Shruthi-1 SDE

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The SDE (Sound Designer Edition) is an extensive mod by user yewtreemagic (Martin Walker), aimed at giving a Shruthi-1 with the SMR-4 MkII VCF a total of up to six extra features. Some of those are present on the 4-pole Mission VCF, others are related to distortion 'flavours', while some are unique to the SDE.

Filter SHAPE

Six filter shapes are offered by the SDE: LP2, LP4, BP2, BP4, HP2, and Notch

SDE Filter Shape Demos

Filter FM MOD

The ‘wobble’ filter character from the 4-pole mission is implemented here as a variable Filter FM control going smoothly from subtle to warbly filth.



This most popular new feature of the SDE extends the Mission/Polivoks ‘MS-20 distortion’ to six distinct Filter Flavours that each react dynamically to incoming audio signals, as well as altering the resonance character:

a) FLAT – the original SMR-4 MkII low distortion filter sound (typically less than 0.3% with most Shruthi-1 patches), but running with lower internal signal levels to provide more headroom, resulting in extra IR3109 acid squelch and resonance.

b) SOFT - smooths the edge off any resonance and adds a tiny amount (~1%) of third harmonic distortion.

c) WARM - offers a healthy 6% second harmonic distortion for a thicker, fatter sound.

d) GERM - adds a massive 20% second harmonic distortion and also kills the resonance, using classic Germanium diode limiting.

e) HARD - adds around 6% of predominantly third harmonic and less second for a harder-edged sound with added sub-octave contribution.

f) EVIL - takes things to the ultimate extreme, using pre-filter half-wave rectification for a huge 35% of extra harmonic content, giving a grungy overdrive with lots of high-end grit.

SDE Flavour Demos SDE Added Squelch Demo

Output DRIVE

Output Drive offers a wide range of overdrive options from subtle to serious distortion, 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.

SDE output DRIVE Demos

Filter FEED

'FEED' is part filter feedback, part circuit bend, offering a variety of tonal changes from subtle warming, through bass boost, to continuous filthy feedback and jumping between different harmonics.

SDE Filter FEED Demos

Filter RES CMP

'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, modeled on the Korg MS-20 type 1 design)


The extra front panel controls in charge of these new features are two 6-way switches for Filter SHAPES and Flavours, and two pots for DRIVE and FMOD. You'll also need the standard Shruthi-1 output level control wired in if you want to use the new DRIVE feature, and its standard input level control if you want to take advantage of the new FEED feature. Finally, a 3-way mini-toggle switch is used for the RES CMP function.

The 6-way switches are tedious to solder up, so it’s not a beginner’s project! In fact, it's for experienced builders only – some of the mods require removal of several existing components on the SMR-4 MkII circuit board and adding various flying leads between points on the front of this circuit board to a small extra circuit board containing various new components.

If you're a beginner and want to have a go at the easiest mods, just try Drive, FM Modulation and FEED.

Filter Shapes

Part: Amount:
2-pole/6-way switch 1
10pf Ceramic cap 1
The following 1% metal film resistors:
10kOhm 4
7k5 3
3k32 2
1k65 1
4k99 3

a) Carefully remove the existing C32 (1nf) from its position between pins 1 and 2 of IC7A and save it for later.

b) Carefully solder a wire link in place of (or across) the existing 10k resistor R6.

c) Remove the jumper currently plugged into the 2/4 Poles ‘switch’.

d) Solder flying leads to the following points, which connect to my circuit as follows:

MIX: centre pin of the 2/4-pole connector

LP1: circuit point labeled BP1

LP2: 2-pole pin

LP3: circuit point labeled BP3

LP4: 4-pole pin

CAP: The ‘outer’ end where C32 was soldered i.e. nearest the edge of the circuit board

Now here’s the circuit:


N.B. The unusual resistor vales are the exact values needed for the ‘perfect’ filter response, but in practice you can probably get away with nearby stock values. However, if you have a selection of resistors on hand to choose from, the important thing is the RATIO of the resistors used in each network.

1. Ideally, for BP2 you should match the values of the two 7k5 resistors as closely as possible by measuring a batch using a multimeter (it doesn’t matter if they are both slightly greater or less than 7k5, as long as they are both the same). Similarly, for BP4 the two 3k3 resistors should ideally be as close as possible in value, and whatever values you find for those two, the 1k6 should ideally be exactly half this. Neither of these two are critical though, as variations will simply change the shape of the filter response slightly and not be particularly audible.

2. For HP2, find two 10k resistors that are very close in value, then for the 4k99, find one that measures exactly half whatever that value is. How close you achieve this will determine how well sound below the cutoff frequency of the filter gets ‘cancelled out’ – a perfect match will achieve maximum ‘thin-ness’. You could even use two resistors in series, for example a 4k7 and a 330 ohm.

3. For Notch, you could use two 4k7 instead of 4k99, as long as they are both the same value, but then the one marked ‘10k’ should be exactly double this (or two resistors in series such as a 9k1 and 330 ohm). The quality of the match here determines how deep your notch is.

Filter FM Modulation (Mk II circuitry)

Part: Amount:
10k log pot 1
15k resistor (5% or better tolerance) 1

a) 10k log pot connected between pin 1 of IC2A and GND.

b) 15k resistor between its slider and FM pad (pin6 IC4B).

  • HOW TO USE : with the FMOD pot at its minimum position your sounds will be exactly the same as before, but as you turn it up you’ll hear the squelchy wobble feedback increasing. The setting Olivier chose for his 4-pole mission Wobble is about 1/3 up on the pot, but here we are allowed much more extreme settings.
  • N.B. Mk I version used 10k linear pot and 47k resistor. Mk II values here give much better control of lower settings up to 50% rotation, as well as three times the previous maximum amount for far more extreme settings

Filter Flavors (MkII circuitry)

Part: Amount:
2-pole/6-way switch 1
1N4148 silicon diodes 5
1N34 (or similar) germanium diode 1
The following 1% resistors:
1k5 1
24k 1
30k 1
33k 1
36k 1
62k 1
68k 1
390k 1

a) Remove the original R13 (33k) and replace with two flying leads long enough to reach the new switch (if you’ve already added the filter fm mod, the ‘top’ of your pot will already be connected to pin 1 of IC2A, and you can extend this wire to the flavour switch) – let’s name it MIXOUT, and label the other flying lead connected to pin2 of IC2A as MIXIN. (this part of the mod tweaks the gain for each of the six flavours so they emerge at about the same level -without it they will be HUGELY different!)

b) A second pair of flying leads needs to be connected to IC7A pins 2 and 1, but again, if you’ve implemented the Filter Shapes mod both wires will already be connected, namely CAP and LP1 (this part of the mod is what alters the resonance characteristics).

Now here’s the circuit:


Output Drive (MkII circuitry)

Part: Amount:
1M linear pot 1
22k resistor (5% or better tolerance) 1

a) remove the original R4 (18k, between pins 6 and 7 of IC2B) and replace with two flying leads long enough to reach the new control.

b) solder a third flying lead to a nearby GND point and then twist all three flying leads together.

c) At the pot, solder your new GND lead to the metal body of the pot (with the DRIVE pot at maximum position the gain of this stage increases by about 50 times, so unless you ground the pot metalwork you’ll probably hear hum when your fingers come near it ;))

d) Solder the flying lead connected to pin 6 of IC2B to the bottom (minimum) end of the new pot.

e) Solder the final flying lead connected to pin 7 of IC2B to the 22k resistor and the other end of that in turn to the slider of the new pot.

Here’s a photo of the correctly wired pot (the rubber sleeve normally covers the resistor):


HOW TO USE You will also need to wire in a 10k linear output LEVEL pot as shown in Olivier’s SMR-4 MkII circuit schematic. Leave the new output DRIVE pot at its minimum position for ‘normal’ output level, then as you increase the new output DRIVE control as much as you need to get the required drive/distortion, simultaneously turn the standard output LEVEL pot down to compensate for it (this is far easier to do than to describe).

From about 50% rotation you'll start to run into slight clipping on the peaks for a more grungy sound, as well as more 'sustain', while at 100% you'll get long and smooth sustained 'fuzz' distortion.

  • N.B. Mk I version used a 12k resistor, but the Mk II value is increased to 22k because the internal signal levels that were dropped before the filter input (for added squelch) now need boosting back up to the same nominal output levels as the original filter.

Filter Feed

Part: Amount:
1uF film capacitor 1

a) An extra feedback loop wired between the switched tip connection on the input jack socket via a capacitor (without the cap you'll get 'motor-boating at about 15Hz which is not very musical), to the LP3 point referenced in the filter shapes section (and labelled BP3 on the original SMR4 MkII circuit board).

b) Since the input socket is already connected to the existing input level control, this control continues to work as normal when you plug in an external signal, but now acts as FEED when no cable is plugged into the input socket.

c) At lower settings you get more bass end, and at higher ones (typically over 50%) this new loop causes continuous feedback and locking onto different harmonics as the filter settings change, giving 'circuit-bent' sounds.

Now here's the circuit:


Filter Resonance Compensation

Part: Amount:
BAT85 Schottky diode 2
1uF film capacitor 1
1k resistor (5% or better tolerance) 1
miniature toggle DP3T On-On-On type 1

Suitable switches include MTS-2033 from Doctor Tweek, or MULTICOMP - 2MD6T1B5M2RE from CPC Farnell

a) First, remove the existing C15 220n capacitor from the SMR4 MkII board, and replace it with two flying leads

b) Then remove the existing R18 1k and R19 10k resistors (if you're careful, these can both be used again in the modded circuit).

c) Solder a flying lead to IC7 pin 14 (we've previously used this as LP4 for the filter shapes mod)

d) Solder another flying lead to IC3 pin 4 (one end of R18 that you've just removed)

Now here’s the circuit:



1. Filter Shapes – instead of shorting out the 10k resistor R6 on the MkII you would short out the 10k resistor R54, and instead of removing the 1nF C32 it’s C25.

2. Variable Filter FM – connect the 10k linear pot between pin 14 of IC3D and GND, with its slider connected by the additional 47k resistor to the FM hack point marked J6.

3. Filter Flavors – Lots of work on the ‘diodes and whatnot’ has been done to provide six distinctly different flavors for the MkII that should again work fine on the MkI (although as pichenettes mentions, internal levels are higher on this earlier version, so it can’t be guaranteed that the relative flavor levels will remain the same.

4. Variable Output Drive – remove R10 on the MkI and replace with a 10k resistor (instead of the 12k for the MkII) in series with the 1M linear pot.