This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
All Acorn machines are equipped with a sound filter designed to remove high frequency harmonics from the sound output. However this does cause a muffled feel to the sound as on some machines the filter is a little too excessive and it filters out valid frequencies. Also the filter is optimised for 20.833 kHz output and has less desirable results when the output rate is changed. Accordingly people who do audio work often want to bypass the filter.
On all machines bar the A3000 there is the Internal Auxiliary Audio Connector (usually called link LK3), which can be easily plugged into to provide the unfiltered output. This connector has 10 pins on it and is usually found near the headphone socket on the motherboard. The pins are :-
1 Unfiltered Left 2 Ground 3 Filtered Left 4 Ground 5 Auxiliary Input 6 Ground 7 Filtered Right 8 Ground 9 Unfiltered Right 10 Ground
Simply hook into the Unfiltered outputs.
On an A3000 you need two 10uF 16V ALEC capacitors. Look for chip LM324 (IC39) and hook the capacitors like this:-
Pin 1 --> --|+ |--- Unfiltered Left Pin 2 --> --|+ |--- Unfiltered Right
The Risc PC & A400 machines have a connector similar to the A5000.
There are several caveats to this procedure. Opening your machine may void your warranty and most definitely should not be attempted if you are unsure of the procedure. Do not unplug/plug the unfiltered audio output while the machine is turned on, by bypassing the filter you also bypass the normal protective circuitry for the audio output.
Finally you will hear higher harmonics present in the audio signal so you will need to connect the signal to a filter of some kind to reduce this extra noise.