Modifying the lowpass filter of the Amiga 4000D

[8bitbubsy]

As you may or may not know, the Amiga 4000D has a lowpass RC filter tuned to 4515Hz (according to schematics, but actual cutoff vary from machine to machine because of degrading and specs/tolerance).
This does help a bit with hiding 8-bit quantization, but it makes the sound very muddy in the upper frequency floor!

The Amiga 1200 and the CD32 are the only Amiga machines to have a cutoff set higher than 5kHz. In fact, it's set to 27205Hz (!) on the A1200 rev 1D.1. According to nyquist, you shouldn't even hear a lowpass filter at all with such a cutoff value. Crystal clear sound. Not Hi-Fi sound but much better than it was for sure. One might want to change the op-amp too if you're hunting for better sound. And no, I'm not using my Amiga for Hi-Fi stuff, because I know that it has rather cheap components inside, and probably not sufficent noise decoupling and the like. Not to mention 14-bit max with Paula.

I modified my "already hacked" A4000D to use the RC components from a broken A1200 rev 1D.1 motherboard I had laying. Two SMD resistors and two SMD capacitors. While I care about having collectors items in an original state, this machine is in bad shape and was already hacked on. Why not?

Here's two audio captures from my A4000D - 14-bit 48kHz AHI:
Before hack: http://16-bits.org/etc/a4000d_27khz_...d__5kHz_lp.mp3 (my A4000D was 5kHz)
After hack: http://16-bits.org/etc/a4000d_27khz_...__27kHz_lp.mp3
(I normalized these in Audacity, and made them start at the same sample, but I can assure you that I did no Audacity lowpass filtering or any sorts on them!)

Here's a reference picture I made:

[prowler]

Worthwhile job!