Collector in me (read hoarder) has bought way too many GB's lately again, so time to clean up the desk a bit:

DMG-01 Yellow, Pro sound mod (both 3.5mm jack and RCA), backlight + bi-vert (or at least that what was the seller told me, does look like plain backlight to me) 50€

Game Boy Color: Oh boy this has seen better days for sure (Please check the pics). Partly spray painted like young Banksy on drugs, Bob Ross did not approve. Un relevant sticker attached. But in the end of the day it has TFT backlight screen, that is something right? 40€

GBA SP 001, has scratches on front lid, otherwise looks even nice to me. Random Ebay "fresh" battery installed, does not come with charger or headphone adapter. 40€

USB boy, small and nice, working unit. If you need just midi via usb to your computer this is the right thing to get. 40€

EMS USB 64 cart with lsdj sticker, what can I say about this? works. 50€

Postage cost will be added, preferably only shipping to EU. If you insist, talk nicely and/or bribe we can talk.
Payment via PayPal or IBAN

There is also deadpanrobot and retrosix selling stuff in UK

DMG is good choice to get you started, i recommend to get one with some sort of backlight or mod one yourself (new IPS screens are really easy to install) also prosound mod is good to have.
People tend to use GBA SP and color when they need more CPU power (complex tables, fast speeds etc)
You also need flash cart if you are going the LSDJ route or buy Nanoloop cart if you prefer that.
You might also want to say where you are located. Ebay and similar are full of pre modified Game Boys, depending what you are looking for.

Hosting the images on behalf of vickysbooties:

Any chance you could upload some pics?

You can also test midi input with keyboard mode in LSDj (Using channel 16 and keyboard mode from arduinoboy) check the note values from arduinoboy repo.
If that fails I can upload you serial print gb rom that can print what gameboy receives (Unless you have GBDK installed and can build the rom yourself)

This problem seems to go down to this commit: … d560e23821
I cannot see this commit in official Arduinoboy repo.
Maybe catskull could shed some light in here, is this cherry picked from somewhere else or was it just removed from master?

Ah, just checked your screenshot. You are missing
#include <PS2Keyboard.h>
line after #else and before // values for the PS/2 Keyboard input

Did you change the code or did you pull this recently? As last change is almost month old to repo.

Can you verify that your board selected is UNO and you have #include <PS2Keyboard.h>
in UNO part of the code (Marked with * Arduino UNO/Ethernet/Nano (ATmega328)) followed with #else

Arduinoboy:249:1: error: 'PS2Keyboard' does not name a type
Error means that compiler does not see PS2Keyboard defined anywhere, so something is wrong with PS2Keyboard include.

Did you install the PS2 library to arduino libs folder mentioned in github?

"Added dependency on this library to handle PS/2 communication. To install, download the library, and extract it into your Arduino Libraries folder (typically Documents/Arduino/libraries). Do not attempt to install a PS/2 library from the Arduino library manager as that version will not work with this code."

Installed that and could flash it on my arduino r3 board.

CleanPower V1 does not work with flash carts. On Hand Held Legend page some one talks about adding one diode to make it work, but no instructions. " Added a (MUR160G) diode, like the instructions said, so that flash carts could get enough power."

It's usually easy to figure out oddities like this just by looking the signal

Probably google drive (check permissions) or dropbox? Audio interface should not have any problems with game boy raw signal.
EDIT: What is your audio interface btw?

Can you post recordings from busking amp and without? Sure you are not recording mono?
I usually record just straight from game boy to audiointerface (scarlett 2i2)

As fast test you can just do two instruments panned left/right and check how it looks on audacity or similar.

Second what frantic said. Check your cables.
Record something from your phone to audio interface, sounds ok? Connect headphones to your game boy, sounds ok?

Resistors do not have polarity, so does not matter. For troubleshooting, have you measured that your opamp gets valid voltage?
You can also check all the connections with continuity test on multimeter. Also add pic of the bottom side of your build.