Simple Op-Amp-based Headphone Amplifiers

Op-amps are the obvious solution, but which ones. While many op-amps could be pressed into service, a few stand out as good candidates, for example the AD822 for low-voltage power supplies is a good choice, although it will not meet our 100mA peak output current specification. The AD842 is a beefier amplifier that works with gains higher than 2, but requires a beefier power supply. If we are willing to cascade a low-output-current op-amp into a high-current unity-gain buffer, then the number of suitable op-amps grows. The advantage of this approach lies in the potential optimization of op-amps for two different tasks: low noise and high output current. Many of the FET input op-amps are wonderfully quiet: AD711, LF351, TL072, and others. And several unity gain buffers are available: AD811, BUF601, HA5002, LH002, LH0033 (discontinued, but excellent), LM310, LM6121H, LT1010, and MAX4178. The Google search words might be “Monolithic, Wideband, High Slew Rate, High Output Current Buffer.” (Because we demanding so much from the op-amp, we should return the favor and apply a small heat sink to its body.) Of course, once we find a suitable candidate, we will have to find a SPICE model for the device. The buffer’s output can be enclosed in the low-noise op-amp’s feedback loop or left independent of it. Including the buffer’s output in the loop will lower the buffer’s noise and distortion, but at the cost of an increased chance of instability at extremely high frequencies. (Download circuit version 4.2 Buffer HP amp.ckt; version 4.0 Buffer HP amp.ckt

It’s only a bit more work to design and test a discrete circuit, using only discrete active devices, although we will give up the ease and the automatic short-circuit protection offered by the op-amp. So the better approach might be to stick to ICs, but ICs have a few problems of their own. Most audio op-amps have been designed to have a minuscule idle current draw, which for the most part is a good, as this approach extends battery life and reduces heat. The disadvantage to the trickle of a quiescent current draw is that class-A operation barely exists and a lean class-AB is the rule. Because the op-amp has so great an open-loop gain, the crossover distortion is all but eliminated. But as we do not plan on powering our loudspeakers or warming our living room with a headphone amplifier, class-A operation has needlessly been discarded. Even the most extravagantly designed headphone amplifier—single-ended, class-A, constant-current source loaded headphone amplifier—is not that big a deal, as even a battery powered amplifier would give a fair battery life expectancy. On the next page is a headphone amplifier that was designed in B2 A/D Spice and shows much promise.