Extensive study of IMD (and THD) distortions on 4 amplifiers with different topology
THD+N (SINAD) at 1kHz and 5W power has become a comparison measure of amplifier parameters here in ASR. I would like to demonstrate my deep persuasion that much more complex view of amplifiers distortion mechanism must be taken into account when trying to compare amplifiers based on their distortion performance. Distortion measurements methods have been developed and standardized for decades and their comparison survey was published for example by Cabot [1].
1. Explanation of intermodulation test methods used
1.1. SMPTE intermodulation
Intermodulation measurements using the SMPTE method have been around since the 1930s (Hilliard 1941, [2]). The test signal is a low frequency (usually 60Hz) and a high frequency (usually 7kHz) tone, summed together in a 4 to 1 amplitude ratio. Other amplitude ratios and frequencies are used occasionally, like DIN 250Hz+8kHz. This signal is applied to the device under test, and the output signal is examined for modulation of the upper frequency by the low frequency tone. The modulation components of the upper signal appear as sidebands spaced at multiples of the lower frequency tone. The amplitudes of the sidebands are root mean square summed and expressed as a percentage of the upper frequency level. Considering the SMPTE test in the time domain it becomes quite easy to understand its operation. The small amplitude high frequency component is moved through the input range of the device under test by the low frequency tone. The amplitude of the high frequency tone will be changed by the incremental gain of the device at each point, creating an amplitude modulation if the gain changes. This test is therefore particularly sensitive to such things as crossover distortion and clipping. High order nonlinearities create bumps in the transfer characteristic which produce large amounts of SMPTE IM. SMPTE testing is also good for exciting low frequency thermal distortion. The low frequency signal excursions excite thermal effects, changing the gain of the device and introducing modulation distortion. One advantage in sensitivity that the SMPTE test has in detecting low frequency distortion mechanisms is that the components occur at a high frequency. In most audio circuits there is less loop gain at high frequencies and so the distortion will not be reduced as effectively by feedback.
1.2. CCIF intermodulation
The CCIF IM distortion test differs from the SMPTE test in that a pair of signals closely spaced in frequency are applied to the device under test. The nonlinearity in the device under test causes intermodulation products between the two signals which are subsequently measured. For the typical case of input signals at 19kHz and 20kHz the intermodulation components will be at 1kHz, 2kHz, 3kHz, 4kHz, 5kHz, etc. and 18kHz, 21kHz, 17kHz, 22kHz, 16kHz, 23kHz, etc. Even order or asymmetrical distortions produce the low “difference frequency” components while the odd order or symmetrical nonlinearities produce the components (“skirts”) near the input signals. The distortion products generated in this test are usually very far removed from the input signal. This positions them outside the range of the auditory systems masking effects. If a test which measures what the ear might hear is desired, the CCIF test is a good candidate. The question of correlation with audibility for this test is covered very well by Stanley and McLaughlin 1977. However, whether one component of the test is more audible than another (second order more than third or vice versa) is not covered. The totally in band character of the test is one of its most attractive attributes. Unlike the sine-square test, the CCIF test may be used to severely band limited system. Distortion both before and after the band limiting point will be tested. Methods which lose their sensitivity with extreme band limiting can only test the circuitry before the band limiting. Some insight into the performance of the CCIF test as an indicator of SID can be obtained by examining the normalized derivative of the signal. The peak amplitude of the CCIF signal is twice that of a single sine wave. If ω1 and ω2 are the two input frequencies, normalizing the peak derivative by the peak amplitude we find that the normalized peak rate of change is (ω1 & ω2)/2. This is equal to that of a sine wave at the average input frequency. If the sine waves are very close in frequency compared to their mean frequency we find that the signal derivative will have the same general shape as the signal itself. Thus, the CCIF test should be similar to a THD test at the mean frequency in terms of its sensitivity to SID, but it will allow all of the resulting high order components to appear in band.
2. Amplifiers under test
3 different amplifiers were put in the test.
1st of them is class D AIYIMA A07 based on TI TPA3255 IC. My review with measurements is on
https://www.audiosciencereview.com/...s-and-review-lm4562-and-opa2134-option.19155/
It is a small cheap class D amplifier with respectable measurements with standard 1kHz THD+N test. However, more deep intermodulation measurements have not been done.
2nd amplifier is based on 52 years old Sinclair Z-30 project, with better output transistors and some other changes described in
https://www.audiosciencereview.com/...-amplifier-and-40-years-of-my-diy-mods.20198/
However, the basic circuit topology of this class AB amp has been preserved and it may be assumed that the well tuned original module would have similar distortion performance. THD+N of this amp at 1kHz is considerably worse that that of AIYIMA A07.
3rd amplifier is my 2x250W/4ohm DIY project described in
http://pmacura.cz/DIY_250W_4ohm_amplifier.html
It is a class AB amplifier with optimum bias of the output stage based on Douglas Self blameless topology. This amplifier has by far highest continuous output power from the amplifiers under test, 2x250W/4ohm and 2x150W/8ohm. THD at 5W is slightly higher at 5W than that of AIYIMA A07, but at higher power it dominates.
Regarding different output power of the DUT amplifiers, AIYIMA was tested at 20W for SMPTE and DIN IMD and 10W for CCIF 19+20kHz, Z-30 was tested at 15W for SMPTE and DIN IMD and 10W for CCIF 19+20kHz, A250W was tested at 50W for SMPTE and DIN IMD and 25W for CCIF 19+20kHz. Always with 8ohm resistor load. Let's see the results.
4th amplifier is Yamaha AX-396 (added on Sep 29, 2021)
class AB amplifier that I bought in the year 2002
www.audiosciencereview.com
3. SMPTE IMD 60Hz+7kHz
4. DIN IMD 250Hz+8kHz
5. CCIF IMD 19+20kHz
6. Summaries
Cumulative result was put in the chart and 1kHz THD and THD+N at 5W and at IMD test power were added as well.
Numbers plotted in the graph are below.
We can see that AIYIMA scores best at 5W/8ohm/1kHz THD and THD+N, followed closely by Yamaha and A250W, with Z-30 worst by far. In SMPTE and DIN IMD, A250W is best. In CCIF IMD test, A250W is best followed closely by Yamaha, with AIYIMA worst by far. In all-round distortion competition, A250W is the best from the amps under test, followed by Yamaha. It is difficult to me to tell the verdict between AIYIMA and Z-30. However, in a controlled A/B and ABX test AIYIMA was distinguishable from other two amps by listening, most probably because of FR modulation by speaker impedance above 8kHz. But it is not excluded that the bad high frequency linearity of AIYIMA might have been a reason as well.
(Yamaha AX-396 was added on September 29, 2021)
Literature:
[1] Cabot, Richard C.: “A Comparison of Nonlinear Distortion Measurement Methods.”, AES Preprint 1638, May 1980.
[2] Hilliard, John K.: “Distortion Tests by the Intermodulation Method”, Proceedings of the IRE, December1941.
[3] Stanley, Gerald and Dave McLaughlin: “Transient Intermodulation Distortion and Measurement”, AES Preprint 1308, November 1977.
THD+N (SINAD) at 1kHz and 5W power has become a comparison measure of amplifier parameters here in ASR. I would like to demonstrate my deep persuasion that much more complex view of amplifiers distortion mechanism must be taken into account when trying to compare amplifiers based on their distortion performance. Distortion measurements methods have been developed and standardized for decades and their comparison survey was published for example by Cabot [1].
1. Explanation of intermodulation test methods used
1.1. SMPTE intermodulation
Intermodulation measurements using the SMPTE method have been around since the 1930s (Hilliard 1941, [2]). The test signal is a low frequency (usually 60Hz) and a high frequency (usually 7kHz) tone, summed together in a 4 to 1 amplitude ratio. Other amplitude ratios and frequencies are used occasionally, like DIN 250Hz+8kHz. This signal is applied to the device under test, and the output signal is examined for modulation of the upper frequency by the low frequency tone. The modulation components of the upper signal appear as sidebands spaced at multiples of the lower frequency tone. The amplitudes of the sidebands are root mean square summed and expressed as a percentage of the upper frequency level. Considering the SMPTE test in the time domain it becomes quite easy to understand its operation. The small amplitude high frequency component is moved through the input range of the device under test by the low frequency tone. The amplitude of the high frequency tone will be changed by the incremental gain of the device at each point, creating an amplitude modulation if the gain changes. This test is therefore particularly sensitive to such things as crossover distortion and clipping. High order nonlinearities create bumps in the transfer characteristic which produce large amounts of SMPTE IM. SMPTE testing is also good for exciting low frequency thermal distortion. The low frequency signal excursions excite thermal effects, changing the gain of the device and introducing modulation distortion. One advantage in sensitivity that the SMPTE test has in detecting low frequency distortion mechanisms is that the components occur at a high frequency. In most audio circuits there is less loop gain at high frequencies and so the distortion will not be reduced as effectively by feedback.
1.2. CCIF intermodulation
The CCIF IM distortion test differs from the SMPTE test in that a pair of signals closely spaced in frequency are applied to the device under test. The nonlinearity in the device under test causes intermodulation products between the two signals which are subsequently measured. For the typical case of input signals at 19kHz and 20kHz the intermodulation components will be at 1kHz, 2kHz, 3kHz, 4kHz, 5kHz, etc. and 18kHz, 21kHz, 17kHz, 22kHz, 16kHz, 23kHz, etc. Even order or asymmetrical distortions produce the low “difference frequency” components while the odd order or symmetrical nonlinearities produce the components (“skirts”) near the input signals. The distortion products generated in this test are usually very far removed from the input signal. This positions them outside the range of the auditory systems masking effects. If a test which measures what the ear might hear is desired, the CCIF test is a good candidate. The question of correlation with audibility for this test is covered very well by Stanley and McLaughlin 1977. However, whether one component of the test is more audible than another (second order more than third or vice versa) is not covered. The totally in band character of the test is one of its most attractive attributes. Unlike the sine-square test, the CCIF test may be used to severely band limited system. Distortion both before and after the band limiting point will be tested. Methods which lose their sensitivity with extreme band limiting can only test the circuitry before the band limiting. Some insight into the performance of the CCIF test as an indicator of SID can be obtained by examining the normalized derivative of the signal. The peak amplitude of the CCIF signal is twice that of a single sine wave. If ω1 and ω2 are the two input frequencies, normalizing the peak derivative by the peak amplitude we find that the normalized peak rate of change is (ω1 & ω2)/2. This is equal to that of a sine wave at the average input frequency. If the sine waves are very close in frequency compared to their mean frequency we find that the signal derivative will have the same general shape as the signal itself. Thus, the CCIF test should be similar to a THD test at the mean frequency in terms of its sensitivity to SID, but it will allow all of the resulting high order components to appear in band.
2. Amplifiers under test
3 different amplifiers were put in the test.
1st of them is class D AIYIMA A07 based on TI TPA3255 IC. My review with measurements is on
https://www.audiosciencereview.com/...s-and-review-lm4562-and-opa2134-option.19155/
It is a small cheap class D amplifier with respectable measurements with standard 1kHz THD+N test. However, more deep intermodulation measurements have not been done.
2nd amplifier is based on 52 years old Sinclair Z-30 project, with better output transistors and some other changes described in
https://www.audiosciencereview.com/...-amplifier-and-40-years-of-my-diy-mods.20198/
However, the basic circuit topology of this class AB amp has been preserved and it may be assumed that the well tuned original module would have similar distortion performance. THD+N of this amp at 1kHz is considerably worse that that of AIYIMA A07.
3rd amplifier is my 2x250W/4ohm DIY project described in
http://pmacura.cz/DIY_250W_4ohm_amplifier.html
It is a class AB amplifier with optimum bias of the output stage based on Douglas Self blameless topology. This amplifier has by far highest continuous output power from the amplifiers under test, 2x250W/4ohm and 2x150W/8ohm. THD at 5W is slightly higher at 5W than that of AIYIMA A07, but at higher power it dominates.
Regarding different output power of the DUT amplifiers, AIYIMA was tested at 20W for SMPTE and DIN IMD and 10W for CCIF 19+20kHz, Z-30 was tested at 15W for SMPTE and DIN IMD and 10W for CCIF 19+20kHz, A250W was tested at 50W for SMPTE and DIN IMD and 25W for CCIF 19+20kHz. Always with 8ohm resistor load. Let's see the results.
4th amplifier is Yamaha AX-396 (added on Sep 29, 2021)
class AB amplifier that I bought in the year 2002
Review and Measurements of vintage Yamaha AX-396 integrated amplifier
Review and Measurements of vintage Yamaha AX-396 integrated amplifier This review shows measurements of Yamaha AX-396 integrated amplifier, ser. no. Y399871TV, made in Malaysia. I bought this amplifier in the year 2002 and have been using it shortly. Then it was sleeping in my stock and 5...
www.audiosciencereview.com
3. SMPTE IMD 60Hz+7kHz
4. DIN IMD 250Hz+8kHz
5. CCIF IMD 19+20kHz
6. Summaries
Cumulative result was put in the chart and 1kHz THD and THD+N at 5W and at IMD test power were added as well.
Numbers plotted in the graph are below.
We can see that AIYIMA scores best at 5W/8ohm/1kHz THD and THD+N, followed closely by Yamaha and A250W, with Z-30 worst by far. In SMPTE and DIN IMD, A250W is best. In CCIF IMD test, A250W is best followed closely by Yamaha, with AIYIMA worst by far. In all-round distortion competition, A250W is the best from the amps under test, followed by Yamaha. It is difficult to me to tell the verdict between AIYIMA and Z-30. However, in a controlled A/B and ABX test AIYIMA was distinguishable from other two amps by listening, most probably because of FR modulation by speaker impedance above 8kHz. But it is not excluded that the bad high frequency linearity of AIYIMA might have been a reason as well.
(Yamaha AX-396 was added on September 29, 2021)
Literature:
[1] Cabot, Richard C.: “A Comparison of Nonlinear Distortion Measurement Methods.”, AES Preprint 1638, May 1980.
[2] Hilliard, John K.: “Distortion Tests by the Intermodulation Method”, Proceedings of the IRE, December1941.
[3] Stanley, Gerald and Dave McLaughlin: “Transient Intermodulation Distortion and Measurement”, AES Preprint 1308, November 1977.
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