Unless you’re from Berkeley, you will most probably consider filters as one of the key sound shaping components in a synthesizer.
What is true for synths in general also applies for modular things like Eurorack. And in the Eurorack world, you have a choice among hundreds of different models.
Rather than giving a big overview, here’s my take on a few of those which I personally use.
Population and Classification
We’ll be looking both at Eurorack modules and filters in semi-modular synths that conform to Eurorack specification.
As for comparing them, we’ll be looking at both hard and soft criteria:
- Form factor (width, standalone/semimodular)
- Filter modes
- Features (controls, inputs, outputs)
- Price (street prices in Germany)
And there’ll be a short verdict and description of the key properties in one statement.
There’s also a Video
A Word of Filter Wisdom
For many of the filters we’ll be looking at, the interaction between frequency, resonance and input level is nontrivial. Resonance may or may not change frequency. Input level may reduce resonance – or increase it. While I’ll be mentioning especially noteworthy aspects, in the end this relationship is often too complex to put into simple words (or videos).
Enter the Contestants
Pretty normal, pretty oldskool
Moog DFAM Filter
It’s only natural to start with what most consider the mother of all filter designs: the Moog transistor ladder filter, here in the implementation in the DFAM.
I already discussed this at length in the DFAM review: the main shortcoming is attenuation of the passband even at low resonance settings. Otherwise you get what you came for – including an almost perfect sine at high resonance.
- Semimodular – standalone or 60TE width for entire synth
- 4th order HP/LP
- Frequency and resonance knobs, frequency CV, no dedicated in/outs
- [€559 for the whole semimodular)
Summary: I wouldn’t get it standalone, but it works in its role unless you want bass and resonance at the same time.
Doepfer A-108 VCF8
What’s better than a 4th order Moog cascade? A 8th order Moog cascade!
With this new product, Doepfer has addressed the needs of all those that want a filter sound that’s often called “buttery”, but without leaving the option for full-bodied distortion or screaming resonance.
There’s a choice among all of the eight stage’s taps, of which four are sent out to the panel (jumpers can change the assignment). Plus a bandpass which looks 2nd-orderish to me.
If there’s one thing to note, than it’s this: this filter sounds beautiful all the time. No, really!
- 12TE, 75mm (!) deep
- 1st-8th order LP, BP
- Frequency, resonance, input level, 3 freq CV inputs (2 with attenuator), 4 LP outs, BP out, normalled feedback in, audio in.
Summary: My #1 lowpass right now. This one sounds just wonderful!
2hp does only one thing: Eurorack modules that are 2TE (or HP) wide. Within that limitation, they do a lot, including three filter designs, among which we’ll look at the LPF today.
I mainly got this to have a small filter for clock removal for my Doepfer BBD, but have since tried what it can do otherwise. And let’s first start that as a normal lowpass (read: no resonance, not driven too hard) it does just that: filter high frequencies (like a clock) and nothing else. Check.
A four-pole lowpass design, this one offers a nice difference between cleanly lowpassing and, at mid resonance values, angrily changing the signal.
- 4th order LP
- Frequency, resonance, freq CV with attenuator, resonance CV, audio in/out.
Summary: Small. And also nice.
MFB Nanozwerg Pro Filter
A very compact and affordable voice module – which also has been discontinued.
Although this one has a small patchbay with only 14 points, all of the filter connectors are available. In case you need it, there’s a dedicated envelope, and filter tracking can be adjusted.
Similar to the 2hp design, this one starts pretty normal but can get pretty angry if driven hard, and especially so in bandpass mode.
- Semimodular voice module, 24TE for entire synth
- 2nd order LP/BP/HP/notch
- Frequency, resonance, freq CV with attenuator, audio in/out
- (€249 for the voice module)
Summary: Again, I wouldn’t get it for the filter alone, but the filter is good in its own right. Great performance at a small price, and in a small package, is what’s true for the entire synth.
Doepfer A-124 WASP
Using the design from the old WASP synth, this one uses digital inverters for its filter circuit. It also offers a two outs: one for bandpass, one where you can sweep with a knob from lowpass over notch to highpass.
This it a perfectly dirty affair, and I intend that statement with nothing but praise. For everything that cuts without losing its foundation, this is the way to go.
As a sidenote, this one doesn’t self-oscillate, so if you’re looking for that: keep looking.
- 2nd order LP/notch/HP (out 1), BP (out 2)
- Frequency, resonance, 2 freq CV (one with attenuator), audio in, LP/notch/HP out, BP out
Summary: A very characteristic filter sound from the cheapest contestant here. Get it!
EricaSynths Pico VCF1
Let’s start from the beginning: odd-numbered widths violate Eurorack spec. For that reason, this filter gets big minus points from me (as does the entire Pico series).
Looking beyond that, it’s a Polivoks-inspired design using Russian opamps (whatever that is worth), and produces a screaming sound with solid foundation, placing it firmly in acid bass territory.
Lacking an input knob, you best dedicate an attenuator to the input, because it seems to have its sweet spot roughly at half the usual oscillator output level – that’s where it really gets that sound usually called “squelchy”.
- 2nd order (?) LP/BP
- Frequency, resonance, freq CV (with attenuator), audio in/outout
Summary: Stupid width. A nice 303 alternative for your acid needs.
Behringer Neutron Moffat Filter
I talked a lot about this synth in the dedicated review.
An original design named after their designer, it doesn’t scream “novel” from its spec page. You get separate outs for different modes, which you can switch between LP/BP, BP/HP and HP/LP. By taking the third mode, you can then get a notch filter as well.
First and foremost, this one sounds great in context with the Neutron’s oscillator and overdrive section. On the other hand, while this is a versatile and flexible filter for other sources, it somehow fails to stand out for me – so while it’s really a great “I need another filter right now” option, it won’t give your sound some truly distinctive character.
- Semimodular – standalone or 80TE for the whole synth
- 2nd order LP/HP/BP
- Frequency, resonance, freq CV (with attenuator), res CV, audio in, two filter outs
- €299 (for the whole semimodular)
Summary: A bread and butter affair – in the good as in the boring sense.
Filters with a Twist
These are the ones that aren’t strictly normal or oldskool – mostly by their circuitry. In some cases, the lines to the preceding sections are a little bit blurred.
Doepfer A-106-6 XP VCF
Oberheim, the first user of Curtis’ CEM3372, and also the first (and only “big”) ones to implement an idea from Doug Curtis: use the separate filter tapes of every stage of this four-pole LP, and by adding them with attenuvertion (does such a word exist?), get different filter designs.
The A-106-6 does offer no less than 16 different ones: from the standard order 1-4 lowpasses, there’s highpass (order 1-3), bandpass (order 2 and 4), notch (order 2), allpass (order 3) and a variety of combinations. A combo of eight (non-configurable) are available at one time on separate outs, and you switch between the two sets of eight.
In addition to that, it also offers resonance CV control.
A truly historic/vintage filter sound, combined with more filter types than you’re typically bound to use. Best combine it with a crossfader or CV-controlled switch!
- 12TE, 61mm deep
- 4th order Curtis CEM3379 with 16 filter modes
- Frequency, resonance, input level, 2 freq CV ins (one with attenuator), Q CV in with attenuator, audio in, 8 filter outs, switch between two sets of 8 filter modes
Summary: Lots of sonic options, all based on a true and tested sound. Not easy to understand, but using it yields great results.
Doepfer A-101-6 OptoFET VCF
This one uses OptoFETs as the frequency-defining resistor in its design, making it not too dissimilar to a vactrol-based lowpass gate. Now those OptoFETs are much faster than vactrols (meaning you don’t have the fast rise/slow fall characteristic as much), but they’re also highly nonlinear. Meaning this thing distorts in a nicely digital-inspired way (don’t think “chiptune”, think “really high-end gear”). And its resonance is a pure scream – that has a tendency to set in suddenly.
As if that weren’t enough, this one offers two allpass topologies, so it can also be used for phaser-like sounds with an unique sonic touch. And the feedback path can be routed through something outside the filter (such as a modulated BBD, to build a completely wicked phaser).
The downsides? First, this thing is rather hard to control with regard to resonance – you’ll have a hard time finding that spot between no resonance and full-on wailing. And secondly, changing the filter mode requries changing a total of typically twelve jumpers in the module and adjusting a trimmer.
And finally, having a 6th order filter is something that might come handy for actual filtering (as opposed to sound-shaping).
- 8th order opamp/OptoFET-based, LP/HP/AP modes per jumper
- Frequency, resonance, input level, freq CV with attenuator, feedback in/out, audio in/out
Summary: A rather unique addition to the arsenal, it tends to be hard to control in more normal applications.
Future Sound Systems Gristleizer Filter TG3
This is part of series of modules replicating a kindof-effects unit called the Gristleizer. The name sounds evil. The red flash indicator on the module’s panel looks evil. And the sound does not fall short of that image.
Let’s make it clear at the beginning. You won’t use this for normal filter duties. And it’s also quite expensive. But if you want to completely screw up your sound, then this is the thing for you. Plus, it’s got a lot of control over the gristle: two kind of frequency controls that affect different parts of the circuit (and sound different), two frequency CV ins, one with attenuverter (!), input and output level…
- 2nd order BP
- Frequency, register, resonance, 2 freq CV (one with attenuverter), drive, output level, audio in/out
Summary: Great as a sound mangling device. Not so much as a filter.
Are these really Filters?
Are lowpass-gates really filters? By their circuitry, yes. By their intended use, they’re rather sidechained gates (hence the “gate” in the name). I’ll include two of those as well.
With everything I’m going to say about these, let’s not forget that their intended functionality is a different one. Meaning that you’d typically would get them to act as LPGs and might abuse them as filter, like when you run out of filters.
Synthrotek Dual LPG
This one is small. Like most Synthrotek thingies, it’s also available as a DIY kit. And different to many competing designs, this one has resonance control, unfortunately only via CV, so if you want to set resonance, you need an external CV source.
The diagram above is a saw sent through the filter at varying CV settings. As you can see, it gives you a clearly defined cutoff point an clear 12dB/oct slope. For resonance, this one is one of the trickier ones: sometimes it doesn’t establish its sine peak at all, while at other times it screams all over the place even on relatively low settings. The latter is especially the case if you employ it in a LPG-like configuration, i.e. apply fast-changing CV ins or even triggers on the frequency CV.
- Dual 2nd order lowpass gate/VCA
- Frequency/level, frequency/level CV, resonance CV, audio in/out
Summary: A great LPG that can also double as a LPF if needed.
In this one, the LPG is hidden in the “Dynamics” building block. This one doesn’t have a proper “frequency” control, rather the dedicated knob attenuates the input to this section.
The diagram was recorded in a similar fashion as for the Synthrotek device. Here, the cutoff isn’t visible as clearly, and also the slope appears slightly flatter, while being decidedly above that of a single-order filter.
From a listening standpoint, this one has more of a “muffled volume knob” characteristic than a closing filter. Which works perfectly well in its intended role as the VCA standin on the 0-Coast, but doesn’t make this one interesting as a filter.
- Semimodular – standalone, can be modded to fit in 40TE case
- 2nd order lowpass gate
- Frequency CV with attenuator
- €512 (for the whole semimodular)
Summary: You wouldn’t use this as a filter.
Ist there a Verdict?
Well, yes and no. Among those twelve contestants, some of them have rather clinical advantages or disadvantages, like size or selling price. The feature count is something that’s highly application specific, and with the sound we’re deep into application-specific and subjective territory.
To me, the Doepfer A-124 WASP is a no-brainer. It’s inexpensive, not too large and offers a nice variety of functionality. The same is true for the 2hp LPF: small size, and a surprisingly nice sound for under €100.
Right behind it is the Doepfer A-106-6 XP VCF for a truly flexible addition. While the interaction between level and resonance is somewhat tricky to tame on this, and 12TE is wider than 8TE (let alone 2), the sheer bandwidth of filter modes and the availability of Q CV makes this a feature monster.
There’s things that you wouldn’t get for their filter function – next to the semimodulars, this is true for the Synthrotek LPG: if you consider getting a LPG anyway, then the Synthrotek DualLPG is also an interesting filter.
Furthermore, while it has that stupid size and is kind of a one-trick pony, you might enjoy the Erica Synths Pico VCF1 if you want a different kind of acid squelch.
And finally, if there’s to be a winner: Doepfer A-108 VCF8. It’s large and expensive, and doesn’t have a lot of features. But it sounds fantastic – and that’s what it’s all about in the end.
Yes, there’s a lot of Doepfer. And there’s a reason for it. Their products are fantastic. Hell, Mr. Doepfer invented Eurorack.