Fixed filter banks offer very useful sound sculpting and shaping in modular synthesiser systems. The new FFB914 features fourteen vintage style fixed frequency, inductor based filters. There is a LP and a HP shelf filter, along with twelve BP filters set at half octave interval spacings, which range from 125 Hz through to 5.8kHz.
The individual bands are split to left and right channels, with an inbuilt cross-fader which allows manual and Voltage Controlled panning between banks.
An additional Wet-Dry cross fader mixes between the treated signal and the dry or external signal , again with manual and Voltage Controlled panning between banks.
A feedback control is also included, this adds a resonance path, which is sourced from the Mix output.
The FFB 914, like many AJHSynth modules, draws its inspiration from the early vintage synthesiser modules of the 1960's and 70's, now brought up to date by using modern components and circuit design. Here we have re-created the Fixed Filter Bank topology from the original Moog™ 900 series modular vintage synthesiser system so that it will now integrate into Eurorack systems, and due to its compact printed circuitboard design it is only 26mm deep, so can even be installed into shallow "skiff" style Eurorack cases..
There are many fixed filter offerings currently available in Eurorack, but generally they use single pole Sallen Key R/C active bandpass filters for the individual bands. These are often single pole, which gives a shallow 6dB slope and fairly low Q factor to the passbands, so that the overall comb filtering of the fixed filter bank at higher settings is fairly gentle, and less defined than filters with more poles and higher Q factors.
For the FFB 914 we have used Gyrator based active inductor circuitry which exactly replicates the vintage passive inductor / capacitor based filter design - this is an important design element, because a major difference between the two topologies is that the phase change versus frequency is lower with L/C filters compared to Sallen Key R/C filters. We have found that this does slightly affect the overall sound and character of the Fixed Filter Bank. Also, just like its vintage predecessor, we have fitted two cascaded L/C filters for each bank, so that we obtain a steeper 12dB passband slope and higher Q factor for the individual filter banks - and to further emulate the vintage behaviour the centre frequency of these cascaded filters is slightly offset against one another, which slightly rounds off the filter peak and give a wider spread.
It is sometimes wrongly assumed that a fixed filter bank is exactly the same as a graphic equaliser but in a modular format– this is certainly not the case, in a graphic equaliser the bands all feature boost and cut of frequencies, and if all of the sliders are set to centre position 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, 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 as the individual filters are taken out of the circuit.
A big difference between the FFB 914 and earlier vintage Fixed Filter Banks is that in addition to having a single output which is summed from all filters, we have split the individual banks alternatively to Left and Right Stereo outputs, so it can also be split into two six band filters (175Hz, 350Hz, 700Hz, 1.4kHz, 2.8Khz and 5.6 kHz plus Low Pass) for the Left Channel, and the alternate six bands (125 Hz, 250 Hz, 500 Hz, 1kHz, 2 kHz, 4kHz plus High Pass) for the Right channel.
This is a very musical split, as it can perform as two separate Fixed Filter Banks with Octave spacing between filters, and the Right channel is offset against the Left channel by a half octave.
In addition to the fixed Left and Right channel outputs, we have added a crossfader and an extra output path, so that it is possible to pan the output between the Left and Right channels, either with the manual Mix control or using an external control voltage patched to the Mix CV Input, which has its own level control.
We have included a second cross fader circuit which can pan between the dry incoming signal and the wet MIX Output, so that the "depth" of the filtering can be controlled. It is possible to sweep maually between wet and dry signals using the WET-DRY control, or alternatively using external control from a control voltage patched to the WD-CV Input, which includes its own level control. The overall level of wet and dry levels can be balanced using the Input 1 and Input 2 level controls respectively.
To offer even more versatility we can choose whether to crossfade from DRY to WET signal mixing or EXTERNAL to WET signal mixing, This is due to Input 2 (IN 2) being routed to the Crossfader rather than the audio input (IN 1) of the FFB 914, and the Input 1 (IN 1) Jack is normalised to the switched side of the input 2 jack socket. It is an important to understand that Input 2 (IN 2) is an input to the crossfader, and not to the Fixed Filters themselves. So, if nothing is patched into IN 2 then the crossfader will pan between the DRY signal from Input 1 and the output of the FFB; however if an external audio signal is patched to the IN 2 jack socket then this normalising from the dry siganl will be defeated and the External signal will take prescedence, so the crossfader will now pan between the External Audio input and the output of the FFB 914. The levels of the external signal can be controlled with the IN 2 Control, and the output level of the FFB 914 filtering can be controlled with the LEVEL 1 control. This is very helpful, because the output level of the FFB 914 can vary hugely depending upon how much boost is added from each filter stage, so a useful mix of the wet / dry or external / wet sounds are always possible. Studying the Block Layout and Signal Flow diagram further down the page should make this signal routing and level control a little clearer.
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…
The FFB 914 is offered in two versions, the Silver panel option blends in well with the majority of regular Eurorack Modules, which are predominentely silver, alternatively we offer a black "Dark Edition" panel option, with a more vintage feel and a perfect match for the MiniMod and other AJHSynth modules.
Having an idea of how everything is connected internally in a module can give a much better understanding of internal signal routings and how to connect the various inputs and outputs to other Eurorack modules,
Below is a simplified Block Layout showing the internal signal flow of the FFB 914:
The module is comprised of comprises twelve individual fixed filters in parallel, each tuned to a different frequency in half octave increments. Each of these filter stages is made up of two single pole 6dB L/C filters cascaded in series, which combine to create a two pole, 12dB band pass filter. Each filter has its own level control on the input, and this arrangement is shared with the Low pass and High Pass stages, which are both 2 pole, 12dB fixed cut off shelving filters.
The combined output of all filters is available from the ALL OUT output, just like in the original vintage 914 Fixed Filter Bank, where this was the only output option. For the stereo outputs the 175Hz, 350Hz, 700Hz, 1.4kHz, 2.8Khz and 5.6 kHz and Low Pass filters are summed and routed to both the LEFT OUT jack socket, and also to the input of the Left / Right mix crossfader. The 125 Hz, 250 Hz, 500 Hz, 1kHz, 2 kHz, 4kHz and High Pass filters are summed and routed to the RIGHT OUT jack socket, and to the second input of the Left / Right crossfader.
The output of the LEFT-RIGHT crossfader connects directly to the input of the WET-DRY crossfader. The other input of this crossfader is fed from the IN 2 Jack socket via the IN 2 LEVEL control, which attenuates the input. The switched side of the IN 2 jack socket is connected to the input of the module (downstream of the input level control), so if there is no external audio signal patched to IN 2 then the input from IN 1 (the DRY signal) will be normalised through the IN 2 jack and on to the WET-DRY crossfader.This configuration allows it to pan between wet and dry signals, and the "depth" of the filtering effect can be varied from zero to 100% filtered.
If an external audio signal (at modular levels) is patched into the IN 2 socket, then the input signal normalising will be defeated and the external audio signal will instead be passed to the WET-DRY crossfader, so in this case it will pan between the External Audio signal and the outout of the FFB 914.
Both the LEFT-RIGHT and WET-DRY crossfaders have manual fade controls, shown as OUT-MIX and WET-DRY above. These can be used to manually pan the output of the crossfader from one input across to the second input, with centre position giving a 50-50 mix of the two signals.
In addition the LEFT-RIGHT and WET-DRY crossfaders can also be controlled from an external Control Voltage (CV), which in the case of the LEFT-RIGHT crossfader would be connected to the MIX CV Input, and the level of the incoming CV can be attenuated with the MIX CV control. A positive CV of up to +12V can be used for control, with zero to +5V giving a full swing - higher levels signals can be attenated with the MIX CV control to allow a full range of control, so if a 0 to +10V CV is used, then this pot would be set at around 50% travel to attenuate this down to the correct level. Negative going CV voltages are ignored. The WET-DRY crossfader CV controls work in the same manner, however the input is to the WD-CV jack socket, and the incoming CV voltage is attenuated with the WD-CV control.
The feedback path is taken from the MIX OUT jack, and feedback level is set using the FEEDBACK control, which is connected to the module input to create a positive feedback loop. It is useful that the feedback is taken from the MIX OUT, as this allows the Feedback to be taken from the mix of LEFT of RIGHT buses determined by the MIX crossfader, so adjusting the OUT-MIX control (or CV) changes the feedback source from the Right or Left channels, or any mix of the two.
Divkid explores the many features of the FFB 914 and creates some of his killer patches with it.
The original Divkid youtube video with timing Index can be found here