Electric Druid surveys rare high-pass filter circuits in vintage synths

Electric Druid published a deep technical post on February 27, 2026 that examines the rare use of dedicated high-pass filters in vintage synthesizers, noting that "Pretty much every synthesizer has a lowpass filter, but far fewer include a highpass filter. Today I’d like to take a quick look at a few that do." The piece is filed under the Vintage synths category and frames its inquiry as a survey of dedicated simple high-pass circuitry rather than broad multimode architectures.

The post explicitly narrows scope with the line "I’m leaving aside synths that have more sophisticated multimode filters that can be set up as highpass, and just looking at the few that included a simple […]," signaling the author’s intent to focus on hardware that shipped with a discrete HPF function. One named case study is the Roland Jupiter 4; Electric Druid reproduces the period context with "The Jupiter 4 was Roland’s first polysynth in 1979. Given that the Prophet 5 had been released the previous year, Roland were definitely playing catch-up, and the design looks notably less ‘modern’ than Sequential’s game-changing instrument. Nonetheless, the Jupiter 4 is well thought of and sounds great, with a character of its own. It’s the […]." The Jupiter 4 section is presented as a concrete example in the survey, though the provided excerpt truncates the Jupiter 4’s full technical description.

Readers will find technical headings that promise circuit-level detail, including "Roland filter designs with the IR3109 or AS3109." The post also treats multimode filter theory in two parts; Part 1 appears as "Multimode filters, Part 1: Reconfigurable filters" while Part 2 is "Multimode filters, Part 2: Pole-mixing filters." Electric Druid reproduces a working definition: "What is a ‘Multimode filter’ anyway? It’s a filter which can provide more than one response. So it might offer a choice of 2-pole or 4-pole lowpass responses, or it might be switchable between lowpass and highpass, or it might have lowpass, bandpass, and highpass outputs. There are basically three techniques for building multimode filters, […]." The author then sets up pole mixing as an alternate technique: "Last time we looked at reconfigurable filters, filters that include switches to rearrange parts of the circuit during normal use. However, that’s not the only way to produce different responses from a single filter circuit. This time, we’ll look at another approach: pole mixing. ‘Pole mixing’? It sounds like stirring a cake with a broom handle… […]."

The post ties the HPF survey to related analogue and early digital designs in a section titled "Analogue Drums: Boss DR-110 Drum Machine." Electric Druid records that "The Boss Dr Rhythm DR-110 was introduced in 1983 and was the second drum machine Roland had made under the Boss label, following the frighteningly basic DR-55 in 1980. It was also the last drum machine that Boss made before the Dr Rhythm series moved onto digital samples. The Linn Drum (LM-2) was released in […]." A separate experimental aside quotes a SynthDIY mailing list thread: "There was a discussion on the SynthDIY mailing list recently about how the early digital drum machines like the LinnDrum or the Oberheim DMX used to change the pitch of drum sounds by simply changing the sample rate. They literally just played the samples back faster or slower, exactly like speeding up or slowing down […]."

Electric Druid’s February 27, 2026 post is a focused technical starting point for anyone restoring or modding period hardware because it collects explicit examples and teases circuit-level analyses under headings such as IR3109/AS3109, multimode filters, and pole mixing. To apply this reporting to bench work, retrieve the full Electric Druid post for the truncated Jupiter 4 material, the IR3109/AS3109 discussion, and any schematics or parts lists before attempting restorations or mods.