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Review and Measurements of Benchmark AHB2 Amp

digitalfrost

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If the clip lights on the AHB2 are not turning on, the transients are not being clipped, attenuated, or distorted. The clip lights have a timer that keeps them on for at least 1/4 second so that every clip event is always visible.

It's details like this that I really appreciate.
 

John_Siau

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So am I if the levels were closely matched. I wonder what happens if the volume is turned up on the stereo version until the cone starts moving, the clip light comes on, or fear of something breaking kicks in.
If the levels are precisely matched, the differences in cone movement are probably due to differences in damping. The larger movement is caused by an under damped response.

When you run the AHB2 in bridged mono, you reduce the effective damping factor by a factor of 2. This is always true when running an amplifier in bridged mono. There are two outputs driving the speakers, and this doubles the source impedance which reduces the damping factor by a factor of 2.

In stereo mode, the AHB2 has a 20 Hz damping factor of 350. In bridged mono, the effective damping factor is 360/2=175 which is somewhat closer to the damping factor of the Accuphase (60) but still significantly higher.

The big "if" in all of this is the precision of the level matching. A 1 dB error will increase the cone excursion by 12%. Due to the low damping factor of the Accuphase, matching levels at 1 kHz does not mean that levels are matched at 50 to 80 Hz. There will be peaks and dips in the low-frequency response of the Accuphase.
 

John_Siau

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One other thing:

The output level of the AHB2 is exactly 6.02 dB higher for a given input voltage when running in bridged mono (4 times the available power and exactly twice the voltage gain). If the input level is not turned down by 6 dB when running mono, the stereo to mono A/B comparison is not valid.

The mono mode can play 6 dB louder before clipping, but must be turned down to match the listening level that was used for the stereo mode if you are attempting to run a valid A/B test.
 

blueone

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If the levels are precisely matched, the differences in cone movement are probably due to differences in damping. The larger movement is caused by an under damped response.

When you run the AHB2 in bridged mono, you reduce the effective damping factor by a factor of 2. This is always true when running an amplifier in bridged mono. There are two outputs driving the speakers, and this doubles the source impedance which reduces the damping factor by a factor of 2.

In stereo mode, the AHB2 has a 20 Hz damping factor of 350. In bridged mono, the effective damping factor is 360/2=175 which is somewhat closer to the damping factor of the Accuphase (60) but still significantly higher.

The big "if" in all of this is the precision of the level matching. A 1 dB error will increase the cone excursion by 12%. Due to the low damping factor of the Accuphase, matching levels at 1 kHz does not mean that levels are matched at 50 to 80 Hz. There will be peaks and dips in the low-frequency response of the Accuphase.

Hi John. You've asserted in your posts that damping factor is a critical determinant of amplifier quality in an audio system. For years I've held output impedance as important when I've gone amplifier shopping, but to be honest I've always felt a little silly about it due to reading many articles which assert technical reasons for the opposite view. Like this one:

https://www.audiofrog.com/community/tech-tips/damping-factor-and-why-it-isnt-much-of-a-factor-2/

I've owned speakers that have had low impedance and simultaneous capacitive phase angles, which at least for my buying decisions have been sufficient justification for demanding very low output impedance. I'm not planning to change my buying criteria at all, even though my current speakers (Salon2s) are an easy load, but how do you respond to these damping-factor-isn't-important assertions?
 

scott wurcer

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Hi John. You've asserted in your posts that damping factor is a critical determinant of amplifier quality in an audio system. For years I've held output impedance as important when I've gone amplifier shopping, but to be honest I've always felt a little silly about it due to reading many articles which assert technical reasons for the opposite view. Like this one:

https://www.audiofrog.com/community/tech-tips/damping-factor-and-why-it-isnt-much-of-a-factor-2/

Some of the data in that article comes from Dick Pierce a highly talented and well known audio professional, you can not dismiss it lightly (he's also right IMO).
 

blueone

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Some of the data in that article comes from Dick Pierce a highly talented and well known audio professional, you can not dismiss it lightly (he's also right IMO).

I've read Pierce's writing many times before, which is why I chose this article out of many others the search returned. His opinions are one of the primary reasons why I've felt silly making low output impedance such an important purchase criteria. But I persist...
 

SIY

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If your speakers dip to 1 Ohm at low frequencies (not uncommon), this would create a 1 dB error in the frequency response of the Accuphase and this would be accompanied by a significant change in the phase response.

I've measured the impedance of quite a few speakers and have never, ever seen that. I've seen midrange dips from Wilsons (a terrible crossover design) and treble dips from ESLs, but those are special cases and would certainly be uncommon. Could you cite a couple examples of a "not uncommon" 1 ohm impedance dip?
 

blueone

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I've measured the impedance of quite a few speakers and have never, ever seen that. I've seen midrange dips from Wilsons (a terrible crossover design) and treble dips from ESLs, but those are special cases and would certainly be uncommon. Could you cite a couple examples of a "not uncommon" 1 ohm impedance dip?

I've never seen a 1 ohm dip either, only some 2 ohm dips. Here's one I saw mentioned on another forum that dips below 2 ohms right in the heart of the range you might really need some power:

https://www.stereophile.com/floorloudspeakers/104legacy/index.html

1588104152696.png
 

SIY

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John_Siau

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Hi John. You've asserted in your posts that damping factor is a critical determinant of amplifier quality in an audio system. For years I've held output impedance as important when I've gone amplifier shopping, but to be honest I've always felt a little silly about it due to reading many articles which assert technical reasons for the opposite view. Like this one:

https://www.audiofrog.com/community/tech-tips/damping-factor-and-why-it-isnt-much-of-a-factor-2/

I've owned speakers that have had low impedance and simultaneous capacitive phase angles, which at least for my buying decisions have been sufficient justification for demanding very low output impedance. I'm not planning to change my buying criteria at all, even though my current speakers (Salon2s) are an easy load, but how do you respond to these damping-factor-isn't-important assertions?
The paper above is technically correct, but it is focused on the wrong problem.

The problem is that a low damping factor can cause significant frequency response problems. As the paper points out, the slight changes in driver damping are relatively insignificant. Unfortunately the paper completely misses the real issue which is frequency response.

This confusion is caused by an unfortunate choice of terminology.

The term "damping factor" is very misleading. It has very little to do with the damping of the driver motion. In contrast, it has everything to do with maintaining a flat frequency response and linear phase response (measured at the amplifier terminals). With adequate speaker cables, this frequency and phase response will be delivered to the speaker terminals.

It would have been better if the audio industry had called this the "impedance ratio" instead of the "damping factor".

Damping_Factor = 8/Output_Impedance, where 8 is the nominal speaker impedance.

Damping Factor can also be specified at other nominal impedances, and this adds to the confusion.

Headphone amplifiers are specified differently:

Fortunately it is common practice to specify the output impedance of headphone amplifiers (instead of the damping factor). The audio industry should do the same with power amplifiers.

So, in answer to your question, the output impedance of an amplifier must be low to maintain a predictable frequency and phase response. This means the "damping factor" must be much higher than an analysis of damping would suggest.

The frequency response of a given speaker is only repeatable, from amplifier to amplifier, when all of the amplifiers have a high damping factor. If you connect the speaker to an amplifier with a low damping factor, you will get a different frequency response. This is especially problematic at the low end of the response and at each of the crossover frequencies.

Here is some math:

Let suppose a speaker with an 8-Ohm nominal impedance has one or more frequencies where the impedance drops to 2 Ohms (this is not uncommon).

At a damping factor of 60, the output impedance of the amplifier is 8/60 = 0.133 Ohms.

The attenuation at the 2-Ohm impedance point will be 2/(2+0.133) = 0.938
Converting to dB: 20*Log(0.938)= - 0.56 dB

At a damping factor of 370, the output impedance of the amplifier is 8/370 = 0.0216 Ohms.

The attenuation at the 2-Ohm impedance point will be 2/(2+0.0216) = 0.989
Converting to dB: 20*Log(0.989)= -0.093 dB

In this example, the higher damping factor keeps the frequency response variations just under 0.1 dB. With the lower damping factor, the variation is 0.56 dB.

0.56 dB may not sound like a lot, but keep in mind that it has been shown that A/B and A/B/X tests should be level matched to +/- 0.1 dB.

But there are more issues:

At the crossover frequencies, the speaker impedance can change very rapidly. If the amplifier output impedance is too high this variation in the load impedance can impact that phase alignment between the two drivers at the crossover point. This change in phase alignment can have a big impact on the amplitude response at the crossover frequency.

How high does the damping factor need to be?

Find your speaker's minimum impedance (lowest impedance on the impedance vs. frequency curve) and then apply one of my personal rules of thumb:

Output Impedance Rule of Thumb:

For a 0.1 dB amplitude variation, the output impedance of the amplifier must be 1/86th of the speaker's minimum impedance.

Example:
If the minimum speaker impedance = 2 Ohms:
2/86 = 0.023 Ohms, maximum output impedance

Lower output impedances will reduce the 0.1 dB amplitude variation, but this difference should not be audible.

Damping Factor Rule of Thumb:

For a 0.1 dB amplitude variation, the required 8-Ohm damping factor = 688/(minimum_speaker_impedance).

Example:
If the minimum speaker impedance = 2 Ohms:
The minimum 8-Ohm damping factor = 688/2 = 344

Higher damping factors will reduce the 0.1 dB amplitude variation, but this difference should not be audible.
 

direstraitsfan98

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Why is damping factor important? In what situation would it become important? Long throw woofers with high qts numbers, and in a speaker cabinet that is a loose, under damped construction? Would that not be more of a flawed speaker design rather a fault of an amp having low damping factor?

My amp is 40 damping factor and I find the bass is just as good if not better then my Akitka which had a damping factor 10 times greater. My subjective opinion too is that the bass actually is stronger, tighter with my new amp. I got asked by my roommate the first day I play music to turn the volume down and that was the first time that happened in two years.

I wonder if my situation is different from others and other people have setups that are more in line with what Dr. Siau is saying about damping factor being important. My JBL seem to be inherently damped, wether it be due to the design of the speaker, the driver itself damping itself how its mounted, the large magnet structure, I don't know. But low damping factor amps sound just as good if not better then high damping factor amps.

Of course bias could all be a part of this. But I have put on roger waters amused to death and was impressed by the dynamics of that recording. Pink Floyd has always had stunning transients, the wall has several intense 65hz drum kicks that will really give your woofers a workout. And I've never heard either of those albums sound as good as they have on my system. Confirmation bias, visual or otherwise? Possibly... I don't have any test bench equipment to really see what's going on though.

I think the relationship between the speakers and the amps is very important and since almost everybody has different speakers, different rooms, enjoys music at different music volumes, its really impossible to say with 100% certainty that someone's experience in their room will mirror your own in your room.
 

blueone

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The paper above is technically correct, but it is focused on the wrong problem.

The problem is that a low damping factor can cause significant frequency response problems. As the paper points out, the slight changes in driver damping are relatively insignificant. Unfortunately the paper completely misses the real issue which is frequency response.

This confusion is caused by an unfortunate choice of terminology.

The term "damping factor" is very misleading. It has very little to do with the damping of the driver motion. In contrast, it has everything to do with maintaining a flat frequency response and linear phase response (measured at the amplifier terminals). With adequate speaker cables, this frequency and phase response will be delivered to the speaker terminals.

It would have been better if the audio industry had called this the "impedance ratio" instead of the "damping factor".

Damping_Factor = 8/Output_Impedance, where 8 is the nominal speaker impedance.

Damping Factor can also be specified at other nominal impedances, and this adds to the confusion.

Headphone amplifiers are specified differently:

Fortunately it is common practice to specify the output impedance of headphone amplifiers (instead of the damping factor). The audio industry should do the same with power amplifiers.

So, in answer to your question, the output impedance of an amplifier must be low to maintain a predictable frequency and phase response. This means the "damping factor" must be much higher than an analysis of damping would suggest.

The frequency response of a given speaker is only repeatable, from amplifier to amplifier, when all of the amplifiers have a high damping factor. If you connect the speaker to an amplifier with a low damping factor, you will get a different frequency response. This is especially problematic at the low end of the response and at each of the crossover frequencies.

Here is some math:

Let suppose a speaker with an 8-Ohm nominal impedance has one or more frequencies where the impedance drops to 2 Ohms (this is not uncommon).

At a damping factor of 60, the output impedance of the amplifier is 8/60 = 0.133 Ohms.

The attenuation at the 2-Ohm impedance point will be 2/(2+0.133) = 0.938
Converting to dB: 20*Log(0.938)= - 0.56 dB

At a damping factor of 370, the output impedance of the amplifier is 8/370 = 0.0216 Ohms.

The attenuation at the 2-Ohm impedance point will be 2/(2+0.0216) = 0.989
Converting to dB: 20*Log(0.989)= -0.093 dB

In this example, the higher damping factor keeps the frequency response variations just under 0.1 dB. With the lower damping factor, the variation is 0.56 dB.

0.56 dB may not sound like a lot, but keep in mind that it has been shown that A/B and A/B/X tests should be level matched to +/- 0.1 dB.

But there are more issues:

At the crossover frequencies, the speaker impedance can change very rapidly. If the amplifier output impedance is too high this variation in the load impedance can impact that phase alignment between the two drivers at the crossover point. This change in phase alignment can have a big impact on the amplitude response at the crossover frequency.

How high does the damping factor need to be?

Find your speaker's minimum impedance (lowest impedance on the impedance vs. frequency curve) and then apply one of my personal rules of thumb:

Output Impedance Rule of Thumb:

For a 0.1 dB amplitude variation, the output impedance of the amplifier must be 1/86th of the speaker's minimum impedance.

Example:
If the minimum speaker impedance = 2 Ohms:
2/86 = 0.023 Ohms, maximum output impedance

Lower output impedances will reduce the 0.1 dB amplitude variation, but this difference should not be audible.

Damping Factor Rule of Thumb:

For a 0.1 dB amplitude variation, the required 8-Ohm damping factor = 688/(minimum_speaker_impedance).

Example:
If the minimum speaker impedance = 2 Ohms:
The minimum 8-Ohm damping factor = 688/2 = 344

Higher damping factors will reduce the 0.1 dB amplitude variation, but this difference should not be audible.

Well, to someone admittedly biased towards very low output impedances in amplifiers, what's not to like? Nice analysis.

But even I have to admit that 0.1db precision at frequencies less than, say, 400Hz, where most speakers will have their impedance minima, is very stringent indeed. Of course, who else would be in the target market for an AHB2?
 

Coach_Kaarlo

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For a colourful plot of left and right channels from the infamous audio follow the link;

https://drive.google.com/file/d/14m2MLVZPjlSo9h7RImwlYehuCvjT0svE/view?usp=drivesdk

Frequencies of the transient attacks range from ~25Hz to ~175Hz at each instant of the drum sound.

So if I have understood the excellent information being shared correctly- the driver is overshooting physically because the old Accuphase amp is loosing control of damping or of it’s physical excursion - simply speaking.

The NS-2000 speaker impedance seems to drop just below 6 ohms - within the conservative window of use for the AHB2.

E71D5ED2-B385-4518-82F3-D27A316DF3F0.jpeg


And finally mismatching the levels will always cause inaccuracies - will re-test and see if there was a change.
 

GoldenOne

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Mine arrived mid-week and I installed them Friday night and Saturday morning. I have to contact support as 1) the speakers pop when the muting circuit engages and disengages and 2) the 12V trigger doesn’t work consistently. If I try it out repeatedly it’ll work, but both times I listened for over an hour it didn’t work on power off. The first time I had to turn all three amps off separetaly, and the second time the second amp turned of the third. Disappointing.

View attachment 27110
Did you get the popping issue sorted?
I've just received my AHB2 and I'm loving it so far (using it to drive hifiman Susvaras and they've never sounded better), but the combination of the pop (even though it is fairly quiet, still makes me nervous it could cause damage) and the fact that seemingly on the EU model you CAN'T disable the 45 minute auto shutoff means I'm having to worry about whether this unavoidable pop is going to damage something.

If I could just leave the amp on it wouldn't be an issue, but this auto shutoff I cannot disable prevents that

@John_Siau Do you know if its normal to have the ahb2 pop slightly when the muting circuit engages/disengages?
And is it possible to disable the 45 minute timer on the EU model of the AHB2?
 

JP

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No - was told it was normal. It's not bad enough to do any damage. Just a bit annoying.
 

GoldenOne

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No - was told it was normal. It's not bad enough to do any damage. Just a bit annoying.
That's a shame.

Do you know if there is any way to disable the auto shutdown "feature" so that I don't need to worry about it regardless?
The manual on the AHB2 website says that you can disable it by holding the power button for 7 secs/until the mute lights turn off.
mBeP8aogt.png


But the manual that came with my unit has nothing about how to disable it. And also following the instructions from the online manual doesn't seem to help.
20200502_233303.jpg
 

digitalfrost

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I'm in EU and I got my amp from SCV Distribution, I could disable the auto-off timer. I have the running pretty much 24/7 controlled via the 12V trigger. Before that, I had it on a power/slave power socket which also worked.
 
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