Effect of measuring bandwidth on class D amplifier measured THD and THD+N parameters
It has been quite long lasting discussion here on measuring bandwidth for THD+N plots vs. amplifier power, measured at various frequencies. Especially for class D amplifiers, THD+N results get rapidly worse, at higher frequencies (>4kHz), when we use higher measuring bandwidth (BW) than 22kHz. 22KHz allows only for 3rd harmonics of 7kHz sine tone, which is insufficient to estimate amplifier's non-linearity. It was usual, in the days of linear amplifiers, to use much wider BW, at least 80kHz, when measuring THD and THD+N. My intent is not to start another pointless discussion if 22kHz BW is enough to measure amplifiers or not, as such discussions become emotional and some commercial interests are hidden under the surface as well. My intention is to show how wider bandwidth is reflected in THD and THD+N vs. power measurements.
As a DUT, my PMA-NC252MP amplifier built with Hypex NC252MP module is used. The amplifier is in class I safety class device, with 3-wire mains cord using L, N and PE conductors. Analog signal ground is connected to chassis at each input XLR connector (pin 1 to chassis). The amplifier is not in the BTL mode. OUT-Hi terminal is amplified signal, OUT-Lo terminal is internally connected with signal ground in the module PCB. The module contains built-in SMPS power supply. All this information is important.
1. Test setup
The test circuit is in the following image. The DUT amplifier is driven from Topping D10s DAC via SE-to-balanced converter, to the amplifier balanced input. This converter must be used to get the best S/N results. 3-wire RCA-to-XLR cable is not enough as it does not fix impedance imbalance. The DAC is connected to the PC via USB-ISO isolated USB link. This is important to cut the signal ground loop. Output of the DUT amplifier is connected by means of a 3-wire method to the balanced RC lowpass filter, resistive divider and then to the input of E1DA Cosmos ADC, which is directly connected to the PC via USB link. The PC is supplied from the class II AC/DC supply adapter, so its ground is not directly connected with mains PE or PEN conductor.
The DUT amplifier is supplied from 230V/50Hz mains via an isolation transformer with 120pF primary/secondary capacitance. This is again important, especially for SMPS supplied amplifiers. The eefect of this isolation transformer will be shown later, its main purpose is to minimize effect of SMPS leakage current.
2. Measurements of THD and THD+N vs. power with 22kHz and 45kHz bandwidth
Measuring bandwidth of 22kHz and 45kHz was used. The reason of 45kHz limit is that the Topping D10s with USB-ISO adapter does not work with higher sampling frequency than 96kHz. The operation without USB-ISO was a subject to additional errors induced by circulating loop currents, so such option was rejected and the DAC was always connected via USB-ISO. The 45kHz BW still allows to evaluate 5th harmonic of the 9kHz sine wave.
The RC lowpass filter, that is necessary to prevent ADC input from high dv/dt induced errors, has some effect in the measuring band. The filter was used in all cases, frequency response of the NC252MP with this filter is shown below. At 20kHz, we can see attenuation of -2.5dB, at 40kHz it makes -7.1dB.
Measurements of THD+N vs. power at various frequencies
The plots for 22Hz, 1kHz, 4kHz and 6kHz are measured with 22kHz BW. In case of 6kHz measurement, the highest harmonic measured is only 3rd, H3.
The plots for 10kHz and 14kHz are measured with 45kHz BW (but let's not forget some effect of the RC low pass filter), the highest harmonic of the 14kHz sine is again 3rd, H3.
The THD+N plots are prone to mix distortion and noise effects, so another measurement of “pure” THD, without “N” noise component, was done and is shown below.
Measurements of THD vs. power at various frequencies
The plots for 22Hz, 1kHz, 4kHz and 6kHz are measured with 22kHz BW. In case of 6kHz measurement, the highest harmonic measured is only 3rd, H3.
The plots for 10kHz and 14kHz are measured with 45kHz BW (but let's not forget some effect of the RC low pass filter), the highest harmonic of the 14kHz sine is again 3rd, H3.
We can see now that it is not only noise that makes the results with higher BW (45kHz) worse, but there is a considerable and inevitable worsening of results due to distortion harmonics that appear above 22kHz.
The effect of isolation transformer in the DUT power supply line
The following spectrum measurements show 100W/4ohm/1kHz spectrum of THD and THD+N, once measured with amplifier directly connected to mains and then via the isolation transformer. The effect of the isolation transformer is indisputable. The main reson of isolation transformer use is to eliminate capacitive leakage currents from SMPS. It is pointless to measure mains voltage “distortion” with and without the isolation transformer, as it is not the reason why this component is used, to improve whole chain S/N.
Please note that the area above 1kHz, with isolation transformer, is now free of usual spurious, SMPS leakage currents induced mess, that we often see in NCore MP modules measurements. Please also an improvement in THD+N and lower amplitude of ultrasound garbage with the isolation transformer. The area below 1kHz now contains only the mains lines resulting from the all-in-one concept (SMPS+amp shared on mutual board).
It has been quite long lasting discussion here on measuring bandwidth for THD+N plots vs. amplifier power, measured at various frequencies. Especially for class D amplifiers, THD+N results get rapidly worse, at higher frequencies (>4kHz), when we use higher measuring bandwidth (BW) than 22kHz. 22KHz allows only for 3rd harmonics of 7kHz sine tone, which is insufficient to estimate amplifier's non-linearity. It was usual, in the days of linear amplifiers, to use much wider BW, at least 80kHz, when measuring THD and THD+N. My intent is not to start another pointless discussion if 22kHz BW is enough to measure amplifiers or not, as such discussions become emotional and some commercial interests are hidden under the surface as well. My intention is to show how wider bandwidth is reflected in THD and THD+N vs. power measurements.
As a DUT, my PMA-NC252MP amplifier built with Hypex NC252MP module is used. The amplifier is in class I safety class device, with 3-wire mains cord using L, N and PE conductors. Analog signal ground is connected to chassis at each input XLR connector (pin 1 to chassis). The amplifier is not in the BTL mode. OUT-Hi terminal is amplified signal, OUT-Lo terminal is internally connected with signal ground in the module PCB. The module contains built-in SMPS power supply. All this information is important.
1. Test setup
The test circuit is in the following image. The DUT amplifier is driven from Topping D10s DAC via SE-to-balanced converter, to the amplifier balanced input. This converter must be used to get the best S/N results. 3-wire RCA-to-XLR cable is not enough as it does not fix impedance imbalance. The DAC is connected to the PC via USB-ISO isolated USB link. This is important to cut the signal ground loop. Output of the DUT amplifier is connected by means of a 3-wire method to the balanced RC lowpass filter, resistive divider and then to the input of E1DA Cosmos ADC, which is directly connected to the PC via USB link. The PC is supplied from the class II AC/DC supply adapter, so its ground is not directly connected with mains PE or PEN conductor.
The DUT amplifier is supplied from 230V/50Hz mains via an isolation transformer with 120pF primary/secondary capacitance. This is again important, especially for SMPS supplied amplifiers. The eefect of this isolation transformer will be shown later, its main purpose is to minimize effect of SMPS leakage current.
2. Measurements of THD and THD+N vs. power with 22kHz and 45kHz bandwidth
Measuring bandwidth of 22kHz and 45kHz was used. The reason of 45kHz limit is that the Topping D10s with USB-ISO adapter does not work with higher sampling frequency than 96kHz. The operation without USB-ISO was a subject to additional errors induced by circulating loop currents, so such option was rejected and the DAC was always connected via USB-ISO. The 45kHz BW still allows to evaluate 5th harmonic of the 9kHz sine wave.
The RC lowpass filter, that is necessary to prevent ADC input from high dv/dt induced errors, has some effect in the measuring band. The filter was used in all cases, frequency response of the NC252MP with this filter is shown below. At 20kHz, we can see attenuation of -2.5dB, at 40kHz it makes -7.1dB.
Measurements of THD+N vs. power at various frequencies
The plots for 22Hz, 1kHz, 4kHz and 6kHz are measured with 22kHz BW. In case of 6kHz measurement, the highest harmonic measured is only 3rd, H3.
The plots for 10kHz and 14kHz are measured with 45kHz BW (but let's not forget some effect of the RC low pass filter), the highest harmonic of the 14kHz sine is again 3rd, H3.
The THD+N plots are prone to mix distortion and noise effects, so another measurement of “pure” THD, without “N” noise component, was done and is shown below.
Measurements of THD vs. power at various frequencies
The plots for 22Hz, 1kHz, 4kHz and 6kHz are measured with 22kHz BW. In case of 6kHz measurement, the highest harmonic measured is only 3rd, H3.
The plots for 10kHz and 14kHz are measured with 45kHz BW (but let's not forget some effect of the RC low pass filter), the highest harmonic of the 14kHz sine is again 3rd, H3.
We can see now that it is not only noise that makes the results with higher BW (45kHz) worse, but there is a considerable and inevitable worsening of results due to distortion harmonics that appear above 22kHz.
The effect of isolation transformer in the DUT power supply line
The following spectrum measurements show 100W/4ohm/1kHz spectrum of THD and THD+N, once measured with amplifier directly connected to mains and then via the isolation transformer. The effect of the isolation transformer is indisputable. The main reson of isolation transformer use is to eliminate capacitive leakage currents from SMPS. It is pointless to measure mains voltage “distortion” with and without the isolation transformer, as it is not the reason why this component is used, to improve whole chain S/N.
Please note that the area above 1kHz, with isolation transformer, is now free of usual spurious, SMPS leakage currents induced mess, that we often see in NCore MP modules measurements. Please also an improvement in THD+N and lower amplitude of ultrasound garbage with the isolation transformer. The area below 1kHz now contains only the mains lines resulting from the all-in-one concept (SMPS+amp shared on mutual board).