I haven't heard the A90 yet, but Dertonarm hit the nail on the head. The thing that matters is matching playback stylus SRA to groove SRA, and a microscope can't help with that.
Groove SRA is different for virtually every LP you own, and you can't see it with any microscope short of an SEM. Even LP's of the same thickness have different groove SRA's. (Exception: unless they were cut on the same lathe with an identical cutting stylus set up to the same parameters - good luck finding that).
Therefore:
1. If playback SRA will be optimized for each LP, using a microcope to set some pre-determined angle is pointless. If I'm going to adjust until each LP sounds right anyway (and record that setting for replays) why waste the time?
2. If playback SRA will NOT be optimized for each LP, using a microcope to set some pre-determined angle is also pointless. For the "set and forget" user, this would serve no purpose except to satisfy OCD tendencies and earn their dealer some fees. If one can't hear or doesn't care about the sonic differences that groove-specific SRA makes, why take pains to set some pre-determined angle that bears little relationship to any particular LP?
The foregoing is especially true for styli with very fine contact lines, like the Ortofon Replicant. With such styli, the window for correct SRA on each LP is so narrow that a generic setting is useless. It won't make much sonic difference whether you're off by a hair or a mile.
Frankly, the microscope suggestion seems like a ploy to tie gullible owners to dealers. If the dealer is going to stand next to your table and adjust your SRA for each LP you play, fine. Having them do it in advance with a microscope (or any other tool) is nonsense.
Groove SRA is different for virtually every LP you own, and you can't see it with any microscope short of an SEM. Even LP's of the same thickness have different groove SRA's. (Exception: unless they were cut on the same lathe with an identical cutting stylus set up to the same parameters - good luck finding that).
Therefore:
1. If playback SRA will be optimized for each LP, using a microcope to set some pre-determined angle is pointless. If I'm going to adjust until each LP sounds right anyway (and record that setting for replays) why waste the time?
2. If playback SRA will NOT be optimized for each LP, using a microcope to set some pre-determined angle is also pointless. For the "set and forget" user, this would serve no purpose except to satisfy OCD tendencies and earn their dealer some fees. If one can't hear or doesn't care about the sonic differences that groove-specific SRA makes, why take pains to set some pre-determined angle that bears little relationship to any particular LP?
The foregoing is especially true for styli with very fine contact lines, like the Ortofon Replicant. With such styli, the window for correct SRA on each LP is so narrow that a generic setting is useless. It won't make much sonic difference whether you're off by a hair or a mile.
Frankly, the microscope suggestion seems like a ploy to tie gullible owners to dealers. If the dealer is going to stand next to your table and adjust your SRA for each LP you play, fine. Having them do it in advance with a microscope (or any other tool) is nonsense.