Review and Measurements of Benchmark AHB2 Amp

[Inner Space]

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?

 

[scott wurcer]

I'll call Benchmark after the holiday, because I'm interested in the reason, but in the meantime, what are your best guesses?

Thermal management maybe, but a quick look at the correction tables does not seem to indicate a problem?

 

[blueone]

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?

Good catch, but 2000 meters is more like 6500 feet. I can't wait to hear the reason for this limitation.

 

[DonH56]

I had not read the altitude limitation. My house sits at about 7500' so that would rule them out for me...

May 24 and snowing like crazy outside at the moment, looking to pick up an inch or two. Just when some of the trees and flowers were budding out nicely.

 

[Inner Space]

"May 24 and snowing like crazy outside at the moment ... "

Here too, Don - thermal management isn't a huge issue here in Wyo ...

 

[PaulD]

Maybe the AHB2 will develop altitude sickness, or even worse faint from the lack of oxygen!

 

[Laserjock]

“Air at high altitude is less dense than air at sea level, reducing its convective capability and overall heat capacity. Therefore, all electronics that rely on natural or forced convection to dissipate heat will experience greater air and component temperature rises for the same amount of power at high altitudes.”

 

[blueone]

“Air at high altitude is less dense than air at sea level, reducing its convective capability and overall heat capacity. Therefore, all electronics that rely on natural or forced convection to dissipate heat will experience greater air and component temperature rises for the same amount of power at high altitudes.”

Yeah, but I suspect the underlying cause with the ABH2 is the small size of the package in relation to the potential power output, especially in bridged mode. The output stage is Class AB. I think some of us are just surprised that the elevation restriction is so low. As a mildly interesting example, for a long time Stereophile magazine was headquartered in Santa Fe, NM, which is at an elevation of 7200 feet.

 

[PaulD]

“Air at high altitude is less dense than air at sea level, reducing its convective capability and overall heat capacity. Therefore, all electronics that rely on natural or forced convection to dissipate heat will experience greater air and component temperature rises for the same amount of power at high altitudes.”

Scott covered this in his post. I also checked the tables and saw that the effective cooling is only marginally less before my trivial post above. I will attach some info with the sort of derating table attached. Heating efficiency is only marginally worse at the altitude specified, but maybe it is just a manufacturer being cautious in case someone tries to launch it into orbit.

 

[Inner Space]

“Air at high altitude is less dense than air at sea level, reducing its convective capability and overall heat capacity ... "

Sure, but the amp has competent thermal protection, so wouldn't the manual simply say, "At high altitudes the protection might kick in a little earlier than it would at sea level"? Yet it says "unsafe". Right now my guess is that it's due to some weird consumer protection rule in one of Benchmark's export markets, probably Europe. The altitude limit is expressed in meters, after all. Judging by the back pages of the manual, Benchmark's compliance paperwork is meticulous. I would expect them to include such a warning, if required by someone, somewhere.

 

[etc6849]

I am betting it is a compliance issue (I think the amps have the CE mark), but you can always call and ask.

It probably just costs a lot more to do high altitude testing, so Benchmark didn't do it as it wouldn't apply to 99.9% of buyers and would only drive the costs up for most of their customers. Thus, for compliance purposes they probably had to have the disclaimer or pony up even more cash for what is already likely a very costly process.

Don't forget about active bi-amping or tri-amping... You could always add more if the two you have go into protection mode (I highly doubt they ever will though)...

“Air at high altitude is less dense than air at sea level, reducing its convective capability and overall heat capacity ... "

Sure, but the amp has competent thermal protection, so wouldn't the manual simply say, "At high altitudes the protection might kick in a little earlier than it would at sea level"? Yet it says "unsafe". Right now my guess is that it's due to some weird consumer protection rule in one of Benchmark's export markets, probably Europe. The altitude limit is expressed in meters, after all. Judging by the back pages of the manual, Benchmark's compliance paperwork is meticulous. I would expect them to include such a warning, if required by someone, somewhere.

 

[etc6849]

Test conditions are typically steady state conditions (e.g. resistive loads only).

You can certainly test amps using a moving load and I would assume this was done internally by a manufacturer as serious with perfection as Benchmark. Benchmark's application notes do this with headphone amplifiers, can't remember if they have published measurements of the AHB2, I want to say they do somewhere.

https://benchmarkmedia.com/blogs/application_notes/12838141-headphone-amplifiers-part-1

Damping factor is the measurement for determining how well a woofer will be controlled. It is the nominal speaker impedance divided by the output impedance of the amplifier. However, as you point out, nominal speaker impedance isn't reality...

Driver control is another reason I tri-amped my speakers and will never go back. My old amps undeniably had better control of the woofers once they were wired directly to them.

So when we are discussing the measuring of amps do we mean steady state conditions or do we mean dynamic live loads?

 

[Laserjock]

The manual actually doesn’t mention from what I can find other than a symbol stating what it means and no symbol that I can see on the back of the amp like the warnings for voltage etc.
Has anyone found a note about the altitude in the environment section?

 

[witwald]

Your general consensus is that amplifiers sound different.
No distortion, no clipping, same room, same speakers, same loudness, same song.

Yes, amplifiers can sound different. Some of that difference can be caused by simple frequency response differences. Consider two amplifiers: Amplifier A is -3 dB at 5 Hz and 80 kHz, while Amplifier B is -3 dB at 20 Hz and 20 kHz. Both amplifiers have a flat response in their pass-bands. The differences in their frequency response will exceed 0.1 dB below about 50 Hz and above about 7830 Hz. The plot that is shown below displays the difference in dB between their responses as a function of frequency. For these calculations, I've assumed that both amplifiers have 2nd-order (12 dB/octave) roll-offs outside their pass-bands. There is still quite a bit of audible sound energy in music programme material below 50 Hz and above 7830 Hz for there to be audible differences between these two hypothetical amplifiers. All other things being equal, which amplifier would someone be inclined to choose?
View attachment AmplifierResponseDifferences.png

 

[RichB]

These two application notes are worth reading:
https://benchmarkmedia.com/blogs/application_notes/ahb2-driving-pmc-mb2s-studio-monitors
(short discussion on audible noise level)

https://benchmarkmedia.com/blogs/ap...preting-thd-measurements-think-db-not-percent
(has a great example calculation on noise level SPL)

I really don't see a point in participating in a thread like the one you mentioned, aside from making one or two posts. Whether you'll hear the noise from an amp really depends on too many factors like playback chain, what you are playing back, speaker sensitivity, listening environment and even how sensitive your hearing is.

In general, Benchmark shows in those two application notes how their amps can offer no audible noise for a very wide range of speaker sensitivities in any listening environment. If you read and study those, you'll be able to do general claculations showing which amps will have audible noise with your speakers.

As for IMD, I think lowering it too matters greatly, and it's one of the reasons I originally actively tri-amped my speakers before I bought the benchmark amps. There is no going back to a traditional amp/speaker setup for me!

I have been using this calculator to convert distortion specifications to convert distortion specifications to attenuation:
http://www.sengpielaudio.com/calculator-thd.htm

Here are the specifications for the newly released A21+ that is a very well made, powerful two channel amplifier.

Power Output – Stereo Mode (0.1% THD)
300 watts x 2, 8 Ω both channels driven
500 watts x 2, 4 Ω both channels driven

Total Harmonic Distortion (THD)
< 0.1 % at full power
< 0.03 % at typical listening levels

Converted to SINAD:
0.1 = -60 dB
0.03 = -70 dB

For reference, the Benchmark ABX the comparison amplifier had 70 dB SINAD that is equivalent to 0.03 THD+N.
For the Parasound, that is specification at typical listening levels.
There are very few amps that provide specifications for the first watt.

For the ATI AT6000 series amplifiers, 0.03 THD+N 20Hz to 20kHz at maximum average power.

- Rich

 

[helom]

“Science, e.g., properly designed double blind testing tests have demonstrated that amplifiers that measure well (low noise and distortion, and enough power and current for a given set of loudspeakers) cannot be distinguished from each other. If you do not understand that fact, you are not accepting of exceptionally strong scientific evidence”

Hope member xulonn don’t mind I use his quote. I want to make my point in proper English!

Where can we find these studies? I've read anecdotal claims of such studies but have yet to find one published by anything close to a credible, peer-reviewed source.

 

[SIY]

Where can we find these studies? I've read anecdotal claims of such studies but have yet to find one published by anything close to a credible, peer-reviewed source.

JAES and BAS. Look for Lipshitz, Vanderkooy, Clark, Carlstrom, and references therein. Mostly late '70s, early '80s, where the issue was put to rest.

To return this to a proper question, rather than an illogical demand for the proof of a negative, not one high end amp manufacturer, marketer, advocate, reviewer, or enthusiast has ever demonstrated an exception under basic controls. Not one. Ever. 40 years have passed, it's safe to infer from this that it's, ahhh, unlikely to happen.

 

[scott wurcer]

I am betting it is a compliance issue (I think the amps have the CE mark), but you can always call and ask.

It probably just costs a lot more to do high altitude testing, so Benchmark didn't do it as it wouldn't apply to 99.9% of buyers and would only drive the costs up for most of their customers.

Possibly, you can rent time on a chamber that will ramp a large room from sea level to near orbit conditions but for audio gear I doubt it is done very often. Cooling electronics in space is a science of its own.

 

[cjm2077]

Thermal management maybe, but a quick look at the correction tables does not seem to indicate a problem?

It's not thermal (although altitude can have an impact on cooling as well), it's for creepage and clearance distances on the PCB. Air breaks down more easily at lower pressure (higher altitude), so you have to increase the distances between traces and part leads on the PCB to reduce the chances of electrical arcing. I design equipment for planes and drones, and the usual crossover limit for us is 10,000 ft under the standards we work with. It looks like the 2000m standard is a Chinese standard for electronics. Since nobody else seems to care about that altitude limit, I wouldn't worry about it too much if you happen to live in a high altitude location.

 

[scott wurcer]

It's not thermal (although altitude can have an impact on cooling as well), it's for creepage and clearance distances on the PCB. Air breaks down more easily at lower pressure (higher altitude), so you have to increase the distances between traces and part leads on the PCB to reduce the chances of electrical arcing.

Interesting possibly switching supplies have internal HV, I would be hard pressed to to believe an ordinary PC trace spacing could be made to arc at 50V levels (see Paschen's law for air).