DirectDrive delivers better audio quality

Most traditional headphone amplifiers used in battery-powered electronics are single-supply devices operating between a positive voltage and ground. This design results in an amplifier that can pass only positive signals. Audio signals, however, inherently swing positive and negative. So before a traditional single-supply amplifier can accept the audio signal, a DC bias must be added. This DC bias is typically half the supply voltage to allow Maximum signal swing.

While a DC bias is necessary for amplification, speakers require a signal without DC bias. When a DC bias is applied to a speaker, the speaker cone physically shifts from its neutral point to a point that is closer to the maximum excursion of the cone on one side. This means that the speaker can no longer generate as much sound pressure without distortion. The DC signal also causes DC power to be dissipated in the voice coil, thus wasting power and unnecessarily heating the speaker. In extreme cases, this heating can permanently damage the speaker.

High-pass filter
To prevent the DC bias from reaching the speaker, a DC-blocking capacitor is typically used. This capacitor combined with the mostly resistive load creates a highpass filter. Since typical headphone loads are equivalent to a 32-ohm resistor, the capacitor must be quite large to avoid blocking the desired portion of the audio bandwidth. If full extension down to 20Hz is required, at least 250μF of capacitance must be used to ensure no more than 3dB of attenuation at 20Hz. If 16-ohm headphones are used, then the DC-blocking capacitor must be at least 500μF. While some systems have enough space for relatively inexpensive aluminum electrolytic capacitors of this size, most portable devices do not have room for such capacitors. In these systems, more expensive tantalum capacitors must be used to save space. But even these capacitors consume significant board real estate. Finally, smaller capacitors are often used to save space and cost, but it sacrifices flat frequency response to 20Hz.

DirectDrive headphone amplifiers eliminate the need for a DC-blocking capacitor on the output by eliminating the DC bias. Even though a DirectDrive headphone amplifier operates from a single supply, the amplifier passes both positive and negative signals. The negative swing is achieved through an onboard charge pump used to generate a negative supply that tracks the amplitude of the positive supply. With a dual supply now available, the amplifier is no longer required to operate from a single supply—it can be ground-biased.

The charge pump used in a DirectDrive design requires only two small ceramic capacitors to operate: one flying capacitor and one hold capacitor. These capacitors are typically 1μF and can be as small as 0402. This specification results in significant space savings compared with a conventional headphone amplifier with 220μF output capacitors, and provides superior performance.

(a)

(b)

Two 1μF charge-pump capacitors (a) represent space savings compared with two 220μF output coupling capacitors (b).

Other advantages
Eliminating the output coupling capacitors required by a traditional headphone amplifier is clearly a significant advantage in solution size and cost. But the DirectDrive approach offers other benefits.

Click, pop suppression—One of the most apparent benefits of DirectDrive is the significant reduction in click and pop. With a traditional headphone amplifier, the output capacitor must be charged up every time the amplifier is enabled and discharged every time the amplifier is disabled. This charging and discharging process can create a noticeable pop because current must be pulled through the headphones. By eliminating the capacitors, DirectDrive thus eliminates a major source of click and pop.

Better bass performance—The highpass filter created by the DC-blocking capacitor and the resistive headphone load also have an audible effect. In most systems, including a capacitor that will allow for full 20Hz to 20kHz frequency response is not possible. To save space and cost, a smaller-than-ideal capacitor is used, thus increasing the low-frequency roll-off point and adversely affecting the system's bass performance. This performance deficiency is exacerbated when 16-ohm headphones are used. Typical systems are designed for 32-ohm headphones; when 16-ohm headphones are connected, the low-frequency corner is doubled, further cutting into the audible bass frequencies.

With the DirectDrive approach, however, the highpass filter is completely eliminated, thus allowing the input coupling capacitor to set the corner frequency. Since the input impedance of the amplifier is typically over 10k-ohm, only a 1μF or smaller capacitor is required to ensure full audio bandwidth.

Low-voltage operation—DirectDrive allows the headphone amplifier to operate directly from the supply used by the system's major digital ICs. By operating from a supply lower than the battery voltage, the headphone amplifier becomes more efficient. While 3.3V or even 2.5V supplies used to be common, 1.8V supplies are replacing these supplies. With traditional headphone amplifiers, only 10mW of output power (into a 32-ohm load) is theoretically possible from a 1.8V supply. DirectDrive amplifiers, however, double the supply voltage available to the amplifier, thus allowing up to 40mW of output power from the same supply voltage. Hence, the amplifier can operate as efficiently as possible in modern systems while still generating sufficient sound levels.

2V_RMS line output amplifiers—The doubled supply voltage that DirectDrive generates has a secondary advantage in systems that require a 2V_RMS audio output. Typically, these systems have a 5V supply readily available. But a 5V supply in conjunction with a standard amplifier is insufficient to output 2V_RMS—a higher supply voltage is necessary to achieve the 2V_RMS output level. With the DirectDrive approach, well over 2V_RMS is possible from a 5V supply, since the available supply voltage is already doubled onboard.

Reduced distortion—Finally, the output capacitor used in a traditional headphone can contribute significant distortion to the audio signal at low frequencies. Near the low-frequency corner, the voltage coefficient of the capacitor causes it to become nonlinear and introduces distortion into the audio signal. In some cases this distortion can be as much as 1%, which is both audible and measurable.

Eliminating the coupling capacitor—By eliminating the coupling capacitor, DirectDrive eliminates this source of distortion.

- Adrian Rolufs
Strategic Applications Engineer, Conventional Headphone Amplifiers
Maxim Integrated Products Inc.