From Jurgen Haible:
The old Moog® Modular System had an option of two different Fixed Filter Bank modules, one with 10 bands, and another one with 14 bands.
("Moog" is a registered Trademark of Moog Mucic (http://www.moogmusic.com).
Both are very unique-sounding, mostly because of the deep notches they produce between adjacent filter bands. Some people also claim it's becuase they used real inductors - wire-wound components that get nonlinear at higher signal levels. I'm not sure if the latter really is such an important factor, because thes efilter banks were designed in a way that in each filter band the signal first passes thru a resonant LC-filter at a certain level, and then passes a second LC-filter at a lower level. So my conclusion would be that the second LC-stage filters out whatever distortion products may or may not be created in the first stage. But then again, a tiny amount of harmonocs may survive, and others may have better ears than I and might hear a difference.
Anyway: I decided to make a PCB that allows both options: real inductors, or electronic inductors, so you can choose for yourself.
This is possible because I closely stick to the circuit topology of the original: There are no sallen-and-key or multi-feedback filters inside (with one exception). It's all passive RLC-Filter design with a an active driver stage and active summing amp. But you can either choose to wind your own inductors for the "L" part, or use an active GIC circuit.
The GIC (General Impedance Converter) circuit is very different from the one-opamp gyrator circuits that are sometimes used for emulating Inductors. The GIC has less parasitcs, and most important: it's less noisy. And in addition to using the GIC technology, which is an advantage all by itself, I'm also making the circuit as low-Z as possible: Using 2.2kOhm resistor arrays and opamps that can drive them without much distortion.
There will be some noise - just like the original hat some noise, even with all channel potentiometers set to zero. This is because the channel attenuators come before the filters. I have kept this topology, too. It has a certain charm, and it allows the control of each channels level and overdrive individually. But if you prefer it the other way, you can also connect the potentiometers after the filters. There are breakout points of the PCB for this.
You can use 2 PCBs to build a 10-Channel Filter Bank, or 3 PCBs to build a 14-Channel Filter Bank.
You can choose between a discrete driver amplifier and summing amplifier, or opamp based amplifiers. With the latter, you can adapt the filter bank to "modern" 5V synthesizer signals. With the former, you can be as close to the original as you want.
There is an on-board power supply that only needs a power transformer and fuses connected, or a wallwart with AC output.
Alternatively, you can use a +/-15V or +/-12V supply voltage as found in many modular systems.
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