How to reduce transient noise Audio Power Amplifier

Transient noise caused TPA1517 What is the reason?

Discussed in this article refers to the transient noise components out of standby mode and when power and lower power sequencing can be heard when the unpleasant noise.

The device into standby mode when the noise is very small, but when the TPA1517 out of standby mode, the noise is heard very clearly. This is caused by the simultaneous two events: the upward bias to the appropriate level of input and output bias level changes.

How to reduce transient noise Audio Power Amplifier

How to reduce transient noise Audio Power Amplifier

Figure 1 and Figure 2 is a typical noise noise describe the capture plan. Here to use Texas Instrument (TI) of the evaluation board (EVM), the load is 4W, power supply voltage of 12V. Note that the output decoupling capacitor before and after the output trajectory shape, sharp wave (sharp transient) is the same. Also, note track 2 DC level slowly rose to the middle rail (midrail) before, in between 40 to 50 milliseconds down to 0V. Figure 2 shows the device from standby to enter the work state for more detailed information, but did not indicate the level of DC bias voltage to reach the appropriate time.

Input bias is how to cause instantaneous murmur?

No matter how much voltage, TPA1517 in the input stage of the DC bias rating of 2.1 V. The TPA1517 placed in standby mode, the input bias will be decreased, and often a few hundred millivolts or more. When the device back to working state, the input bias will quickly return to its rating 2.1 V. Standby input bias and the greater the gap between the 2.1 V, return to work state, the greater the noise generated instantaneous. Figure 3 graphically describes the 12 V, 4W of the system input noise. Track 1 is "STANDBY" pin voltage. Track 2 is the output decoupling capacitors (DC-coupled) voltage in the load area.

Output offset is how to cause instantaneous murmur?

TPA1517 output stage of the rated DC bias of VCC / 2. This setting, the output signal will be able to have a high positive and negative output rate up, but will not happen to be on the other side while limiting situation. TI audio power amplifier with many other different, TPA1517 placed in standby mode, the output is not grounded, but at the DC location of the middle rail. However, from the standby state of the conversion process to work, the output DC voltage shown in the brief but significant transient increase. The voltage of the sudden increase (spike) (size may reach several volts) to a speaker to generate high transient noise. This happens because the voltage change so fast, so that DC blocking capacitors are unable to identify the changes it is DC, thus allowing signals.

Figure 4 is a capture map, describes the 12 V, 4W output bias of the system caused by the transient noise. Note that track 2 and 3 have a voltage close to 5V, sudden increase substantially.

Reduce the transient noise

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

Input stage muting

DC input offset problems caused by noise problem is not as big as the output bias, but the contrast is even more complex, input-bias problem discussed here before.

Because the input DC bias noise generated is due to go into standby mode when the device input DC bias caused significantly reduced, so it is obviously a solution of any state in the device is in force when they enter to keep the 2.1 V.

This solution is not as at first it seemed so simple. Simply add in the input circuit of a resistor divider to get from the power supply 2.1 V bias, this is not a good solution. Although it provides a constant DC bias required, but it also requires the input capacitor device side permanently install two resistors, the effect is greatly weakened the input signal.

We need such a solution: When the device is in standby mode, the input from an external source applied bias, but when the device is in normal working condition, the external source will be disconnected. To this end, must be resistive voltage divider (its size should be suitable for power supply) with the use of a switch. The first switch and the "STANDBY" pin-connected, since the role of the inverter. The second switch for connecting or disconnecting "INPUT" pin and the formation of the 2.1V voltage resistor divider.

How to reduce transient noise Audio Power Amplifier

TPA1517 input bias current is relatively large, it is necessary to use the resistor divider impedance less resistance. This input bias current of the divider can generate the 2.1V voltage to minimize the impact. Use 10kΩ resistor in series worth more than it would be unwise, because the input bias current will be large enough to significantly change the voltage divider. However, the resistance is too low can lead to high current through the resistor, which will generate unnecessary heat. For example, if R1 is 1 kΩ, it would consume about 100 mW of power, R2 will consume about 25mW of power, the divider current is 9.84mA. If the R1 resistance from 1kΩ down to 100Ω, 12 V under the divider current will jump from 9.84 mA to 98.4 mA. This means that R1 and R2 will consume approximately 1W, and 1 / 4 W of power! See Table 1, to obtain the recommendations of the input voltage divider resistance. In the choice of resistance, we should pay attention to selecting an appropriate power rating of resistors.

Output instantaneous value of the noise impact is very large. Shown in Figure 2, the maximum voltage output stage is the formation of the reasons for the sudden increase, which could hear the noise and the ear directly.

Caused by noise on the output stage of the solution when the device into standby mode, quickly (but not instantly to) the output ground, and then return to work when the device mode, allowing the output value back to the middle rail.

If you are interested in the output ground, then began to return to work when the device mode, the output will not fluctuate. Output back to the appropriate level, and only switch off when the output can only drive the speaker (transistor Q2 and Q3 is the output switch, see Figure 5).

Considered together with input and output mute

Around the input and output circuits must have the right to obtain the best noise solution. In addition, because TPA1517 is a stereo amplifier, the noise suppression circuit must be modified so that the number of components with at least two channels can work on. To achieve this, you can only use one inverter to drive the left and right sides of the input switch and output switch left and right sides.

Figure 5 is about a complete stereo solution detailed graphic. Figure 5 describes the use of bipolar circuits, it is generally cheaper than the FET. If you prefer FET, Figure 6 depicts a similar circuit. "Standby control" should be pulled most low-grade, which can ensure that the VBE of the change will not accidentally activate the circuit.

How to reduce transient noise Audio Power Amplifier

Audio Performance

Described in this article TPA1517 transient noise solution will not increase the overall system total harmonic distortion and noise of the and (THD + N). Figure 7 and Figure 8 includes the TPA1517 EVM with THD + N for the two scans. Figure 7 is a THD + N scan compared with the output power of scanning, while Figure 8 is a THD + N scanning and frequency scanning contrast. Figure 8, lower distortion and higher frequency of the input capacitor is formed with the input resistance caused by high-pass filter.

How to reduce transient noise Audio Power Amplifier

How to reduce transient noise Audio Power Amplifier

Instantaneous power and power noise reduction

The application of noise reduction scheme described can also be used to reduce the impact of electric power sequencing.

In normal operation, TPA1517 in power during power outages often subject to greater noise disturbance. Noise suppression circuitry, can be used to solve this problem. Noise suppression circuit in the electric power alone can play a big role, because the power supply from the noise suppression circuit and equipment to eliminate. However, TPA1517 to power in the standby mode power. When powering up to a noise suppression circuit enough time to impose appropriate bias, let TPA1517 remain in standby mode, so that when the device placed in working condition, noise can be greatly reduced. Similarly, the noise suppression circuit in the standby mode, the output to keep grounded, so that when power is turned off, it in fact did not murmur.

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