For the most part the toroidial transformer has replaced the traditional wax and paper transformer. There are some classic designs that cling to the latter. I I lack the technical expertise but I see no reason why we should still utilize fuses. In deed if you already own equipment it may be good idea to have it modified for coircuit breakers. This is especially true since it seems my fuse always blows at night or on the weekend when the store is closed.
What say you?
Not sure what a toroidal transformer has to do with your question. However, if your question is about performance potentials between a simple fuse and circuit breaker interal to the component, on its face, I'd opt for the internal circuit breaker.
But that's also not knowing much about the internals of internal circuit breaker, quality of materials, etc. nor with a fuse for that matter. Non-audio grade standard fuses are cheap $0.50 retail parts I think mostly of aluminum. I've had audio-grade $50 fuses and I've had $0.50 cent fuses both cryo-treated and non-cryo-treated.
My Class D monoblock amps each employ an internal circuit breaker as do I think each of my little dedicated line conditioners and I'm comforted knowing that this difference alone most likely contributes to their performance and it's potentially one less perforamnce bottleneck. So it's only my source and sub that use fuses.
Then again. From a performance perspective, a fuse say even the very best aftermarket fuse available is a very very simple design whereas an internal circuit breaker must be far more complex. IOW, the electrical current flow is most likely having to travel through more parts via an internal circuit breaker than a fuse so I suppose the internal circuit breaker by its more complex design alone could potentially induce more performance compromises. But I'm reaching. In the end, whichever way a designer goes, I think the simplicity of design, quality of construction and materials and whether or not the electrical object is or could be cryo-treated determines greatest performance potentials.
I'm pretty sure my amps' internal circuit breakers are not cryo-treated and if not, that should be a little compromise in performance. But the line conditioners' internal circuit breakers are cryo'ed.
I've also tried inserting for a short time a standard 8-gauge I think Home Depot copper solid core wire which happens to be roughly the same gauge as a standard fuse. I replaced a single audio-grade fuse with an 8-gauge copper wire blank cut to the same length as the fuse. As I recall, after a few days of burn-in, I thought I heard an ever so slight improvement but re-installed the audio-grade fuse as it was not worth the risk to what was then an $8500 amp. I'm all for performance at MOST any cost but risk mgmt from all perspectives much take precendence.
To my ears, just replacing a single fuse with an upgraded version, I generally cannot distinguish an audible difference. However, if I upgrade all my fuses say 4 or 5 at the same time, I can easily discern an audible difference AFTER burn-in. Just remember that even a tiny fuse will take roughly 2 1/2 days of round-the-clock burn-in / play time before hearing any difference.
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As for modifying existing components to replace internal fuses with internal circuit breakers? Not a chance. Sure, there are a few who are able to do so while keeping a performance-oriented mindset e.g. soldering types, quality of circuit breakers, etc but there's also hacks and also-rans out there and I would not take such a risk as again risk mgmt must always come into play. I'd much rather purchase a new component where the designer already has accounted for such performance issues from the ground up.