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Review and Measurements of Benchmark AHB2 Amp

RichB

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I saw this StarQuad video on the Benchmark site. However, reading and watching about it and actually hearing it are different. I will be able to hear this myself next week after the cables are delivered. My guess is that I won't hear a difference. A audio designer on A'gon who helped me understand some of this, atmasphere, told me that balanced connections that support the AES48 standard should not have much variability in sound for different cables. Benchmark gear does support this AES48 standard on the DAC + HPA4 + AHB2. The designer's gear also supported the AES48 standard and he said cables do not make a difference with his gear. He said that the cables must have a minimum level of quality but that is still a low cost cable.

When I had Benchmark DAC3 HGC and AHB2 a few years ago I tried some Canare XLR cables I had received from Bryston about 20 years ago. I compared them to the Audience Au24SE XLR (or just AU24) cables and I could not hear a difference (as I am expecting with the Benchmark StarQuad). However, trying the same cables on a Parasound A21 amp I could tell a big difference in sound. The Audience sounded better to me.

The equipment is in a credenza which makes it difficult to have neat cable management.
When I had the Marantz AV8805 and Parasound A21 I was able to reduce noise at the tweeter using XLR versus RCA interconnects. I have all star-quad XLR cables (from bluejeans) . These are not designer cables and nothing is lost providing additional immunity, so why not?

Note: the A21 is a not a balanced amplifier and measured slightly worse via the balanced connectors but I would still use them for immunity.

- Rich
 

Coach_Kaarlo

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An application note by John Siau (VP at Benchmark) on damping factor published today, which also describes Benchmark's approach to damping factor and discusses the AHB2 and Benchmark/Canare cables related to the note's topic, is available here.

Having followed the discussion. and sometimes lack of, regarding the AHB2's dynamic performance I will read this with much interest. Thanks for the share!


PDF link below;

https://cdn.shopify.com/s/files/1/0...actor_isn_t_Much_of_a_Factor.pdf?v=1591291523

Spreadsheet for damping factor calculation here;

https://cdn.shopify.com/s/files/1/0321/7609/files/DampingFactorCalculator.xlsx?v=1591278482


[edit] So after quickly reading the excellent Benchmark application note, and hastily plugging in some values, my speaker cables have ok resistance values - 0.58 ohms per 100m. Not sure where the spreadsheet calculates the cable resistance value shown (0.031764 Ohms) but long story short the AHB2's damping factor combined with speaker and cables seems reasonable.

When 2 x AHB2's are used in mono presumably the damping factor is different? Am I correct in saying it's halved?

1591314785206.png



So after all the discussion about slew rates and damping factors it seems something else yet to be measured is going on. Back to the search for ways to measure what I continue to observe in the dynamic performance of the AHB2.
 

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restorer-john

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An application note by John Siau (VP at Benchmark) on damping factor published today, which also describes Benchmark's approach to damping factor and discusses the AHB2 and Benchmark/Canare cables related to the note's topic

Not sure why John Siau feels is necessary to re-invent the same wheel. There are absolutely no "myths busted" or new information presented. It's all been known and understood for 50+ years or more.

So, the AHB-2 has a decently low output impedance and one that remains respectable across the audible bandwidth and beyond. So do tens of thousands of amplifiers from the past and present. Nothing to see here folks.
 

RichB

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Not sure why John Siau feels is necessary to re-invent the same wheel. There are absolutely no "myths busted" or new information presented. It's all been known and understood for 50+ years or more.

So, the AHB-2 has a decently low output impedance and one that remains respectable across the audible bandwidth and beyond. So do tens of thousands of amplifiers from the past and present. Nothing to see here folks.

There are amplifiers with higher damping factors, lower output impedance, so I suspect Benchmark felt the need to provide their own analysis to address the damping factor specification touted by competitors in the marketplace.

This was also address by @John_Siau in this post:
https://www.audiosciencereview.com/...f-benchmark-ahb2-amp.7628/page-82#post-385011

- Rich
 
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GXAlan

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The one setup that Benchmark doesn’t account for is Kenwood Sigma Drive which enabled 15,000 damping factor at the terminals since it ran a second sensing pair of wires to the speaker terminals. They were the first company to eliminate the effect of speaker cables.

Although 0.1 dB is the threshold of audibility, I think we could say that pride in measurements is part of the appeal of audiophile equipment. The Apple USB-C dongle is pretty much transparent for 16/44 for any type of casual listening but I am sure we all spend more for that nth degree of performance.

Likewise, a speaker that dips to 2 ohms for that frequency may have a different damping factor at that frequency than our assumed 8 ohm speaker.
 

restorer-john

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The one setup that Benchmark doesn’t account for is Kenwood Sigma Drive which enabled 15,000 damping factor at the terminals since it ran a second sensing pair of wires to the speaker terminals. They were the first company to eliminate the effect of speaker cables.

Kenwood's Sigma Drive was a cool concept although dreadfully unreliable and not consumer friendly. The combination of DC-Daylight frequency response and two feedback paths, along with confusing terminals (for the average consumer) meant many of them landed on my bench. Combined with their commutating rail, high dynamic performance topology things went awry fast.

Of interest was Kenwood's L-O series where they essentially removed the speaker cable from the equation by using a specially braided 1Metre length for the monoblock amplifiers to be placed right at the speaker.

Here's some excerpts from the manuals (circa 1977)

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1591337202696.png
 

GXAlan

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Kenwood's Sigma Drive was a cool concept although dreadfully unreliable and not consumer friendly.

yup. Dead amp if you short the sigma drive terminals.

I have an L-08c and L-08m pair.
 

Aerith Gainsborough

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An application note by John Siau (VP at Benchmark) on damping factor published today, which also describes Benchmark's approach to damping factor and discusses the AHB2 and Benchmark/Canare cables related to the note's topic, is available here.
Interesting stuff, especially the effect of the cables thank you for posting.

After successfully asking google to convert your annoying American units, I got a total error of 0,19dB for my system.
Replacing my 230€ budget AVR with a 4K€ AHB-2 would push that down to 0,12dB.
I'd wager both values are inconsequential in normal listening.
 

samsa

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An application note by John Siau (VP at Benchmark) on damping factor published today, which also describes Benchmark's approach to damping factor and discusses the AHB2 and Benchmark/Canare cables related to the note's topic, is available here.

There is something a bit messed up about that blog post.

Siau says that his 10 feet of his 11 AWG cable has a return resistance of 0.0252 Ω. If you look up the resistance/foot of 11 AWG copper cable and multiply by 20 feet (for the return resistance), you indeed get 0.0252 Ω.

But he says that the same length of 12 AWG cable has a return resistance of 0.04 Ω, 1.6 times larger than his 11 AWG cable. From the same table, the return resistance of a 10 foot length of 12 AWG copper cable is 0.0318 Ω. Only 1.26 times larger than his 11 AWG cable. For the purposes of his discussion, this is not a small correction! Maybe, in citing the 0.04 Ω figure, he's talking about copper-clad aluminum, rather than copper. But, if so, he should say so.
 

Aerith Gainsborough

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Siau says that his 10 feet of his 11 AWG cable has a return resistance of 0.0252 Ω

But he says that the same length of 12 AWG cable has a return resistance of 0.04 Ω, 1.6 times larger than his 11 AWG cable.
From the same table, the return resistance of a 10 foot length of 12 AWG copper cable is 0.0318 Ω.
Hmm the spreadsheet they linked, uses the correct values for both. Maybe it was just an honest typo?
 

John_Siau

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thx @pjug.
So in normal language a well designed and engineered amplifier working not in clipping or distortion have inaudible differences? Can we say this?
Here is an A/B/X test that we did in our listening room that clearly identified differences between an AHB2 and another amp with reasonably low distortion specifications. The difference was produced by zero-crossing distortion when playing a single tone at a fraction of a watt. The other amplifier produced harmonics that could be clearly identified in the A/B/X test. Test results show a perfect score, meaning the differences were very clearly audible:
https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt

Also, my latest application note explains why damping factor can produce audible changes in the frequency response of the system:

https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor
 

John_Siau

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There is something a bit messed up about that blog post.

Siau says that his 10 feet of his 11 AWG cable has a return resistance of 0.0252 Ω. If you look up the resistance/foot of 11 AWG copper cable and multiply by 20 feet (for the return resistance), you indeed get 0.0252 Ω.

But he says that the same length of 12 AWG cable has a return resistance of 0.04 Ω, 1.6 times larger than his 11 AWG cable. From the same table, the return resistance of a 10 foot length of 12 AWG copper cable is 0.0318 Ω. Only 1.26 times larger than his 11 AWG cable. For the purposes of his discussion, this is not a small correction! Maybe, in citing the 0.04 Ω figure, he's talking about copper-clad aluminum, rather than copper. But, if so, he should say so.
Thanks for catching that error. It is good to see that some people are reading carefully! I fixed the error. BTW, I used the correct number when calculating the resulting damping factor of 200 (due to the cable resistance). The 0.4-Ohm number was for a 12 foot 12-AWG cable. We don't sell 12-foot cables, so I had changed the calculations to reflect 10-foot cables, but missed that number.
I am very impressed that you found the error!

Five gold stars to samsa!
 

QMuse

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John_Siau

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One thing I wasn't expecting, and haven't seen before - I bought a pair of AHB2s for a system I'm setting up in Colorado, and the (excellent) manual says they're unsafe to operate above 2,000 meters, which is about 5,600 feet. Not a problem for their eventual destination, which is at 5,000 feet, but while I'm waiting for construction to finish, I had them delivered to my place in Wyoming, which is at 8,400 feet. I'll call Benchmark after the holiday, because I'm interested in the reason, but in the meantime, what are your best guesses?
Amplifier RMS power ratings are based on the ability to run a 1/3 power continuously for one hour, before testing at full output. The 1/3 power preconditioning usually brings an amplifier up close to its thermal limits. High altitudes reduce the cooling and the amplifier may reach the thermal shutdown limit in less than an hour. Above 2000 meters (6562 feet), the maximum temperature will be reached in a little less than an hour. This has no impact on normal listening and and no impact on safety. The amplifier has multiple thermal sensors that will shut the unit down before an unsafe temperature is reached.
 
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ex audiophile

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Here is an A/B/X test that we did in our listening room that clearly identified differences between an AHB2 and another amp with reasonably low distortion specifications. The difference was produced by zero-crossing distortion when playing a single tone at a fraction of a watt. The other amplifier produced harmonics that could be clearly identified in the A/B/X test. Test results show a perfect score, meaning the differences were very clearly audible:
https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt

Also, my latest application note explains why damping factor can produce audible changes in the frequency response of the system:

https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor
Thanks for this info. Wouldn't it be more applicable if you did an ABX comparison using actual recordings of music at low power levels, rather than a test tone?
 

QMuse

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Here is an A/B/X test that we did in our listening room that clearly identified differences between an AHB2 and another amp with reasonably low distortion specifications. The difference was produced by zero-crossing distortion when playing a single tone at a fraction of a watt. The other amplifier produced harmonics that could be clearly identified in the A/B/X test. Test results show a perfect score, meaning the differences were very clearly audible:
https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt

What is the THD of your SMS1 speaker at 1kHz at 0.01W?

What was the noise level in the room in which you were doing ABX test with 0.01W and 1kHz test tone?


Quote from your paper: "If amplifier distortion is audible with a test tone, it may also be audible while playing music. "

That really is a bit of a stretch and should be proved by at least one music material with ABX test.
 

John_Siau

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Here's a comparison I did for using an AHB-2 to my now sold Emotiva XPR-1 monoblocks I used to drive some Klipsch P-17b bookshelf speakers. I don't have any speaker data for the P-39f drivers or I'd do similar calculations for them. The bookshelf speakers still have the factory crossover in them and sit in my office.

From: https://hometheaterhifi.com/reviews...fier/emotiva-xpr-1-monoblock-power-amplifier/
View attachment 65452

View attachment 65447


Emotiva XPR-1 monoblocks (max output and 1 watt output):
View attachment 65464
View attachment 65465

AHB-2 bridged mode (max output and 1 watt output):
View attachment 65466
View attachment 65467

Alternate method of calculating pressure for a given voltage to prove to yourself that Benchmark's rule of thumb makes sense (which it should provided the speaker specs are measured at 2.83Vrms):
View attachment 65468

Note that for the noise SPL figure, it will likely depend on your environment.

A while back I did an accurate measurement of my room in this thread:
https://www.audiosciencereview.com/...hat-level-is-noise-heard-in-your-system.1013/

The spectrum below is for my room (measured with Earthworks M30, Benchmark MPA1, Benchmark ADC1 sending AES to my RME soundcard). Note that the room's inherent noise varies with frequency over the audible range by A LOT! My room is actually very quiet in the md-range.

View attachment 65469

@RichB
I love the photos of your hand-written calculations!

One correction:

Don't assume that something is inaudible if it is lower than the room noise. We have the ability to hear a 3 kHz tone that is 30 dB lower than white noise. I often do this demo for visitors to Benchmark. Our ears are amazing.

If you do an analysis, you will find that many of the musical details that you are hearing in a recording, are playing at levels below that of your room noise. This is especially true of natural reverberant sounds.

Distortion does not normally sound like random noise and this makes it easier for our ears to detect underneath background noise. Most of the masking of distortion is due to the fact that musical instruments already have significant harmonic content. IMD is not harmonic and is not masked by the harmonic content of the instruments.

I find that solo piano recordings are very good for exposing harmonic distortion. The reason for this is that the overtones of the piano are slightly non-harmonic. The overtones are stretched to a slightly higher than integer ratio above the fundamental. Harmonic distortion from the electronics are always spaced by exact integer ratios. The electronically-generated harmonics audibly beat against the piano's overtones, making the piano sound like its strings are out of tune with each other. I first noticed this effect after tuning a piano for the first time. I am not a professional piano tuner, but I tune our home piano from time to time. It is a good learning experience.
 

pjug

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Here is an A/B/X test that we did in our listening room that clearly identified differences between an AHB2 and another amp with reasonably low distortion specifications. The difference was produced by zero-crossing distortion when playing a single tone at a fraction of a watt. The other amplifier produced harmonics that could be clearly identified in the A/B/X test. Test results show a perfect score, meaning the differences were very clearly audible:
https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt
Do decent Class AB amplifiers really have distortion of the magnitude shown in that app note? For example, Amir's test of the Outlaw M2200 showed distortion below -100dB (5W).
 
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