Finally it’s time to select my favourite part of a synthesizer, the filters!

A little about filters

Filters are used to reject certain frequencies and shape the sound. They commonly come in low pass (high frequencies cut off, low pass through), high pass, band pass, and notch.

The Cutoff is used to determine at which frequency the filter takes effect. Some modules like the Synthesis Technology E440 allow this to be controlled via CV control voltage. Resonance, aka peak or emphasis, is used to boost the signal at the cutoff frequency.

The slope at which the frequency are attenuated are typically 12 or 24 db per octave (aka 2 pole and 4 pole filers).

Famous filters include the Moog ladder filter used on synths like the Minimoog, the Oberheim SEM filter and the ESP Wasp filter, all of which have been cloned or copied and modified on multiple occasions.

Ask Audio have this great article if you want to know more.

What I need

So far my custom modular synth has a great mix of analogue and digital oscillators, 4 in total, so I’d like to add two different filters to allow a couple different signal paths and add different flavours.

My basic specifications are as follows. Although CV inputs and controls are important, the deciding factor will be the sound.

• Two different analogue low pass filters
• At least one filter with high pass function (band pass I can achieve with the two in series if I really need to)
• The modules must be skiff friendly, i.e. not that deep. So no modules where the circuit board is perpendicular to the front panel.
• Good audio demos of the units must be available
• Ample low frequency response

The list and initial assessment

I compiled a long list of filters for consideration (in no particular order). Thankfully restricting myself to skiff friendly modules cut a lot out, otherwise the task would be endless! Here’s my initial assessment.

What made it:

• Fonitronik TH VCF-VCA – A nice sounding 12db filter based on a Thomas Henry design, it has LP/HP/BP, linear FM (frequency modulation) and an in-built VCA. It’s only around 50 euros for the PCB, ICs (and panel which I don’t need).
• Manhattan Analog Discrete SVVCF – A single LP/HP/BP/notch based on the based on the J. Haible design (like the Living VCOs I selected). It also has a handy 3 input mixer. This demo shows that this can make some nice acid sounds, or fat synth-wave bass.
• Din Sync VCF303 – A low pass filter based on the famous Roland TB-303 filter, it provides that squelchy acid sound with the overdrive, and has an envelope mod with decay and accent built in. One of the fattest filters I’ve heard.
• River Creative Technology Nucleus OTA VCF – A 12/24db low pass filter with an OTA core, it has a bass boost to maintain bass when resonance is applied. There’s a nice demo available and the low end response sounds nice.
• God’s Box Humpback Filter – A 12db LP/HP/BP/notch filter, this is one of the nicest filters I’ve heard thus far and there’s multiple decent demos available.
• RYO Aperture – This different module is a LPF, VCA and LPG (low pass gate) inspired by the Buchla 292. It has some odd features like Klang which generates a decay signal. The sound from the demos seems pretty fat and with the unique features this makes it.
• Fonik SSM2044 VCF – A filter based on the classic SSM2044 IC used in cool synths such as the PPG Wave and the Korg Trident. It’s a 4-pole (24db) and the SoundCloud demo sounds pretty varied and fat so this is in, even thought the chip is quite expensive.
• Neutron Sound Duofonik – This is a dual LPF (low pass filter) based on the Fonik SSM2044. It has an acidic TB-303 type sound, and the filters can be used in unison which can also create a HPF.
• Random Source Serge Variable Slope Filter (VCFS) – Random Source brought us the Haible Living VCOs and the Haible Wasp, and this beauty is from the Serge Modular system. It’s a 12db LP/HP/BP filter, and it sounds amazing! I can already imaging the Living VCOs running through this; definitely a contender.
• Bastl Instruments Cinnamon – For whatever reason I’ve never taken to the Bastle modules so I was happy to have a critical listen of their LP/HP/BP. It features character and drive functions to shape the sound, and I was surprised at how good it sounded. So this is in!
• Noise Reap VCF – Another LP/HP/BP, this has 3 CV inputs and sounds. It sounds good and the resonance in particular has a nice distorted squelch when pushed.
• Mutated Ripples – Mutable Instruments are one of the more well known Eurorack manufacturers, with DIY provided via the “Mutated” PCBs. I hadn’t yet selected any of their modules for my custom synth (although I own a Peaks) so I was keen to check out the Ripples. It’s a 24db low pass filter with 12db HP/BP also available. Even though it’s light on knobs (only 3), the sound is on par with anything else I’ve heard, so this rounds on the list.

What didn’t:

• Befaco BF-22 Dual Sallen VCF – Dual low/high pass filter based on the Korg MS-20, although doesn’t really sound like it. Although the bass extends nicely, the sound of the resonance isn’t to my taste (see demo) as it sounds too harsh and distorted, so this is out.
• L-1 Mutant Vactrol VCF – Provides 12 or 24 db of band and low pass filtering. I’m not a fan of this layout for a filter and not convinced by the demos I’ve heard (there’s not enough). Bit harsh as this decent but there’s too many others on this list.
• L-1 2180 VCF – A basic LP/HP/BP filter with bass boost and FM. I couldn’t find any decent video demos and the SoundCloud ones weren’t impressive, so this is out.
• NLC FK1T VCF – A high pass and low pass filter in series based off an old Korg pedal. Not a fan of the audio demo or layout so this is out.
• Bubblesound DiOD Filter – A classic Steiner Parker diode filter with LP/HP/BP, looks promising but the audio demos don’t have basic bass examples (mostly distorted sounds) so unfortunately this is out.
• Haible Dual Wasp Filter – I already chose the Haible Living VCOs, so I was keen to hear their Wasp filter. I already own a Doepfer Wasp filter, so it was interesting to hear a different version. The Wasp filter is what I’d call an effects filter, as in it greatly changes the sound and is very distinct. It’s great to have in your arsenal, and I do have a Doepfer version in my Eurorack, but for a modular with only two filters I can go without the Wasp sound.
• MST LP Filter – This is a 12/24 LPF. The demo doesn’t have much bass at all so this is out. Bleeps are fine but I need to hear the full range of frequencies.

Round 2 many filters

I’ve ended up with 13 filters on my “short list”. This is way too many so I’m going to do another pass, listening closely to each and being stricter on the sound characteristics.

There’s two SSM2044 filters, the Fonik and the Neutron. The Fonik demo sounds better, and without the possibility of an apples for apples comparison, the Neutron is out. The Ripples kind of reminds me of the SSM filter as well, and is fat enough to make it though.

On second pass the Fonitronik doesn’t sound as good as the other filters here, and the VCA is handy but takes up more real estate, so this is also out. The Manhattan Analog has one of the nicest layouts, although I’m not convinced by the tone of the resonance. Although I can’t fault the Nucleus, it’s an expensive kit at around US$200.

The Aperture is different and likeable, but for this project I’m favour others; this gets cut. The same goes with the Bastl Cinnamon as I already have distortion in my Optodist so the advantage of this unit is gone.

The VCF303 passes. Even though I don’t want to make TB-303 acid sounds, I think this filter has a lot of potential to do otherwise as it has unique controls. The God’s Box is smooth, and despite the small CV knobs has a nice layout, so this is through, as is the brilliant sounding Serge. The squelching resonance of the Noise Reap is attractive. All these are through to the final round.

So I’ve cut it down to the following…

Din Sync VCF303

A brilliant sounding Roland TB-303 filter clone! The VCF303 was one of the front runners in the back of my mind, but at EU197.34 for the full kit, this may be a deciding factor, not to mention the fact that it seems the kit is unavailable. Regardless, what I do like about this is the decay, accent and overdrive, which sets it apart from other filters.

Manufacturer’s link: http://www.dinsync.info/search/label/VCF303

God’s Box Humpback Filter

This covers ground with the LP/HP/BP/notch outputs. It has two CV inputs, a solid layout, and a good all round sound. EU139 for the full kit.

Link: https://www.thonk.co.uk/shop/humpback/

Fonik SSM2044 VCF

Who doesn’t want an SSM2044 filter in their system! Not only did the awesome Korg Trident use one, it was in the famous sampler the E-MU SP-1200. The Fonik is a nice sounding implementation of this chip. The layout is a little odd with the horizontal alignment, and the features are basic, but the sound is where this excels. The PCB (and panel) is only US$25, but the chips go for around GBP25. Either way this should be one of the cheaper filter builds.

Link: http://modularaddict.com/timo-ssm2044vcf-pcbpanel

Random Source Serge Variable Slope Filter (VCFS)

Possibly the sweetest sounding filter here, where resonant sweeps leave you wanting more. Although having the outputs at the top is a little odd, the layout for Eurorack is quite spacious. There’s a nice demo below but it would’ve been good to hear a basic synth bass sound.

89 euros for the two PCBS (and panel) plus the fancy parts which is decent. I can also trust them given my delivery of the Living VCOs PCBs.

Manufacturer’s link: http://randomsource.net/serge_euro

Noise Reap VCF

Like the Humpback filter this covers some ground with the LP/HP/BP all available. As mentioned above, the squelch factor of the filter is where this excels, although as per the Serge some standard bass demos would’ve been nice. The PCB is only US$20 and apart from the VCA chip it uses all the parts aren’t particularly costly. It’s low on tactile control but has a nice layout nonetheless.

Manufacturer’s link: http://noisereap.com/?product=12db-vcf

Mutated Ripples

This is possibly the most polished sounding filter with the most variety. It has a great SSM like bass response like the Fonik, but still provides squelchy acid tones like the VCF303 (albeit not as exaggerated). It covers LP/HP/BP, is only US$7 for the PCB and surprisingly only AU$29 for the parts (am presuming the potentiometers are extra as I cannot see them on the BOM). This is an SMT build which I haven’t done before (but need to).

Manufacturer’s link (for off the shelf product):  http://mutable-instruments.net/modules/ripples

Final selections

This will be the most difficult decision. In order to help decide, I can categorise the filters as character filters which have a more unique sound, and classic bass filters which I feel are more general but provide a desirable sound. I feel this is biased by the demos available as which shapes the perception of these modules.

Character filters: The VCF303, Noise Reap and to a less extent the Humpback all offer a lot of character, acidic type sounds and well rounded bass.

Classic bass filters: The SSM2044, Serge and Ripples each provide the classic fat bass sound; think the bass line at the end of the Stranger Things intro.

The VCF303 I’m cutting out due to cost. The God’s Box Humpback isn’t cheap either and I’m having doubts about the character of the resonance compared to the others here, so this is also out. That leaves the Noise Reap to challenge the rest with its dirty sound.

The SSM2044 sounds awesome, however the bass drops when resonance is introduced. This is common but not desirable, therefore the SSM2044 is out. Although this is apparent on the Ripples too, the overall bass extends deeper, at least in the demos, which gives it the nod over the SSM2044. It’s hard to tell from the Serge demos what it’s like regarding this phenomenon.

So I’m left with the Noise Reap, Serge and the Ripples. Each provide the LP/HP/BP functions I want.

Between the Serge and Noise Reap, the decision is very difficult. None of the demos were satisfying me either way, although the Soundcloud demo if the Serge has pushed that to the lead. The Noise Reap has amazing character. It reminds me of my Korg MS-20’s filter; dirty and ballsy. The Serge has a pure sound quality that I believe tops this list here, and will sound amazing with its brother Living VCO from Random Source.

The Ripples demonstrates pure deep bass lines, but can I select it alongside the Serge? The Ripples is versatile but perhaps more clinical. I’ve recently been going off some Dave Smith products for that reason. I was going to select the Ripples, but I think a better combination would be the Serge and the Noise Reap! The Serge for sweet tones the Noise Reap for the aggressive. Imagine the Orgone Accumulator through the Reap, amazing!

This has been the most difficult selection by far, but I’m happy with the outcome! Here’s what my custom modular synth now looks like with the native Eurorack panels. Next I’ll select the LFOs.

DIY Modular Synthesizer Preview