The AJH Synth Fixed Filter Bank 914:
This 30HP wide module features twelve fixed frequency, inductor based bandpass filters, set at half octave interval spacing and covering from 125 Hz through to 5.8kHz. It also features low pass and high pass shelf filters, again they are fixed frequency and both are active inductor based circuits. It has a vintage pedigree that harks back to the original and highly sought after 914 Fixed Filter bank from the 1960’s Moog™ 900 series modular synthesiser system.
However, we have brought this design up to date and made it compact enough to fit into skiff friendly Eurorack cases by using gyrator based active inductors, and have added some other very useful features too: the bands are split alternatively to left and right channels, each having their own individual output; and there is also an inbuilt Voltage Controlled cross-fader that allows panning between the left and right banks – with a mix of all bands available in the central position.
In addition, we have a second cross fader that can mix between the dry incoming signal, or an external signal fed into input 2, and this is cross-faded against the wet MIX Output – this allows Voltage Controlled crossfading from wet to dry signals, or between wet and external signals – and the level of each can be adjusted using the Input 1 and Input 2 level controls.
A further addition is the feedback control, this is similar in design to the resonance control on a regular VCF, but because of the multiple fixed filters topology it is rather different in operation – it would normally be added in small amounts to thicken up the sound, however we have allowed the feedback to traverse to quite extreme settings for howling feedback and even soft clipped self-oscillation, especially for the noise merchants out there…
Using L/C inductor topology bandpass circuits rather than the usual Sallen Key R/C active bandpass filters does give a slightly different sound and this is part of the magic of the original 914 filterbank, as the LC based circuit exhibits less phase shift against frequency that the more common Sallen Key R/C based bandpass filters that are used in many Modular synths.
Many people often wrongly assume that a fixed filter bank is exactly the same as a graphic equaliser – however this is certainly not the case, if all of the controls of a graphic EQ are set to centre then a flat frequency response is expected and the output will sound very similar, or identical to the input – this does not happen with a fixed filter bank, the frequency response rises and falls as it passes between each stage (but to a lesser extent than if the filters are all set to maximum output), so it acts as a soft slope comb filter, and the resulting output sounds very different from the input signal. Also, with all of the individual filters turned to zero there will be no output at all.
Power: 180mA @+12v / 170mA @-12v