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Old 04 December 2021, 17:18   #1
ugrnm
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Line level of the Amiga 600 (and possibly other models)

Hello,

I'm going through some maintenance of my Amiga 600, with a special focus on the audio. I'm getting to the point where everything sounds great, no more cracks, hums, CPU activity leak weirdness, etc.

However, while plugging the Amiga on a mixer I noticed that the audio levels were quite low. By low I mean that they are probably perfectly fine for an Amiga, but that in comparison to other gears like synths, it is significantly lower. I want to mix in the sound output of the Amiga with other music gears, and ideally it would be great to not have to boost the signal too much on the line channel where the Amiga is plugged, to avoid adding to the noise floor (and also to give me more room to increase/decrease the sound of the Amiga in relation to other channels).

What followed was a rabbit hole into the world of line level. And in particular this graph found on Wikipedia:



As a result I'm trying to figure out how the Amiga was designed, I would assume for consumer line level? Anyone knows or ever measured this?

I'm also wondering if the audio circuit could be modded to so-called pro line level or at least boost it a little bit. This is where I'm also a bit stuck because when I look on the Amiga 600 schematics of the 1.3 (the model that I have), I see that I could switch R334 and R335 for the right channel (and do the same for left)? I'm suggesting switching because I don't have smd resistors, so that could be a lazy hack.



What I noticed is that later revisions (1.5) have the same voltage divider as the A1200, where the ground resistor of 390 was replaced with a 2K resistor, which makes me also wonder, does it mean they realized eventually that the line level was too low, or that the circuitry before the final output had changed and became too high? (I did not compare the full diff between 1.3 and 1.6 audio circuitry so this is just speculation).

Ultimately, could I also simply get rid of R335? Even.... both R335 and R334?...

Any advice would be very much appreciated
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