Generally speaking classic Arc's have very good sound output - the A3000 even came as standard with internal stereo speakers.
Unfiltered Sound for the A3000.
Like a lot of computers from back in the day Arc's had quite aggressive sound filters for screening out high frequency harmonics on the assumption that owners will be using some pretty rubbish speakers, but this can give output a muffled quality with a decent set-up.
For the A3*0-A5*0 range Acorn gave their machines headers for unfiltered output to get round this potential issue but the A3000 has no such facility...
The A3000's sound is output via IC39 (LM324 Low Power Quad Operational Amplifier) prior to being filtered and thence on to the standard 3.5mm output.
Unfiltered Left and Right channels can be found on pins 1 and 14, while there's a convenient Ground to be found on pin 11. Ah-ha! A Plan...
...L+R+G give our sources but there are one or two issues:
Without some kind of 'protection' we'll kill that LM324 pretty quickly
It would help if we had some kind of socket for a stereo jack
A good place to mount said socket would be helpful
To the left is the main part of the cunning solution to these issues. It's a Frankenstein combination of a 3.5mm stereo socket removed from an old motherboard with two 100uF electrolytic capacitors super-glued to it (Anywhere from 10-100uF in the range of 16-50v will do).
That's one cap each for L and R with the negative side pointing to the output. I've soldered them to the L and R pins of the socket and used three cores of a ribbon cable to connect L, R and GND to the IC.
IC ---> ---|+ |--- ---> Output, is the basic scheme.
The other end is soldered to IC39. Yes, I had intended the 'red' line to be 'right' but ended up soldering it the wrong way round - never mind, as long as both ends are the same.
It's important to make sure you've got L and R the correct way round, especially as the plan is to glue the socket to the motherboard upside-down, so transposing the L and R pins.
Why upside-down? because you don't want those pins digging in to tracks on the motherboard and causing shorts. Besides, this gives you two flat surfaces to glue together...
I found the most convenient place to mount the socket was glued to a fairly empty place on the motherboard behind a little recess in the plastic case where I could Dremmel a hole for the socket - through not pushed flush against the case as that could make removing the motherboard at a later stage, um, tricky.
Now, Super-glue is great but a bit 'thin' if the surfaces aren't 100% flat. This has got to be a pretty solid bond so the sensible solution would be something like hot-glue - Pah! Mix Bostick with an equal amount of Super-glue and you get strong + goopey + fast. Hurrah!
Some shaving of the latch lug on that side will be needed...
Once done, plug your best speakers or hi-fi into your new socket and enjoy your Arc's output at it's best.