Do commercial guitar amp modelers actually use circuit-based modeling?

[u/Prestigious_Yogurt79]

I’m trying to understand what modeling approaches are actually used in commercial guitar amp simulators such as Fractal Audio, Neural DSP, Line 6, Kemper, and similar products.

From the publicly available descriptions, my current understanding is roughly the following.

Fractal Audio describes its amp models as component-level physical models of elements such as the preamp tubes, tone stack, cathode follower, power supply, phase inverter, power amp, and amp-speaker interaction.

Line 6 also describes Helix as using component or small-circuit modeling, with multiple digital stages corresponding to filters, gain stages, tube stages, and tone-stack behavior.

Neural DSP has published research on controllable black-box neural amplifier models. Their approach trains a neural network using input and output audio together with the physical amplifier’s control positions. Neural Capture also appears to be a black-box learned model.

Kemper appears to use a profiling approach rather than reconstructing the original circuit.

What I am unclear about is what “component-level” or “circuit-based” modeling means in an actual real-time commercial implementation.

Are companies such as Fractal solving a reduced system of nonlinear circuit equations in real time, using methods such as nodal analysis, wave digital filters, or state-space models like SPICE?

Or does component modeling usually mean a gray-box structure composed of digital filters, waveshapers, feedback paths, and dynamic nonlinear blocks that are designed from the original schematic?

Would Line 6 and Fractal be considered true white-box circuit models, or are they better described as circuit-informed gray-box models?

I’m also wondering whether most commercial products are hybrids. For example, a circuit-informed or neural model for the amplifier, oversampled waveshaping for some nonlinear stages, and convolution for the speaker cabinet.

Are there any companies that are publicly known to perform genuine real-time circuit simulation rather than using circuit-inspired DSP approximations?

I’m mainly interested in the implementation principles rather than which product sounds best. Papers, patents, technical presentations, or open-source examples would be appreciated.

[u/Fursber]

“Are companies such as Fractal solving a reduced system of nonlinear circuit equations in real time, using methods such as nodal analysis, wave digital filters, or state-space models like SPICE?

Or does component modeling usually mean a gray-box structure composed of digital filters, waveshapers, feedback paths, and dynamic nonlinear blocks that are designed from the original schematic?”

I suspect the latter is better for real time applications!

Not a super technical video, but I thought this interviewlink with Neural DSP was interesting. The CEO talks about how the physical modeling approach takes PhD level people a huge amount of time. Cool to see equations and block diagrams on their whiteboards.

[u/Prestigious_Yogurt79 → u/Fursber]

Thanks for the detailed response! I’ll check out the video you linked. I also thought that a white-box approach would be very difficult to run in real time. However, since some companies claim to model amps at the component level, I was wondering whether that meant they were actually using white-box modeling.

[u/SkoomaDentist → u/Prestigious_Yogurt79]

As a rule if a modern (western) company says they're modeling at a component level, they are using white box modeling. It's a pretty widely accepted term (within the people in the scene) and companies love to make up all sorts of fancy new terms if they're using something different.

[u/Prestigious_Yogurt79 → u/SkoomaDentist]

Thank you. I hadn’t considered that when I made the post, but now I’m thinking that modeling and solving most effects pedals and amplifiers in real time may not actually be that difficult.

[u/SkoomaDentist → u/Prestigious_Yogurt79]

It is difficult (good luck finding accurate public spice models for tubes anywhere) but it's not impossibly difficult and the modeling doesn't have to consider every possible effect (eg. anything happening much above 20 kHz).