Shruthi-1 CEM3379 board

Schematics and PCB

The CEM3379 board replicates the audio signal processing section of the original Shruti-1, with a couple of changes:

The control signals are properly filtered. As a result, the output signal is free of high frequency artifacts that might be folded into the audible spectrum when sampled into a lo-fi sampler.
Trimmers have been added to tune the frequency scale of the filter, so that you can “tune” the sine wave produced when the filter self-oscillates. On the Shruti-1, cutoff tracking was implemented in software with a x0.9 correction – which wouldn’t apply to other filter boards if done similarly on the Shruthi.
On the Shruti-1, self oscillation was reached for resonance = 34. Now, self oscillation is reached for resonance = 60 (a value consistent with the behavior of the other filter boards).
The power supply has been revised to use the same circuit as the other analog boards. This new circuit is more tolerant to higher input voltages, and its switching frequency is higher and easier to filter.
A protection 68 ohm resistor has been added at the audio output.
AC-coupling caps have an increased capacitance, for beefier bass.

Note that this module will only be distributed as a PCB (no kit).

You can find the Eagle files for this board in the shruthi/hardware_design/pcb directory of the source code hosted on github.

The schematics in PDF format are here.

The VCA envelope clickiness can be reduced by using a 100nF capacitor in place of the 22nF capacitor C5.

In bold, the editors’ picks

Removed middle spacers holes.
Increased range of cutoff adjustment trimmer.
Changed layout of power supply section to match that of the other filter boards.
Increased hole sizes for audio and DC connectors.