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Each of the SID6581s I have tested sound different, particularly when it comes to filter cutoffs and distortion. What are the causes of these variances?

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SID6851 specification quotes:

Cut-off frequency variation may occur from chip to chip due to process variations, and power supply voltage. Capacitor values and voltage regulation can compensate for these variations

We're talking about a 30-some years old chips, the manufacturing process wasn't very consistent back then, and also MOS was known to have issues with some process steps such as passivation layer that was "leaking" and allowing external elements to "age" the chip.

Also, SID is in big part an analog chip, there are plenty of analog components there on a die (this is why it's so hard to reimplement say in FPGA), each and every one of those analog components can vary between chips slightly.

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    Sometimes more than slightly — even to this day getting consistent resistances on silicon is one of the hardest things, which is why you get designs that depend only on the ratios of resistances, increased use of current mirrors, etc. To my knowledge the SID wasn't especially clever about this, it just took a "good enough is good enough" approach. – hobbs May 30 '16 at 8:03
  • @hobbs: I wonder if it would have been practical to have the SID chip use duty-cycle modulation with some off-chip resistors. That would have reduced the variability if the internal resistance of the SID was low relative to the external resistance. – supercat Jul 1 '16 at 22:54

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