Amiga 1200 Rev 1A Distorted Audio Output - Part 3

Here's the same mod file being captured at 0dB after the following motherboard modifications:

R331 changed to 340R (2 x 680R in parallel)
R321 changed to 340R (2 x 680R in parallel)
C331 changed to 6800pF
C321 changed to 6800pF

Gain is significantly reduced although still quite a bit higher than the A600 (Rev 1.5 board). I'd say the playback is 99% correct and I could easily use it like this but the gain is still just a tiny bit too high for my taste.

Another notable difference between the A600 and A1200 is the aliasing present when playing the 1kHz test tone with the low pass filter disabled (dim power LED).

Here's the full volume 1kHz test tone captured at 0dB.  Note the level is now much reduced and there's visible aliasing on the waveform.

Here is the same test with the low pass filter switched on (bright power LED).  Note the nice clean waveform.

I might have another go at reducing the gain a little more and trying to clean up the default filtering to remove the aliasing.

Amiga 1200 Rev 1A Distorted Audio Output - Part 2

Here's the same mod file being captured at 0dB after the following motherboard modifications:

R331 changed to 680R
R321 changed to 680R
C331 changed to 6800pF
C321 changed to 6800pF

Gain is slightly reduced but is still very high compared to the A600 and it sounds distorted. It's significantly better than the unmodified board but still not quite right.

I think the gain is definitely reduced as I was able to record at 0dB level without having to heavily attenuate the line input to stop it clipping so badly.

AudioTest

Using the Amiga 1kHz Audio Test program (copy available here AudioTest.lha), here's the audio capture at 0dB from both the A600 and A1200 for comparison.  This is the 1kHz tone at maximum volume output with the filter turned off.  The difference in output level is clear to see.

I'll follow up with Part 3 after I've done some more troubleshooting.  I'm not done yet!