OK so I spent some time today digging into the audio problems.
I can't speak for all of these bootleg boards, but I'm kind of impressed by the component quality of my board (Red Sun version with the JAMMA notch), it appears to have Legit Yamaha chips, the Amp is a legit Toshiba chip and all of the electrolytic caps appear to be legit Suncon caps.
From what I can hear there are 4 problem with the audio (from most important to least important)
1. There's a very loud hum
2. the speaker +/- appear to be wired backwards
3. bass sounds like in-game explosions and the kick drum sound like they're being over-driven by the amp/clipped.
4. mid range sounds are really muffled/muddy
Despite the appearance of high quality Japanese caps I did recap it anyway but it made no difference what so ever.
After that I decided to dig into the Audio Amp Circuit
the Amplifier is a Toshiba TA8201AK:
http://pdf.datasheetcatalog.com/datasheet/toshiba/1144.pdf
it's a mono amp designed to run on 12V, so at least that's correct. looking at the data sheet the recommend wiring for the amp looks like this:
poking around the PCB it appears that the way it's using the amp is like this:
Right off the bat the first thing I notice is that Speaker + and - are NOT actually backward at all. Speaker - is routed correctly, it's just the speaker + is not I've actually seen some legit PCBs wired this way. So technically if swapped speaker + and - to "fix" this you'd be inverting the audio... not that it really matters with mono audio.
Another interesting thing is C1 cap is actually 2x 10uF caps run in series to act like a single 5uF cap... I think a big problem with the audio setup here is actually that they decided to only use 3 values of caps (10uF, 22uF and 220uF), and then just picked the nearest value out of the 3 available. Kind of clever way to get close to 4.7uF, I left this as is but you could replace then with a 4.7uF easy enough.
Following the data sheet Cap C2 is the "feedback condenser" and determines the "low cut-off frequency" so this will likely fix our bass clipping.
C3 isn't even equipped at all, in-fact the whole of pin 3 doesn't seem to go anywhere.
Again looking at the data sheet the function of C5 is "for filtering power supply hum and ripple" this cap is WAY out of the recommended spec no wonder I'm getting hum
and that leaves us with the addition of a cap in-line on the speaker -, I'm not sure at all why it was added but I'd venture a guess that it's responsible for our muddy mid-range sound.
Since none of the caps are labeled on this PCB here is how they map to the above circuit:
One other thing to note is that the square via on the Amp is actually Pin 7 NOT Pin 1 like you'd expect. so keep that in mind if you're going to hack around with this.
What I've fixed so far:
So far I replaced C5 with a 1000uF 16V cap and it's eliminated 95% of the hum, hooking up Pin 3 and adding C3 into the circuit by adding a 220uF cap across pins 3 and 4 of the Amp takes away that last 5% so there is 0 hum remaining.
Replacing C2 with a 47uF 16V cap has fixed the bass clipping issue as well.
What I plan to try next:
I'd also like to get rid of the C? cap and add a resistor and replace C4 for a proper output circuit I think this will fix the muddy mid-range audio I'm still trying to work out a clean way to do that and I'll probably have to order some parts as I don't have the right sized caps and resistors on hand.
I'd also like to properly tie the output + to the speaker + pin on the JAMMA edge (and leave the speaker - hooked up as it is). I think doing that could give much clearer output and potentially 0 hum since then the speakers will be completely isolated from the PCB's power circuit.
Other areas of investigation:
so there are a lot more caps on here not related to the Amp, given how off the values were of the amp caps I'd suspect the others might be off too, so it could be worth investigating the proper values for the rest of them.
for now though I'm super happy with the world of difference just replacing C2, C3, and C5 has made.
TLDR; here is how you de-shittify your Audio:
Replace C5 with a 1000uF 16V Cap
Replace C2 with a 47uF 16V Cap
install a 220uF 16V cap between pins 3 and 4 of the amp (negative on pin 4, positive on Pin 3 and note that pin 7 is the one nearest the JAMMA edge)
More upgrades may follow but for now this will make a substantial improvement.