The reason you see aluminum in interconnect cables is because it isn’t a current source. DCR isn’t as big an issue. The aluminum’s fragility has to be managed with smaller wire to see the benefits and not have the wire break. Termination is a problem, and some use copper covered aluminum to use a solder solution.
In speaker cable that deliver higher current, the CMA has to go up as it is higher resistance, and this offsets the better skin depth as the wire gets bigger so it is a catch-22. DCR offsets the better skin depth penetration.
Hmm, about termination…
In what proportion does the plating of a conductor’s termination affect signal transfer, relative to the base metal? I see that this is dependent on whether it’s a power cable, interconnect or speaker cable.
Rhodium plating is never described as audibly detrimental to sound quality despite being significantly more resistive than gold - actually it’s often described as more analytical sounding than gold plating.
PS Audio’s power cables are nickel plated copper. Nickel has relatively poor conductivity so I guess in a power cable the plating really doesn’t contribute that much to current transfer and should be robust, right?
Then what about using such platings as rhodium in, say, phono cable terminations where there’s a small voltage? Is this where conductivity should be maximized to avoid losses? Interconnects’ ends are quite universally gold-plated, right?
In speaker cable terminations again, there’s rhodium plating available and it’s apparently not any worse than gold, just a different signature. Again, in a high current transfer application.
Rhodium is a soft metal that deforms under pressure to fill all the nooks and crannies between mating surfaces. This is also why it scatches easily…it is soft.
The thin coating isn’t enough, or used over a long enough length, to really impact the sound but it does keep connection DCR in check as it squeezes out air under pressure on spade connectors and the like. Poor contact DCR changes the sound worse than any suitable platings.
Most plating are used to control oxidation and manage contact DCR stability.
Nickel is a poor choice most of the time ($$$) unless it is used where high heat is applied to a copper wire and it would oxidize on the spot, so Nickel is used over the copper as it won’t oxidize at the needed temperatures for specific fluorocoplolymer insulation materials like ram extruded TFE. Silver is also used where DCR is more important at higher frequencies. Low frequency power can use Nickel just fine and it is cheaper than silver and low frequencies diffusion couple through the wire, not just travel on the surface, so the DCR isn’t an issue as thin as nickel is applied.
I have compiled a rather extensive cheat sheet on materials and their properties. Easy to see why to use one over another. Chasing material to be magical for sound has proven to be pretty poor in return for the money spent. Better geometrical designs yield for more benefits than touting fancy materials in ordinary designs. Using material is easy, designing a Ferrari with those materials isn’t. Design is first, always.