PultEQFx

A circuit modelled passive program equalizer, built with nice-plug and Vizia. Builds as CLAP and VST3 everywhere and as an Audio Unit (AUv2) on macOS, plus a standalone application for trying it out.

PultEQFx models the passive equalizer circuit of the classic 1950s tube program equalizer, the Pultec® EQP-1A. It is not affiliated with, endorsed by, or connected to Pulse Techniques, LLC; Pultec® and EQP-1A are their marks, used here only to say which circuit is modelled.

What it models

The original is a passive LC/RC equalizer followed by a tube make-up amplifier that undoes the network's ~22 dB insertion loss. The passive network is simulated as a circuit rather than approximated with a bank of shelving filters, which matters because the boost and attenuate controls of each band tap the same divider chain at different points. Using both at once is not two filters added together, it is one network rearranged, and that is where the "low end trick" comes from: boosting and attenuating the same low frequency lifts the bottom and digs a dip an octave or two above it. The original manual told you not to do it.

• Topology and values are taken from the original schematic: a 10k high boost pot with the LC tank bridged off its wiper, a 1k high attenuate pot shunting the top end to ground through the selected capacitor, a 100k low attenuate pot and a 10k low boost pot with their own pairs of frequency capacitors. Pot tapers follow the Pultec service documentation, so the low boost and low attenuate knobs are audio taper and the rest are linear. The knobs are calibrated 0 to 10 like the hardware, and the non-linear relationship between knob position and decibels falls out of the circuit rather than being dialled in by hand.
• Solved per sample by nodal analysis with trapezoidal companion models, the way SPICE solves a linear circuit. The conductance matrix only changes when a knob moves, so it is factorised at control rate and each sample costs one forward and one back substitution.
• Frequency prewarped per element, so the 16 kHz bell sits at 16 kHz even at a 44.1 kHz sample rate instead of sliding down towards Nyquist.
• The make-up amplifier contributes the rest of the character: mostly odd order harmonics from the push-pull stage with a little second order from its imbalance, plus the bandwidth limits of the iron at both ends. It is clean at the usual −18 dBFS operating level and runs gently out of headroom as the peaks approach full scale. The DRIVE control is the one liberty taken with the hardware: it hits the amplifier harder, as more level into the hardware would, so the sound gets louder and dirtier together.

Measured against the published curves, with the tests in tests/response.rs checking each one:

Setting	Response
Low boost 10 @ 100 cps	+16.3 dB at 20 Hz
Low atten 10 @ 100 cps	−19.2 dB at 20 Hz
Low boost 10 + atten 10 @ 30 cps	+5.1 dB at 80 Hz, −4.2 dB at 200 Hz
High boost 10, sharp	+18.0 dB on the selected frequency
High boost 10, broad	+12.9 dB, wider
High atten 10	−16 dB on the selected frequency

Controls

The front panel is the hardware's. The left hand knob is the EQ IN/OUT switch, which lifts the passive network out of circuit but leaves the amplifier in it. The OFF/ON knob at the right takes the whole unit out of circuit, and the pilot lamp follows it. Switched off, the signal still comes out as late as the reported latency says, so the track stays in time with the rest of the session whichever way the switch sits.

The strip above the panel is not on the hardware. It carries the preset drop down, a save button and the settings button.

Nor are the two level meters, one either side of the controls, there for gain staging: INPUT on the left reads what arrives, and OUTPUT on the right what leaves, after the output trim, or the dry signal with the power off. They read in dBFS, on the scale DAW peak meters use, from −60 to +6, with a bar per channel. Each bar rises to a peak at once and falls back at 20 dB in 1.7 s, with a line holding its highest point for two seconds; it is solid up to the RMS level and fainter from there to the peak. A lamp above each bar lights at the first sample at or over full scale and stays lit. Under each meter are the figures a level is set by, to a tenth of a decibel: PEAK, the highest sample since it was cleared, which turns red once full scale has been reached, and RMS, the average over the last 300 ms, which a full scale sine reads as −3.0 and which moves five times a second so it can be read. Clicking a meter or its peak figure clears them. They are sample peaks; a peak between samples is not measured.

Right of the output meter, where they can be set against it, are the amplifier's two trims, also not on the hardware: DRIVE above, up to 18 dB more level into the amplifier, and OUTPUT below, ±24 dB, each with its setting lettered under it. Turning DRIVE up makes the sound louder and dirtier, as it would on the hardware; bring the level back down with OUTPUT, watching the output meter.

Up to 0.11, DRIVE was a saturation amount from 0 to 100 % that kept the level and squashed only the peaks, so turning it up made loud material quieter. A session saved with it opens with DRIVE at 0 dB, and Low End Punch has been moved across to settings that sound as it did.

Presets are the panel's parameter values, minus the oversampling setting, which is a choice about the machine rather than the sound. The loaded preset's name is saved with the session, and an amber dot appears next to it as soon as the panel no longer matches what was loaded. That is decided by comparing the values rather than by tracking an edited flag, so turning a control back to where it was clears the dot again. Low End Punch is built in: the low end trick at 100 cps with a little 10 kc air, which measures +7.7 dB at the bottom, +6.4 dB at 100 Hz, a scoop through 500 Hz and +6 dB of air on top. Saving asks for a name, and confirms first if that name is already one of yours, whatever its case, and then replaces that preset in whichever file it lives. A new name that would come out as an existing file's name, as "A/B" and "A_B" both would, gets a numbered file of its own rather than taking the other one over. Saved presets are one JSON file each, under ~/.config/pulteqfx/presets on Linux and macOS and %APPDATA%\PultEQFx\Presets on Windows, so they can be copied between machines or edited by hand, and each carries a cross to delete it, which asks before removing the file.

A built-in preset has no file, so it cannot be deleted, and saving under its name writes a preset of your own beside it rather than replacing it in the list. Replacing it would put it out of reach for good: there would be no file left to delete to get it back.

The settings button holds:

• Window size, from 50 % to 200 %. The faceplate, lettering and scales are drawn rather than pictured, so they stay sharp at any size, and the rendered metal is generated large enough to hold up at the top of the range.
• Oversampling, off through 8x. The equalizer is prewarped and accurate without it, and so is the amplifier's frequency response, which stays within a tenth of a decibel of the analog circuit at every setting; oversampling is there for the amplifier's saturation. The plugin always reports 74 samples of latency and pads the shorter settings out to match, so switching quality never changes the reported latency while the host is running. A new setting is brought up alongside the old one and faded in a fifth of a second later, rather than dropping out while it fills.

Knobs respond to drag, scroll, shift for a finer grip and double click to reset. The switches turn by drag or scroll, or by clicking one of the values engraved round them, and the two way ones, EQ IN/OUT and OFF/ON, also throw with a click on the knob.

The panel

The faceplate, the lettering, the dial scales and every pointer are drawn with vector paths, so they stay sharp at any window size. Everything that is metal is a render: the five large knobs, the five knurled switch knobs, the pilot lamp and the four mounting screws.

The renders come from assetgen, a renderer in this repository. It takes either parametric geometry or a glTF model, bakes occlusion into it by casting rays against itself, and shades it under a lighting rig the whole panel shares. The large knobs are built from geometry; the switch knobs come from assets/knob_metal.glb and the lamp from assets/led_ring.glb. glTF states base colour, metalness and roughness but has no way to state a finish, so the brushing and micro-texture of the aluminium are assetgen's.

Nothing on the panel is ever drawn by rotating a picture of itself. Rotating the picture rotates the light baked into it, so the highlight would travel round with the part instead of staying where the panel lamp is. The three consequences are worth naming, because each one is why a part is made the way it is:

• The large knobs are filmstrips — 48 frames across the 250 degree sweep, each lit from the same place, and the widget picks the frame matching its value.
• The switch knobs are a single frame. The knurled cylinder has no indicator on it and looks the same at every angle, so the widget draws the pointer on top at whatever angle the switch is thrown to.
• The screws are four separate renders, each already driven to its own angle, because a screw sits where it was tightened.

The pilot lamp is rendered twice, once with the lamp behind the jewel burning and once with it out, rather than dimming one render in the widget. A dark jewel is not a bright one with less light on it: its specular stays exactly where it was while the body of it goes out, and that is what tells the eye the lamp is off rather than the room.

One light

The panel is lit from a single lamp, hung a little above the top edge and a little in from the left. Four separate pieces of code have to agree about it: the faceplate paints its highlight there, assetgen keys every render from up and to the left, each control casts a contact shadow down and to the right, and each control is tinted by how far it is bolted from the lamp.

That last one is the part a render cannot carry. A render knows the direction its light came from, but nothing about the panel it ends up on — so without the tint a knob at the far end of a nineteen inch panel is as bright as one under the lamp, which is the giveaway that a panel is a collage rather than a photograph. The falloff is linear in distance rather than inverse square, because a faceplate is lit by a broad source at a distance, and an inverse square across this width puts the right hand end in the dark.

tests/lighting.rs holds the four pieces to it: that shadows fall away from the light, that the panel's brightest point is under the lamp and its darkest is the opposite corner with no rise in between, that the corner-to-corner falloff is visible without being theatrical, and that no control is tinted brighter than the render it was made from.

Regenerate the renders with:

./assetgen/render.sh

The renderer is deterministic, so re-running it without changing assetgen reproduces the same files byte for byte.

Building

Needs a Rust toolchain, the usual X11 development packages, and fontconfig's, which Skia uses; .github/workflows/build.yml lists them all. The first build downloads Skia's prebuilt native library.

./install.sh

That builds the plugin and installs the CLAP and VST3 into ~/.clap/BurningTreeC and ~/.vst3/BurningTreeC. Pass --no-build to install what is already built, or set CLAP_PATH and VST3_PATH to install somewhere else.

To build without installing:

cargo xtask bundle pulteqfx --release

This writes PultEQFx.clap and PultEQFx.vst3 to target/bundled.

The Audio Unit is built on macOS only, from the same universal binary, through nice-plug-au2 (vendored in vendor/nice-plug-au2):

cargo xtask bundle-universal pulteqfx --release
bash tools/package_au2.sh

That adds PultEQFx.component to target/bundled, ad-hoc signed. It is an aufx effect with subtype PEQf and manufacturer BrTC, which auval -v aufx PEQf BrTC validates once it is in ~/Library/Audio/Plug-Ins/Components.

To try it without a host:

cargo run --release --features standalone -- --backend auto

Licensing

PultEQFx is under the GNU General Public License version 3 or later, whose text is in LICENSE.

It was not a free choice when the plugin was built on NIH-plug, whose VST3 export linked the GPLv3 vst3-sys bindings. nice-plug exports VST3 through the permissively licensed vst3 crate instead, so nothing the plugin links requires the GPL any more; the licence is unchanged.

Some dependencies are worth naming directly:

• nice-plug, its companion crates and nice-plug-au2 are under the ISC licence.
• Skia, which draws the panel through Vizia, is BSD licensed; its native library's licence is reproduced separately from the Rust bindings'.
• Noto Sans is compiled into the binary for the panel lettering, from assets/fonts. The font files are under the SIL Open Font License 1.1, copyright The Noto Project Authors. That licence requires it travel with the binary, so it is reproduced in full.

THIRD-PARTY-NOTICES.md reproduces the licences and copyright notices of all 313 crates PultEQFx links on any platform it ships for, grouped by licence. Where a crate offers a choice, the licence taken is named, and where a crate bundles assets under a different licence than its own, that is called out too. Regenerate it after changing dependencies:

python3 tools/third-party-notices.py

Layout

Path
src/dsp/eqp1a.rs	the passive network, its component values and pot tapers
src/dsp/nodal.rs	the nodal solver and the companion models
src/dsp/tube.rs	the make-up amplifier
src/dsp/oversample.rs	linear phase halfband oversampling
src/editor/	the front panel
src/editor/style.rs	the drawn controls and the panel's colours
src/editor/sprites.rs	the rendered knob and the screws
assetgen/	the renderer that generates the knobs
src/presets.rs	built-in and saved presets
src/meters.rs	the input and output levels the meters read
src/editor/meter.rs	the meters themselves
tests/response.rs	frequency response against the published curves
tests/latency.rs	the reported latency, and switching without a gap
tests/drive.rs	what DRIVE does, and Low End Punch against the old drive
tests/presets.rs	preset storage round trip
tests/state.rs	what survives a save and reload
tests/scaling.rs	the window size, and how a zoom reaches the host
tools/linux_gui_smoke.py	opens the real CLAP editor in an X11 window
tools/package_au2.sh	wraps the macOS binary as an Audio Unit
tools/third-party-notices.py	regenerates the dependency licence file
vendor/	the GUI stack and AU bridge, with their local changes in PATCHES.md