Has anyone ran into any open source/pre-built guitar amplifier DSP code?
I’m essentially looking to start working on a amplifier plugin but would love things to start from and learn from. The cabinet IR convolution is the obvious easy part. The effect suite behind the amplifier is easy too. The complex part is the DSP for the drive or distortion. Any leads?
To be fair, the sound is kind of wonky. Not exactly a “boutique tube warmth vintage British” whatever, but it is a guitar amp.
It uses an unconventional pre-amp in that it does a multiband split and distorts low and high end separately. But this should give you a good idea of how you could implement a tone stack using IIRs and basic waveshaping.
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Not a full answer to your question but you could look for transfer functions that are asymmetrical. It’s a more “tube-y” sound and gets away from that typical digital distortion sound you get from functions like tanh/arctan/etc. Asymmetrical transfer functions will give you both even and odd harmonics while symmetrical ones will give odd harmonics only.
These functions come from machine learning but I find they make nice tube sounds haha. It’s like a soft rectifier. You can control the amount of rectification by biasing the input by adding DC offset. You can remove the DC offset after by adding a high pass filter set to like 10hz.
You can also tweak some variables in those transfer functions to get different curve slopes for more/less aggressive sounds.
If you look at Ableton’s tube distortion device you’ll see the shape is similar.
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This amp sim was recently released and is written in JUCE:
Pirkle published an addendum to this FX book about guitar amp modeling: Tube Amp Modeling Addendum: Chapter 19 – Will Pirkle Audio Technology
Airwindows has a few guitar amplifier plug-ins that come with source code: airwindows/plugins/WinVST at master · airwindows/airwindows · GitHub
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Yeah there is that one I made… I’m currently working on a new one heavily influenced by the WickerAmpCombo from the Pirkle addendum, should be finished soon when I get the time. Will be much more ‘true’ to analog modelling than the AA50. This is a good place to start for some simple distortion using waveshapers https://docs.juce.com/master/tutorial_dsp_convolution.html - this is what I use in the AA50.
And there is this too https://github.com/GuitarML/SmartGuitarAmp if you wanted to go the AI route… Let me know if you have any questions I’d be happy to help you. Not too much of the guitar amplifier juce stuff is open source, so can be a struggle to wrap your head around it from zero 
A few more examples. I haven’t tested any of them though.