How to solve the biggest challenge in headphone dr

How to solve the biggest challenge in headphone driving

nowadays, when connecting headphone amplifiers, we often hear such ostentatious emphases as "zero capacitance" or "no capacitance". At present, several such solutions have appeared in the market, all of which are quite radical based on several different technologies. The advantages and disadvantages of these solutions are not always so obvious - ironically, compared with traditional circuits in the past, some of the most attractive solutions actually require more capacitors, but they have advantages in some aspects, such as power consumption, burst suppression and startup time. This paper will discuss these problems in depth and give a reasonable choice of solutions

1. The problem of using capacitors

Figure 1 shows a traditional headphone driver circuit. Its left and right channel output amplifiers use a single power supply VDD, and the DC voltage at its output is located at the midpoint of the power rail, that is, vdd/2. In order to eliminate this DC voltage, two capacitors are inserted behind the amplifier

Figure 1: the traditional Headphone Drive circuit

usually uses electrolytic capacitors or tantalum capacitors, while the common capacitance value is 220 μ F。 The frequency response of the circuit to the low-frequency signal is determined by the two electric (2) measuring the size of the sample with a vernier caliper, the capacity of the container and the impedance of the headset, and the tone below the cut-off frequency fc is attenuated. For 220 μ For the capacitance value of F, when the impedance of the earphone used is 16 ohms, the cut-off frequency of the circuit is 45Hz, while when the impedance of the earphone used is 32 ohms, the cut-off frequency is reduced to 22.5hz. Less than 220 is not expected μ F capacitance value, because it will refer to the low-frequency cut-off frequency of the circuit, resulting in the loss of the bass part, which is a problem. Even if the most advanced signal processing technology is adopted, the loss can only be partially compensated and corrected

although the capacitor manufacturing technology is also constantly improving and improving, it still lags behind the pace of rapid reduction in the volume and cost of consumer electronics caused by Moore's law. As a result, only these two 220 μ The capacitor of F occupies most of the space on the personal media player or circuit board. Nowadays, although some compromises can be made in the physical size, height and cost of capacitors, traditional solutions ultimately cannot meet the requirements of most applications. This is the main problem of the circuit shown in Figure 1

there is another less obvious problem when starting this circuit. Before starting, the voltage on all circuit nodes is 0V, and the two capacitors begin to be charged. However, during normal operation, the left voltage of each capacitor is vdd/2 (dc term), while the right voltage stays at 0V. To achieve this state, a current must be driven to charge it through the capacitor. In this way, if the output of the amplifier instantly jumps from 0V to vdd/2 during startup, there will be a large short-term current spike in the charging current. Because any current passing through the capacitor will pass through the headset, which will produce a large explosion noise, which is unacceptable in today's market. Of course, by delaying the increase of the output level of the amplifier, the amplitude and swing rate of the charging current can be reduced, so as to reduce the noise to an inaudible level, but the cost is to greatly increase the startup time. This is a big defect, because voice playback is usually part of the user interface, for example, to confirm whether a key is pressed or whether an option is selected. The long delay between such user input events and the expected confirmation tone will make the system clumsy and slow to respond

for end users, both blasting noise and the response speed of user interface are crucial, which makes the system designer in a dilemma. However, it's a little surprising that many people keep the power of the headphone amplifier on, even when they don't need it, in order to avoid excessive startup delay. This practice undoubtedly increases the standby power consumption, thus violating the strict and refined power management criteria that have become common in battery powered systems. During audio, the provider responds to active power management by providing a low-power standby mode, offsetting the output of the amplifier to vdd/2, thereby consuming less power than during playback. However, this is only an imperfect anti-corrosion solution, because it requires a condition that the VDD power supply voltage must always be provided, that is, the voltage regulator that generates the power supply voltage VDD cannot always be turned off, which will also shorten the service life of the standby battery

in general, the traditional headphone driver circuit forces system designers to adopt a compromise, which should be increasingly accepted. The first is to make a compromise between the physical size of the capacitor, the cost of the capacitor and the low-frequency response of the system; Secondly, we have to make painful choices between blasting noise, long startup time, high standby power consumption and increased additional costs

2. "Virtual grounding" scheme

an alternative solution without capacitors is shown in Figure 2. Here, a third amplifier is added and connected to the ground wire post of the headset (that is, the socket of the general TRS connector). As a virtual ground, it provides vdd/2 DC voltage without AC component. The left and right channels are unchanged from the traditional circuit in Figure 1. Since the DC voltage difference between the left or right channel and the virtual ground is 0, it is no longer necessary to isolate the DC capacitor

Figure 2: capacitive Headphone Drive circuit with virtual grounding

this solution has three advantages. First, it is smaller than the traditional circuit, lower in height and cheaper in price; Secondly, its low-frequency response is flat, which ensures the accurate reproduction of bass; Finally, the starting time is small, because there is no need to charge the isolated DC capacitor. Audio device providers have provided this virtual grounding solution for several years, which is known in the market as "Pseudo differential", as well as "no output capacitance" and "virtual ground". At present, many OEM manufacturers have adopted this solution, including some well-known brand companies

however, this solution is not without problems. One of the disadvantages is the increase in power consumption caused by the addition of a "virtual ground" amplifier. Assuming that the class B amplifier with small output amplitude and resistive load is used, its power consumption is equal to the sum of the left channel amplifier and the right channel amplifier, that is, the total power consumption of this circuit is twice that of the traditional circuit under similar conditions. Even with full-scale sine wave, the power consumption of the virtual grounding solution is more than 64% (2/pi) higher than that of the traditional circuit. This result will greatly shorten the playback battery life, no matter how much volume is used

another problem occurs when headphones are used as line output. In portable systems, there is no separate line output socket. End users often use commercial adapter cables to connect the headphone output to the line input of home high fidelity systems or docking stations, where components made of composite materials are composite materials. Since both ends of the adapter cable must be grounded (the portable system will be grounded through a charger), it is virtually connected directly to the real ground. As a result, the audio signal in case of short circuit cannot be transmitted correctly. At present, the headphone amplifier can withstand short circuit for any long time, and the probability of permanent damage is very small. However, from the perspective of reliability, it is obvious that this is still not an ideal solution. In general, virtual grounding is an available alternative to traditional circuits in many applications, but it still has its own shortcomings, so it cannot become the standard solution in the industry

3. Reference ground solution using reverse charge pump

in order to solve the problems of traditional Headphone Drive, it does not bring new problems. It needs a reference ground amplifier whose output voltage is centered on 0V. This amplifier needs a symmetrical power supply, which consists of a positive power supply and a negative power supply, and the voltage amplitude of the positive and negative power supplies is equal. Since there are few negative power rails in consumer electronics systems, some component providers have integrated charge pumps into their audio ICs, as shown in Figure 3. This solution is currently being adopted by several manufacturers' brands

Figure 3: reference ground Headphone Drive circuit

ironically, this solution requires more capacitors than traditional solutions, including a capacitor at the input and output of the charge pump, as well as a "flyback" capacitor. (sometimes, the capacitance at the input end is not indicated in the short IC data page, which is actually needed to compensate for the non ideal transient response in the actual power supply that is not directly caused by the charge pump). Obviously, for this kind of circuit, "no capacitance" is inappropriate. However, since the capacitance of these capacitors is only a few Micromethods, they are much better than the two 220 Micromethods in the traditional circuit. Unlike the virtual earth solution, this circuit provides a real ground output, which can be used for various line outputs without any restrictions. With this technology, we can produce very attractive packaging products. This circuit can work even when the battery voltage is very low, because the charge pump doubles the voltage swing of the amplifier

for the reference solution, the main remaining problem is power consumption. At low volume, the efficiency of the charge pump is limited to a lower value by the switching loss, while at high volume, its efficiency is limited by the interconnection resistance on the chip and the physical size of the switching device (increasing the chip means increasing the cost). In addition, some amplifier designs cannot tolerate the power ripple generated by the charge pump, making some providers add LDO voltage regulators to eliminate the ripple. The output voltage drop of LDO voltage regulator further introduces losses. In general, the power efficiency of most reference solutions is only about half that of traditional solutions, which shortens the life of playback batteries

4. More advanced reference ground solution

how to solve the problem of low power efficiency in the reference ground Headphone Drive solution is becoming a hot topic in the field of low-power audio. Class G amplifier architecture is used to solve this problem uniquely. In this architecture, the power supply voltage will be adjusted according to the volume of the audio signal. However, the reverse charge pump with fixed output voltage does not support this class G amplifier architecture. Wolfson microelectronics company introduced the first reference ground class G amplifier -wm8900, which uses a unique charge pump design with two inputs to solve this problem. They are connected to different power supply voltages in most battery powered devices today, so that the charge pump can produce two different output voltages

as the latest solution of Wolfson company, it is called "W" and is integrated into wm8903 audio modem. In this solution, the charge pump has only one single power input, which is usually connected to the 1.8V power rail, but it has two power outputs, VPOS and vneg, respectively, so as to provide power for the amplifier