SMR4 mkII mods

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VCA bleed fix

There is a slight VCA bleed that can be heard when resonance is set to its maximum value. The CVs generated by the Shruthi-1 control board cannot swing as low as ground — there is always a tiny bit of residual voltage. This will prevent the VCA to be completely silenced. Most of the time this is not a problem, because the Shruthi-1’s digital oscillators switch themselves off when the VCA envelope reaches 0.

To prevent this bleed, solder the optional 1M resistor. This will shift the VCA CV down by a few mV. You find this resistor above IC4 between C8 and C9.

Pole selector switch


You can connect those three points to the 3 terminals of a SPDT switch. This will allow you to switch between the 2 and 4 pole modes with the switch.

Here's an easy to follow "Banana chart" by fcd72:


Additional filter modes


Solder a 10k resistor vertically below the pad labelled Mode on the PCB.


Connecting the point labelled Mode to the points labelled BP1 and BP3 will give different flavors of band-pass filtering. It is recommended to use a SP3T switch (pole to Mode, throw1 to BP1, throw2 to BP3, throw3 left floating) to easily control this extra connection. This graph shows the 6 different responses that can be obtained by switching the pole selector to either positions (LP2 or LP4) and by patching BP1 or BP3 to the “Mode” pad (or leaving it unpatched in the first row).


Here's another "Banana Chart" by fcd72:


Extra CV inputs for filter cutoff control

Each row is for an extra CV input controlling the filter cutoff. A resistor should be mounted vertically across the first two pads (try 15k to 68k… depending on your CV source!), and the last pad in the row is the CV input itself.

Interesting FM effects can be obtained by feeding audio signals at these points, when the filter resonance is high!

Shruthi-1 SDE (Sound Designer Edition)

This is an extensive mod that gives a Shruthi-1 many of the features present on the 4-pole mission filter board, as well as various others not seen on any other filter board. It was developed by forum member yewtreemagic (Martin Walker).

Using the existing Audio IN as an FM Mod Input

This mod maintains the Audio IN to the filter as well as an Audio IN for the FM Mod by forum member qp.

ATTENTION: This mod requires cutting a trace on the PCB!

This mod works best when the Audio IN Potentiometer is installed. It doubles as an FM Mod Amount Control. Very handy. The chart also shows other mod points of the PCB that are easy to install.

NOTE: If the Audio IN Potentiometer is NOT installed, you will need to add a 15k to 68k resistor… depending on your CV source! The potentiometer is a variable resistor, so the installed resistor for this mod is the lowest value.