The DSM01 Curtis Filter is the first Eurorack module from Dave Smith Instruments. DSM 01 brings the classic sound of the highly regarded Curtis Filter chip into the modular world and adds some very interesting modulation capabilities as well.
All little history
Pioneered by Doug Curtis, electronic instrument pioneer extraordinaire his line of analogue chips and IC’s have been used by pretty much every single synthesizer manufacturer around the world, from Korg, Moog, Sequential and E-MU. He’s range of chips consisted of VCO’s, VCA’s and most notably VCF’s.
The Curtis filter circuit was used by many synthesizer manufacturers in the 80's, it found its way into many models, form factors and has been revised countless times. Sequential circuits were one of the notable makers who adopted the SSM filter chip, the first synth they produced utilized these VCF chips was the Prophet 5, as found in Rev 1 and 2. Heralded by many as the greatest polyphonic synth ever made, the Prophet 5 owed much of its signature tonality and charm to its choice of filter design.
Early revisions use the SSM2044 chip and are considered to be amongst the best sounding variants of the Prophet 5 as such they are highly sought after in the vintage synth market.
Fast forward in time and Dave Smith Instruments begin to release a wide range of cool, interesting synthesizers that utilize the Curtis filter chip (albeit a newer design) in-keeping with their rich heritage.
Dave Smith and Curtis are synonymous with one another as DSI their selves put it, the Curtis filter is the heart and soul of the Dave Smith sound, so it’s no surprise that their first eurorack module is a Curtis Filter.
Bring forth the DSM01
The DSM 01 is a seemingly simplistic module at first glance, an 8HP dual mode low pass filter with 12dB and 24dB slopes encrusted with the soft touch control knobs from the prophet 12, but delve a little deeper past its dark mysterious exterior and you’ll be pleasantly surprised with the modulation capabilities onboard.
The standard fair of modulation parameters apply, of course you have the access to modulate the cut off frequency, but more uniquely you’re able to modulate the resonance also. This ability is pretty unheard of for most filters and it allows for some diverse sound design capabilities and subtle rhythmic evolution as each section can be controlled at different rates. Very nice.
The DSM01 also include a boost circuit which allows you to add some pre-filter drive into the signal and really push the harmonic content out of the filter.
Another interesting addition is the onboard VCA, which includes its own dedicated CV input for controlling the amplitude of the module. This can also be struck and modulated at different rates to the other CV inputs. Which makes for some very interesting sounds.
To test the DSM01, we used the Mutable Instruments Edges to generate some subtly detuned pitches and chiptune “plink” and also used the triangle wave VCO output from the Intellijel Atlantis. This means we’ve got some clinical digital sources with Edges and a well rounded analogue source in Atlantis To begin we patched the summed output of the Mutable Edges into the input of the filter, and began to work with the frequency sweep.
The sweep in the four pole 24dB mode yields all of the brassy, creamy response that the Curtis Filter is so well known for. The feel is very smooth and instantly recognisable as a Curtis Filter. This module will speak to any avid Dave Smith or Sequential fan.
As you move through the resonance settings, as you would expect a portion of the low end is lost but this can be counteracted using the onboard boost circuit if desired. This makes for some interesting high frequency action and odd harmonics.
At very high resonance settings you can obtain self oscillation / sine wave, which tracks exceptionally well when a 1 volt/oct control source is applied at the frequency CV input.
In the 2-pole 12dB mode the character of the filter is more subtle and subdued; it feels more like a colouration tool when used sparingly and definitely smoothes off harsher waveforms and audio sources, an ideal pairing with something like edges when you’re working with multiple waveforms running at different rates.
With high resonance settings there’s no self oscillation available, but that’s not to say things don’t get squeamish or aggressive. There is plenty of tonal diversity available in both modes, but things get really interesting when you start to apply modulation.
Modulation + Filter = ?
For modulation we used a combination of sources, each running at very different rates to see what bizarre or sonically pleasurable results we could obtain. We used the Intellijel Atlantis, 4MS QPLFO, Mutable Instruments Frames and Synthrotek Mattson Sample and Hold modules to insert three different sources into the three CV inputs.
Each section requires a maximum of 10 volts to fully strike the mod input, our modulation sources vary in term of CV generated, so our results varied depending on that source struck which destination – but we’ll try to keep the finding and results short and sweet.
We inserted an LFO signal from the MOD oscillator of the Atlantis into the frequency CV input and set it to VCO mode. What was instantly noticeable was how nice this filter sounded when being modulated by and audio rate source, under heavy audio modulation the filter was extremely stable and produced a super grungy tone with lots of harmonic content.
Slower modulation rates paired with opposing / offset modulation rates inserted into the resonance input yielded some poly rhythmic results, with the high resonance kicking in at random intervals that made certain phrases of our sequence pop out in the mix.
We applied an envelope from the Studio Electronics Shapers Dual EG to get some envelope control over the filter, this allowed us to craft squelchy pseudo 303 type filter tones and vocal format style sounds with ease. The raw filter capabilities of this circuit are well known in most circles, but we were pleasantly surprised with just how good it sounded.
VCA modulation is what interested me the most about this module. Not only can you route the VCA output to another source post filter to be used elsewhere, you can also modulate the VCA itself to generate amplitude based effects.
We send a downbeat ping to the 4MS QPLFO and routed that into the VCA input, since the LFO was triggering against the kick drum pattern, we were able to use the VCA almost as a sidechain allowing our synth signal to duck under the kick drum each time it triggered, a really handy capability when working in the EDM arena.
Applying audio rate modulation to the VCA extends the sonic capabilities, when working with audio rate modulation or just under, you can rapidly switch the VCA on and off to get a whole different sound into your system. When the VCA is being struck or modulated, the onboard LED will blink in time with the signal its receiving so you always have visual feedback on what’s exactly going on. Pretty handy when working with mountains of spaghetti.
So what do we think?
The DSM01 might not be the only filter claiming to be a Curtis circuit, but it’s clear that the sound and behaviour of the classic circuit is completely intact here. It’s a superb filter and sound source with added modulation capabilities that any eurorack user would find pleasing to work with.
Dave Smith Instruments have done a great job with the DSM 01, it sounds great, performs well and brings a vintage charm to any modular users system. Heck, it’s 8HP and under £150, what more do you want!
We have the DSM01 and all of the other modules mentioned in this article in stock and available to play with in our Whetstone synth city demo area.
For more information about the Dave Smith DSM01 call us on 020 8445 2446 or e-mail us firstname.lastname@example.org
By Tom Lewis