I assume you try to get it as simple as possible. IN this case You are already there, Just try and see how it sounds.
the buffer operational amplifiers it is recommended to have socketed, so you can try different ones too. NE4570,
NE5532, TL072.

(A): for line output is sufficient, but if the clipping on the oscilloscope is visible then higher ..9V voltage rail should cure the problem.
It can drive headphones, and if not then just add the 20ohm resistor between so it would not overload the op-amp. However, for headphones just have a extra buffer op-amp with transistors at the end.

B: it is just to try. If it sounds all right then it is not needed. Especially for just first test. The high frequency filter is more concern when connected to recording equipment or amplifiers.

Thanks for your reply!
Yeah i am indeed trying to make it simple and hopefully achieve a better sound quality. I did as you suggested and just ordered a handful of OPA1688 so i can test and have spares for if/when i break something 😀

From what i have been able to find online it seems like i can add a low pass filter with even just the one op-amp. I am very new to this field so unfortunately it will mostly be trial and error getting it right.

If you want just to get sound then you are already there and can admire it several days ;D 😉 If you want to have a low noise floor then it is much more than complicated but very rewarding.

to have a higher signal to noise ratio then...
every item should be considered the higher voltage versions. So if you have one DAC which is meant for 3.3V and another DAC which is for 5V then choose the 5V one. ALso to prefer the symmetrical output. The synth chips also is recommended to choose based on the output voltage swing, packaging, voltage rails.

The point is that the models with higher voltage rail have higher signal output, and with symmetrical signal output versions have higher noise immunity! Also the symmetrical op-amps which are +9 / -9V while inside computer (stabilized from +12/-12V voltage rails ). but outside computer with +15/-15V recommended, 12..15V symmetrical voltage rails is recommended becouse with these parameters the op-amps work best. 5V for op-amps (especially when using it as one polarity) is still too low.
It needs more work, and is more complicated than the 1-2 op-amps, but the positive side is that all is possible if to take time for it and it can be done as a upgrade module and it does not resist making music with the existing version. 😉

The OPL2 chip and the DAC are designed for each other and both meant for 5v, though the spec sheets allow for some margin so i probably wont change this.
I am perfectly satisfied with a good line level signal, as I am going to hook it up to an external amp afterwards anyway as a synth module. Many instruments use 9v, which i plan on powering this with in the end, so i suppose i could make a +4.5/-4.5 supply for the amp stage, and keep the arduino/pi and OPL2 as 5v (i hope the op-amp does not send -4.5v down the input line!)

Good info about the dual supply though! I thought it would be a no-brainer to just use a single supply and save the inconvenience building circuitry for dual supply, but now there is a good reason to do it.

Single supply is an inconvenience if you actually got dual supply available as you have to start mucking around with virtual grounds, you will need a number of more components compared to dual supply and you will always have worse noise floor.
For 5V only operation you will need to use rail to rail I/O opamps to maximize dynamic range, most chips inputs and outputs will be at least 1V shy from the rails, i.e with 0...5V power supply you won't get output more than 1...4V on the output nor can the opamp input accept more than this. Some opamps are worse than others about it, you will have to look at the datasheets to be sure.

Yamaha's DACs are not great however and using top of the line components only gets you so far.
One thing that can help is use of a very high PSRR 5V regulator to power the DAC such as LT3042 or ADP7118.

Making a dual supply myself (or using a split rail converter IC like TPS65133) seems a lot easier than using single supply based on what the two of say and from what I have read online afterwards. And when it will have the benefit of higher dynamic range and better sound quality, then i am convinced on going in this direction.

Apologies for my ignorance, but should the LT3042 ground be connected to the same ground as the rest of the board or how would you recommend? (as in YM3812 gets 5V from the regular PSU, YM3014B gets 5V from LT3042).
A lot of the material i have read online mentions separating analogue ground from digital ground but i do not know if/how i can accomplish this with the YM3014B.

Ps. Awesome sound card in your signature

The grounds will have to converge at some point, for most devices preferably near the point where power enters the whole thing (dedicated power connector, IO connector's power pins etc.). The whole point of the separation is to keep digital return currents of digital side away and thus provide smoother reference on the analog bits, it will reduce noise and interference potential.
For the DAC chip you keep the power reg as close as possible to it and treat it as if it was a fully analog part. Do not use output capacitors larger than datasheet suggests as it will make it harder for the reg to react to the power supply wiggles caused by the DAC. The remote sense signal should go straight to the DAC power supply pin for best effectiveness at suppressing any fluctuations, end result should be fairly low noise output.

Thänk you ~

Thats some very good tips, thanks!

I saw that the LT3042 can be bought as a completely assembled module (although sold by chinese vendors with some risk for surprises), which i can just connect to 9V in parallel with the power regulator that supplies 5V for the digital parts of my circuitry.

I am wondering if the YM3014B DAC will work with an MP value (the analogue reference voltage input - photo in my first post) coming out of my op-amp with +-5V dual supply, since the OPL2 designs i have seen used single supply op-amps for both bias reference and buffer/amp stage. I mean, wouldnt that require that the DAC can make the output signal swing negative relative to the dual supply ground instead of relative to the Vdd/2?

Which DAC and op-amp(s) did you use in your card? (cant see it on the photos very clearly)

The ready made modules will greatly diminish the advantages of the regulator unless you can set the sense line to the destination. Any length of wire will greatly diminish high freq PSRR where things really matter and why one wants to use the regulator in the first place.

The reference output on the DAC is somewhere between GND and VCC. You really only care about it if you're gonna do full DC coupled setup but that will complicate gain setting and other factors so the general connection will be DAC out > DC blocking capacitor > resistor to GND > gain+filter stage > output. After the cap the signal will swing around GND. Offset voltages are very low on the good opamps you can even omit output capacitors.

I use NJM/RC4580 opamps. NE5532 would also be very decent to use.