Omicron Headphone Amplifier: Background and Intro

Omicron is a compact, low distortion headphone amplifier that I have been developing jointly with @Rus2000 from the RCL-electro.ru electronics forum. 

I wanted a good sounding headphone amplifier that would meet a few formal requirements:

• Distortion meaningfully - say by an order of magnitude - lower than that of a reasonable digital source, across the audio band. That means not more than -130dB of distortion.
  
• Suitable for headphones from 32ohm (i.e. Grado) to 600ohm (Beyerdynamic T1)
• At least 100mW of output power to deliver 120dB SPL peaks with the typical 100dB/mW headphones
• Stable with reasonable capacitive loads, say up to 10nF
  
• Common, easily available, inexpensive parts that will not disappear from the market tomorrow (or in a year)
• Usable by itself, without a hodgepodge of boards - that is, cross-feed, fast DC protection and turn-on delay and  EMI protection are all integrated on board
• Compact - doesn't need to fit into an Altoids can, but shouldn't be a full 17-inch box either
• Simple and easy to build, a comfortable weekend project

There are quite a few headamp projects around, yet it is not easy to find something that ticks all the boxes above. Low distortion and garden-variety parts seem to be two most difficult and often conflicting requirements. After looking around for some time, I decided to roll my own.

This is what we got:

• Outstanding linearity: Distortion better than 100 parts-per-billion (-140dB, 0.000 01%)
• Compact: Just one IC and two transistors per channel
• Inexpensive, commonly available parts (NE5532, BD139, BD140), easy through-hole construction
• Functionally complete: One 80x110mm board carries two channels, DC protection and an optional cross-feed circuit
  

Simplified schematic:

Some distortion measurements:

In the next post, I will discuss Omicron's amplifier circuit.