Model of the classic Ibanez Tube Screamer 808 using Nodal Analysis and WDF with a few new features added!
Download Windows vst3 and Mac vst3 and AU files here: https://github.com/JamesStubbsEng/TS-808-Ultra/releases/tag/v1.0.0
Basis of circuits simulated
The circuit schematics of the TS-808-Ultra are based off Electrosmash's analysis here: https://www.electrosmash.com/tube-screamer-analysis
Jatin Chowdury's WDF library was used to simulate the clipping stage of the TS808. The library is found here: https://github.com/jatinchowdhury18/WaveDigitalFilters
The clipping stage of the TS808 can be broken down into 3 WDF stages based on ideal opamp circuit analysis. This is done in the following paper and is shown below:
The tone circuit analog transfer function can be found by nodal analysis, and is shown in section 4.3 on page 6 of the paper below: http://dafx.de/paper-archive/2007/Papers/p189.pdf
The analog transfer function can then be converted into a digital transfer function using the bilinear transform (BLT). No frequency warping was used in the plugin, as it was found no frequency warping produces a better sound.
A comparison between the analog and digtial transfer function for the tone circuit is seen below:
To make the iconic tube screamer circuit more usable in applications other than in front of a guitar amp the following is added to the plugin: input gain, mix, direct LFP, direct compression, and output gain! These extra features are implemented in the digital domain.
The drive pot is modelled with the following function, which emulates a classic audio taper pot:
According to Electrosmash's analysis, the physical Tube Screamer uses a G taper pot. However, during development, it was found an inverse log taper allows for more control of the upper settings, so the inverse log taper pot emulation below is used in the plugin.