How to reduce the audio noise of the audio power amplifier

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Abstract The TPA1517 audio power amplifier is a powerful general-purpose device that can input stereo power above 6 W into loads as low as 4 W.
However, there is a phenomenon in the TPA1517 that an unpleasant "click" sound is heard when the device exits from standby mode.
This article can help circuit designers better understand what is transient noise, why this happens with this device, why a transient pop-reduction circuit works, and the use of different components in this circuit can occur. What are the pros and cons.
What is the cause of the TPA1517 transient noise?
The transient murmurs discussed herein refer to the unpleasant noise that can be heard when a component exits standby mode and powers up and powers down.
The noise when the device enters standby mode is very small, but when the TPA1517 exits standby mode, the audible noise is extremely noticeable. This is caused by two simultaneous events: an upward bias to the appropriate level of input and output bias level changes.

Figures 1 and 2 are capture diagrams depicting typical noise noise. Texas Instruments' (TI) evaluation board (EVM) is used here with a load of 4W and a supply voltage of 12V. Note that the shape of the output trace before and after the output decoupling capacitor, the sharp transient is the same. Also, note that the DC level of Track 2 drops to 0V between 40 and 50 milliseconds before slowly rising to the midrail. Figure 2 shows more details when the device goes from standby to active, but does not indicate the time it takes for the DC voltage to reach the appropriate bias level.

How does input bias cause transient noise?
Regardless of the supply voltage, the TPA1517 has a DC bias rating of 2.1 V during the input phase. 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 operating, the input bias quickly returns to its nominal value of 2.1 V. The larger the difference between the input bias voltage and 2.1 V during standby, the greater the transient noise generated when returning to the operating state. Figure 3 graphically depicts the input noise in a 12 V, 4 W system. Trace 1 is the voltage on the “STANDBY” pin. Trace 2 is the voltage at the output of the decoupling capacitor (DC coupling).

How does the output offset cause transient noise?
The rated DC bias of the TPA1517 output stage is VCC/2. With this setting, the output signal has a higher output amplitude in both the positive and negative directions without the side being clipped 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 grounded, but is in the DC middle rail position. However, during the transition from standby to active, the output exhibits a brief but significant transient rise in DC voltage. These voltage spikes (which may reach a few volts in size) are transmitted to the speaker, producing extreme transient noise. The reason for this is that the voltage changes too fast, so that the DC blocking capacitor cannot recognize that this is a change in the DC current, thus allowing the signal to pass.
Figure 4 is a capture plot depicting the instantaneous noise caused by the output bias in a 12 V, 4 W system. Note that there is a large voltage spike of nearly 5V on tracks 2 and 3.

Reduce transient noise
The transient noise is caused by the DC bias problem at the input and output stages of the 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 problems. This is basically equivalent to two separate solutions, as any one solution can be used alone.

Input stage squelch
The noise problem caused by the DC input bias problem is not as large as the output offset, but it is more complicated than this, so the input bias problem is discussed here first.
Because the noise generated by the input DC bias is caused by a significant drop in the input DC bias when the device enters standby mode, one 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 might seem at first glance. Simply adding a resistor divider to the input circuit to get a 2.1 V bias from the power supply is not a good solution. Although it provides the constant DC bias required, it also requires two resistors to be permanently mounted on the device side of the input capacitor, the effect of which is to greatly attenuate the input signal.
We need a solution that biases the input from an external source when the device is in standby mode, but the external source 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 (the size of which should be suitable 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 a resistor with a small impedance value 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 not wise to use a resistor with a total value in series of more than 10kΩ because the input bias current is large enough to significantly change the voltage divider voltage. However, too low a resistance value will cause a high current through the resistor, which will generate unnecessary heat. For example, if R1 is 1 kΩ, it will consume approximately 100 mW, R2 will consume approximately 25 mW, and the divider current will be 9.84 mA. If the R1 resistance is reduced from 1kΩ to 100Ω, the divider current at 12 V will jump from 9.84 mA to 98.4 mA. This means that R1 and R2 will consume approximately 1W and 1/4 W, respectively! See Table 1 for the recommended resistance values ​​for the input voltage divider. When selecting a resistor, care should be taken to select a resistor with the proper power rating.

Output stage squelch
The effect of the output instantaneous value on the noise is very large. As shown in Figure 2, the output stage is the cause of the maximum voltage spike, which is directly related to the audible noise that the ear can hear.
The solution to the noise caused by the output phase is to ground the output quickly (but not immediately) when the device enters standby mode, then allow the output value to return to the intermediate 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 operating mode. The output value returns to the appropriate level and the speaker can only be driven when the output switch is turned off (transistors Q2 and Q3 are output switches, see Figure 5).

Input and output squelch are considered together
Appropriate circuitry must be available around the input and output to get 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. To achieve this, you can use only one inverter to drive the left and right input switches and the left and right output switches.
Figure 5 is a detailed illustration of a comprehensive stereo solution. The circuit depicted in Figure 5 uses a bipolar tube, which is generally less expensive than an FET. Figure 6 depicts a similar circuit if the FET is preferred. The “standby control” should be pulled to the lowest level, which ensures that changes to the 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 of the entire system (THD + N). Figure 7 and Figure 8 contain the results of two THD + N scans with the TPA1517 EVM, respectively. Figure 7 compares a THD + N scan with an output power sweep, while Figure 8 compares a THD + N scan with a frequency sweep. The higher distortion at lower frequencies in Figure 8 is due to the high-pass filter formed by the input capacitor and the input resistor.

Power-on and power-off transient noise reduction
The noise reduction scheme described in this application can also be used to mitigate the effects of power-up and power-down sequencing.
During normal operation, the TPA1517 often suffers from large noise disturbances during power-off. A noise suppression circuit can be used to solve this problem. The noise suppression circuit alone cannot play a large role in power-on and power-off because the power supply has been eliminated from the noise suppression circuit and the device. However, the TPA1517 can be powered down in standby mode. When the power-up operation applies a proper bias to the noise suppression circuit for a sufficient amount of time, the TPA1517 is kept in standby mode so that the noise can be greatly reduced when the device is placed in operation. Similarly, the noise suppression circuit keeps the output grounded in standby mode so that when the device is powered down, it is virtually silent.