You're right that it's even simpler with all the gain in the first stage, I just thought that it's easier to fine-tune the gain to taste by changing the feedback on the second stage, but since it's all about making it as simple as possible, this is hard to beat:
To make it even simpler, only 1 X 5532 is needed for the gain stage, it will "see" only 1K loading at its output with 10X 10K vol. pots in parallel, still well above its 600 Ohms limit. Infact I would use single 4556 for the gain stage too, considering its 100KHz BW limit, so less prone to oscillations.
He wants separate inputs for the 8 channels, each fed from a mixer aux output. I assume the idea is to have the typical setup with an independent monitor mix for each musician.
That is OK for the input, but do not short the H.P output to GND.
It certainly simplifies the circuit and has an advantage in S/N. However, we do not know how much output the Behringer XR18 has in actual use, but the specifications say MAX +16dBu (4.9Vrms). If increase the gain in the first stage and distort it, will not be able to recover by turning down the volume pot. I think need to carefully distribute the gain between the first stage and the H.P. amplifier section here (especially if follow availlyrics' suggestion and lower the power supply voltage for the entire circuit).
Yes, that's something to watch out for. Standard procedure is to run a test to see what the maximum level without distortion is and then give yourself a healthy margin when setting up the mixes for the aux outputs. If the mixer software allows it (most do), you can also put limiters on those mix buses for extra safety.
Thanks everyone, I bought the components based on Cabirio's first design, I will run some tests once they get here from China and let yoh know. Cheers!
BEWARE - Your design uses dual supply, any failure and you will have DC at output frying the headphones. Measure twice, cut once!
By the way, regarding the power supply, I have bought one of these (sm-fla17a):
Do you think it is powerful enough to drive all the 8 channels at the same time?
Do you think it is powerful enough to drive all the 8 channels at the same time?
Interesting project. 
I have an AKG HP4E headphone amp that has 4 stereo outputs, each switchable to either A or B input. The amp runs off a small 12V wall wart power supply. I have never opened it to see what's in it.
I have an AKG HP4E headphone amp that has 4 stereo outputs, each switchable to either A or B input. The amp runs off a small 12V wall wart power supply. I have never opened it to see what's in it.
Here is the KiCad project, if anyone is interested: https://drive.google.com/file/d/1t4JfHiVtvQ3Sk6bFNjiezdE6RzpeT-7r/view?usp=sharing
I will make a prototype using a perfboard, but it is easier to visualize it using the PCB design.
But you're only using half of each opamp, so the other half is wasted... Why don't you try putting two channels in each PCB, so you only need 4 x 5532 and 4 x 4556 for the whole project, instead of twice as many?
It is everything except the power supply. Right?
I would like to know if the PCB design has apparent issues. I have tried to place the 100nF as close as possible to the ICs.
All of them except for the supply. If you start with a layout like this:
Then you can put one channel at the top and the other at the bottom of the PCB (connectors and pots facing left, as you have drawn it). Passing the supply traces between the opamp legs as shown gives you easy access to all the signal pins on either side of the opamps. Those extra ground traces out of the supply header is where you would connect to the audio grounds of each channel.
Then you can put one channel at the top and the other at the bottom of the PCB (connectors and pots facing left, as you have drawn it). Passing the supply traces between the opamp legs as shown gives you easy access to all the signal pins on either side of the opamps. Those extra ground traces out of the supply header is where you would connect to the audio grounds of each channel.
Behringer is poor mans brand, you can't get cheaper and better than them.
If you want poor man's Behrinher, that means aliexpress trash.
If you want poor man's Behrinher, that means aliexpress trash.
I haven't checked all the traces but, as a general observation, you have plenty of room to make the PCB much narrower by bringing the opamps closer together and moving components to all that empty space behind the top and bottom connectors. It's not so much a matter of PCB waste or aesthetics (you may not care about either) as of keeping the traces as short as possible, reducing parasitics and the chance of picking up noise.
Also it looks like even a small knob on those pots may block the connectors. Figure out which size knob you want to use, bear in mind that jack plugs can be quite thick and then space the connectors and pots accordingly.
Also it looks like even a small knob on those pots may block the connectors. Figure out which size knob you want to use, bear in mind that jack plugs can be quite thick and then space the connectors and pots accordingly.