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Measuring the "sound signature" of two different integrated amplifiers.

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GXAlan

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If I summarize properly the root cause issue around 4khz, if means that the amplifier needs to be matched with the speaker impedance.
This is very difficult/impossible to verify with the available marketing information.
Imagine what is the user experience with the Onkyo RZ50 on board amps going to safety mode without and warning.
In the old time, on some amplifiers, there was a vu-meter and a Led indicating an overload.
At list nowadays the amplifier manufacturers could put back the overload Led.

We actually do NOT know the root cause. While we can speculate about what I heard and how easy it might be heard, all of the numbers and measurements are done with a resistive load.

The power is <0.2 watts RMS so we weren’t running into overload either.

All I have been able to “prove” is that the differences are real, measurable, and meet the threshold of audibility.

What I thought I heard, described as “attack” for the piano is the lack of compression of about 1 dB into a resistive load.

The difference when compared to the raw digital file does in fact suggest the better amplifier is the more accurate amplifier.
 
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GXAlan

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@KSTR figured it out! The left and right channels had been swapped during recording! Presumably this must have been the case when listening too!

The calculations had been done in stereo. He also identified that the phase was inverted between the two -- this makes sense now because the PM-11s2/SA-11s2 use the older standard not the current XLR standards.


The tape loop also seems to have a bigger difference since crossing over the channels did not make a difference.

SA11s2-direct to-PA5 vs. SA11s2 direct to PM11s2
The individual channels drop to indistinguishable levels but the L+R is different

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And now SA-11s2 -> PM11s2 -> tape out -> PA5 vs SA-11s2 ->PM-11s2
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KSTR

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This is what I get after some pre-processing of the files and using (IME) adequate settings in DW. I've been using the files for System A and System B (so, tape loop being in place), kindly provided by @GXAlan.

- manual trim of start and end alignment in an audio editor
- channel swap for one of the recordings
- polarity inversion for one the recordings
- drift correction enabled in DW (even though the accumulated drift is really low with less than 3ppm)
- 20Hz and 20kHz filters enable on reference and compare, and before and after processing
- non-linear calibration turned off as the and phase differences are not that large and less than optimum settings make result often worse
- there still is a channel level difference of 0.7dB in the left channel
- using L (or R) channels in the compare, not L+R(mono mix) or stereo. Mono mix cannot work because of the level difference and Stereo analyzes only left channels but applies results to both, cannot work either, obviously.


Right channels compare:
1665321762120.png

Note the rather good RMS null.

Other data generated by DW now also is very good match

Left channel is very similar, only slightly worse and from that I would conclude that System A and System B are "close to identical" in measurements and I'm convinced the perceived difference will be absent when comparing apples to apples.. that is, one channel of System A to the other channel of System B, connecting only one (and the same) speaker, and the polarity flip and precision level matching.
 
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GXAlan

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The other lesson that is newly understood is that channel imbalance can affect the PK Metric.

When testing individual channels, Deltawave can use different algorithms to match the two contents. But when using L+R, it has to apply the same corrections to both.
 

pkane

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The other lesson that is newly understood is that channel imbalance can affect the PK Metric.

When testing individual channels, Deltawave can use different algorithms to match the two contents. But when using L+R, it has to apply the same corrections to both.

L+R is simply the calculated mono signal from left and right channels. It's neither left nor right.
 

tw99

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So, as an executive summary, it seems like there were some measurement and processing issues, and when these are corrected, actually there is no measurable difference between the two amps?

So the original contention that one amp has more "attack" is now just back to being an unconfirmed impression ?
 
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GXAlan

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So, as an executive summary, it seems like there were some measurement and processing issues, and when these are corrected, actually there is no measurable difference between the two amps?

So the original contention that one amp has more "attack" is now just back to being an unconfirmed impression ?

Maybe. We still see -65 dBFS for the setup with the tape loop in the chain. And we did identify phase differences.

1) we are back to confirming that two amplifiers that are in the blue zone can be matched perfectly when running at low power
2) The differences no longer cross confident thresholds for audibility
3) The channel error affects PK metric but doesn’t affect the RMS and peak volume —> so we do know there was a compression effect with the tape loop was on the system.
 

HarmonicTHD

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Maybe. We still see -65 dBFS for the setup with the tape loop in the chain. And we did identify phase differences.

1) we are back to confirming that two amplifiers that are in the blue zone can be matched perfectly when running at low power
2) The differences no longer cross confident thresholds for audibility
3) The channel error affects PK metric but doesn’t affect the RMS and peak volume —> so we do know there was a compression effect with the tape loop was on the system.
Either way. The work you did and the way you handled the discovery of KSTR speaks volumes about your integrity and scientific mindset. I can only applaud and thank you for it. (Others in your position might have defended their results to the bone, you instead kept an open mind and critically reviewed the facts once pointed out by KSTR).

I also guess it is correct to assume that now that the measured „difference do not cross confident thresholds for audibility“ only an elaborate controlled ABX could confirm or not if there is indeed a difference in audibility.
 
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GXAlan

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Either way. The work you did and the way you handled the discovery of KSTR speaks volumes about your integrity and scientific mindset. I can only applaud and thank you for it. (Others in your position might have defended their results to the bone, you instead kept an open mind and critically reviewed the facts once pointed out by KSTR).

I also guess it is correct to assume that now that the measured „difference do not cross confident thresholds for audibility“ only an elaborate controlled ABX could confirm or not if there is indeed a difference in audibility.

Well it’s reassuring // it means I did hear a difference, but it was configuration error to show these numbers. :) which was the whole start of this system.

All of the actual listening had the tape loop in the system which still has higher difference numbers but not the easy thresholds.

I cannot say for a fact whether or not I listened in proper stereo or reversed.

Now that we also identified the inverting phase, it may be important to test that because that would correlate with “attack” potentially.
But yeah, this is the value of the scientific process. An error was made, but in the process we learned a lot and know how to better test the next round of comparisons.

Edit: What I don’t know is if the reversal was at the E1DA hookup or if it was between the source and amp. But this pushes the PK
Metric from -50 dB to -65 dB give or take. I would lean toward the E1DA hook up since switching from tape loop to no tape loop required looking at the jacks again.
 
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KSTR

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Either way. The work you did and the way you handled the discovery of KSTR speaks volumes about your integrity and scientific mindset. I can only applaud and thank you for it. (Others in your position might have defended their results to the bone, you instead kept an open mind and critically reviewed the facts once pointed out by KSTR)
I was also very pleased to see Alan's swift reaction to my findings, very professional. To note, everyone working in a field of engineering or science knows how easily one can overlook simple issues with setups and procedures despite all efforts of being thorough, it just happens... two steps forward, one step back... or sometimes even full reset.

I may add that it took me several minutes staring on the waveforms in the files (and scratching my head about the origin of those unusual extremely large and non-systematic differences) before I finally had the enlighting moment and saw the correlation to the other channel and vice versa. It's been there from the begining but selective attention kept me from spotting it right away.

I also did not notice the channel swap in the informal listening carried out simultaneously even though it's very obvious once your attention focusses on that.
Our brains can be tricked so easily...
 
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