter schematic main board 02 — 40 Band (String) Filter: Schematic main board 02

40 Band (String) Filter schematic filter board — 40 Band (String) Filter: Schematic filter board

Description:

Top

Calibration

Filter calibration: I recommend testing and calibrating the filters standalone before soldering them in. Power them up, apply a sine to the input and adjust the trimmer to maximum output
Adjust Th DC offset of the OTA with the usual procedure
Adjust the feedback to your needs and liking

Top