Technical Info.

THD measurements

This section introduces the process for measuring the total harmonic distortion of the CDS-4700 amplifier.

The THD measurement system used by Olas audio, although it is somewhat simple and could even be catalogued as archaic by many, it is a system that 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.

The sine wave generator with intrinsic total harmonic distortion less than or equal to 0.000015% @ 1KHz was designed and tested by Olas audio. The signal from this generator is injected into the input of the system under test, in this case the CDS-4700 amplifier. An 8 Ohms load (to be precise an 8.3 Ohms) was connected to the output port of the amplifier where the distortion measurement will be taken.

The input of the attenuator buffer and Notch filter (also designed by Olas audio) is connected to the 8 Ohms load and the output of the filter is connected to the input of an audio interface and this in turn with a computer through a USB port. A software capable of delivering the Fourier spectrum of the analyzed signal (ARTA, REW and so on) must be used.

The buffer voltage attenuation factor can be selected between 1X (zero attenuation) and 0.38X. The Notch filter attenuation is -66.5 dBV at the fundamental frequency (1 KHz). Furthermore, said filter inserts a loss of 9 dBV and 5 dBV in the second and third harmonics respectively. Finally, a 40 dBV amplification of this result is performed (voltage gain equal to 100).

The following image corresponds to the Fourier spectrum of the test signal with an amplitude equal to 1.1 Vrms (-25.63 dBV – 40 dBV + 66.5 dBV = 0.87 dBV).

Since the buffer voltage attenuation factor was selected as null or 1X, the displayed result is the signal from the generator passing through the Notch filter. From the image, the fundamental value of the signal is taken as 0.87 dBV (-25.63 – 40 dBV + 66.5 dBV), the second harmonic is immersed in the noise of the system, the third harmonic has a value of -137.1 dBV (-102.1 dBV – 40 dBV + 5dBV), the fourth harmonic has a value of -142.4 dBV (-102.4 dBV – 40 dBV). The rest of the harmonics are immersed in the noise of the system. Background noise is around -150 dBV (-110 dBV – 40 dBV).

The RMS voltages corresponding to these harmonics (having already applied the correction for the insertion loss in the third harmonic and corrected for the 40 dBV of the amplification by the Notch filter) are: 1.1 V, 139.64 nV, 75.86 nV and 31.6 nV for the fundamental, third and fourth harmonics and the background noise of the system respectively. Then, the total harmonic distortion result of the sine wave generator including also the entire measurement system is less than or equal to 0.0000147%..

Shown below is the Fourier spectrum of the CDS-4700 amplifier output signal. The power consumed is 2.31 W measured into a resistive load of 8.3 Ohms.

In this case, the 0.38X attenuation was selected in the input buffer of the measurement system.

Harmonic	Amplitude dBV	Amplitude Vrms
Fundamental @ 1KHz	4.44 (-22.06 – 40 + 66.5)	1.667
2th	-124 (-93 – 40 + 9)	630.96E-09
3rd	-127.7 (-92.7 – 40 + 5)	412.1E-09
4th	-136 (-96 – 40)	158.49E-09
5th	N/A. Immersed in the background noise	0
6th	-130.8 (-90.8 – 40)	288.4E-09
Background noise	-140 (-100 – 40)	100E-09

Therefore the THD for this test is less than or equal to 0.0000487%.

Since the measured signal is attenuated, the actual value at the output of the CDS-4700 amplifier is equal to 1.667 / 0.38 = 4.38 Vrms). So the power dissipated in the 8.3 Ohms load is equal to 2.31 W.

The following image shows the Fourier spectrum of the CDS-4700 amplifier output signal at a power of 47.18 W into a resistive load of 8.3 Ohms.

Harmonic	Amplitude dBV	Amplitude Vrms
Fundamental @ 1KHz	17.53 (-8.97 – 40 + 66.5)	7.52
2nd	-101.4 (-70.4 – 40 + 9)	8.51E-06
3rd	-96.6 (-61.6 – 40 + 5)	14.79E-06
4th	-131.5 (-91.5 – 40)	266.07E-09
5th	-128.6 (-88.6 – 40)	371.54E-09
6th	-130.8 (-90.8 – 40)	288.4E-09
Background noise	-140 (-100 – 40)	100E-09

Therefore the THD for this test is less than or equal to 0.000227%.

Since the measured signal is attenuated, the actual value at the output of the CDS-4700 amplifier is equal to 7.52/0.38 = 19.79 Vrms). So the power dissipated in the 8.3 Ohms load is equal to 47.18 W.

The following diagram shows the THD measurement sequence for the SP-DR1200 Audio Wave Speaker Protection Board.

The following Fourier spectrum corresponds to the THD measurement performed on an 8.3 Ohm load connected to the SP-DR1200 loudspeaker protection board with a TIANBO HJR-3FF-S-Z relay. The dissipated power is equal to 2.53 W..

Harmonic	Amplitude dBV	Amplitude Vrms
Fundamental @ 1KHz	4.83 (-21.67 – 40 + 66.5)	1.74
2nd	N/A. Immersed in the background noise	0
3rd	117.2 (-82.2 – 40 + 5)	1.38E-06
4th	-122.4 (-82.2 – 40)	758.58E-09
5th	-131.5 (-91.5 – 40)	266.07E-09
6th	-125.4 (-85.4 – 40)	537.03E-09
Background noise	-140 (-100 – 40)	100E-09

Therefore the THD for this test is less than or equal to 0.000097%.

Since the measured signal is attenuated, the actual value at the output of the SP-DR1200 board is equal to 1.74/0.38 = 4.58 Vrms). So the power dissipated in the 8.3 Ohms load is equal to 2.53 W.

The following Fourier spectrum corresponds to the previous THD measurement but now the power dissipated is equal to 51.68 W.

Harmonic	Amplitude dBV	Amplitude Vrms
Fundamental @ 1KHz	17.92 (-8.58 – 40 + 66.5)	7.87
2nd	-100.9 (-69.9 – 40 + 9)	9.02E-06
3rd	-92.7 (-57.7 – 40 + 5)	23.17E-06
4th	-124.6 (-84.6 – 40)	588.84E-09
5th	-116.3 (-76.3 – 40)	1.53E-06
6th	-125.1 (-85.1 – 40)	555.9E-09
Background noise	-140 (-100 – 40)	100E-09

Therefore the THD value for this test is less than or equal to 0.00031%.

Since the measured signal is attenuated, the actual value at the output of the SP-DR1200 board is equal to 7.87/0.38 = 20.71 Vrms). So the power dissipated in the 8.3 Ohms load is equal to 51.68 W.

Shown below is the Fourier spectrum of the THD measurement performed on an 8.3 Ohm load connected to the SP-DR1200 loudspeaker protection system with a GOODSKY GZ-SS-112L relay. Dissipated power is equal to 1.95 W.

Harmonic	Amplitud dBV	Amplitud Vrms
Fundamental @ 1KHz	3.7 (-22.8 – 40 + 66.5)	1.53
2nd	N/A. Immersed in the background noise	0
3rd	-117.9 (-82.9 – 40 + 5)	1.27E-06
4th	-127.3 (-87.3 – 40)	431.52E-09
5th	-130 (-90 – 40)	316.23E-09
6th	-129.1 (-89.1 – 40)	350.75E-09
Background noise	-140 (-100 – 40)	100E-09

Therefore the THD for this test is less than or equal to 0.000093%.

Since the measured signal is attenuated, the actual value at the output of the SP-DR1200 card is equal to 1.53/0.38 = 4.03 Vrms). So the power dissipated in the 8.3 Ohms load is equal to 1.95 W.

The following Fourier spectrum corresponds to the previous THD measurement but now at a dissipated power equals to 40.88 W.

Harmonic	Amplitude dBV	Amplitude Vrms
Fundamental @ 1KHz	16.91 (-9.59 – 40 + 66.5)	7
2nd	-99.2 (-68.2 – 40 + 9)	10.96E-06
3rd	-96.1 (-61.1 – 40 + 5)	15.67E-06
4th	-127.6 (-87.6 – 40)	416.87E-09
5th	-119.5 (-79.5 – 40)	1.06E-06
6th	-124.6 (-84.6 – 40)	588.84E-09
Background noise	-140 (-100 – 40)	100E-09

Therefore the THD value for this test is less than or equal to 0.00027%.

Since the measured signal is attenuated, the actual value at the output of the SP-DR1200 board is equal to 7/0.38 = 18.42 Vrms). So the power dissipated in the 8.3 Ohms load is equal to 40.88 W.