Review and Measurements of Benchmark AHB2 Amp

[Wombat]

Out of curiosity: is there still a critical mass of manufacturers who adhere to this FTC rule in 2019? I remember in the golden age of car audio there was CEA-2006...

Personally speaking, doesn't matter if the AHB2 doesn't meet FTC. It's current rating for real world musical transients (a few seconds?) is a whole lot already given it's size, price and measurements. I think doing 190W @4 ohms for 5 mins while staying clean will already defy physics given the size of this amp. The declared specs doesn't say "FTC" so IMO no deception here.

FTC Amplifier Rule compliance is mandatory. It seems that enforcement is less so.

It seems many sellers are not aware of it or ignore it.

 

[blueone]

Consider this, and tell me a comparison is not fair.

John_S is clearly aware and worried about tweeter lead-in wires getting vaporized when amplifiers burst into a few cycle oscillation on or about the onset of overload or when the amplifier is run at high power, high frequencies for an indeterminate period not disclosed to the owners.
He speaks of tweeter protection. He's also acutely aware of how little time a typical tweeter can survive a high power, HF burst.
He speaks of clipping causing tweeter damage. We know clipping itself doesn't damage tweeters, too much power damages tweeters. Where that power comes from, and what frequencies that power is manifested is the issue. Some amplifiers do it well, others, not so much.
He speaks of transient front end input overloads from AP range switching relays (shifting the blame), and yet no other amplifiers seem to be upset by it in any of Amir's tests. How his amplifier clips as it runs into its tightly regulated SMPS rails is not documented. How does it behave in an overload situation? What is its recovery like?

We don't know, because there is a system that prevents the amplifier reaching its rated power for more than a few seconds. The owners are not in control, the FPGA protection system is and apparently, it knows best...

You have to believe the manufacturer when they say it cannot be triggered by music and would only activate in an overload or series of events the owner was not made aware of. Sound rather similar to MCAS if you ask me...

(using MCAS in this discussion this in no way diminishes the terrible loss of life, it is merely illustrative and I certainly hope no-one reading this was touched by loss)

John, I know your comparison to MCAS was not intended to diminish the impact of the MCAS decisions by comparing them to an audio amplifier.

We're still on different pages about the comparison and whether it's fair, but no matter. I don't know why the AHB2 needs the current-limiting protection mechanism, in an FPGA no less, which is hardly a cheap or easy solution. Other amplifiers I've seen have used regulated DC power supplies (certain older Krell and Mark Levinson products come to mind), but they were regulated linear power supplies, not switched designs, and they had no reported behavioral anomalies. I'm not an amplifier design expert, but I'm having trouble visualizing how the power supply could cause oscillation, but there is the matter of the feed-forward error correction, which is uncommon (though certainly not unique), and it at least makes me wonder...

The AHB2 is a very sophisticated analog amplifier design, and clearly the end product has produced some very impressive results. Perhaps there is a (small) price that had to paid to realize it with safety. Personally, if I were in the amplifier market I'd be willing to live with the theoretical limitation.

 

[restorer-john]

but there is the matter of the feed-forward error correction, which is uncommon (though certainly not unique), and it at least makes me wonder...

My experience with commercial FFWD designs in the past also led to tweeter destruction in a few cases. I'll save that for another thread...

 

[spigot]

You have to believe the manufacturer when they say it cannot be triggered by music and would only activate in an overload or series of events the owner was not made aware of.

They've sold a few, has anyone complained about the protection circuit preventing them from listening to music, or is it still zero people?

 

[restorer-john]

They've sold a few, has anyone complained about the protection circuit preventing them from listening to music, or is it still zero people?

 

[Wombat]

They've sold a few, has anyone complained about the protection circuit preventing them from listening to music, or is it still zero people?

See post #1003, above.

Now who is going to take this thread to an incredible post #1009?

 

[RayDunzl]

Now who is going to take this thread to an incredible post #1009?

 

[Frank Dernie]

I finally appreciate an answer.

To me, it's a consumer electronics version of the MCAS software debacle in the 737MAX-8. As shipped from the factory, we have a system that takes over, can't be shut-off easily and prevents the product behaving like the tens of thousands of amplifiers tested before it, and worse still, not meeting its own advertised specifications which require 5 minutes at advertised power, not a mere and non-specific 'several seconds' before it crashes and shuts down.

Unless the user can easily defeat the time/frequency integrator part of the protection, you should either amend the specifications to reflect the true continuous ratings in accordance with the FTC's Amplifier Rule or remove the reference to continuous average power and call it 'several seconds' power.

Consider since 1974, manufacturers have played by those rules. Their amplifiers were big, hot, had tons of heatsink area, tons of dissipation in silicon, massive power supplies and little or no current limiting apart from short circuit protection, DC and sometimes over-voltage. Those amplifiers met or exceeded their ambitious specifications, year after year. They were built to do it. In short, consumers were getting what they paid for.



Here is the relevant part of the FTC's Amplifier Rule:

View attachment 28460

The rule was retained after industry consultation:

View attachment 28462

This Benchmark power amplifier is no doubt a lovely product, built well and performs admirably in all areas except meeting its continuous power output ratings. Rather important wouldn't you say?

I must say that the old, old standard does seem a bit pointlessly like a definition for a welding station to me.
Music, certainly the type I listen to, never requires high continuous power.
Certainly when this sledgehammer of a suggestion surfaced it made sense to do something about the absurd peak power claims being bandied about and amplifiers were capable of 10 to 12 watts per channel under these conditions, a 30 watt amplifier was a powerful one and 100 watts all but unheard of. Making sure all of that was available all the time was probably well worth while.
It does now seem hopelessly antidiluvian with speaker busting power available from even modestly priced amps.
Yes, better splendid amps were designed back in the day as a result of these standards. I have had some of them and still have one or two.
Today what Benchmark have done to prevent damaging the tweeter seems eminently sensible, well throught through engineering to me.
Bravo to Benchmark for engineering in a feature which would have been impossible to do so cleverly only a short time ago.
Just IMHO.

 

[typericey]

Maybe about time the FTC ruling gets revisited and amended? I know not of any music/movie/recorded material with 5 minute continuous peaks*. Oh except that album Pink Noise - The Wall.

*edit: just realized this is an oxymoron

 

[Sal1950]

I remember the good ole days when my Phase Linear 700B would go Chernobyl on me and take out a driver or two. All in a good days fun, no?

 

[restorer-john]

I remember the good ole days when my Phase Linear 700B would go Chernobyl on me and take out a driver or two.

Pink Floyd The Wall, eh Sal?

PS. Chernobyl wasn't even planned, let-alone constructed back then was it?

Ouch..sorry. ouch. That hurts...

 

[Sal1950]

Pink Floyd The Wall, eh Sal?

Tear down the wall, tear down the wall, BOOOOOM
LOL

 

[KSTR]

FTC power rule is outdated as it's a dumb rule for dumb amplifiers. A smart rule would state that only the amplifier core must fulfil the power spec at any frequency without *amplifier* protection kicking in, not the complete product which may (and does, in this case) contain *user* protection. Which means, the user protection should be turned off for the measurement (which Benchmark did). But, disabling the feature certainly shall not be user accessible, at least not as easy as setting an internal DIP switch or jumper...
Maybe the best option is to restrict bandwith of the testsignal to avoid cut out or limiting of intelligent amp designs .

 

[DonH56]

<Not Benchmark-specific but then most of this thread is not...>

My problem with tossing the FTC rule is that it throws out the baby with the bathwater. Fine, get rid of it, what do we have? The 20 ms burst test? A 1 s, 10 s, how long for a full-power test? What about thermal management, and the poor guy not using 100 dB/W/m speakers who actually listens in a large room with an amplifier putting out 10 W average and consistent 100 W peaks into a low-Z speaker load? Do we care if his 100 W amp goes into thermal shutdown halfway through a CD or movie?

A couple of decades ago there was an effort to refine the FTC specs, update compliance, add multichannel standards, etc. It basically got shelved at the behest of the manufacturers, as they did not want to be held to a standard their new products could not meet, and the whole effort seems to have stalled.

I'm in favor of standards, at least they provide some sort of reference, even if ad-hoc. At this point I am not sure there are even ad-hoc standards for multichannel and the stereo tests are being ignored. It's easier for manufacturers; they can make up any sort of spec they want to impress the consumers. Back to the 70's and "max peak instantaneous dynamic transient output power" and the like, oh joy...

 

[Kal Rubinson]

FTC power rule is outdated as it's a dumb rule for dumb amplifiers. A smart rule would state that only the amplifier core must fulfil the power spec at any frequency without *amplifier* protection kicking in, not the complete product which may (and does, in this case) contain *user* protection. Which means, the user protection should be turned off for the measurement (which Benchmark did). But, disabling the feature certainly shall not be user accessible, at least not as easy as setting an internal DIP switch or jumper...

Sounds like something inspired by Volkswagen.

 

[neutralguy]

The FTC specs were created to be a standard for comparison, exactly so manufacturers can't invent their own test conditions with unspecified self selected criteria for what constitutes real word usage. Just because you disagree with a rule, doesn't mean you're allowed to break it.

If Benchmark truly believes that their test criteria are more reflective of real world conditions than FTC's, then why not proudly specify what it is and why that's superior, instead of specifying only 190W@4ohms leaving the consumer to assume incorrectly that it's the FTC's? Benchmark's website and manuals go into great details about their specs and testing, so surely there's room for such a discussion.

I'm also not convinced that the only reason for the shutdown is to protect the tweeters. As pointed out previously, Stereophile review tested with only 1 channel driven and was able to achieve 210W@4ohms. Perhaps it was because with both channels driven it also shut down in Stereophile's testing. If so, why didn't the protection kick in with 1 channel driven? Is it ok to blow out one tweeter but not two?

 

[Sal1950]

I'm in favor of standards, at least they provide some sort of reference, even if ad-hoc. At this point I am not sure there are even ad-hoc standards for multichannel and the stereo tests are being ignored. It's easier for manufacturers; they can make up any sort of spec they want to impress the consumers. Back to the 70's and "max peak instantaneous dynamic transient output power" and the like, oh joy...

I'm with you Don. I clearly remember the days before the FTC spec and the chaos in amp measurements. What's going on today with multich gear is despicable. I always felt it was a good rule that held the manufacuters feet to the fire for making claims.

But, disabling the feature certainly shall not be user accessible, at least not as easy as setting an internal DIP switch or jumper...

Why not? If some idiot user opens the amp and throws the switch and then fries a speaker that's his problem. That's what he gets for disregarding a builders recommendation.

 

[blueone]

...there is the matter of the feed-forward error correction, which is uncommon (though certainly not unique), and it at least makes me wonder...

Reading through this long thread I noticed that in post 126 John S says that forward error correction is inherently stable.

 

[DonH56]

That is not strictly true though is in practice. But there is also feedback in the circuit.

 

[RichB]

I am a bit confused by this chart.
Does this indicate that the sweep was performed high frequency to low and shutdown at about 45 Hz?

Why would tweeter protection kick in a low frequencies?
This amp is rated at 180 WPC into 4 Ohms.

I agree with the sentiment that it should be rated and shipped using the same configuration or at least be settable.

That said, I have not had any AHB2 shutdown driving the Salon2's (bi-amped) when the clipping lights were barely blinking.
I suppose I could put on some ear defenders and let it play Lorde's entire album.

- Rich