Saturday 27 May 2017

Amiga 1200 Rev 1A Distorted Audio Output - Part 4 (Final)

This is the final instalment of my quest to get nice sounding audio out of an Amiga A1200 Rev 1A.

Here is the same mod file captured at 0dB with the following modifications:

R331 changed to 680R
R321 changed to 680R
C331 changed to 6800pF
C321 changed to 6800pF
R335 changed to 360R
R325 changed to 360R
R345 changed to 680R

(R335 and R325 are on the bottom side of the board)

These modifications are the same as Commodore implemented between the Rev 1 and Rev 2 boards according to the schematics I have.


For comparison with previous posts, here's the 1kHz sound test with the LED filter disabled (jaggies) and enabled (smooth)...


The audio output sounds about perfect to me and I'm more than satisfied with the result.  It does appear the changes Commodore specified for the Rev 2 board onwards were well engineered.

The output level is still a little higher than the A600 (Rev 1.5 board) and could perhaps be lowered just slightly (say changing R331 and R321 to ~600R).  Also, with the optional (LED) filter switched off there's noticeably more aliasing present than on the A600.  However, I think these slight differences give the A1200 its own characteristic sound verses the other models and I rather like it.

In theory this machine should sound the same as a stock Rev 2 board but I don't have one to compare with.

That's the end of this mod.  Thanks for reading!

Saturday 13 May 2017

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.

Thursday 11 May 2017

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!