A variety of headphone amplifier types are available, but little guidance is provided in their selection. This article discusses the types available, and covers their relative strengths and weaknesses. Real world examples are discussed to demonstrate good guidelines for selecting the best amplifier type for many applications.
DESCRIPTION OF HEADPHONE AMPLIFIER OUTPUTS
Headphones have been a versatile method of listening to music since the earliest days of amplified sound. Over time the form factor of headphones evolved into a relatively common setup. Almost 100% of headphones use a 3.5mm (nominally 1/8") three-terminal connector known as a TRS (tip-ring-sleeve) connector. To a much lesser extent some headphones are provided with a larger ¼" (nominally 6.3mm) format TRS connector.
A diagram of a TRS connector is shown in Figure 1. The commonality of this connector is one of the key strengths of headphones; they are interchangeable between virtually every modern piece of audio and multimedia electronics.
Figure 1. A tip-ring-sleeve connector.
Unfortunately, this commonality makes one of the weaknesses of this approach just as widespread as the usage of these headphones: the two speaker elements share a single common (or ground) connection. The reason this is a weakness is that the vast majority of portable consumer electronics operate with a single positive power supply source. The AC nature of audio forces the electronics to perform some sort of level shifting between the single polarity of the power supply and the dual polarity of the audio signal.
Capacitor-Coupled Amplifier Outputs
A headphone amplifier operating from a single positive supply can easily drive a ground-referenced headphone by using coupling capacitors. An example of this approach is shown in Figure 2. A dual amplifier, biased at mid supply, drives the stereo headphone via a pair of coupling capacitors.
Figure 2. A capacitor-coupled headphone amplifier.
Selecting the value for these capacitors is relatively straightforward, given by the following equation:
CMIN = 1 / (2 π fC ZL)
where CMIN is the minimum capacitance value, fC is the desired low frequency cutoff, and ZL is the impedance of the headphones. For example, if a cutoff frequency of 100Hz is desired, with a load impedance of 16Ω, then use a 100µF coupling capacitor. In practice, the next larger value in a standard size is often selected to ensure adequate bass response and to push the distortion at frequencies near the cutoff to a lower frequency.
Because the polarity of the level shifting done by the output capacitor is known, a polarized capacitor is usually selected. Even though polarized capacitors have high C-V density, they still are the largest object in the amplifier system, larger than the connector itself.
Although a standard operational amplifier can be used for a capacitor-coupled headphone amplifier, it would typically have a loud click or pop when the amplifier is powered up and powered down. An amplifier intended for this application, such as an LM4810, has a controlled power-up and power-down sequence to virtually eliminate click and pop.
A capacitor-coupled amplifier is typically the cheapest headphone amplifier IC, although the cost savings for the amplifier are somewhat offset by the cost of the coupling capacitors. A capacitor-coupled headphone amplifier is typically used where space is not of primary concern.