MATRIXSYNTH: Analog Renaissance Tests Out the New Sound Semiconductor SSI2164

Wednesday, May 08, 2019

Analog Renaissance Tests Out the New Sound Semiconductor SSI2164

You might recall Sound Semiconductor announced the availability of the SSI2164 QUAD VCA chip back in March of 2018 (data sheet here). The SSI2154 is technically an updated design of the classic SSM2164 found in the Octave Voyetra Eight, Siel DK 600, Opera 6 & Kiwi, and the full blown Waldorf Wave. Analog Renaissance was commissioned by Sound Semiconductor to write an application note on the chip in practical use. The following are some of his findings including a couple of demos and a cool oscilloscope synth cat. :)

via Analog Renaissance:

"Sound Semiconductor (basically SSM risen from its ashes) hired me a few months back to write an application note AN701: 'Designing Voltage Controlled Filters for Synthesizers with the SSI2164' for them.

This job quickly escalated because while analyzing this IC's design, it became apparent that it is far more capable, due to its advanced design over its predecessor the SSM2164 (and its cheap knockoffs), than what I expected from reading the datasheet.

Its exceptional THD and noise specs allow building ultra clean filters that are virtually distortion and noise free, its excellent channel tracking means that it is very suitable for multi-mode filters which rely on precision and inherent in its design is the same type of asymmetric distortion responsible for the legendary SSM2040’s sound and more.

The 32-page manuscript is now being reviewed and edited to match their house-style, and should be made public soon. Imho it's quite extensive and covers next to SSI2164 specific content quite a bit of VCF design in general, circuit history and some novel stuff.

As there are already more than enough datasheets, application notes and books filled with non-working (read: tested in spice) nonsense, I accepted this job on one condition: build, test and since it's about synthesizers, hear everything before writing about it.

The small building blocks were breadboarded, but as this type of construction quickly becomes impractical for larger designs, certainly when they need multiple sources and control voltages, and impossible to do meaningful measurements on beyond a certain level, due to coupling and EMC issues, I built 3 filters in eurorack format, which also allows for quick connection to other synth and audio equipment.

SSI2164 - 2040 VCF Published on May 5, 2019 dnaab136

The first one, on the left, is an ultra-clean 4-pole VCF with a SNR better than 107dB (A-weighted) and THD is less than 0.0066%. Both these numbers are the limit of my current test equipment.

The second one, in the middle, is a VCF that employs the SSI2164's SSM2040-like non-linearities and for the first time since the early 80's done monolithically.

The 3rd one, on the right, expresses my contempt for what can and can't be done by ignoring a few mathematical constraints and is a self-oscillating 6dB/oct (1-pole slope) VCF.

SSI2164 - Mathematically impossible self-oscillating 6dB/oct VCF Published on May 6, 2019 dnaab136

Math wasn't happy with this, so it sounds a bit angry and, one of its main features is its capability of transforming sine waves into cats.
one of its main features is its capability of transforming sine waves into cats.

All 3 modules borrow 2 things from power electronics: A FET-bridge that allows plugging in the power cable in any orientation, without the voltage drop associated with diode bridges, and an S-Curve dimmer, originally intended for incandescent lamps but here used for a reversing attenuator with a dead zone in the middle.

The first 2 are multi-mode and have next to the standard 4-pole output a few extra outputs: 2 & 4-pole band-pass and a 2-pole low-pass. All outputs have the exact same phase response and can be mixed to obtain variable slope filters.

I'll bring these to Superbooth in one of these waldorf KB37 keyboards, with some basic VCO, LFO, ADSR and mixer modules for you to play with along with a printout of my manuscript and what is already edited (about 2/3rd) for publication."

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