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Extensive study of IMD (and THD) distortions on 4 amplifiers with different topology

Head_Unit

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TIM distortion measurement ...15 kHz sine and 3.18 kHz square wave using 4:1 amplitude ratio.
Is this truly transient? Like a burst? I'm curious because almost all testing whether of amps or speakers is steady state, but various possible difference mechanisms are transient.
 
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pma

pma

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I would also include TIM distortion measurement using the high performance amp test signal version.

This is 15 kHz sine and 3.18 kHz square wave using 4:1 amplitude ratio. Frequency response of the test signal is 1st order low-pass with fc=100kHz. (see AES paper of Leinonen/Otala/Curl from 1976)


(I've started using this test signal for ADC/DAC testing too)

I was a fan of the TIM test in the past but ceased it. As shown by Cordell, Jung and Cabot (see results in [1] post #1) both CCIF 19+20kHz and THD 20kHz reveal potential SID issues and are more sensitive to various non-linearities than the Leinonen/Otala/Curl test. This test is the past, to me. It was good in its time to point to low slew rate design issues.
 
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pma

pma

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Without psychoacoustic weighting I don't find IM measurements useful and the same could be said about THD but normally the product spurs are low enough not to be an issue.

I find IM measurements very useful as they indicate to serious circuit design issues. IM can be predicted from HD only in case that the nonlinearity is 'static' (not dependent on frequency) and smooth (without dead zone, hysteresis etc.). This is not the case and especially not the case of cheap class D amps based on TPA32XX like circuits.
The audibility is also in question as there are many possible IM combinations. The amplifier simply should not behave like the A07 if it is expected to be considered seriously.

A07_4R_TDFD.png


A07_4R_Klingelnberg.png
 

Miska

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I was a fan of the TIM test in the past but ceased it. As shown by Cordell, Jung and Cabot (see results in [1] post #1) both CCIF 19+20kHz and THD 20kHz reveal potential SID issues and are more sensitive to various non-linearities than the Leinonen/Otala/Curl test. This test is the past, to me. It was good in its time to point to low slew rate design issues.

I still use the standard 19+20k IMD test, but I find that I get more extensive results by also using the TIM test. You should especially find differences in class-D amps where you can have aliasing problems. (HypeX and similar also suffer from these)
 
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pma

pma

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For those who are not familiar with the IMD test signals I am posting their shape in the time domain, as they appear at the output of the AIYIMA amplifier. 1kHz sine is added as a comparison. The amplitudes shown now are not the same as those used during measurements in post #1.

THD_1kHz_time.png


SMPTE_IMD_time.png


DIN_IMD_time.png


CCIF_IMD_time.png
 
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pma

pma

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One more amplifier measured, a small class AB with TMC frequency compensation, 2x40W/4ohm. This amp is a clear winner in my test regarding harmonic and intermodulation distortions, but it is limited with its smaller power when compared to A250W.

classAB_TMC_thdampl_60-15k.png


classAB_TMC_SMPTE.png


classAB_TMC_DIN.png


classAB_TMC_CCIF19.png


Please note that distortion products in the 19+20kHz IMD test all lay down below -100dBr related to F1 and F2 amplitudes.
 
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