Headphones have been ubiquitous since the very early days of audio electronics, and today’s trend towards portability makes them even more popular. Millions of headphone amplifiers in portable audio systems play a crucial role in determining the sound quality that these systems achieve in everyday use. However, because they are now usually built into audio ICs along with many other functions, their importance is often overlooked. What, then, makes a good headphone amplifier?
Firstly, since portable audio systems are often used in noisy environments, adequate output power is critical to ensure that users can hear the audio signal at all. Insufficient volume can be a problem with amplifiers running on low supply voltages—below around 3V, it becomes difficult to generate a signal that is strong enough. On the other hand, too much power can be a bad thing, too. Some governments have enacted legislation to limit headphone volume in consumer equipment, and manufacturers intending to comply need to restrict the maximum output power. Keep in mind that the electrical power delivered to headphones depends on their impedance (power P = Vrms2/Z, where Vrms refers to the AC voltage across the headphone and Z is impedance). Since 16Ω and 32Ω headphones are both very common, designers of systems with standard headphone sockets need to accommodate both impedances. Moreover, the actual sound pressure level (SPL) produced by headphones depends not just on their electrical power input, but also on their efficiency, which varies widely.
The second factor in creating a pleasant user experience is sound quality, which is measured in metrics like signal-to-noise ratio (SNR) and total harmonic distortion (THD). A headphone amplifier with poor SNR or THD can ruin the sound quality of an otherwise excellent system. However, since the amplifier is just one part of a signal chain, improving its characteristics only makes a difference as long as overall performance is not limited by another part of the chain--e.g. the quality of the recording being played, the MP3 decoder, the digital-to-analogue converter (DAC) and most importantly, the headphone itself. Ideally, the SNR and THD values for each element should be at least a few decibels better than for the weakest link in the signal chain. THD specifications should be read very carefully, as they often depend a great deal on the output power. For example, THD figures quoted with a 10kΩ load are meaningless for a headphone output because a 16Ω headphone load will draw much more power and thus increase distortion (see
Figures 1a and 1b).
Click to See Image
Figure 1(a and b): THD strongly depends on output power, as shown in the graphs of THD versus power for 16Ω and 32Ω loads.
After establishing the basic specifications for the amplifier, designers then need to select appropriate external components. Many headphone amplifiers require a capacitor in series with each transducer in order to remove the DC part of the amplifier’s output. This prevents large DC currents from damaging the headphones, and reduces the amplifier’s power consumption. These capacitors need to be quite large in order to ensure that bass frequencies can pass through (Figure 2).
Figure 2: Headphone output design using DC-blocking capacitors.
For example, a bass cut-off frequency of 22Hz with a headphone impedance of 32Ω requires a capacitance of around 220μF on each channel (cutoff frequency fc = 1/2πRC). However, as soon as a 16Ω headphone is plugged in, the cut-off frequency doubles, deteriorating the bass response. In order to achieve hi-fi performance on both types of headphones, the capacitance must be doubled. Each of the capacitors then takes up more PCB space than the audio IC itself, and their height makes the PCB difficult to fit into thin enclosures. It is possible to counteract the effect of small headphone capacitors with a bass boost circuit. However, in order to produce a perfect linear frequency response, the bass boost characteristic must be exactly the opposite of the headphone-capacitor combination. Since the headphone impedance is usually not known, it becomes impossible to know where the bass-boost cut-off should be, and the end result is a non-linear frequency response. Moreover, bass boost circuits often increase distortion at high volumes.
Headphone capacitors can be eliminated with headphone amplifiers where the DC voltage between each channel and the headphone ground is zero, or at least very close to zero. One approach is to center the amplifier’s output voltage around 0V, so that it does not have a DC component. However, such amplifiers need symmetrical positive and negative supplies, which are usually not available in portable systems. Although a charge pump or DC-DC converter circuit can generate an extra supply voltage specifically for the headphone amplifier, the extra cost, switching noise problems and higher power consumption often make such solutions unattractive. An alternative method is to bring the headphone ground up to the same DC level as the left- and right-channel outputs (Figure 3).
Figure 3: DC-coupled headphone output design.
This has the advantage of increased power supply rejection and works with a single supply voltage, but requires an additional output buffer inside the audio IC, which consumes extra power and thus reduces battery life.
It is worth noting that the issue of headphone capacitors only arises with three-pole headphone plugs where the left and right transducers share the same ground wire. Manufacturers who use proprietary plugs can avoid the problem altogether by driving headphones differentially. This is common practice with loudspeakers in portable systems, and has the additional advantage that higher output power can be achieved with the same supply voltage and power supply noise rejection is improved. However, the pressure to maintain compatibility with standard connectors is too high for most manufacturers to take this route.
Powering the amplifier
Analog audio circuitry is notoriously sensitive to the quality of the power supply. Since ripple and switching noise generated by nearby digital circuits can degrade the audio signal, high-fidelity designs generally use a dedicated voltage regulator to produce a clean supply for the analogue section of audio ICs. This solution suits audio ICs with modest power requirements. However, headphone amplifiers draw relatively large peak currents, increasing the size, cost and heat dissipation of the regulator.
Separating the headphone amplifier’s power supply from the rest of that of other analogue circuitry can solve this problem. In systems powered by single-cell lithium batteries, for example, headphone amplifiers can run directly from the unreg-
ulated battery voltage. This voltage varies between roughly 3 and 4.5V depending on the battery charge, and contains noise from other circuit blocks. The headphone amplifier must therefore be able to withstand supply voltages above the 3.3V typically used for audio functions. It also needs a good power supply rejection ratio (PSRR) to prevent supply noise from affecting the headphone signal.
Regardless of how the headphone amplifier is powered, it is always a good idea to place a decoupling capacitor as close to its supply pins as possible. This ensures that sufficient electrical charge is available to accurately reproduce signal peaks and transients. The net result is lower distortion, especially at high volumes.
Using headphone sockets as line-outs
End users often connect portable systems to their home hi-fi systems by plugging an adaptor lead into a standard 3.5mm headphone socket. The performance requirements for the headphone amplifier are more stringent in this case because home environments are less noisy than outdoors, and because overall sound quality is no longer limited by the quality of the headphones. Fortunately, since distortion decreases at higher load impedances and most audio amplifiers have high-impedance inputs, the headphone amplifier’s THD figure improves “automatically” when it is used in this way. Noise, on the other hand, varies much less with loading.
Another concern arises with capacitor-less headphone amplifiers where the headphone ground is not at the same DC level as the system ground. In the unlikely event that the portable system is grounded through a battery charger, and its headphone socket connected to a home stereo whose input is referenced to the same ground level, the headphone ground is effectively short-circuited to the system ground. Depending on the amplifier design, this may cause, at best, a malfunction, or at worst, damage to the headphone amplifier.
Selecting a headphone amplifier
Although headphone amplifiers in portable systems are often just one function in a highly integrated audio IC, their characteristics are still vital to the end user’s listening experience. Headphone amplifiers need to deliver sufficient power into headphones of different impedances, mainly 16Ω and 32Ω. They must also achieve SNR and THD figures appropriate for mobile use at these same load impedances and at the intended output power. If it is anticipated that the system is used as a component in a home stereo, the headphone amplifier should also achieve the higher performance levels that users expect of home systems. External component requirements should be evaluated carefully, as they can have a strong impact on a design’s physical size and weight. Their cost can also contribute significantly to the final bill-of-materials cost. Headphone capacitors are the prime culprits, and although no capacitor-less circuit architecture is perfect, designers should always examine the available options for eliminating these capacitors.
About the Author
Yan Goh is Product Marketing Engineer at Wolfson Microelectronics, www.wolfsonmicro.com with specific responsibility for portable products. Prior to joining Wolfson in 2004, he worked at NewLogic Technologies for four years in sales and marketing positions, in the US and Austria. He has also held IC design positions at Infineon and Tritech Microelectronics. He holds a B.Eng Hons in Electrical and Electronic Engineering from the University of Edinburgh, and a Diploma in Electronics and Communication Engineering from Singapore Polytechnic.