What to do about the ABX test?

[xnor]

For SS amps, output impedance is not sufficient to characterize an amplifier driving loads.
Here are two amp measurements into load with frequency variation based on the output impedance.

The relevant bit of information regarding my first point is the deviation of the dark gray line from each other when comparing both graphs. After all, you're not switching from 8 ohm speakers to 2 ohm speakers when trying to compare these amps, you're using the same speakers.

Regarding your point, besides some curves in these graphs looking like the were manually inserted (though this might be due to lossy compression), I don't know how they measured the output impedance.
If from 8 to 2 ohms there's a 0.75 dB drop then the (effective) output impedance is about 0.25 Ohm.
For a 0.25 dB drop it is about 0.08 Ohm.

 

[levimax]

See below some actual FR measurements I took when doing an ABX test of a tube amp with ~0.8 ohm output impedance and a SS amp with vanishing low output impedance and then the SS amp with a 0.9 ohm resistor in series. The FR behaved as expected and the 0.9 ohm resistor almost perfectly "converted" the FR of the SS amp to that of the tube amp. Even though the FR differences were measurable due to the output impedance I was not able to reliably ABX them, maybe someone could. I guess if you had pathological cases like a 5 ohm output impedance SET and a speaker with 1 ohm minimum you could get some large and hard to predict FR changes but for most amps and speakers the FR differences due to output impedance differences are small and easily compensated for if need be.

 

[RichB]

The relevant bit of information regarding my first point is the deviation of the dark gray line from each other when comparing both graphs. After all, you're not switching from 8 ohm speakers to 2 ohm speakers when trying to compare these amps, you're using the same speakers.

Regarding your point, besides some curves in these graphs looking like the were manually inserted (though this might be due to lossy compression), I don't know how they measured the output impedance.
If from 8 to 2 ohms there's a 0.75 dB drop then the (effective) output impedance is about 0.25 Ohm.
For a 0.25 dB drop it is about 0.08 Ohm.

The black lines relate to your point on output impedance.
The drop in output at 4 Ohms and 2 Ohms shows the amp with lower output impedance having a greater drop.
Since most speakers are reactive loads, the effective load placed on the amplifier can have an effect. It may even trigger limiting when the voltage.
Nothing in the output impedance measurements of the above amplifiers explains the drop in output at 2 Ohms.
In fact, the frequency variation in the black line is far less that the load.
There is also no reason to believe that a combination of load and phase could not produce greater deviations in linearity.

This is an interesting article:

Heavy Load: How Loudspeakers Torture Amplifiers​

Heavy Load: How Loudspeakers Torture Amplifiers

Why, in loudspeaker reviews, is impedance measured (assuming that the magazine in question bothers to measure anything)? Generally, for one principal reason only: to establish whether the speaker presents an "easy" or a "difficult" load to its partnering amplifier. In the design context, much...

www.stereophile.com

Output impedance influences linearity with resistive loads but EPDR can be used to get a better understanding of the load presented by a speaker.

EPDR is simply the resistive load that would give rise to the same peak device dissipation as the speaker itself. Adopting the EPDR view, the red traces of figs.5–7 become those of figs.8–10.

Taken together, these figures confirm that the orders of EPDR identified in figs. 8–10 are of real, practical significance when playing music signals: speakers really can make these high demands of amplifier output-device dissipation in normal use. If the amplifier's protection is invoked as a result, then its output will be clipped, even though the speaker's voltage and current demands may be within its capability.

This article is useful if you want to understand why the B&W 802 "has a reputation for being an amplifier ball-breaker."
Output impedance of an amplifier is an important metric, but is insufficient to describe an amplifiers performance driving reactive loads.

With respect to amplifiers, we have objectivists that can read these charts, see that even with simple tests, deviations can delve into the audible range so must rely on ABX to explain why it does not matter. I'd expect a call for more and better measurements.

- Rich

 

[vavent_]

Hi @solderdude can you please help me with question, can I perform correct ABX test having Mojo 2 and Qudelix 5k? Or it only possible with separate DACs and Amps connected via RCA/etc?

 

[DVDdoug]

can I perform correct ABX test having Mojo 2 and Qudelix 5k?

Yes... probably... but you'll need someone to help you. Someone to do the randomizing and blind switching so you don't know which one you're listening to.

Level matching might be tricky and if one unit makes a "click" or "pop" or something like that when switched-in or plugged-in, that's a "give away" to which one you're listening to and it invalidates the test.

You may not be able to switch back-and-forth instantly and slower switching or a pause between units makes it harder to hear subtle differences. But in real life you wouldn't be switching quickly and if you can't hear the difference between one on Monday and the other on Tuesday the differences aren't worth worrying about (or worth paying more for).

 

[vavent_]

Yes... probably... but you'll need someone to help you. Someone to do the randomizing and blind switching so you don't know which one you're listening to.

Level matching might be tricky and if one unit makes a "click" or "pop" or something like that when switched-in or plugged-in, that's a "give away" to which one you're listening to and it invalidates the test.

You may not be able to switch back-and-forth instantly and slower switching or a pause between units makes it harder to hear subtle differences. But in real life you wouldn't be switching quickly and if you can't hear the difference between one on Monday and the other on Tuesday the differences aren't worth worrying about (or worth paying more for).

Do I need to invest additional money? I have a friend that could help with blind part of the test, but I don't have this volume meter to calibrate the loudness of each one, also to switch back-and-forth fast, do I need some switcher/box that will have like a button that will quickly allow to listen to one DAC/amp or another? Or it's still ok that friend will manually plug/unplug the 3.5 jack into each DAC/amp?

 

[somebodyelse]

A multimeter is cheap (a few $) and most of them are accurate for AC up to 400Hz. Send the same sine wave to both and match them - you should be able to get it within 1%. Various bits of software can make the test signal for you, like REW. I usually use sox.
https://www.audiosciencereview.com/...tor-in-rew-to-take-measurements-please.43308/
https://www.audiosciencereview.com/...to-sox-audio-tool-as-a-signal-generator.4242/

A switcher would be ideal, but if your helper can plug and unplug without a 'tell' within a couple of seconds that should be fine too. That can be tricky though - the sound of unplugging and plugging can be different for example. It can be a problem with switch boxes too - one paper describes one participant who got perfect scores and explained the sound made by the relays in the automated switch box was different when connecting to each source.

 

[dlaloum]

Do I need to invest additional money? I have a friend that could help with blind part of the test, but I don't have this volume meter to calibrate the loudness of each one, also to switch back-and-forth fast, do I need some switcher/box that will have like a button that will quickly allow to listen to one DAC/amp or another? Or it's still ok that friend will manually plug/unplug the 3.5 jack into each DAC/amp?

For this purpose, a level meter app on your phone will do the job...

Switching will still be manual though

 

[JP]

For this purpose, a level meter app on your phone will do the job...

Nope.

 

[antcollinet]

For this purpose, a level meter app on your phone will do the job...

Switching will still be manual though

Not accurate to eliminate level difference as a confounding factor for the test.

 

[dlaloum]

Not accurate to eliminate level difference as a confounding factor for the test.

It doesn't have to be a calibrated mic... as long as it has the same reading between two readings - the actual value is irrelevant

If a higher level of accuracy is required, then it should be measured using a multimeter to measure the V out of the amp... microphones will always suffer various issues, including precisely identical placement in the room between readings (!).

P.S. - the well known microphone placement issues... also imply that unless the listening head/ears, are placed in a vice for precise positioning - there are further confounding elements (the room/speakers) which imply that slight head movements could impose "error bars" in the measurement, that may be larger than the error level of a cheap phone used as a mic...

Sometimes, all you need is "accurate enough" - and the really really hard part, is determining, what is "enough"

 

[Blumlein 88]

Much easier and is reliable to use a voltmeter.

 

[dlaloum]

Much easier and is reliable to use a voltmeter.

Hmm and how much V variation would calculate out to 0.2db? - as that is the required minimum audible variation level...

ie: how precise does the V reading need to be?

 

[danadam]

Hmm and how much V variation would calculate out to 0.2db? - as that is the required minimum audible variation level...

ie: how precise does the V reading need to be?

Let's say measured output is 1 V, then:

• 0.2 dB less is 0.977 V
• 0.1 dB less is 0.989 V
• 0.1 dB more is 1.012 V
• 0.2 dB more is 1.023 V

So two decimal places should be enough.

 

[Blumlein 88]

Hmm and how much V variation would calculate out to 0.2db? - as that is the required minimum audible variation level...

ie: how precise does the V reading need to be?

I often tell people get it within 2% of each other. That is about .17 db which is a bit larger than optimum. So one volt is 1.02 or .98 volts. So yes 2 decimal places. It can be a cheap meter. Absolute accuracy for these purposes is not important. Just relative accuracy between two measurements. So even cheap meters will do just fine. Within 1.2% is right at .1 db which is the usual guideline. Certainly you want to be within .2 db or better. If you are measuring at speaker terminals you'll often run a -20 db tone thru and be between 1 or 2 volts depending upon speaker in use etc. You can go a little higher if it helps.

If you had 2.54 volts on one source, then multiply and divide by 1.02 for the limits.
2.54V x 1.02 = 2.59 volts
2.54V / 1.02 = 2.49 volts

 

[voodooless]

Sometimes, all you need is "accurate enough" - and the really really hard part, is determining, what is "enough

A phone app, mic or db meter is not accurate enough. We’ve seen this go wrong in the forum recently: eventually it turend out that the error by mic level matching was more than a dB. That is way off! This method is just too unreliable.

 

[antcollinet]

It doesn't have to be a calibrated mic

The problem isn't the calibration, it is the impact of mic position in the sound-field of the room - and the impact on the sound-field of the mc moving - or anything in the room - such as the person taking the measurements - moving.

unless the listening head/ears, are placed in a vice for precise positioning - there are further confounding elements (the room/speakers) which imply that slight head movements could impose "error bars" in the measurement,

Not a problem - because head movement is accounted for in the claim. And we are not making a measurement - we are testing perception - in that room and with those speakers.

I can hear a difference between these two devices (even when my head isn't clamped).

The blind test can still show if he can genuinely tell the difference or if that difference vanishes when he doesn't know what device is playing.

 

[mixsit]

Let's say measured output is 1 V, then:

• 0.2 dB less is 0.977 V
• 0.1 dB less is 0.989 V
• 0.1 dB more is 1.012 V
• 0.2 dB more is 1.023 V

So two decimal places should be enough.

Gads. I would offer never seeing voltage readings -or even one for that matter anywhere near consistent enough levels like that simply from the contact variations from my leads.
Fluke 75s, 88s typically. Admittedly not High End here.

 

[antcollinet]

Gads. I would offer never seeing voltage readings -or even one for that matter anywhere near consistent enough levels like that simply from the contact variations from my leads.
Fluke 75s, 88s typically. Admittedly not High End here.

Contact resistance plays little part in voltage readings since the current is tiny.

I've just tried my aging fluke 75 on two alkaline AA batteries. Handholding the test leads on each end of the battery while also holding the battery.

One consistently read 1.620V over multiple readings.
The other read 1.569V or 1.570V On one reading it alternated between the two.

So repeatable to +/- 1 on the third decimal place. Clearly good enough to measure the voltages suggested above, where even for 0.1dB there are +/- 11 of the third decimal places available.

 

[SIY]

Gads. I would offer never seeing voltage readings -or even one for that matter anywhere near consistent enough levels like that simply from the contact variations from my leads.
Fluke 75s, 88s typically. Admittedly not High End here.

I have no problem getting reliable level matching to 0.1 dB using a $10 Harbor Freight multimeter.