Shruthi Fun

Well, I dug my collection of Shruthi-1s out of storage the other day. This little synth that could was a clever amalgamation of a digital control, display and CPU board stacked onto an analogue board containing power regulators, an external input, and a VCF and VCA, with just about everything routable in a 12-slot modulation matrix. 

Between 2010 and 2012 Emilie delivered an insane amount of creativity only rivalled by her Eurorack designs in the five years to follow. Pictured below are four of the nine different editions made available, Clockwise from top left: Dual SVF (with two independent LP/BP/HP filters stackable in series or parallel); 4-pole Mission (a Matrix-12-style pole-mixing filter with 15 different responses, plus three different resonance pathways); Red Alert (Polivox programmable-power opamp filter with nasty switchable filter fm and overdrive); and Yellow Magic (Korg MS20 MkII lowpass filter with low-fi Princeton PT2399 digital delay).

In particular the Red Alert sounded much better than I remembered. Émilie had clearly based it on the Marc Bareille adaptation of Vladimir Kuzmin's 1982 design, and had added in some fun things like an overdrive function, which complimented the original filter's wild sound that was caused by its lack of limiting circuitry in the resonance path. However I also noticed significant bleed from the oscillators at full cutoff that went away just as the filter was about to open up at a setting of about 10 or so. Time to take a look...

Oh dear...

Right off the bat I can see a few issues that were no doubt due to my eagerness to finish the job with the parts I had to hand and get the filter in place. Firstly, there were bypass capacitors missing from the two programmable power opamps that provide voltage-controlled gain AND replace the filter capacitors-not a good idea. They were promptly added.

Secondly, there was a missing capacitor from the Shruthi's LT1054 switching charge pump IC, which provides a negative rail from the input. Turns out this was always an optional extra-it may have been specified in early data sheets for the (very old ie 1990) LT1054 charge pump IC, but not in current data sheets. As the switcher only runs at about 25kHz things are not super-critical.

So, it was time to apply my experience designing my own programmable opamp filter, the Metro Modular Steel Falcon to see if it could be possible to mitigate the noise issues...