How to reduce transient noise of audio power amplifier

　　What is the cause of transient noise in TPA1517?
　　
　　The transient buzz discussed in this article refers to the unpleasant noise that can be heard when the part exits standby mode and when the power-up and power-down sequencing is performed.
　　
　　The noise when the device enters standby mode is very small, but when the TPA1517 exits standby mode, the noise heard is extremely noticeable. This is caused by two events occurring simultaneously: an upward bias of the input to the appropriate level and a change in the output bias level.

　　Figures 1 and 2 are captures depicting typical murmur noise. An evaluation module (EVM) from Texas Instruments (TI) is used here, with a load of 4W and a power supply voltage of 12V. Note the shape of the output trace before and after the output decoupling capacitor. The sharp transients are the same. Also, note that the DC level of trace 2 drops to 0V between 40 and 50 milliseconds before slowly rising to the midrail. Figure 2 shows more detail as the device transitions from standby to active, but does not show the time it takes for the DC voltage to reach the appropriate bias level.

　　How does input bias cause transient noise?
　　
　　The TPA1517 has a DC bias rating of 2.1 V at the input stage regardless of the supply voltage. When the TPA1517 is placed in standby mode, the input bias voltage drops, often by a few hundred millivolts or more. When the device returns to operation, the input bias voltage quickly returns to its nominal value of 2.1 V. The greater the difference between the input bias voltage in standby and 2.1 V, the greater the transient noise generated when returning to the operating state. Figure 3 graphically describes the input noise in a 12 V, 4 W system. Trace 1 is the voltage on the "STANDBY" pin. Trace 2 is the voltage across the output decoupling capacitor (DC coupled) at the load.

　　How does output bias cause transient noise?
　　
　　The rated DC bias voltage of the TPA1517 output stage is VCC/2. After this setting, the output signal can have a higher output amplitude in both the positive and negative directions without one side being limited by the other side. Unlike many of TI's other audio power amplifiers, when the TPA1517 is placed in standby mode, the output is not connected to ground but is at the DC center rail. However, during the transition from standby to active, the output will show a brief but significant transient increase in DC voltage. These voltage spikes (which may be several volts in size) are transmitted to the speakers and produce extremely loud transient noise. This happens because the voltage changes so quickly that the DC blocking capacitor doesn't recognize it as a change in DC and therefore allows the signal to pass through. 　　Figure 4 is a capture depicting the transient noise caused by output bias in a 12 V, 4 W system. Note the large voltage surge of nearly 5V on traces 2 and 3.
　　

　　Reduce transient noise
　　
　　Transient noise is caused by DC bias problems in the input and output stages of TPA1517. In order to reduce noise as much as possible, it is necessary to find a solution that can solve the input and output bias problem. This essentially amounts to two separate solutions, as either solution can be used on its own.

　　Input Stage Noise
　　
　　Squelch The noise problem caused by the DC input bias problem is not as big as the output bias, but it is more complicated in comparison, so the input bias problem is discussed here first.
　　
　　Because the noise generated by the input DC bias is caused by a significant decrease in the input DC bias when the device enters standby mode, an obvious solution is to force the input to remain at 2.1 V during any state of the device.
　　
　　The solution is not as simple as it seems at first glance. Simply adding a resistor divider to the input circuit to obtain a 2.1 V bias from the supply is not a good solution. While it provides the required constant DC bias, it also requires two resistors to be permanently installed on the device side of the input capacitor, with the effect of significantly attenuating the input signal.
　　
　　We need a solution where the input is biased by an external source when the device is in standby mode, but is disconnected when the device is in normal operation. To achieve this, a series of switches must be used in conjunction with a resistor divider (sized appropriately for the supply voltage). The first switch is connected to the "STANDBY" pin and acts as an inverter. The second switch is responsible for connecting or disconnecting the 2.1V voltage formed by the "INPUT" pin and the resistor divider.

　　The input bias current of the TPA1517 is relatively large, so it is necessary to use smaller value resistors in the resistor divider. This minimizes the effect of the input bias current on the 2.1V voltage generated by the voltage divider. It is unwise to use resistors with a total value of more than 10kΩ in series because the input bias current will be large enough to significantly change the voltage divider. However, a resistance value that is too low can also cause a high current through the resistor, which can generate unnecessary heat. For example, if R1 is 1 kΩ, it will dissipate approximately 100 mW of power, R2 will dissipate approximately 25mW, and the voltage divider current is 9.84mA. If the R1 resistor value is reduced from 1kΩ to 100Ω, the voltage divider current jumps from 9.84 mA to 98.4 mA at 12 V. This means that R1 and R2 will consume approximately 1W and 1/4 W respectively! See Table 1 for recommended resistor values ​​for the input voltage divider. When selecting a resistor, be careful to choose one with the appropriate power rating.

　　The instantaneous value of the squelch output in the output stage
　　 has a very large impact on noise. As shown in Figure 2, the output stage is responsible for the largest voltage surge, which is directly related to the noise that the ear can hear.
　　
　　The solution to the hum caused by the output stage is to quickly (but not instantly) ground the output when the device goes into standby mode, and then allow the output value to return to the mid-rail when the device returns to active mode.
　　
　　If the output is intentionally grounded, the output value will not fluctuate when the device begins to return to operating mode. The output value returns to the appropriate level and can drive the speaker only when the output switch is turned off (transistors Q2 and Q3 are the output switches, see Figure 5).

　　Consider input and output squelch together.
　　
　　There must be appropriate circuits around the input and output to obtain the best noise solution. Additionally, since the TPA1517 is a stereo amplifier, the noise suppression circuit must be modified to function on both channels with a minimum component count. To achieve this goal, only one inverter can be used to drive the left and right input switches and the left and right output switches.
　　
　　Figure 5 is a detailed diagram of a comprehensive stereo solution. The circuit depicted in Figure 5 uses bipolars, which are generally less expensive than FETs. If FETs are preferred, Figure 6 depicts a similar circuit. "Standby Control" should be pulled to the lowest setting. This ensures that changes in VBE do not accidentally activate the circuit.

　　Audio Performance
　　
　　The TPA1517 transient noise solution described in this article does not increase the total harmonic distortion and noise (THD + N) of the entire system. Figures 7 and 8 respectively contain the results of two THD + N scans performed with the TPA1517 EVM. Figure 7 is a comparison of a THD + N sweep and an output power sweep, while Figure 8 is a comparison of a THD + N sweep and a frequency sweep. The higher distortion at lower frequencies in Figure 8 is caused by the high-pass filter formed by the input capacitor and input resistor.

　　Reduction of transient noise during power-on and power-off.
　　
　　The noise reduction scheme introduced in this application can also be used to reduce the impact of power-on and power-off sequencing.
　　
　　During normal operation, TPA1517 often suffers from large noise during power-on and power-off periods. Noise suppression circuit can be used to solve this problem. The noise suppression circuit alone cannot play a big role during power-on and power-off, because the power has been eliminated from the noise suppression circuit and the device. However, the TPA1517 is capable of powering up and down in standby mode. When the power-up operation gives the noise suppression circuit enough time to apply the appropriate bias voltage, leaving the TPA1517 in standby mode allows the noise to be significantly reduced when the device is put into operation. Likewise, the click-suppression circuitry holds the output to ground in standby mode so that when the device is powered off, it is virtually click-free.