IEC 62368-1 Test requirements for equipment containing audio amplifiers

According to the ITU-R 468-4 (Measurement of audio noise levels in sound broadcasting) specification, the 1000Hz frequency response is 0dB (see the figure below), which is suitable as a reference level signal and is convenient for evaluating the frequency

response performance of audio amplifiers. Peak response frequency signal. If the manufacturer declares that the audio amplifier is not intended to operate under 1000Hz conditions, the audio signal source frequency should be replaced by the peak response frequency. The peak response frequency is the signal source frequency when the maximum output power is measured on the rated load impedance (hereinafter referred to as the speaker) within the intended operating range of the audio amplifier. In actual operation, the inspector can fix the signal source amplitude and then sweep the frequency to check that the signal source frequency corresponding to the maximum effective value voltage appearing on the speaker is the peak response frequency.

Output power type and regulation - maximum output power

The maximum output power is the maximum power that the speaker can obtain, and the corresponding voltage is the maximum effective value voltage. Common audio amplifiers often use OTL or OCL circuits based on the working principle of Class AB amplifiers. When a 1000Hz sine wave audio signal is input into the audio amplifier and enters the saturation region from the amplification region, the signal amplitude cannot continue to increase, the peak voltage point is limited, and flat-top distortion appears at the peak.

Using an oscilloscope to test the speaker's output waveform, you can find that when the signal is amplified to the effective value and cannot be further increased, peak distortion occurs (see Figure 2). At this time, it is considered that the maximum output power state has been reached. When peak distortion occurs, the crest factor of the output waveform will be lower than the sine wave crest factor of 1.414 (as shown in Figure 2, crest factor = peak voltage / effective value voltage = 8.00/5.82≈1.375＜1.414)

Figure 2: 1000Hz sine wave signal input condition, speaker output waveform at maximum output power

Output power type and adjustment - non-clipped output power,Non-clipped output power refers to the output power at the junction of the saturation zone and the amplification zone when the speaker is operating at maximum output power and without peak distortion (the operating point is biased towards the amplification zone). The audio output waveform presents a complete 1000Hz sine wave with no peak distortion or clipping, and its RMS voltage is also less than the RMS voltage at maximum output power (see Figure 3).

Figure 3 shows the output waveform of the speaker entering the non-clipping output power state after reducing the amplification factor (Figures 2 and 3 show the same audio amplifier network)

Because audio amplifiers operate at the interface between the amplification and saturation regions and are unstable, signal amplitude jitter (the upper and lower peaks may not be equal) can be generated. The crest factor can be calculated using 50% of the peak-to-peak voltage as the peak voltage. In Figure 3 , the peak voltage is 0.5 × 13.10V = 6.550V , and the RMS voltage is 4.632V . The crest factor = peak voltage / RMS voltage = 6.550 / 4.632 ≈ 1.414. Output Power Type and Regulation - Power Regulation Methods. Audio amplifiers receive small signal inputs, amplify them, and output them to the speakers. The gain ratio is typically adjusted using a detailed volume scale (for example, a television's volume adjustment can range from 30 to 100 steps). However, adjusting the gain ratio by adjusting the signal source amplitude is much less effective. Reducing the signal source amplitude, even with the amplifier's high gain, will still significantly reduce the speaker's output power (see Figure 4). In

Figure 4: Output waveform when the speaker enters a non-clipped output power state after reducing the signal source amplitude.

(Figures 2 and 4 show the same audio amplifier network)

Figure 3 , adjusting the volume returns the speaker from maximum output power to a non-clipping state, with an RMS voltage of 4.632V . In Figure 4 , by adjusting the signal source amplitude, the speaker is adjusted from the maximum output power state to the non-clipped output power state, and the effective value voltage is 4.066V . According to the power calculation formula

Output power = square of voltage RMS / speaker impedance

The non-clipped output power of Figure 3 exceeds that of Figure 4 by about 30%, so Figure 4 is not the true non-clipped output power state.

It can be seen that the correct way to call back from the maximum output power state to the non-clipping output power state is to fix the signal source amplitude and adjust the amplification factor of the audio amplifier, that is, to adjust the volume of the audio amplifier without changing the signal source amplitude.

Output power type and adjustment - 1/8 non-clipped output power

Normal operating conditions for audio amplifiers are designed to simulate the optimal operating conditions of real-world speakers. Although real-world sound characteristics vary greatly, the crest factor of most sounds is within 4 (see Figure 5).

Figure 5: A real-world sound waveform with a crest factor of 4

Taking the sound waveform in Figure 5 as an example, crest factor = peak voltage / RMS voltage = 3.490 / 0.8718 = 4. To achieve distortion-free target sound, an audio amplifier must ensure that its maximum peak is free of clipping. If a 1000Hz sine wave signal source is used as a reference, to ensure the waveform remains undistorted and the 3.490V peak voltage is not current-limited, the RMS signal voltage should be 3.490V / 1.414 = 2.468V. However, the RMS voltage of the target sound is only 0.8718V. Therefore, the reduction ratio of the target sound to the RMS voltage of the 1000Hz sine wave signal source is 0.8718 / 2.468 = 0.3532. According to the power calculation formula, the voltage RMS reduction ratio is 0.3532, which means that the output power reduction ratio is 0.3532 squared, which is approximately equal to 0.125=1/8.

Therefore, by adjusting the speaker output power to 1/8 of the non-clipped output power corresponding to the 1000Hz sine wave signal source, the target sound with no distortion and a crest factor of 4 can be output. In other words, 1/8 of the non-clipped output power corresponding to the 1000Hz sine wave signal source is the optimal working state for the audio amplifier to output the target sound with a crest factor of 4 without loss.

The operating state of the audio amplifier is based on the speaker providing 1/8 non-clipping output power. When in the non-clipping output power state, adjust the volume so that the effective value voltage drops to about 35.32%, which is 1/8 non-clipping output power. Because pink noise is more similar to real sound, after using a 1000Hz sine wave signal to obtain non-clipping output power, pink noise can be used as the signal source. When using pink noise as the signal source, it is necessary to install a bandpass filter as shown in the figure below to limit the noise bandwidth.

Normal and abnormal working conditions - normal working conditions

Different types of audio amplifier equipment should consider all of the following conditions when setting normal operating conditions:

- The audio amplifier output is connected to the most unfavorable rated load impedance, or the actual speaker (if provided);

——All audio amplifier channels work simultaneously;

- For an organ or similar instrument with a tone generator unit, instead of using a 1000 Hz sine wave signal, depress the two bass pedal keys (if any) and the ten manual keys in any combination. Activate all stops and buttons that increase the output power, and adjust the instrument to 1/8 of the maximum output power;

- If the intended function of the audio amplifier is determined by the phase difference between the two channels, the phase difference between the signals applied to the two channels is 90°;

For multi-channel audio amplifiers, if some channels cannot operate independently, connect the rated load impedance and adjust the output power to 1/8 of the amplifier's designed non-clipped output power.

If continuous operation is not possible, the audio amplifier operates at the maximum output power level that allows continuous operation.

Normal and abnormal working conditions - Abnormal working conditions

The abnormal working condition of the audio amplifier is to simulate the most unfavorable situation that may occur on the basis of normal working conditions. The speaker can be made to work at the most unfavorable point between zero and maximum output power by adjusting the volume, or by setting the speaker to short circuit, etc.

Normal and abnormal working conditions - temperature rise test placement

When conducting a temperature rise test on an audio amplifier, place it in the position specified by the manufacturer. If there is no special statement, place the device in a wooden test box with an open front, 5 cm from the front edge of the box, with 1 cm of free space along the sides or top, and 5 cm from the back of the device to the test box. The overall placement is similar to simulating a home TV cabinet.

Normal and abnormal working conditions - noise filtering and fundamental wave restoration The noise of some digital amplifier circuits will be transmitted to the speaker along with the audio signal, causing disordered noise to appear when the oscilloscope detects the speaker output waveform. It is recommended to use the simple signal filtering circuit shown in the figure below (the method of use is: points A and C are connected to the speaker output end, point B is connected to the audio amplifier reference ground/loop ground, and points D and E are connected to the oscilloscope detection end). This can filter out most of the noise and restore the 1000Hz sinusoidal fundamental wave to a large extent (1000F in the figure is a typo, it should be 1000pF).

Some audio amplifiers have superior performance and can solve the problem of peak distortion, so that the signal will not be distorted or clipped when it is adjusted to the maximum output power state. At this time, the non-clipping output power is equivalent to the maximum output power. When visible clipping cannot be established, the maximum output power can be regarded as the non-clipping output power.

Electric energy source classification and safety protection

Audio amplifiers can amplify and output high-voltage audio signals, so the audio signal energy source must be classified and protected. When classifying, be sure to set the tone controller to a balanced position, allowing the audio amplifier to operate at maximum non-clipped output power to the speaker. Then, remove the speaker and test the open-circuit voltage. The audio signal energy source classification and safety protection are shown in the table below.

Pink Noise Generator