SamplerDPM2Ancestral_SUN

Let's get the awkward part out first: SamplerDPM2Ancestral_SUN doesn't exist in the current pack anymore. It was one of the six per-sampler nodes in bvhari/ComfyUI_SUNoise's early releases, and the July 2024 "Complete rework, breaking changes" update removed every one of them in favor of the dropdown nodes SamplersSUNoise and SamplersSUNoiseAdvanced. Install the pack today and you'll look for this node in vain.

What it was

DPM2 ancestral - dpm_2_ancestral - is the second-order cousin of Euler a: it takes a midpoint step before updating, which gives cleaner trajectories than plain Euler at the same step count, at the cost of being roughly twice as expensive. It was a respectable pick in the SD 1.5 era, then DPM++ 2S a basically ate its lunch in A1111 circles, and by now it's a sampler most people only encounter in old workflow JSON. This node wrapped that sampler and swapped the per-step Gaussian noise for scaled uniform noise on [-sigma_down, +sigma_down], seeded deterministically from your sampling seed - the SUNoise idea that gives the whole pack its name.

Its inputs were eta (default 1.0, the ancestral jump size) and s_noise (default 1.0, noise amplitude). Output was a SAMPLER wired into SamplerCustomAdvanced.

What to do if it's in your workflow

ComfyUI will flag it as a missing node when you load a workflow saved between April and July 2024. The fix is trivial: swap in SamplersSUNoise, set sampler_name to dpm_2_ancestral, pick your noise_type. Same solver, same uniform-noise mechanism, current code - plus the v2.0.0 improvement where SUNoise alternates with standard Gaussian noise instead of running pure uniform on every step.

If you really need byte-for-byte old behavior, pin the pack to a pre-rework commit (git checkout efc6e38 in the custom_nodes folder), but honestly there's no reason to. This was never the most popular sampler in the pack, and its modern form is one dropdown entry.

Install, for completeness

ComfyUI Manager, search "SUNoise", or:

cd ComfyUI/custom_nodes
git clone https://github.com/bvhari/ComfyUI_SUNoise

Restart, no extra dependencies, no model files, GPL-3.0. The pack is small and has essentially no community discussion, so there are no crowdsourced troubleshooting threads - the versioning trap is the one real gotcha, and you're already past it.

Bottom line

If you're on this page because an old workflow references SamplerDPM2Ancestral_SUN, you're one dropdown swap away from a supported equivalent. If you were genuinely going to pick DPM2 ancestral on purpose, you do you - but the rest of us are on DPM++ 2S a, and the SUN selector node has that too. Either way: 30 steps or more, per the README.