The number of portable devices has increased explosively, and many new functions have been added. Portable devices with external speaker audio playback functions are increasing, such as MP3 players, mobile phones with MP3 functions or speakers, and portable CD players. The output of these systems varies depending on the configuration and drive. But the output of these systems differs in configuration and drive. In this article, the author discusses several methods for detecting and deploying single-ended / differential audio amplifiers that can amplify input signals to output equal signals.
The output of an MP3 player or mobile phone is a single-ended signal that can drive a 32Ω headphone speaker. The speaker impedance of a typical external speaker system is 4 to 8Ω, and there may be multiple speakers per channel. However, 32Ω drivers are difficult to drive these low-impedance speakers and cannot provide sufficient volume for end users.
The external speaker system differs depending on the quality, volume, and number of speakers, so general-purpose amplifiers are not suitable for driving these speakers. For example, the speaker system of an MP3 player has a headphone jack input and can support a single-ended stereo signal. Some new high-end speaker systems can support differential signals. For backward compatibility, these new systems must also support single-ended signals.
Because the differential signal is twice the single-ended signal, the single-ended signal and the differential signal will produce different volume. Since the relationship between human hearing and sound volume conforms to the law of the logarithmic curve, linear control cannot be used (Figure 1).
Figure 1: There is not a linear relationship between the volume and the output power of the audio system.
For single-ended / differential audio amplifiers that can amplify the input signal to a signal equal to the output, there are several ways to detect and implement it. The interface connector between systems should have at least 5 pins to provide differential signals. It should be noted that the common ground connection between the two devices is very important. On the surface, because the signal is DC-isolated, the AC coupling capacitor does not need to be grounded, but actual experience proves that grounding is necessary to provide ideal noise performance.
The first problem to be solved is to detect whether the input is single-ended or differential. In many circuits, there are two circuits that use an external pin of the connector to test the DC level of the input signal. It is easy to specify an external pin of the connector, but it is not feasible for space-constrained applications. The source device either opens the pin or shorts it to ground.
The second method of detecting differential signals is to use a comparator to detect the DC level of the signal, either grounded or differential. The input signal of the above two methods must pass through a low-pass filter. The original signal must be separated from 50% to 25% of its DC level. If the system is in the differential mode of low-frequency, peak-to-peak AC signals, it will cause erroneous detection results. This technique cannot be used if the DC level of the original signal is ground. Figure 2 shows an example of such a circuit.
The second part of the circuit is the audio amplifier. The solution of this circuit is related to the required sound quality. A true differential input can provide higher sound quality than a differential signal entering an amplifier. The actual differential amplifier requires an additional circuit to convert a single-ended signal into a differential input signal.
The easiest way to implement an audio amplifier is to input the signal into an amplifier (Figure 3). In single-ended mode, this differential input does not generate a signal, allowing the positive input to be set to 0.5Vcc, which is the standard single-ended input configuration. The analog switch is kept off, so that the amplifier outputs a gain of twice. In differential mode, the analog switch is closed and the gain becomes 1. Therefore, for different input modes, the two input signals will produce the same output signal amplitude.
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