ABSTRACT
This chapter focuses on techniques for driving loads heavier than can be handled by a single 5532 or LM4562 opamp, basically any impedance less than 500 Ohm. This is typically a headphone load, the impedance of headphones varying widely from 600 Ohm to 16 Ohm or less, with sensitivity inversely related so the higher impedance models need more drive voltage. When lower headphone impedances are to be driven, the number of multiple opamps required can become unwieldy, taking up an excessive amount of PCB area, and it becomes more economical to adopt a hybrid circuit. Most headphones will need to be rewired for balanced use, as the typical arrangement for high-quality headphones is to have a 3-wire cable running from a 1/4-inch jack plug to one ear-piece, with a 2-wire cable running from there to the other ear-piece. Four-pole jack plugs do exist, but these are usually in the 3.5 mm size for mobile phone use.
