Moog 904B Voltage-Controlled Highpass Filter
A $275 module that does the unglamorous half of the filtering job — and does it under voltage control, which is the whole point.
Overview
Everybody remembers the ladder lowpass. The 904B highpass is the other half of the Moog modular filter story, and it's the one that gets overlooked until you actually patch a system and realize the lowpass alone can't carve a hole in a mix — it can only take the top off. The 904B does the opposite job: per Moog's own module documentation, it lets the upper portion of a signal's harmonic spectrum through while varying the low-frequency cutoff point, controlled by the sum of a manually set voltage and external control voltages, across a range running from 20 Hz to 30 KHz.
That range is the number worth pausing on. A cutoff that reaches 30 KHz means the 904B isn't just a rumble-remover — swept downward it can strip a bright, buzzy waveform down to a thin harmonic sliver, and swept upward it opens the full spectrum back up. Because the cutoff tracks the sum of manual and CV inputs, the sweep is patchable: an envelope, a sequencer, an LFO, another module's output. That's the difference between a highpass filter and a voltage-controlled highpass filter, and it's the reason the module exists in a modular system at all.
Moog's price list put the 904B at $275.00 — sitting between the 904A lowpass at $395.00 and the 904C filter coupler at $215.00, which is a useful way to read the lineup. The highpass was the cheaper of the two main filters, and the 904C was the cheap add-on that tied them together. Owners and collectors note that the 904C is permanently connected to the 904A and 904B, and that's the detail that makes the trio interesting: the coupler combines the two filters into a single manually invertible function. In bandpass mode the signal paths of the 904A and 904B are placed in series, passing a variable center-frequency band; in bandreject mode they're placed in parallel, producing a void between the high- and low-frequency cutoff points. The width and center frequency of that band or void is set by the sum of manually set and external control voltages — which is to say, the 904B isn't just a standalone highpass, it's one half of a patchable parametric bandpass/notch when paired with its lowpass sibling and the coupler.
That's the context in which most 904Bs were actually sold. Moog's system documentation lists the 904B as standard equipment in the larger modular complements — the Synthesizer IIc included one 904B alongside a 904A and a 904C, and the bigger system lists in the same manual show the same 904A/904B/904C trio repeated. So a 904B on the used market today is almost always a module pulled from a system, not a standalone purchase someone made in isolation. Collectors note the practical consequence: a loose 904B without a 904A and 904C nearby is a filter without its partner, and the coupler functions that make the trio special simply aren't available.
The module's control set, as documented, is straightforward in the way modular gear of the period tended to be: cutoff frequency, an input, an output, and modulation inputs. Sound on Sound (UK) (July 2003, p.55), surveying the Moog modular range, listed the 904B's front-panel facilities exactly that way — cutoff frequency, input, output, modulation inputs — a reminder that this is a single-function module with no menu, no presets, and no hidden modes. What you patch is what you get.
One point of confusion worth heading off for anyone shopping: the 904B is not the 904A. The 904A is the voltage-controlled lowpass filter at $395.00; the 904B is the voltage-controlled highpass at $275.00. They share a model number prefix and a filter coupler, and they turn up in the same systems, but they are distinct modules with distinct jobs. Period survey coverage of Moog systems sometimes rendered the highpass as "9048" in OCR-degraded print — Studio Sound (May 1975, p.58), surveying a system's filter complement, listed an "additional 9048 vc highpass filter, vc or manual control, 60 to 20k Hz" alongside a 904A lowpass, a 907A fixed filter bank, and the 921/921A/921B oscillator set. That 60 Hz–20 kHz figure is a system-level survey description and doesn't match the 20 Hz–30 KHz range in Moog's own module documentation; collectors should treat the manufacturer's module spec as the authority and read the magazine figure as a survey-era approximation. The "9048" is a scanning artifact for 904B.
What the 904B asks of an owner is the same thing every Moog modular module asks: a powered frame, the correct supply, and enough neighboring modules to make the CV inputs useful. The module itself is a filter — the patching, the envelopes, and the sequencer that make it musical live elsewhere in the system. That's not a flaw, it's the design premise, and it's why the 904B is best understood as a component rather than a product. Buyers chasing a single highpass box will be disappointed; buyers assembling or completing a Moog modular system will find it's the piece that makes the 904A/904C combination work as intended.
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