The Korg 35 filter
Back in the 1970’s when Korg, Roland and Yamaha were bringing out their now classic synths and drum machines one of the hallmarks of their design was the supremely clever little circuits that the Japanese engineers came up with to fulfill the requirements of making affordable, reliable instruments with signature sounds that have over time become icons.
Drum machines like the Roland TR-808 and TR909 had ingenious little discreet transistor based circuits that produce for example the distinctive Roland “triple shot” hand clap sound. The circuits in these drum machines generated drum sounds by combining analog waveforms generated by oscillators mixed with white noise via little transistor VCA circuits.
Around the time that Solid State Microelectronics (SSM) and Curtis Electromusic (CEM) were working on creating their famous synth chips that became so popular in many synthesizers of the late 70’s and the 1980’s a Korg engineer, Mr. Mieda devised a little voltage controlled filter module designated the Korg 35. It was used first in the large PS series analog polysynths and then in the MS-10 and the earlier MS-20 monosynths as well as the X-911 guitar synthesizer. It is a tiny black module with a row of pins along one side, it looks similar to the infamous Roland Juno-106 80017A “voice chip” but shorter and a little higher. Click or tap on the images for a larger view.
The Korg 35 is a masterpiece of simplicity. Inside the chip there are five transistors and six resistors. By adding a small number of external components around the Korg 35 it can be configured as a high pass or low pass music filter. There is one in the MS-10 configured as low pass and the MS-20 adds a second Korg 35 for it’s high pass function. Thus was born the distinctive and quite aggressive Korg filter sound.
The Korg 35 is a form of Sallen Key filter that uses clever transistor biasing to enable the filter to become voltage controllable. Later MS-20’s for some reason discarded the 35 module and replaced it with a chip-based OTA transconductance filter. If you happen to have access to an original Korg MS-20 you can tell which type it has by looking at the area around the top centre of the synth’s front panel – in the space marked “Voltage Controlled Amplifier” there is a smooth panel if the filter is a Korg 35 and there will be a black Phillips head screw in that area if the synth contains the later chip based filter. The screw is there to mount the little daughter circuit board that holds the much larger IC filter component set.
The tiny Korg 35 filter was key to enabling the huge PS series poly-synths to have been able to contain so many separate synth channels. These epic machines have an entire synth voice on every key and in the case of the PS3300 there are three complete synth channels on every key. That’s three Korg 35’s per key! The photo in this post shows but one of the 12 gate boards from a PS-3300 for a total of 144 Korg 35’s in one synthesizer. We will be taking an in-depth look inside the PS-3300 in another post. Miniaturisation is the key characteristic of the PS series and the Korg 35 filter fits the bill perfectly.
Presumably the Korg 35 was designed specifically for the PS series and found it’s way into the MS series that was released a year after the first PS Polysynth. Interestingly the Korg 35 filters in the PS series are set up so as not to be able to be driven into self-oscillation. You cannot get the wild, high resonance sounds from the PS series that you can from the MS-10 and MS-20. This difference is due to the selection of the external components around the Korg 35, not because of the module itself. They are rare to find as a spare part but due to their simplicity they have proven to be very reliable.
Korg has resurrected the basic design of the Korg 35 filter in their Monotron a few years ago using modern components and so the little Korg 35 circuit is still going strong into what will soon be it’s 4th decade.