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Review and Measurements of Benchmark AHB2 Amp

typericey

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Folks, not sure if you read the TAS review but I think it’s worth 3 minutes of your existence. No, wait, hear me out: the review was refreshingly quite objective.

[Spoiler alert] Here’s the conclusion:

“Because accuracy allied to absolutely reliable performance is the goal of all the Benchmarks, they are not products that tend to attract cults or other sorts of starry-eyed enthusiasts, wholly lacking any of the quirks, foibles, idiosyncrasies, sonic flavorings, euphonic distortions, and so on that characterize the objects of most audio cults. Professionals buy Benchmark because they know the products work and are reliable and accurate—indeed, reference caliber. Music lovers buy them because they are neutral and accurate and thus reproduce the tonal character of voices and instruments correctly (and also, I presume, because they are reasonably priced, most musicians, like most other people, being typically not wealthy). But audiophiles? Well, the longer I’m in this racket, the less I sometimes think I understand what audiophiles really want except that a lot of dallying about with components, equipment swapping, and coloration matching seems to be what amuses them. I’m not sure I can in good conscience recommend this amplifier to them as I am not sure they are in search of what it offers: a precision instrument designed to perform the precisely defined task of reproducing music and sound accurately, which it does essentially to perfection. But to anyone else, the AHB2 gets as high, enthusiastic, and confident a thumbs up as my arm is capable of reaching.”


@orangejello here’s an excerpt that may explain your listening experience:

“That said, there is one aspect of the design that must be emphasized: The power supply is very strictly regulated, the 100 watts of continuous power generating only about 110 watts on instantaneous peaks. This is salutary because it keeps the amp within its noise and distortion specifications vis-à-vis its power output. But there are other amplifiers with less tightly regulated power supplies that allow for much greater power output on instantaneous peaks (this is a design feature on the NAD amps). The price there is increased distortion, but the argument goes that because the music is so loud, and the duration so brief, the extra distortion goes essentially unnoticed.”
 

DonH56

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Except the power cube test is CEA-2006 or basically the 'maximum power' (old EIA/IHF A-202) burst test using 20 cycles at 0dB and 480 cycles at -20dB. Hardly stressful for an amplifier. It's a nice test, nothing gets too hot, it's easy to determine peak clipping on a scope because it is precise and clear, unlike the continuous sine where the waveform can flatten, sink, recover etc and stuff gets hot, fast. It gives you arguably the best numbers you can get. It's also kinder on older gear, which this Benchmark is not. But continuous it is not.

It's already been shown to fail to hit its rated specifications in Amir's testing (it shut-down at 133W@4R both channels driven in considerably less than 5 minutes on a purely resistive load). I also note, that Stereophile only tested the amplifier with one channel driven. Something I have never seen done before for a stereo power amplifier.
"and with just the left channel driven, the AHB2 clipped at 108W into 8 ohms (20.3dBW) and 210W into 4 ohms (20.2dBW), both powers slighter greater than those specified by Benchmark, which are with both channels driven."
I am wondering whether JA also discovered the amplifier shut-down at ~133W@4R on a continuous basis, so he tested one channel driven only. Considering he apparently had two examples of the amplifier to test, this seems very strange.

John_S has completely ignored my post on a possible overly sensitive protection and seemingly left the thread. He hasn't returned since this, and the questioning of the Xover distortion plots of 'competitor's amplifiers'.

I wrote:


He had said earlier that :

Apparently, testing the amplifier in accordance with standard procedures in order to verify its rated continuous specifications is classed as 'test bench abuse'. I think not. There is a big difference between 133W and 190W.

View attachment 28133
(AP's sweep time for the test is considerably less than 5 minutes)

So, what can the amplifier actually achieve, both channels driven in accordance with standard continuous testing procedures? We don't know. Stereophile didn't test it properly and Amir didn't precondition the amplifier or run the tests for 5 minutes in case he damaged the unit.

Maybe the FPGA running the protector needs a firmware update to allow the amp to hit its specification on the bench?

Doesn't anyone have the guts to run an amplifier at full power as per the FTC requirements into its rated loads these days or am I the only one?

Good, reasonable points. I should have emphasized "stability" in my post; short-term burst or not, I have seen few amplifiers do so well over the Cube's load (impedance) space. But, by this time I have forgotten most of the testing I did decades ago, and back then I was using my own home-brew set of loads. And I guess I was giving them a pass on long-duration output; knowing it's class G I tend to expect less ability to sustain long-term loads, and by and large tend to agree with those arguing short-term peaks are more like the "real world".

That said, the tightly-regulated supply is a mixed blessing for power ratings. It means little headroom whereas a more conventional design may spec lower continuous power but have 2x or more the power for short peaks, albeit at higher distortion, but the AHB2 probably clips not far above its steady-state (don't know for sure, didn't take time to search this morning). Was it NAD who used to spec 6 dB of headroom?

Edit: I had not read @typericey ' s post with the TAS excerpt before responding to John -- the last bit is the same as my last paragraph.
 

STUDIO51

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As far as I know, the AHB2 has distortion detection for speaker protection.
스크린샷 2019-06-23 오후 11.19.07.png


When I measured using a dummy load rather than an Activeload box to demonstrate a measurement lecture using AHB2.
AHB2 entered protection mode at significantly lower output than when connected to active load box.

Later on, the dummy load could not sustain the high current, resulting in high distortion, and AHB2 detected it and entered protection mode.
This is also true work for speaker protection. Because the speaker moving coil has high excursion, it produces high distortion.


And in AHB2's torture test, there was a case of entering protection mode at a lower level than indicated in the spec sheet.
Later I asked John Siau. The APx 555 operates the relay when changing the output level, which creates a transient and inputs a level higher than + 22dBu.
 

zalive

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Protection is truly great.
But continuous power rating specs are fake.
 

orangejello

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You, and most of the rest of us, it's normal behaviour, but it does make meaningful comparisons impossible. Try to keep an SPL meter handy so you know what's going on. I would think to really get to the bottom of these 2 amps you would need to level match very closely, and done blind I predict a draw.
Maybe, but these amps sound too different to me even at moderate volumes. I will try to borrow my friends spl meter soon and check though.
 

orangejello

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@orangejello :

1. How much working hours behind the AHB2 by now? Unlike guys who cling on science only, my experience is that burn-in period should be finished for the assesment as particularly during in-between time electronics can sound funny. Though as some audiophiles have stated, if it doesn't sound really good from the beginning, it will never likely sound great. Still this has to be done.

2. Since you listened at high volume it's possible you provoked some nasty distortion on peak power transients...the one which protection circuits which shut the amp down didn't allow to show at measurements (protection from bad measurement, huh? :oops:).

3. Now this is why audiophiles I know prefer plugging equipment in their own system at their own home, because this is what will get the final result which they will be listening to. All is being taken into account, synergy/interoperability between components and the room as is, resulting in a sound. It doesn't really matter to me whether the final result is from a specific distortion profile or from higher fidelity - as long as it sounds like more fidelity in my perception. However, objectivists gonna tell you that you shouldn't trust your ears at all but instead you should trust graphs even if you don't enjoy that much in your subjective listening impression. Kind of completely absurd to me.

1. About 25-30 hours on the amp. I am letting it play while I am not home just in case...
2. I wasn't playing it that loud.
3. Yeah. I tend to agree with your last statement. It was curiosity about that the brought about this little experiment.
 

orangejello

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Folks, not sure if you read the TAS review but I think it’s worth 3 minutes of your existence. No, wait, hear me out: the review was refreshingly quite objective.

@orangejello here’s an excerpt that may explain your listening experience:

“That said, there is one aspect of the design that must be emphasized: The power supply is very strictly regulated, the 100 watts of continuous power generating only about 110 watts on instantaneous peaks. This is salutary because it keeps the amp within its noise and distortion specifications vis-à-vis its power output. But there are other amplifiers with less tightly regulated power supplies that allow for much greater power output on instantaneous peaks (this is a design feature on the NAD amps). The price there is increased distortion, but the argument goes that because the music is so loud, and the duration so brief, the extra distortion goes essentially unnoticed.”

My strongest impression is that the NAD controls the speakers better and therefore produces a more lively sound. My friends called the AHB2 "dull". The above comment may be related to those perceptions. On the other hand, the AHB2 might be a smidge more detailed. That also correlates. But I am no EE, so I would have to defer on that.
 

orangejello

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Or levels weren't matched. Which is exactly what that description sounds like.
You are correct. The levels were not matched. But this is the impression I get for levels at or above medium volume - and after a number of listening sessions where I was not AB'ing, just listening to music though one amp and then the other.
 

SIY

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Yep, you did things in the way most likely to lead to an erroneous conclusion. Blind (preferably double blind) and level-matched is the only way to avoid that.

There are some simple ways to do that easily and quickly, if you're feeling open-minded and adventurous.
 

zalive

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1. About 25-30 hours on the amp. I am letting it play while I am not home just in case...
2. I wasn't playing it that loud.
3. Yeah. I tend to agree with your last statement. It was curiosity about that the brought about this little experiment.

Well IMO you can wait to get pass the 150 hours mark, then make a final judgement.
As for loudness, it's said transients go 10-20x higher than the average power. So while it's still significant power, 5-10W output gets you in territory where transients go pretty high. However, if it's true that AHB2 limits transients to avoid distortion, in which case you should hear a compression when you crank it up a little, rather than a distortion.
 

maty

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@orangejello

Seriously speaking, do you notice depth, 3-D sound in the two amplifiers? Is the timbre/tonality correct?

There are factors that can not be measured and you have to rely on ears used to appreciate good/very good recordings with acoustic instrumentation and high/very high DR.
 

SIY

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There are factors that can not be measured

Nonsense. The contribution of the electronics to this is very easily measurable as long as your listening comparison is ears-only, no peeking.
 

SIY

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If you find any measurement in this regard I would appreciate a link.

In audio I have a saying, appropriate of mathematics: good measurements are a necessary but not sufficient condition.

They're known as "frequency response" and "crosstalk." I suspect you can find them here and there.

Again, this assumes that differences in "depth" or "timbre/tonality" are actually heard ears-only, no peeking.
 

maty

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Frequency response and crosstalk are not enough. Audio is a complex system and requires a multitude of measurements, many of which can not be done heuristically. Only the resource of blind listening remains.
 

SIY

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Frequency response and crosstalk are not enough. Audio is a complex system and requires a multitude of measurements, many of which can not be done heuristically. Only the resource of blind listening remains.

Good. Could you provide one example of a difference in "depth" or "timbre/tonality" of an amplifier that is not reflected in basic measurements?
 
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