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Class D amplifier's THD+N above 1KHz test result? | Simplify

Guddu

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Seeing multitone test output on reviews, specially for class D amps (like TPA3255, etc.), THD+N % below 1KHz isn't as good as 1KHz or below.
Would you see it -55/60/70/80 DB for higher freq. vs -100/102 DB for mid/bass/lower freq.? How and What would you interpret it into your conclusion?

.......with the fact that most of us probably own many of such amps with this performance and probably are daily drivers for most of us with acceptable audio output.

1735932028172.png


How would the SINAD chat look if the test was taken at 15 KHz rather than 1 KHz?
 
FAQ no.7.
 
 
FAQ no.7.

Thanks, but are you referring to measurement bandwidth here (being kept 45KHz for this measurement)?
The parameter I am referring is between 1KHz - 20 KHz.

Post Edit: So 1 - 10KHz is to be taken into consideration, with the fact that over 10 KHz harmonics goes beyond hearing?
 
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Thanks, I have gone though it (and some other posts/threads as well) and mostly it has ended with unintentional arguments which is why I thought I might give it another attempt to request simpler explanation.

So, the question:
Would you see it -55/60/70/80 DB for higher freq. vs -100/102 DB for mid/bass/lower freq.? How and What would you interpret it into your conclusion?

From your linked post, let's ignore 100+ watts results. What about up to 100w, considering I personally haven't seeing my meter showing any of my amplifiers ever going over 100 watts.
 
I'm curious about those higher test frequency results too.

What would be the outcome if the bandwidth was limited to 0-20kHz?

Could it be that lesser amplifiers could be identified by attenuation of the (eg 15kHz) fundamental? (Because energy has been transferred into harmonics that are then filtered out).
 
I'm curious about those higher test frequency results too.

What would be the outcome if the bandwidth was limited to 0-20kHz?
For frequencies >10 kHz, there will be no harmonic distortion, because all their harmonics will be >20 kHz.
Could it be that lesser amplifiers could be identified by attenuation of the (eg 15kHz) fundamental? (Because energy has been transferred into harmonics that are then filtered out).
If the amplitude of the fundamental is lowered, it will show up in the frequency response measurement.
 
I'm curious about those higher test frequency results too.

What would be the outcome if the bandwidth was limited to 0-20kHz?

Could it be that lesser amplifiers could be identified by attenuation of the (eg 15kHz) fundamental? (Because energy has been transferred into harmonics that are then filtered out).
Then the best you can do to compare apples to apples is a 6.6KHz signal,at least this will have H2 and H3 computed in the result and it's well into the audible range.
There a new trend in some stuff though having H4,H5,etc higher than the first couple ones :facepalm:

I would stick with Amir's 45KHz BW for those too.
 
For frequencies >10 kHz, there will be no harmonic distortion, because all their harmonics will be >20 kHz.

In theory, but I think it would be nice to see some experimental verification for a couple of different amplifiers with the filter bandwidth set to 20kHz.

I assume that the filter isn't actually a brick wall but rather a rolloff with a certain rate of dB per octave. So some fraction of harmonics might actually still be measured. This could show differences between different amplifiers, that might be more meaningful than the 45kHz ones.

Does the argument about the typical frequency distribution of music content perhaps omit testing the capability of an amp to support very rapid attack (eg leading edge of an explosion). (There was another recent thread asking about testing slew rate).
 
Could it be that lesser amplifiers could be identified by attenuation of the (eg 15kHz) fundamental? (Because energy has been transferred into harmonics that are then filtered out).
Yes but adding a few harmonics that are 60db down will add (or take away) very little power, like -60 db or 1/1000000 of the power in the fundamental.
 
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How would the SINAD chat look if the test was taken at 15 KHz rather than 1 KHz?
Just find the 5W point on the graph for that frequency and you'll have your answer:

1735938009663.png


As noted, though, above 10kHz it's far less critical due to several factors: harmonics being out of the audible frequency range, very low power levels involved (even 5W is quite a high power level at those frequencies), and our hearing resolution being pretty low at those high frequencies (if you can even still hear them at all). There is some question about IMD though. I'm trying to remember if that's what the 19kHz/20kHz test is supposed to capture?

1735938269881.png
 
Summing the response till now, 15/20 KHz THD+N appears to be totally ignorable if it is misleading.
Why not request Amir to just take it off and/or use (may be) 5KHz for SINAD chart where most of these amplifiers show their weakness?
Will it be unreasonable?
 
Just adding a few examples for illustration...

Denon AVR-X8500H (amp section):
SINAD: 86dB
Gain: 29dB
index.php

(Note: Testing goes up to 20kHz, others are only 15kHz)

Buckeye NC252MP:
SINAD 97dB
Gain 26dB
index.php


BoXem 4222/E1 (Purifi 1ET7040SA):
SINAD 106dB
Gain 20dB (medium setting)
index.php


Soncoz SGP1:
SINAD 107dB
Gain 21dB
index.php

(20Hz result is claimed to have been fixed by manufacturer).

Benchmark AHB2:
SINAD 111dB
Gain 9dB (low setting)
index.php


As Amir says, the (1kHz) SINAD figure seems to be a good measure of the overall cleanliness.

On the other hand...
Purifi 1ET7040SA and Soncoz SGP1 have pretty much the same SINAD but the 10kHz and 15kHz results differ a fair bit. Does it tell us anything?
 
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On the other hand...
Purifi 1ET7040SA and Soncoz SGP1 have pretty much the same SINAD but the 10kHz and 15kHz results differ a fair bit. Does it tell us anything?
What’s your interpretation?

How do you define linearity of amplifier? In simpler less-technical way!
 
How do you define linearity of amplifier? In simpler less-technical way!
Ok, I'll have a go!

The job of an amplifier is to make an electrical signal bigger. Ideally, the output signal should have exactly the same shape as the input signal, but with the voltage at each instant multiplied by the same scale factor (the gain). If the shape remains exactly the same then the amplifier is linear.

In reality the shape can get a bit distorted (i.e. the peaks might get slightly clipped, in which case they have been amplified by a lower gain factor). This is nonlinear behaviour because the amount of gain (the multiplication factor) depends on how large the input signal is.

When we calculate an FFT of a time domain signal, it is decomposed into its frequency domain components. If we added together sine waves in the proportions shown on the FFT (and with the correct phase) then we could construct a signal with the same shape in the time domain.

So for a perfectly linear amplifier with a pure 1kHz input signal the FFT of the output would only show a single spike at 1kHz. If the output is slightly distorted then this shows up as contributions at other frequencies in the FFT. When the input signal is a pure tone, the strongest distortion components typically occur at harmonics of the input signal frequency (i.e. 2kHz, 3kHz etc if the input is at 1kHz).

The other factor discussed is the bandwidth of the measurement. I.e. Which range of frequencies is considered when calculating the SINAD. I think Amir's default for this is 45kHz. Consider an amplifier that has a large distortion component at the 3rd harmonic.

If the test is run with a 1kHz input signal then the distortion at 3kHz is easily within the measurement bandwidth of 45kHz. The SINAD value will be degraded.

But if the input signal is at 20kHz then the 3rd harmonic at 60kHz is outside the measurement bandwidth and will not be taken into account in the SINAD value.


What’s your interpretation?
A highly simplified explanation of a Class D amplifier is that its output voltage is controlled by a feedback loop and two switches. One connects to the high voltage rail (increasing the output voltage) and the other the low voltage rail (decreasing it).

Class D amplifiers (the Buckeye NC252MP and the BoXem Purifi 1ET7040SA) have high internal switching frequency, but given the mode of operation, perhaps it wouldn't be too surprising if their amount of distortion increased as a function of input signal frequency.

The differences between the Purifi 1ET7040SA (class D) and Soncoz SGP1 (class AB) 15kHz SINAD results are perhaps due to differing spread of distortion components. Eg. The way that the Purifi module distortion seems to reduce at higher power levels is perhaps because its distortion tends to shift to higher harmonics and therefore gets pushed outside of the measurement bandwidth.

I don't think distortion components that are >20kHz are going to be an audible concern which is why I was wondering if limiting the measurement bandwidth to 20kHz might make sense.
 
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