NAM A2 explained
What is NAM A2?
A2 is a newer Neural Amp Modeler architecture. Captures trained with it generally model an amplifier more accurately for the same file size, but they need a player built to run them. Older players load only the earlier A1 captures, which is why some A2 files appear to fail.
What changed
Neural Amp Modeler works by training a small neural network to reproduce what an amplifier did to a signal. The architecture is the shape of that network, and it sets the trade-off between how accurately the capture reproduces the amp and how much processing it costs to run.
A2 is a later architecture than the original. Broadly, it gets closer to the amp for a comparable amount of work, which is exactly the trade that matters on a phone: there is a hard ceiling on how much processing you can spend before the audio starts breaking up, and a more efficient architecture buys headroom.
Why do some players refuse to load A2 captures?
Because running a capture is not like opening a picture. The player has to implement the architecture the capture was trained with. A player written before A2 existed has no code for it, so the file loads and fails, or does not load at all.
Nothing is wrong with the file, and there is nothing to convert. The app has to add support. This is the single most common confusion in the format, and it produces a steady stream of forum posts from people convinced they downloaded a corrupt capture.
Should I care which one I download?
Only enough to check your player supports it. Beyond that, the architecture is far down the list of things that decide whether a capture sounds good. What amp it is, how it was set, how carefully it was captured, and — most of all — whether you are hitting it with the right input level, all matter more.
A capture is a photograph of an amp at one setting. A better camera does not make it a photograph of a different amp.
How does this affect apps on a phone?
The wave of Neural Amp Modeler players that arrived on iOS landed either side of A2 becoming common, so support is uneven. The community treats prompt A2 support as a signal that a developer is actually maintaining their app rather than having shipped it and moved on, which is a reasonable heuristic.
If you are choosing a player, check its release notes rather than its marketing.