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Audibility of Low Damping Factor? - Benchmark Myth-Busting White Paper

witwald

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The question is whether the magnitude of the frequency response changes are significant enough to be audible.
If you take a look at my frequency response plots as a function of damping factor, they should provide enough detail to be able to equalise a test signal. This will enable a before/after switchover to be done while monitoring the signal. For example, take a pink noise source, and apply parametric equalisation at the required frequencies to it. It's then possible to have equalised and unequalised versions of the signal in a program such as Audacity. It would be possible to monitor the results on headphones/loudspeakers and see if one can hear any difference using an instantaneous switchover testing approach.
 

ayane

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Very easy to test at home if you have an equalizer of any kind. Have a friend change the settings while you are out of the room, then come back and write down what you hear. After 10 trials at different levels (or not) unveil the results.
That's not a good methodology to test, because it will lower the chances of being able to detect any differences. Head position can change the perceived frequency response, and having time between settings changes will make it harder still, because our memory is worse than our hearing. It needs to be instantaneous and at the flip of a switch.
 

ayane

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My memory is a bit vague on this but by adding series resistance, 1 to 5 ohms, to the speaker cables of SS amps a reasonable emulation of 'tube sound' is purported. This may be more appropriate for the large woofers of old, e.g. Altec, JBL, Jensen and their overseas equivalents, drivers developed for movie theatre and movie industry soundstage(not HiFi soundstage meaning) use. I have lost the references. Can anyone provide information?
Tube output stages tend to have a high output impedance, so adding resistance in series to the amp makes sense in that it colors the output similarly. There's also the question of harmonic distortion, which is rather subtle, all things considered.
 

Alexanderc

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March audio specifies its Purifi monos as having an output impedance of 70 uohms. So that would be a damping factor in the neighborhood of 114,000? Have I screwed up the math?
 

DonH56

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March audio specifies its Purifi monos as having an output impedance of 70 uohms. So that would be a damping factor in the neighborhood of 114,000? Have I screwed up the math?

No, your math is fine. I get 114,285.7 referenced to 8 ohms.
 
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Eetu

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March audio specifies its Purifi monos as having an output impedance of 70 uohms. So that would be a damping factor in the neighborhood of 114,000? Have I screwed up the math?
The NC252MP, as @samsa mentioned in post #20, has output impedance that translates to damping factors of ~2300-5300 but when measured by Archimago he got 340 into 4 ohms (~700 into 8 ohms).

In any case, seems like damping factor/output impedance is not an issue in well designed Class D amps. :cool:

No. I get 114, 285.7 referenced to 8 ohms.
I believe the specs are 70 micro ohms, not milli. So 8 / 0.00007

Edit: Yeah, so exactly as you calculated. Never get used to the American use of commas, my bad :)
 
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NTK

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No. I get 114, 285.7 referenced to 8 ohms.
I believe the specs are 70 micro ohms, not milli. So 8 / 0.00007
LOL. @DonH56 was just having some fun by calculating DF to 7 digits of precision to @Alexanderc 's 3 digits.

The confusion, I think, is with the decimal separator. US uses the comma (,) as thousands separator and period (.) as decimal separator. European countries tend to do the reverse.

From Wikipedia:

Decimals_1.JPG
Decimals_2.JPG
 

pjug

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Do I have the right info that 2 AWG wire is 156u-ohms per foot? So 70u-ohms is about 6 inches of 2 AWG wire.
 

samsa

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The NC252MP, as @samsa mentioned in post #20, has output impedance that translates to damping factors of ~2300-3300 but when measured by Archimago he got 340 into 4 ohms (~700 into 8 ohms).

I suspect that's entirely the intervening cables and equipment. With output impedance that low, the impedance of cables and whatnot become the dominant effect. The return resistance of 2.5 feet of 12AWG cable, added to the output impedance of the NC252MP gives you a damping factor of 700.
 

Eetu

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I suspect that's entirely the intervening cables and equipment. With output impedance that low, the impedance of cables and whatnot become the dominant effect. The return resistance of 2.5 feet of 12AWG cable, added to the output impedance of the NC252MP gives you a damping factor of 700.

Thanks, good point. I read the Benchmark paper but thought that perhaps Archimago had taken the cable impedance into account.

LOL. @DonH56 was just having some fun by calculating DF to 7 digits of precision to @Alexanderc 's 3 digits.

The confusion, I think, is with the decimal separator. US uses the comma (,) as thousands separator and period (.) as decimal separator. European countries tend to do the reverse.

From Wikipedia:

View attachment 67564 View attachment 67565
Yes, thanks realized that too. What confused me was the space after the comma and at first glance I thought the 114 was the 4 ohm load and 285 was the 8 ohm figure using some advanced formula.. :facepalm:

So - as pointed out on the Benchmark paper - while the damping factors on these Class D amps sound impressive there's little to be gained with damping factors over 300 (or even 100).
 

samsa

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So - as pointed out on the Benchmark paper - while the damping factors on these Class D amps sound impressive there's little to be gained with damping factors over 300 (or even 100).

Where the law of diminishing returns sets in depends on the return resistance of your speaker wire.
The return resistance of 10 feet of 12 AWG copper wire is 31.8 mΩ. What matters is how that compares to the output impedance of the amp.
  • The Benchmark ABH2 has an output impedance of 21.6 mΩ.
  • The Hypex NC252MP has an output impedance of 1.5 mΩ (at 1 kHz)
  • The Purifi 1ET400A has an output impedance of 0.065 mΩ (at 1 kHz)
With that 10 foot run of speaker wire, there's a factor of 0.6 in the output impedance seen by the speakers (damping factor of 251 versus 150) between the Purifi and the ABH2.

@John_Siau argues that you want to see a damping factor >200. The Purifi and the Hypex both get you there easily. With the ABH2, you need better speaker cable.
 
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DonH56

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The NC252MP, as @samsa mentioned in post #20, has output impedance that translates to damping factors of ~2300-5300 but when measured by Archimago he got 340 into 4 ohms (~700 into 8 ohms).

In any case, seems like damping factor/output impedance is not an issue in well designed Class D amps. :cool:


I believe the specs are 70 micro ohms, not milli. So 8 / 0.00007

Edit: Yeah, so exactly as you calculated. Never get used to the American use of commas, my bad :)

That's OK, I'll never get used to the European use of commas... :D

1 1 4 2 8 6 -- integer number with no decimal/comma/whatever point.
 
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MediumRare

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That's not a good methodology to test, because it will lower the chances of being able to detect any differences. Head position can change the perceived frequency response, and having time between settings changes will make it harder still, because our memory is worse than our hearing. It needs to be instantaneous and at the flip of a switch.
What you are describing is the difference between detectability and audibility. If, in the lab, under perfect conditions, a trained ear can detect a difference, that's nice to know. But that's not reality as experienced by a normal person in a normal situation. My little single-blind in-home test is easy to execute and useful from the POV of a normal listener.

I'll give you an example: If my car does zero to 60 in 5.1 seconds and I can upgrade to one that can do it in 4.7 seconds, how much should I pay for that upgrade? If I'm a professional racer, maybe a lot. If I'm getting on the highway on my way to work, I'd be wasting my money. Make sense?
 
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Alexanderc

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My thinking is, those purifi amps seem like they do everything well. This is just more confirmation that I can buy a couple and not have to worry about them.
 

witwald

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I've seen that article before; it concentrated on the potential effects of amplifier damping factor on loudspeaker transient response, with examples using tone bursts. However, it didn't provide a detailed analysis of the effects of damping factor on amplifier frequency response. This has been covered to a greater extent by the articles by Pierce and Benchmark Audio. It has also been repeatedly demonstrated by measurements of amplifiers during reviews undertaken by publications such as Stereophile and Australian Hi-Fi (there may be others)
 

tifune

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Simply put, if you have a speaker that dips to low impedances (hence the reason I and others publish them), the amplifier output impedance and that of cables becomes more important. The lower the amplifier impedance and that of the speaker cables, the less chances it can change the frequency response of the speaker.

Q - how audible is this, generally speaking? For awhile I had a McIntosh MC152 that had 2, 4, and 8 ohm outputs. I swore my Focal 948s sounded best on the 4 ohm output, but I never could figure out why. Muddy on the 2 ohm, weak low end on the 8 ohm.

Compounding the issue, the difference was extremely subtle - almost like the old trick of making a signal louder by 0.5 dB, consciously perceived as an inaudible difference in most rooms but "sounds better for some reason."

I inevitably chalked it up to some psychoacoustic placebo because the spec sheet shows nominal of 8 but, unlike most manufacturer, also lists the minimum impedance of 2.5 making 4 the middle ground
 
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MediumRare

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Q - how audible is this, generally speaking? For awhile I had a McIntosh MC152 that had 2, 4, and 8 ohm outputs. I swore my Focal 948s sounded best on the 4 ohm output, but I never could figure out why. Muddy on the 2 ohm, weak low end on the 8 ohm.

Compounding the issue, the difference was extremely subtle - almost like the old trick of making a signal louder by 0.5 dB, consciously perceived as an inaudible difference in most rooms but "sounds better for some reason."

I inevitably chalked it up to some psychoacoustic placebo because the spec sheet shows nominal of 8 but, unlike most manufacturer, also lists the minimum impedance of 2.5 making 4 the middle ground
I also run my "8 ohm" speakers off the 4 ohm tap. Mine drop to 2 ohms in deep bass. Amp runs a lot cooler, too.
 
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MediumRare

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@amirm What’s the output impedance of the amp in the Klippel system and what’s the total round-trip impedance? I suspect your cables must be on the long side.
 

solderdude

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You cannot see 0.1dB variations in Amir's plots.
Wy worry about it ?

When one can hear differences in amps between DF 20 and DF 1000 one can also hear differences in speaker cables.
What benchmark aims at is 0.1dB with worse case speakers and short cable runs.
The real world question is whether you could actually hear a local 0.5dB boost difference when you don't have anything to compare it with as one cannot change the output R of an amplifier instantly.

personally I am not really worried if speakers, that vary many dB's over the entire frequency range, would have a 0.3dB 'pump' at some frequencies and a 0.2dB dip at other frequencies.

What one can measure doesn't mean one can also hear.
 
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