Technical Info.
THD measurements
This section introduces the process for measuring the total harmonic distortion of the CSS-4700 amplifier. The THD measurement system used by Olas audio has proven to be very useful and precise, especially for making THD measurements quickly and safely. The following diagram shows the stages of the harmonic distortion measurement system used by Olas audio.
Figure 1. THD measurement system for the CSS-4700 amplifier.
Olas Audio has designed and tested a sine wave generator with an intrinsic total harmonic distortion (THD+N) ≤ 0.000015% at 1 kHz. The signal from this generator is fed into the input of the system under test, in this case, the CSS-4700 amplifier. An 8-ohm load is connected to the amplifier’s output, where the distortion measurement is taken.
The input of the attenuator buffer is connected to the 8-ohm load, while the notch filter’s output is connected to the input of an audio interface, which in turn connects to a computer via a USB port. Software capable of providing the Fourier spectrum of the analyzed signal (such as ARTA, REW, etc.) should be used.
The buffer voltage attenuation factor can be set between 1X (zero attenuation) and 0.5X. The notch filter provides an attenuation of -58.4 dBV at the fundamental frequency of 1 kHz. Additionally, this filter introduces a loss of 9 dBV for the second harmonic and 5 dBV for the third harmonic. Finally, a 40 dBV amplification is applied to this result, corresponding to a voltage gain of 100.
The following image corresponds to the Fourier spectrum of the test signal with an amplitude equal to 1.044 Vrms (-18.02 dBV – 40 dBV + 58.4 dBV = 0.38 dBV).
Figure 2. Fourier spectrum of the test signal with an amplitude of 1.044 Vrms and a THD of less than 0.000007%.
With the buffer voltage attenuation factor set to null (1X), the displayed result in the previous figure reflects only the signal from the generator passing through the notch filter. From the image, the fundamental signal value is measured at 0.38 dBV (-18.02 – 40 dBV + 58.4 dBV). The second harmonic is submerged in the system noise, while the third harmonic registers at -145.5 dBV (-110.5 dBV – 40 dBV + 5 dBV). The fourth harmonic measures -151.2 dBV (-111.2 dBV – 40 dBV), and the fifth harmonic is at -151.9 dBV (-111.9 dBV – 40 dBV). The remaining harmonics are also buried in the system noise, with background noise recorded at approximately -153 dBV (-113 dBV – 40 dBV).
The RMS voltages corresponding to these harmonics—after applying corrections for the insertion loss in the third harmonic and the 40 dBV amplification from the notch filter—are as follows: 1.044 V for the fundamental, 53.08 nV for the third harmonic, 27.54 nV for the fourth harmonic, 25.41 nV for the fifth harmonic, and 22.38 nV for the system’s background noise. Consequently, the total harmonic distortion of the sine wave generator, including the entire measurement system, is less than or equal to 0.000007%.
The Fourier spectrum of the CSS-4700 amplifier’s output signal is shown below. The power consumed was measured at 1.09 W into a resistive load of 8 ohms.
Figure 3. Fourier spectrum of the CSS-4700 board’s output signal with 1.09 W dissipated in an 8-ohm resistive load and a THD of less than 0.000045%.
In this case, the input buffer of the measurement system exhibited no attenuation (1X).
Table 1
| Harmonic | Amplitude dBV | Amplitude Vrms |
| Fundamental @ 1KHz | 9.43 (-8.97 – 40 + 58.4) | 2.96 |
| 2th | -118.6 (-87.6 – 40 + 9) | 1.17E-06 |
| 3rd | N/A. Immersed in the background noise | 0 |
| 4th | -127.6 (-87.6 – 40) | 416.87E-09 |
| 5th | -128 (-88 – 40) | 398.1E-09 |
| 6th | N/A. Immersed in the background noise | 0 |
| Background noise | -132 (-92 – 40) | 251.19E-09 |
Therefore, the total harmonic distortion (THD) for this test is less than or equal to 0.000045%, and the power dissipated in the 8-ohm load is equal to 1.09 W, calculated as [(2.96/8)×2.96].
The image below shows the Fourier spectrum of the CSS-4700 amplifier’s output signal at a power of 43.8 W into a resistive load of 8 ohms.
Figure 4. Fourier spectrum of the CSS-4700 board’s output signal with 43.8 W dissipated in an 8-ohm resistive load and a THD of less than 0.000115%.
Table 2
| Harmonic | Amplitude dBV | Amplitude Vrms |
| Fundamental @ 1KHz | 19.43 (1.03 – 40 + 58.4) | 9.36 |
| 2nd | -100.2 (-69.2 – 40 + 9) | 9.77E-06 |
| 3rd | -107 (-72 – 40 + 5) | 4.47-06 |
| 4th | -118.5 (-78.5 – 40) | 1.18E-06 |
| 5th | -136 (-96 – 40) | 158.5E-09 |
| 6th | N/A. Immersed in the background noise | 0 |
| Background noise | -138 (-98 – 40) | 125.89E-09 |
Therefore the THD for this test is less than or equal to 0.000115%. Since the measured signal is attenuated (0.5X), the actual value at the output of the CSS-4700 amplifier is 9.36/0.5=18.72 Vrms. Therefore, the power dissipated in the 8-ohm load is 43.8 W.
The following diagram shows the THD measurement sequence for the SP-DR1200 Speaker Protection Board.
Figure 5. THD measurement system for the SP-DR1200 speaker protection board.
The following Fourier spectrum corresponds to the THD measurement performed on an 8 Ohm load connected to the SP-DR1200 loudspeaker protection board with a TIANBO HJR-3FF-S-Z relay. The dissipated power is equal to 1.47 W.
Figure 6. Fourier spectrum of the output signal of the SP-DR1200 board with a TIANBO HJR-3FF-S-Z relay and a power of 1.47 W dissipated in a resistive load of 8 Ohms and a THD of less than 0.000053%.
Table 3
| Harmonic | Amplitude dBV | Amplitude Vrms |
| Fundamental @ 1KHz | 10.71 (-7.69 – 40 + 58.4) | 3.43 |
| 2nd | -116.3 (-85.3 – 40 + 9) | 1.53E-06 |
| 3rd | -123.1 (-88.1 – 40 + 5) | 699.84E-09 |
| 4th | -127.5 (-87.5 – 40) | 421.7E-09 |
| 5th | -126 (-86 – 40) | 501.18E-09 |
| 6th | N/A. Immersed in the background noise | 0 |
| Background noise | -132 (-92 – 40) | 251.19E-09 |
Therefore the THD for this test is less than or equal to 0.000053% and the power dissipated in the 8 Ohms load is equal to 1.47 W.
The following Fourier spectrum corresponds to the previous THD measurement using the same relay but now the power dissipated is equal to 43.43 W.
Figure 7. Fourier spectrum of the output signal of the SP-DR1200 board with a TIANBO HJR-3FF-S-Z relay and a power of 43.43 W dissipated in a resistive load of 8 Ohms and a THD of less than 0.0001%.
Table 4
| Harmonic | Amplitude dBV | Amplitude Vrms |
| Fundamental @ 1KHz | 19.39 (0.99 – 40 + 58.4) | 9.32 |
| 2nd | -101 (-70 – 40 + 9) | 8.91E-06 |
| 3rd | -113.1 (-78.1 – 40 + 5) | 2.21E-06 |
| 4th | -120 (-80 – 40) | 1E-06 |
| 5th | -133.4 (-93.4 – 40) | 213.79E-09 |
| 6th | N/A. Immersed in the background noise | 0 |
| Background noise | -138 (-98 – 40) | 125.89E-09 |
Therefore, the THD value for this test is less than or equal to 0.0001%. Since the measured signal is attenuated by 0.5X, the actual output value from the SP-DR1200 board is 9.32 Vrms / 0.5 = 18.64 Vrms. As a result, the power dissipated in the 8-ohm load is 43.43 W.
Shown below is the Fourier spectrum of the THD measurement performed on an 8 Ohm load connected to the SP-DR1200 loudspeaker protection system using a GOODSKY GZ-SS-112L relay. The dissipated power is equal to 1.18 W.
Figure 8. Fourier spectrum of the output signal of the SP-DR1200 board with a GOODSKY GZ-SS-112L relay and a power of 1.18 W dissipated in a resistive load of 8 Ohms and a THD of less than 0.0000619%.
Table 5
| Harmonic | Amplitud dBV | Amplitud Vrms |
| Fundamental @ 1KHz | 9.75 (-8.65 – 40 + 58.4) | 3.07 |
| 2nd | -115.7 (-84.7 – 40 + 9) | 1.64E-06 |
| 3rd | -124 (-89 – 40 + 5) | 630.95E-09 |
| 4th | -126.3 (-86.3 – 40) | 484.17E-09 |
| 5th | -126.3 (-86.3 – 40) | 484.17E-09 |
| 6th | N/A. Immersed in the background noise | 0 |
| Background noise | -132 (-92 – 40) | 251.19E-09 |
Therefore the THD for this test is less than or equal to 0.0000619% and the power dissipated in the 8 Ohms load is equal to 1.18 W.
The following Fourier spectrum corresponds to the previous THD measurement using the same relay but now the power dissipated is equal to 43.33 W.
Figure 9. Fourier spectrum of the output signal of the SP-DR1200 board with a GOODSKY GZ-SS-112L relay and a power of 43.33 W dissipated in a resistive load of 8 Ohms and a THD of less than 0.000118%.
Table 6
| Harmonic | Amplitude dBV | Amplitude Vrms |
| Fundamental @ 1KHz | 19.38 (0.98 – 40 + 58.4) | 9.31 |
| 2nd | -99.5 (-68.5 – 40 + 9) | 10.59E-06 |
| 3rd | -111.5 (-76.5 – 40 + 5) | 2.66E-06 |
| 4th | -118.7 (-78.7 – 40) | 1.16E-06 |
| 5th | -130.6 (-90.6 – 40) | 295.12E-09 |
| 6th | N/A. Immersed in the background noise | 0 |
| Background noise | -132 (-92 – 40) | 251.19E-09 |
Therefore the THD value for this test is less than or equal to 0.000118%.
Since the measured signal is attenuated by 0.5X, the actual output value from the SP-DR1200 board is 9.31 Vrms / 0.5 = 18.62 Vrms. Therefore, the power dissipated in the 8-ohm load is 43.33 W.
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