Devlog
FK Rig: a guitar rig calculated from the schematic
Devlog with video: how FK Rig calculates amps, pedals, cabs and mics from schematics and data sheets, and where the FK Audio Suite stands as a project today.
Transcript
The video shows the guitar plugin FK Rig with its English interface. Every sound is FK Rig, played on an electric guitar in E minor at 120 BPM. A "Now playing" panel always names the amp, cab and effects.
0:00 Intro: the Tweed 59 amp, clean, with reverb, a ringing E minor chord.
0:06 Overview of the signal chain from left to right: input, pedals, amp, cab, effects, output. A click opens a module, the power icon bypasses it. Tweed 59 crunch with gate and Green Drive plays the main riff.
0:14 Step 1, input, then an empty chain. A compressor, an overdrive, a fuzz and a vintage phaser are added one after another, each audible.
0:30 Five overdrive models, one second each: Blues Drive, Clear Drive, Yellow Drive, Green Drive, Gold Drive. Then the drive knob of the Green Drive is turned from 1 to 9.
0:38 Step 2, amps: ten amps, one second each, from Tweed 59 and Blackface through Class A 30, Plexi 100, British 800 and California to Rectified, German Savage, FK Doom 80 and FK Tight Modern 100. On the Plexi 100 the gain goes from 3 to 8, then mid and treble are raised.
0:54 Step 3, cabs: the same amp through a 1×12 open, 2×12 open, 4×12 stack and 4×10 tweed cab.
0:58 Step 4, mics: the cab seen from above, a dynamic mic is dragged from the centre to the edge and back and moved from 2 to 20 cm away; in the centre it sounds bright and direct, at the edge softer, close in with more bass. Then four mic types: dynamic, condenser, ribbon, large dynamic.
1:10 Step 5, effects: a lead lick with delay, then with delay and reverb on the California Mark.
1:16 Step 6, Amp Builder: an amp as a graph of blocks such as triode stage, gain control, tone stack and power amp. A block is selected and its values (tube, voltage, resistors) are changed; the resulting Plexi 100 Normal plays in the rig.
1:30 Effect Builder: a pedal of my own, an overdrive made of circuit blocks, placed in the chain.
1:36 Outro: Tweed 59 clean with reverb, chords E minor, C, G, D, ringing out. FK Rig Lite is free, Pro unlocks everything; part of FK Audio Suite.
Almost two weeks ago I wrote here about why my instruments calculate their sound instead of playing recordings. A lot has happened since: FK Rig is finished at version 1.0, the FK Audio Suite is available as a public beta, and there is a video that shows the whole rig in under two minutes. Every sound in it is FK Rig, played with my own guitar.
This post goes deeper: what happens inside FK Rig between input and output, how the video was made, and what lies behind the FK Audio Suite as a project.
The chain: just like a pedalboard
FK Rig is a complete rig for guitar and bass: input, pedals, amp, cab, mics, effects, output. The signal flows from left to right, and every link can be clicked, moved or bypassed. That sounds obvious, but it is a deliberate choice: guitarists think in exactly this order, so the software should look that way too.
Amps from their circuits
Most amp simulations work with recordings of a real amplifier or with a neural network trained on them. FK Rig takes a different route and calculates the circuit itself:
- Preamp stages built from triodes at their real operating points: supply voltage, plate and cathode resistor, cathode capacitor. The distortion happens where it happens in the real amp.
- Tone stack from the actual resistors and capacitors. That is why bass, mid and treble interact the way they do on the original, and the curve in the plugin shows exactly that.
- Power amp with phase inverter, output transformer and sag, the supply voltage dropping when you dig in.
- The speaker as a load: the power amp does not see a fixed resistor but the impedance of the cab, which changes with frequency.
Today that adds up to 24 amps, from clean American combos through British crunch to modern high gain, plus four FK designs of my own. None of them is a special case in the code: every amp is a data set that describes which stages it has and which component values they contain. A new amp means a new data set, not a rebuilt engine.
Pedals after their original circuits
For the pedals I wrote my own circuit solver in C++. It calculates, in real time, the parts you typically find in pedals: op-amps, diodes as clippers, transistors and JFETs, OTAs in compressors and light-dependent resistors in phasers. An overdrive therefore sounds like its circuit, and turning up the drive does not just shift a level, it changes how the diodes behave.
On top of that come the effects you expect after the amp: delay, reverb, chorus, phaser, EQ, noise gate and compressor.
Cabs and mics
The cabs are not recorded impulse responses either. A cab is built from its enclosure (open or closed back, size, number of speakers) and from the speaker's data sheet, with the frequency response taken from the manufacturer's figures. There are 12 cabs, from a small 1×8 to the 4×12 stack and a bass cab.
In front of every cab are two mics that you drag across the speaker with the mouse. In the centre it sounds bright and direct, towards the edge softer, closer in with more bass. Dynamic, condenser and ribbon mics are each described by their frequency response and polar pattern. In the video you can see and hear the tone change while the mic moves.
Build your own: Amp Builder and Effect Builder
Because everything is made of building blocks, you can build your own. The Amp Builder is a graph editor: tube stage, gain control, tone stack, power amp, wired like in a schematic. Every block has its values (tube type, voltage, resistors), and while you change them you hear the result in the rig straight away. You save the finished model as a file, in exactly the same format as the built-in amps.
The Effect Builder does the same for pedals: plug op-amp stages, diode clippers, filters, compressor cells and modulated delays together into a pedal of your own and drop it into the chain. At the end of the video a home-built overdrive plays in front of an amp from the Amp Builder.
One interface for all plugins
FK Rig is the first of several plugins, and they all share the same interface: a UI library of my own, fully vector-based and bound directly to the parameter state. Undo, automation in the DAW and presets come with it for free instead of being rebuilt for each plugin. The interface is available in English and German and scales to any window size without blurring.
How the video was made
The video is not a screen recording and was not edited by hand. The interface is rendered frame by frame from a test program that operates the plugin like a user would: clicking modules, turning knobs, moving the mic. I recorded the sound as a dry guitar signal (DI) and then ran it offline through exactly the same engine, with every switch happening at the moment it happens on screen. What you hear is exactly what the plugin does at those settings. The "Now playing" panel always shows the amp, cab and effects.
FK Audio Suite: the project behind it
FK Rig is part of the FK Audio Suite, my own audio workstation for Linux and Windows. It started as a hobby project because the big music programs do not run on Linux. By now it has grown into a small ecosystem:
- The DAW for recording, editing and mixing, with audio and MIDI tracks, a mixer, automation and export. It is free software under the GPLv3 and costs nothing, with no subscription and no account. It checks third-party plugins in a separate process, so a faulty plugin does not take the DAW down with it. Version 0.6 is out now as a public beta.
- The plugins FK Rig, FK Grand, FK Wave, FK Orchestra and FK Drums, each as VST3 and as a standalone program, so they also run in Reaper, Bitwig, Cubase and any other DAW. Each has a free Lite version; FK Rig Lite (Tweed 59 and British 800, three cabs, gate, compressor and overdrive) comes with the DAW.
- Licences without a customer account: a Pro key is checked offline, the plugin needs no internet connection. Licences, downloads and updates go through a small licensing service of my own that stores as little data as possible.
- A release pipeline per product: every plugin is built, tested, packaged and published separately for Linux and Windows.
Under the hood it is C++23, JUCE and the Tracktion Engine. The requirements, the architecture and the technical decisions are mine; an AI assistant helps with the implementation, and as a Java developer I am learning C++ on this project.
What comes next
For FK Rig the main task is comparing it with real amps, ear to ear, and adding more models, which are now just data sets. The DAW is heading for the next beta, and FK Grand and FK Orchestra keep growing their physical engines.
What this has to do with business software
The core idea behind FK Rig is the same one I follow in applications for businesses: behaviour lives in data, not in special cases in the code. A new amp is a data set, just as a new branch or a new duty roster in a well-built application needs no programming. And real-time audio enforces a discipline in testing and processes that every interface benefits from.
FK Rig Lite and the DAW are free at fk-audiosuite.com, and there is more on the project in the portfolio under FK Audio Suite. I am glad to hear from guitarists and developers.