First of all, I want to thank Olivier Gillet for his incredible Shruthi-1 hardware synth with its excellent SMR-4 MkII filter board, both of which I built about a month ago and which I love to bits for exploring new sounds. However, as a sound designer I’ve always been interested in pushing the boundaries, so having dusted off my electronic engineering skills, polished my soldering iron and retrieved my component boxes from the shed I’ve spent many spare hours over the last few weeks seeing what other sounds I could coax from the SMR4-MkII filter board.
The result of these hardware mods and additions is the Shruthi-1 SDE (Sound Designer Edition ;-)), with three new features:
1. First of all, I was green with envy when Olivier’s stunning 4-pole mission filter was released, so I cherry-picked my favourite six filter shapes from it and reverse-engineered them for the SMR-4 MKII: LP2, LP4 BP2, BP4, HP2, and Notch
2. Second, I explored the ‘wobble’ filter character from the 4-pole mission and implemented it as a variable Filter FM control going from none to serious filth.
3. Finally (and what took most time, energy and listening sessions) I extended the Mission/Polivoks ‘MS-20 distortion’ feature so that the SDE has six Filter Flavours that each react dynamically to incoming audio signals:
a) FLAT - the original squelchy SMR-4 MkII filter sound with low distortion (typically less than 0.3% with most Shruthi-1 patches).
b) SOFT - smooths the edge off any resonance and adds a tiny amount (~1%) of third harmonic distortion.
c) WARM adds a healthy 10% second harmonic distortion for a thicker, fatter sound.
d) GERM adds a massive 25% second harmonic distortion and kills the resonance using a classic Germanium diode limiter.
e) HARD adds a similar amount of distortion as GERM, but this time predominantly third harmonic and less second for a harder-edged sound with added sub-octaves.
f) EVIL takes things to the ultimate extreme, using hard limiting to add loads of extra harmonic content for a grungy overdrive.
ADDED 2ND JUNE 2012
4. I couldn’t resist adding yet another feature to the Shruthi-1 SDE after hearing the Polivoks in action: Output: Drive offers some subtle to serious overdrive and distortion options, along with a tweak to output level to make the SMR-4 MkII presets all run within -10dBV input sensitivity on your audio interfaces. On my setup some presets previously needed turning down or my interface switching to +4dBu to avoid digital clipping. Now everything runs nicely at -10dBV so you don’t have to watch the meters
ADDED 30TH MARCH 2013
5. I’ve been exploring some internal feedback/circuit bending mods, and the result is FEED, with a new set of demos on my Soundcloud page to demonstrate the possibilities.
ADDED 15TH JUNE 2013
6. The final mod is in place! ‘RES CMP’ is a three-way switch with the following settings:
1. CMP (full bandwidth compensated filter resonance, with lots of low end even at high resonance settings like classic Roland filters)
2. OFF (uncompensated filter resonance (bass falls off at higher resonance settings like the classic Moog ladder filter)
3. BITE (a screaming distorted resonance with increasing resonance at lower frequency settings, like on the Korg MS-20 type 1 design)
The extra front panel controls in charge of all these new features are now two 6-way switches, two pots, one 3-way toggle switch. I’m well pleased with the results!
See what you think on my SOUNDCLOUD PAGE
I’m happy to pass on the circuit-bending modifications to anyone who’s interested in trying them, although I filled a small circuit board with the added components and the 6-way switches are tedious to solder up, so it’s not a beginner’s project