Because a few milliwatts will drive headphones to full volume, a great headphone amplifier design can be relatively simple. Yet, there are any number of reasons for experimenting with more complex topologies, such as improved performance and the ability to incorporate custom options. Of course, some DIYers like to try different circuits just for the fun of it. The major disadvantage of complex circuits is that they are complex. It can take a DIYer months to locate and purchase the parts, not to mention the time to assemble and troubleshoot the project.
Integrated circuit opamps are both complex and simple. They may contain hundreds of components on a chip, but are relatively easy to configure. For DIYers short on time and patience (and few people have the luxury of both), opamps are a convenient entre into the world of complex design. The audio cognescenti have attacked opamps as being one of the major causes of "mid-fi" sound, but if the truth be known, they are lurking everywhere - even under the covers of prestigious high-end gear. Opamps are not all the same. Building a headphone amplifier with good sound is a matter of careful selection and design.
This article discusses several opamp-based headphone amplifier circuits, including suggestions for selecting opamps, input coupling and filtering, high current output stages and power supply options. There are no recommendations for specific opamp brands or models. For tube devotees, there is also an introduction to designing with tube amp-blocks. Tube amp-blocks (AC feedback amplifiers and tube opamps) are not as compact as their silicon brethren, nor do they measure as well, but they do offer smooth tube sound with the ease of feedback configuration.
SELECTING SOLID STATE OPAMPS
Entire books cover the subject of interpreting opamp specifications. Here are a few guidelines for choosing opamps when designing headphone amplifiers. Opamps inch closer to the "ideal" with every succeeding generation. Modern devices are internally compensated for stability, have slew rates going through the roof and noise and distortion numbers at threshold of measurement. There are even opamps that will run off a 1-volt supply. For portable devices, the power supply requirements should be the first consideration. The majority of modern opamps will run with as little as ±4V, but low voltages may degrade performance. Check the manufacturer´s VCC specs to confirm that low voltage operation is, in fact, recommended. The most common battery supply voltages are ±1.5V, ±3V, ±4.5V and ±9V. Single supplies are another possibility. Keep in mind also that the idling current for the entire amplifier must also be low - around 10mA or less for good battery life. For more information, see the section on battery power options below.
Opamp performance specifications are an unreliable indicator of sound quality. So long as the numbers are below audibility thresholds, specs that are magnitudes better than the averages will not necessarily translate into better sound. Regardless of type (bipolar or FET), modern opamps do very well on the test bench. Total harmonic distortion figures are so low (typically less than 0.1%) that datasheets have stopped listing them. Look for noise specifications, listed as "noise density" in units of nV/Ö(Hz), of 25 or less, slew rates of 5uV/sec or more and "wide" unity gain-bandwidths of 3 MHz and higher.
When reviewing the gain-bandwidth specification of a bipolar-input opamp, also examine the open-loop bandwidth. The gain-bandwidth defines the amount of small-signal gain at any frequency and is the product of the open-loop bandwidth and the open-loop gain. Most opamps have a high open-loop gain (100dB or more) and a relatively narrow open-loop bandwidth (100Hz or less). In a multi-stage system with overall feedback, if the opamp has a bipolar input stage and narrow open-loop bandwidth, it can manifest dynamic phase shifts and other response non-linearities with high level, high frequency input signals.
To reduce this type of distortion, choose a bipolar-input opamp with a wide open-loop bandwidth (into the kHz range) or use a FET-input opamp. FET input stages are more linear and so less susceptible to this type of distortion. Finally, the open-loop bandwidth of the voltage-gain input stage can be effectively extended with local feedback (see the section on output stages below).