How to reduce transient noise in audio power amplifier

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How to reduce transient noise in audio power amplifier [Copy link]

　What causes the TPA1517 transient noise?
　　
　　The transient noise discussed in this article refers to the unpleasant noise that can be heard when the part is taken out of standby mode and during power-up and power-down sequencing.
　　
　　The noise is very small when the device enters standby mode, but when the TPA1517 comes out of standby mode, the noise is extremely obvious. This is caused by two events happening at the same time: the input biased up to the appropriate level and the change in the output bias level.

　　Figures 1 and 2 are captures depicting typical pop and pop noise. A Texas Instruments (TI) evaluation module (EVM) was used here with a 4W load and a 12V supply voltage. Note the shape of the output traces before and after the output decoupling capacitors. The sharp transients are the same. Also, note that the DC level in trace 2 drops to 0V in 40-50ms before slowly rising to midrail. Figure 2 shows more details of the device going from standby to active, but does not indicate the time required for the DC voltage to reach the proper bias level.

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

　　How does output biasing cause transients?
　　
　　The TPA1517 output stage is rated for a DC bias of VCC/2. This allows the output signal to have high amplitude in both the positive and negative directions without one side being clipped by the other. Unlike many other TI audio power amplifiers, the output of the TPA1517 is not grounded when in standby mode, but rather is at the DC mid-rail. However, during the transition from standby to active, the output exhibits a brief but noticeable increase in DC voltage. These voltage spikes, which can be several volts in size, are transmitted to the speaker and cause large transients. This occurs because the voltage changes so quickly that the DC blocking capacitors cannot recognize the change as a DC current and allow the signal to pass. 　　Figure 4 is a capture of transients caused by output biasing in a 12 V, 4 W system. Note the large voltage spikes of nearly 5 V on traces 2 and 3.
　　

　　Reducing transient noise
　　
　　Transient noise is caused by the DC bias problem of the TPA1517 input and output stages. In order to reduce the noise as much as possible, it is necessary to find a solution that can solve the input and output bias problems. This is basically equivalent to two separate solutions, because either solution can be used alone.

　　Input Stage Noise
　　
　　The DC input bias problem is not as big of a problem as the output bias, but it is more complex, so the input bias problem is discussed first.
　　
　　Since the input DC bias noise is caused by the input DC bias voltage being significantly reduced when the device enters standby mode, an obvious solution is to force the input to remain at 2.1 V when the device is in any state.
　　
　　This solution is not as simple as it may seem 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. Although it provides the required constant DC bias, it also requires two resistors to be permanently installed on the device side of the input capacitor, which has the effect of greatly attenuating the input signal. What is needed
　　
　　is a solution where the input is biased by an external source when the device is in standby mode, but this external source is disconnected when the device is in normal operation. To achieve this, a series of switches must be used in conjunction with the resistor divider (which should be 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 "INPUT" pin from the 2.1V voltage formed by the resistor divider.

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

　　Output Stage Quieting
　　 The transient value of the output has a significant impact on the noise. As shown in Figure 2, the output stage is responsible for the largest voltage surge, which is directly related to the noise that can be heard by the ear. The
　　
　　solution to the noise caused by the output stage is to quickly (but not instantly) ground the output when the device enters standby mode, and then allow the output value to return to the middle rail when the device returns to the operating mode.
　　
　　If the output is intentionally grounded, the output value will not fluctuate when the device begins to return to the 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 switch, see Figure 5).

　　Consider input
　　
　　and output noise together Appropriate circuitry must be placed around both the input and output to achieve the best noise solution. In addition, since the TPA1517 is a stereo amplifier, the noise suppression circuitry must be modified to work on both channels with a minimum number of components. This can be achieved by using only one inverter 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 bipolar transistors, which are generally less expensive than FETs. If FETs are preferred, Figure 6 depicts a similar circuit. The "standby control" should be pulled to the lowest position, which 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 contain the results of two THD + N sweeps performed with the TPA1517 EVM. Figure 7 compares a THD + N sweep with an output power sweep, while Figure 8 compares a THD + N sweep with 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.

　　Reducing transient noise during power-on and power-off
　　
　　The noise reduction scheme introduced in this application can also be used to mitigate the effects of power-on and power-off sequencing.
　　
　　In normal operation, the TPA1517 often suffers from large noise problems during power-on and power-off. A 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 is removed from the noise suppression circuit and the device. However, the TPA1517 can be powered on and off in standby mode. When the power-on operation gives the noise suppression circuit enough time to apply the appropriate bias voltage, keeping the TPA1517 in standby mode, so that when the device is put into operation, the noise can be greatly reduced. Similarly, the noise suppression circuit keeps the output grounded in standby mode, so that when the device is powered off, it is actually noise-free.