Wednesday, June 8, 2022

Omicron Headphone Amplifier: Background and Intro

Omicron is a compact, low distortion headphone amplifier that I have been developing jointly with @Rus2000 from the 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.