[gastro54]

I'm doing this project as a personal learning experience and plan to open-source everything (with the requisite disclaimer: if you build this and destroy anything, its not my fault)
 
Most of my design experience is in the digital realm.  I'm trying to improve my analog layout and circuit design - please provide feedback!  It is a dual headphone amplifier based on nwavguy's O2 amp. I wanted to make something nice for listening to music or watching a movie together in an airport / plane / quiet place.  There are two headphone outputs with independent volume control and output-stage opamps. 
 
Some notable differences: Only one 4556 opamp per output jack - so the potential output current per headphone jack is roughly 1/2 of what the O2 is capable of.  Since this is designed to be used in portable situations where battery life is critical, I thought this was an OK concession.  Use of surface mount components.  This means thin film SMT resistors such as RT0603FRE07100RL and others from that series in the signal path.  I was really excited about getting an initial prototype built, so I omitted some of the more practical features present in the O2 such as:

1. switchable gain

1. wall power
2. low voltage battery protection: this device could become dangerous near the end of uneven battery discharge
3. power LED indicator
4. the ability to remove power from one of the headphone output stages (n/a to the O2)

Schematic
https://docs.google.com/file/d/0B1UlVoMq1FqlQXNoekRCZ1hRZmc/edit?usp=sharing
 
Bill of Materials
https://docs.google.com/spreadsheet/ccc?key=0AlUlVoMq1FqldFVGcDd6SzY4SGcwOGVFU01aUFpJb3c&usp=sharing

Layout
Bottom layer is a minimally interrupted ground pour.


3D render

 
I'm getting 3 boards made by oshpark.com.  They are being fabbed as we speak!

 

[gastro54]

 

[HolyCheese]

It looks neat! How does it sound?

 

[gastro54]

It sounds great!  I've only tested it with Visang VS-R03, Vsonic GR06, MDR-V6, and Koss DJ100s.  I'm in the market for purchasing some nicer headphones that will really benefit from dedicated amplification.  I'd like to get it measured as well.  I have a lot of ideas for improving the next revision of the PCB.
 
One of the secondary functions of the battery protection circuit that I omitted was to minimize turn-on transients, so as it currently stands, this version of the amplifier has a pretty bad turn on transient that I discovered when powering it on with a set of sacrificial MDR-V150s connected to the output.  This results from uneven rise times of the dual positive and negative rails after they are connected to the rest of the circuit with the mechanical latching switch.  Knowing this, I was mindful to insert my more valuable headphones into the output jacks only after the amp was powered on.  This problem is obviously unacceptable and will be corrected in future revisions.

 

[Punnisher]

Very cool project. I love my O2 so this is exciting.
 
If I had the electrical engineering knowledge I would design an amplifier similar to this but with an emphasis on desktop performance where saving power isn't an issue.
 
Keep up the good work and post updates!

 

[proid]

Hi all! I want to make a custom o2 that is as small as possible too (only using batteies) but i dont use smd part so i want to ask if the filter section for batteries is really need or can i omit it? Like in the Grado Ra1, there are no filter capacitors too.