Doepfer A-106-6 VCF

Doepfer A-106-6 XP Filter

The Doepfer A-106-6 VCF is a multimode voltage controlled filter based on the filter design in the Oberheim Xpander. Like the Oberheim synthesizer the A-106-6 has 15 filter modes. In the analogue filter architecture different poles build up a filter, where every pole attenuates frequencies above or below (depending on filter type) the cut off frequency with 6 dB per octave. So, for low pass filters a 1-pole, or a 6 dB per octave, filter attenuates less high frequencies than a 4-pole, 24 dB per octave, filter does. By that means different filter architectures creates different sounds when they attenuate different amounts of undesired frequencies. Of the 15 filter modes provided by the A-106-6 8 filters are available simultaneously. The filters are paired to one output, and a switch selects group of filters.

There are 11 symmetrical filters:
– one-, two-, three- and four-pole low pass
– one-, two and three-pole high pass
– two- and four-pole band pass
– two-pole notch
– three-pole phase shift or all pass as Doepfer calls it
And 4 asymmetrical filters:
– two- and three-pole high pass plus one-pole low pass
– two-pole notch plus one-pole low pass
– three-pole phase shift (all pass) plus one-pole low pass

As usual the Doepfer layout is perspicuous, even if the row of filter outputs might appear a bit mazy at a first glance. Build quality and knob feeling are very nice. As for all Doepfer modules the A-106-6 VCF comes in bubble wrap with the mounting screws in a zip bag. For once, at least in the writing moment, there’s no manual to download at the Doepfer web site. The A-106-6 VCF is 12 HP and is easily mounted with the two supplied mounting screws. The filter has one signal input, with attenuation, two CV inputs for the cut off frequency whereof one with attenuation, controls for cut off frequency and resonance (called Q), one CV input with attenuation for the resonance, a switch to select the left or the right filter group, and 8 filter outputs (the two-pole high pass plus one-pole low pass filter mode has one output in each filter group).

The filter sounds absolutely great, creating any sound you could ever dream of. No, it will not substitute your craving for a MiniMoog, or your avidity for a TB-303. However, the A-106-6 will always fill your need for a voltage controlled filter and it will do that with its own sound, maybe with some flavor of the Oberheim Xpander and who could really ask for more?

The filter goes nicely into self-oscillation, and the control scale for the cut off frequency is good enough to use this VCF as a voltage controlled sine wave oscillator. The resonance sounds great, and adds extra power to the cutoff frequency while attenuating frequencies around, giving rather nice 303-ish acidic sounds if desired. A great feature that I often miss in other VCFs, is the possibility to voltage control the resonance. With the A-106-6 you can add some growl to your sound whenever you want to, and then go back to a more subtle sound again. This makes this multimode filter design even more usable. A nice thing to try out is using a mixer between your sound source and the VCF input, and rerouting one of the outputs (for example the one-pole low pass) to the mixer, giving some feed back to the signal path, for even more interesting sounds.

However, the multimode design, the 15 filter types, and the 8 outputs might make the VCF less intuitive since not all filter types are available at the same time. You also need to understand and remember Doepfer’s abbreviations, like 3A1L and 2N1L, and in turn how the cut off frequency control affects all filter modes. A slight drawback with the A-106-6 filter design is that the outputs have slightly different levels and noise floor. This is caused by the different internal amplifications and numbers of stages that are required to generate the different filter modes. The noise is not very prominent and the different output levels are, according to me, not a big problem, it rather gives the filter a true analogue feeling.

To summarize, the A-106-6 VCF is a great VCF. Its many filter modes and fantastic sound makes it, in my opinion, a must buy. But maybe, it’s not that suitable as the first VCF for the more inexperienced modular geek.

Ratings (1 = poor, 5 = excellent)
Build quality: 4
Functionality: 4
Audio quality (if applicable): 5
Ease of use: 3
Cost/Bang for your buck: 4

Comparison chart
Category: Filters
Controls: 1x Input level, 1x Cut off frequency, 1x Attenuator for cut off frequency CV 2, 1x Resonance, 1x Attenuator for resonance CV, 1x Switch for filter group.
Inputs: 1x in, 2x Frequency CV in, 1x Resonance CV in.
Outputs: 16x filter type outputs.
Size: Width: 12 HP, Depth: 50 mm
Power consumption: +12 V = 50 mA, -12 V = not specified, +5 V = not specified
Price: 150.00 € / $245.00

Doepfer A-124 VCF

Doepfer A-124 Wasp Filter

The Wasp was a black and yellow quirky little synthesizer manufactured by EDP (Electronic Dream Plant) in the UK between 1978 and 1981. The peculiar synth had a great sounding filter with high pass, low pass, and band pass options. The Doepfer A-124 Wasp filter is a voltage controlled multimode filter based on the filter in the Wasp, and likewise treats digital inverters like analog operational amplifiers. This misuse of the digital circuits is what creates the rather unique distorted filter sound. But, when the Wasp was impudent yellow, the Doepfer is boringly gray. A bit dull maybe, but the layout is nevertheless easy to understand and work with. However, I’m not going to compare the Wasp and the Doepfer filter further in this review.

All input and output jacks are located to the left with the control knobs on the right. Personally I would prefer to have all jacks at the bottom and controls above, since the patch cables might interfere with knob tweaking and live adjustments. Even though the VCF is rather cheep, the feeling of the knobs and the build quality are great. The A-124 VCF comes in bubble wrap with the mounting screws in a zip bag, not very sexy but efficient enough. A manual can be downloaded from the Doepfer web site.

The A-124 VCF is 8 HP and mounts easily with the two supplied mounting screws. The filter has one signal input, with attenuation, two CV inputs for the cut off frequency whereof one with attenuation, controls for cut off frequency and resonance, a mix control between low pass mode and high pass mode, and two outputs: one for band pass and one for the mix between low pass and high pass. The filter setup makes sure of easy patching and good flexibility. The mix control outputs, in the leftmost position only the low passed filtered sound and in the rightmost position only the high passed filtered sound. When adjusting the mix control different notch-filter settings, symmetrical as well as asymmetrical, are obtained.

The filter sounds great, creating superb bass sounds, screaming lead sounds, and clacking synth sounds. The control scale for the cut off frequency is not very precise. However, this is not a big problem since the filter won’t go into self-oscillation, and you can’t use the VCF as a sinusoid signal source. The resonance sounds good but distorts the filter, too much and in a not very musical way, at maximum setting. The resonance is also affected of the input level, and sounds better when driven with a slightly lower level.

To summarize, the A-124 VCF is a low price but great sounding VCF. Its multimode options, offering low pass, band pass, notch, and high pass filter modes, makes it a competent and useful filter at a very affordable price.

Ratings (1 = poor, 5 = excellent)
Build quality: 4
Functionality: 4
Audio quality (if applicable): 4
Ease of use: 4
Cost/Bang for your buck: 4

Comparison chart
Category: Filters
Controls: 1x Input level, 1x Cut off frequency, 1x Attenuator for cut off frequency CV 2, 1x Resonance, 1x Mix (Low pass and High pass).
Inputs: 1x In, 2x FSrequency CV in.
Outputs: 1x Band pass out, 1x Low pass/High pass out.
Size: Width: 8 HP, Depth: 45 mm
Power consumption: +12 V = 30 mA, -12 V = not specified, +5 V = not specified
Price: 75.00 € / $110.00