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The opamp wired "as buffer" means that it has OUT and IN- shortcut with each other?
Yes. The gain should ideally be 1:1.
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Did you comapre the LM324 wiring to SNES mainboard schematic? It's having LM324, too (with each channel having two opamp's in series, in lack of knowing a better name, I've called them "pre-amp" and "post-amp").
Yes, they have similar configurations - One "pre-amp" (the buffer stage) with some RC filters and the "post-amp" (the gain stage) that has connections for mixing three separate inputs with an added transistor for muting the output. There are some differences.
I should draw up a schematic...
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PS. just curious, as the LM324 is a quad opamp, and needing only two opamp's for the stereo audo... are the other two opamp's used for something, too?
After doing some quick probing the op-amp setup appears to be
almost the reverse of what the SNES has - being that the gain stage ("post-amp") with the muting signal (I am going to assume it is a muting signal) comes prior to the buffer ("pre-amp"), the buffer ("pre-amp") then connects to expansion port.
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In the satellaview unit it's probably wired to the 64pin or 100pin chip. Ah, and/or... it might also go to one of the six "???" pins on the 38pin CN2 connector.
Decided to probe those out.
Code:
EXT_port --- Chip.Pin
23 --- Q5.Base (Collector of Q5 to /IRQ on SNES Expansion port, Emitter to GND)
29 --- U5.75
30 --- U5.74
31 --- U5.72
33 --- U5.82
34 --- U5.83