This phi-L Tube Preamp emulation has been developed by Philipp Bulling as thesis work. Philipp has been offering the plugin in VST format for some time on his own website. He has now moved on to new areas for some time already but didn’t want to let down his users. Hence, DDMF took over the code and will continue to offer the plugin as freeware, with the intent to further improve it and maybe add some more bells and whistles in the future.
The plugin is very unique in the sense that it gives you full control over all aspects of the modelled circuit. All values of all the capacitors, resistors etc. can be tuned by the user. Here is what Philipp had written on the original page.