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This analog filter is inspired by the legendary Russian Polivoks synthesizer, originally designed by Vladimir Kuzmin. Known for its aggressive, raw character, the Polivoks filter stands out for its unusual architecture and bold sonic footprint — qualities captured in this unique design.
Offering both Lowpass (LP) and Bandpass (BP) modes, the filter uses an unconventional approach that sets it apart from most analog designs. Instead of relying on the typical combination of a fixed capacitor and a variable resistance element (like OTAs or opto-couplers), it employs a pair of programmable low-power operational amplifiers (NTE888) at its core.
At a glance, it may resemble a state-variable filter (SVF), but there's a major twist: no capacitors are used in the filter core. Instead, control voltage (CV) is applied to the Iset pins of the op-amps, dynamically adjusting gain-bandwidth and simulating variable RC time constants. The result is a 12dB/oct filter with a powerful and distinctive voice.
At high resonance settings, the filter takes on a wild, almost unruly character — reminiscent of the MS-20 but with its own Soviet-style edge.
Features:
Lowpass and Bandpass modes
Bipolar attenuated CV input for dynamic frequency modulation
Second non attenuated CV input (V/Oct)
Raw, expressive tone with character that shines at extreme resonance
Perfect for modular users looking to add a bold, nontraditional filter with roots in vintage Eastern European synthesis.
** 4U Modular is a term used for the format most commonly known as “Serge Format” or “Loudest Warning Format”. Out of respect for the ever growing format, 4U Modular is the easiest way to refer to it. More Specifically it refers to the panel height and mounting hole style. 4U Modular will patch up just fine with other 4U "Serge" formats such as "Random Source", but it will not mount in RS boats or Buchla Boats (or power off Buchla power for that matter). An info page about this will be added to the website soon to make this a lot more easy to understand.
LGE078 DIY Build Documentation:
LGE078 Mouser Cart (Does not include Knobs, Pots, board to board headers, screws/split washers or NTE888)
LGE078 Main Board Bill of Materials
LGE078 Main Board Schematic
LGE078 Main Board Designator Layout
LGE078C I/O Board Bill of Materials
LGE078C I/O Schematic (Wiring Diagram)
Build Notes:
Best location to get NTE888M’s at the moment is SynthCube via this link.
Output Clamp Mod:
Similar to other filters, when you turn up the resonance the feedback amount tends to get wild and the output signals boost up to the power rails or to the level the op-amps can handle. In this case, it will clip the resonant harmonics at around +/-10.5V. This has always
bothered me because I prefer my signals to stay at around +/-5V. I played around with a favorite trick of mine using two zener diodes in various clamping configurations and I managed to get it to work with minimal effort!
Using two 5V1 Zener Diodes (1N5231) back to back (wired in parallel, going opposite directions) connected between ground and the Resonance Pot Wiper. This is probably easier done on the I/O PCB. I recommend doing this mod over the resistor changes listed above.
Here’s an updated Schematic
Here is a photo of the modification using the I/O Board for simplicity. Sorry for the cotton fibers, was in a hurry to document :)
Mechanical Parts BOM generally required for building Low-Gain Electronics Modules
Details:
PCB Size: 6” x 1”
Current Draw: TBD