I designed the O2 Booster Board to plug in and replace the standard NJM4556A output chips (U3 & U4) in NwAvGuy's O2 Headphone Amplifier. The board fits in the same standard O2 B2-080 case, in the top slot above the O2 PC board. The batteries and ODAC can still fit in the case with the O2 Booster Board installed.
In NwAvGuy's writeup about his O2 Headphone Amplifier he noted that the NJM4556A output current-buffer chips are the limiting factor on THD+N since the first-stage NJM2068 gain chip has lower levels. He didn't state it explicitly but the O2's output chips are also the limiting factor on slew rate, at 3V/uS vs. 6V/uS for the NJM2068 gain chip. I discovered that space existed in the existing O2's case for another PC board in the top slot, if it was shaped just right. Then I discovered that there was just enough space between that top slot board and the top of the O2's case. With those discoveries the O2 Booster Board was born to address the shortcomings of the O2's NJM4556A chips and a few other O2 shortcomings.
The O2 Booster Board provides several benefits:
The build instructions for the O2 Booster Board covers each of these features in more detail and explains why they matter, along with providing further information about the project.
The O2 Booster Board FAQ is here..
The O2 booster board project is a do-it-yourself project I've posted out at diyaudio.com in the headphone forum. All the project files are at the Google Drive link below including schematic, layout, bill of materials (BOM) for purchasing parts from Mouser or Digikey on the internet, build instructions, photos, Gerber files (to fabricate the PC board), and part ID diagram. In the DIY Audio forum I've posted a full photo build guide.
Bare PC boards, PC boards with all the parts soldered on, and entire O2 Headphone Amplifiers with the O2 Booster Board installed are available. If Mouser and Digikey are out of stock of any part for the Booster Board I probably have that too, although I don't sell complete parts kits.
For prices on O2 Booster Boards please see here.
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