Review and Measurements of Benchmark AHB2 Amp

[DonH56]

@amirm can you also check measurements in bridged mono?

He has, e.g. https://www.audiosciencereview.com/...of-benchmark-ahb2-amp.7628/page-2#post-178716

 

[John_Siau]

Sometimes a bare wire just won't work.

As everyone already knows, I use (now defunct) BellCore approved power cable (we are delivering power, aren't we), terminations, and antioxidant...

View attachment 26594

Scale reference: The nuts are 1/2"/13mm

Ok, it's stupid, but it was all in my scrap pile. The cable was manufactured in 1992, the connectors at least the same vintage.

Same thing at the amplifier, a whole five cable feet away.

Thanks for posting the photo. The problem with this setup will be inductance. Inductance increases when the spacing between the conductors increases. The large conductors provide a low series resistance, but the spacing creates a large inductance in series with the speakers. This inductance creates a low-pass l-r filter which will roll off the high frequencies. Your solution is excellent for sub woofer applications but will cause some attenuation of high frequencies. It will also cause a phase shift that will delay the high frequencies.

These effects can easily be measured. You can measure the inductance of the cable assembly and you can measure the frequency response of the cable when it is loaded by the speakers or with a dummy load. The solution is to use multiple conductors that are closely spaced to reduce the inductance. We use Canare 4S11 star-quad speaker cable because the star-quad configuration reduces the inductance of the cable. The photo below shows the cross section of the cable. The two red conductors are wired in parallel and are connected to the + terminal. The two white wires are wired in parallel and are connected to the - terminal. The magnetic field produced by the red conductors is canceled by the return currents in the white conductors. This cancellation of the magnetic fields is what minimizes the inductance. You should use 4 wires in a tight bundle where the paralleled conductors are on opposite sides of the quad.

The Canare 4S11 uses four 14 AWG conductors. When the conductors are paired as descrived above, this is equivalent to 11 AWG.

Here is a link to an application note on Star Quad Cable:

https://benchmarkmedia.com/blogs/ap...-the-importance-of-star-quad-microphone-cable

The application note talks about star-quad microphone cables being immune to magnetic fields. The use of star-quad cables for speaker connections is the reverse of this, but the principles are the same. With speaker cables we are trying to prevent the emission of magnetic fields because these are what produce inductive losses.

Audio cables do not need to be made from exotic and expensive materials. They just need to be designed using sound engineering principles that can be proven with simple tests and measurements. I have seen some outrageously expensive cables that measure very poorly.

 

[DonH56]

That is my doubt about such amazing numbers (Benchmark and NC400/NC500), if the feedback has been abused by sacrificing musicality. What good is a spectacular sound if it does not excite? Since I have not heard them, I can not comment.

I read this all the time these days, that somehow feedback (often any feedback) is somehow inconsistent with "musicality", whatever that means. Given such a wide bandwidth it seems unlikely that something like TIM would be a problem; it seems more likely the better-performing ("cleaner") amp is simply not coloring the sound the way the listener likes. One of the things the audiophile press has done is to make technical excellence somehow "bad", and the garage-shop mentality of a lone-wolf designer (engineer or not, preferably not if you read some of the reviews) swapping components based solely on how they "sound", trumps all engineering. It is pretty much the antithesis of what ASR stands for.

IMO - Don

 

[maty]

The solution is to use multiple conductors that are closely spaced to reduce the inductance. We use Canare 4S11 star-quad speaker cable because the star-quad configuration reduces the inductance of the cable...

I have a thread in a very audiophile forum about Star-quad geometry. I have recommended your video numerous times on the net.

Star quad, the best geometry to build audio and power cables

https://www.audiocircle.com/index.php?topic=156793.0

 

[John_Siau]

The numbers are AMAZING.

I have a question: how much global feedback? More or less 30 dB?

The AHB2 uses feed-forward correction and some feedback. The feed-forward correction is not subject to loop delays and can provide correction at very high frequencies. In contrast, feedback systems have a limited correction bandwidth. If the bandwidth of a feedback system is not adequately limited, the amplifier will go unstable and begin to oscillate. Feed-forward correction is inherently stable.

 

[maty]

I read this all the time these days, that somehow feedback (often any feedback) is somehow inconsistent with "musicality", whatever that means...

IMO - Don

There is a middle ground between a lot and no feedback in class A and AB.

Some diyers say that less than 30 dB the sound is more holographic, with more differentiated instruments. There are two threads in the diyaudio forums about it.

 

[John_Siau]

Congratulations Benchmark.
I remember reading about this amp when it first came out. I even read the Stereophile review.
Interesting thing about Stereophile is they still went on to praise some other pieces of audiophile jewelry at 100K without reference to the Benchmark.
It seems to me that there isn't a better buy. I agree in general with those who say it performs way past what is audible. If the amp was priced at 50K it would be an expensive excess; it isn't, it's 3K, which in audiophile terms is nothing and for a buy once to have the best for the average audio enthusiast, not much either.
If Benchmark made it five times the sized and at a weight that required and sack barrow to move it, they would probably sell more.
All the people I've come across using Benchmark products have had nothing but praise for the products, the service and the reliability.
If I buy another amp this will be it. There is nothing on the market that comes close.

Many years ago, a man who had been in the business of selling audio amplifiers visited Benchmark. He said "If you ever build an amplifier, our market research shows that it must be big, very heavy, run hot, and have a high price tag". The AHB2 is none of those things. It is a labor of love created by a group of THX and Benchmark engineers that wanted to build an amplifier that was as transparent as possible. We wanted to build an amplifier that could match the performance of our D/A converters. If it was big, heavy, hot and expensive we might have even more trouble keeping these in stock.

 

[John_Siau]

I'm reminded of an anecdote where Peter Aczel and (I think it was) David Rich heard a Max Wilcox DAT copy of one of his masters, and thought it was great. When the CD came out both were chagrined, accusing DGG of mucking up the original mix. Aczel called up Wilcox to complain. Wilcox said that it was impossible that DGG would screw up his recording. The two then found the DAT that Max had given them from his master, did a controlled listening, and found that they were indeed the same.

How can a DAC 'strip away huge amounts' of what is on a recording? Does that even make sense to anyone? How could that even happen?

Some DACs add delay to the high frequencies and this seems to make reverberation more audible. The delay is produced if linear-phase filters are not used when reconstructing the analog signal.

The Benchmark DACs use linear-phase filters. Some DACs use filters that change the phase response and this can cause an audible effect.

 

[lichie]

Mine just came in the mail on Wednesday . For now it is my dedicated Hifiman HE-6 amp.

 

[svart-hvitt]

https://www.stereophile.com/content/nothing-what-i-want:

Thanks!



This Herb Reichert quote deserves some hot light:

«I use Chesky recording sessions to review headphones because I can compare what I hear live to the sound coming off the so-called “mike feed.” The Border Patrol DAC reproduced the church walls, the reverb, the positions on the floor where the musicians were standing, and all the subtle breathiness of Macy Gray’s voice. With the Benchmark, the majority of that information (which is definitely on the master file and appears via David’s $100K MSB DAC and via my Holo Spring DAC) disappeared !!! Your neutral DAC “stripped” away information that is unquestionably on the master file. Not to mention the BM DAC made it sound hard cold and harmonically threadbare. I call this subtractive distortion. Did you measure any of that?

My definition of accurate is: the DAC that makes a real piano in a real room sound the most like a real piano in a real room is the most accurate.

My definition of a neutral DAC is: the DAC that preserves the most information I know is on the file or tape.

What is yours?

Peace, love, and jangling keys,
herb».


Source: https://www.stereophile.com/content/nothing-what-i-want#JT6ykJu1xKKl42dy.99

 

[RayDunzl]

Thanks for posting the photo. The problem with this setup will be inductance. Inductance increases when the spacing between the conductors increases. The large conductors provide a low series resistance, but the spacing creates a large inductance in series with the speakers. This inductance creates a low-pass l-r filter which will roll off the high frequencies. Your solution is excellent for sub woofer applications but will cause some attenuation of high frequencies. It will also cause a phase shift that will delay the high frequencies.

These effects can easily be measured. You can measure the inductance of the cable assembly and you can measure the frequency response of the cable when it is loaded by the speakers or with a dummy load. The solution is to use multiple conductors that are closely spaced to reduce the inductance. We use Canare 4S11 star-quad speaker cable because the star-quad configuration reduces the inductance of the cable. The photo below shows the cross section of the cable. The two red conductors are wired in parallel and are connected to the + terminal. The two white wires are wired in parallel and are connected to the - terminal. The magnetic field produced by the red conductors is canceled by the return currents in the white conductors. This cancellation of the magnetic fields is what minimizes the inductance. You should use 4 wires in a tight bundle where the paralleled conductors are on opposite sides of the quad.

The Canare 4S11 uses four 14 AWG conductors. When the conductors are paired as descrived above, this is equivalent to 11 AWG.

Here is a link to an application note on Star Quad Cable:

https://benchmarkmedia.com/blogs/ap...-the-importance-of-star-quad-microphone-cable

The application note talks about star-quad microphone cables being immune to magnetic fields. The use of star-quad cables for speaker connections is the reverse of this, but the principles are the same. With speaker cables we are trying to prevent the emission of magnetic fields because these are what produce inductive losses.

Audio cables do not need to be made from exotic and expensive materials. They just need to be designed using sound engineering principles that can be proven with simple tests and measurements. I have seen some outrageously expensive cables that measure very poorly.


View attachment 26611

Excellent advice.

I should dump my MartinLogans, too, since they are non-conformant with "good speaker" wisdom.

Too bad I've never heard and never will be able to hear those high frequencies. Discovered in 1961 listening with Dad to a stereo test LP. I blame Mom. She had the same defect.

Then again, with all the dissatisfaction I hear others describe when talking tweeters, maybe it's not such a debilitation.

I still feel I can hear all of the music, just miss some of the sound. Ok, I miss the top of a glockenspiel, and some other tinkly stuff. But I'm unable to even imagine sound higher than I can hear, so...

The first time I (unknowingly) heard an original DAC1 (or any standalone DAC) at Audio Buddy's house, I thought it shrill. "What did you doooo?"

I have a DAC1 USB and a DAC2 HDR.

You can buy me lunch sometime.

What's in your pipeline for the future?

 

[RayDunzl]

*scratches head, wonders "Is that a lot? It doesn't look like a lot. Could be more."

 

[DDF]

*scratches head, wonders "Is that a lot? It doesn't look like a lot. Could be more."

I design speakers so I characterized my test set up (MLSSA) using wire constructed like star quad and wire constructed with a gap between conductors (perhaps ~ 1cm), using a NAD 3020 amp or a Bryston 2BLP Pro amp. The resultant change in frequency response with the inductive wire was as high as +/- 0.5 dB over the audio range (broad bandwidths) depending on the load condition (I tested open cct, 7ohm resistor, D2905 9300, and B110 and they were all different).

 

[estuardo4]

I have never seen an amp that measures this way. I don't know that anybody has. It isn't the first amplifier to use feedforward -- indeed, John Siau points out that feedforward is actually an older technology than negative feedback -- but it's rare in doing so.

It's great to see someone breaking all the "rules" of amplifier design, because it's been in a rut for some years now, with the best designers squeezing pretty much everything that could be squeezed out of available devices using conventional topologies.

Anyone who thinks all amps sound the same will be in for a rude but happy awakening when they compare an inexpensive amp to an amp like the Benchmark.

That said, anyone who thinks that the Benchmark is a *perfect* amp because of its steady state measurements will learn that that doesn't yet exist.

I've owned an AHB2 for several years now, and it's the cleanest amplifier I've ever heard. I was listening to it last night and I can only describe the sound as ravishing. Like silk. Or like Class A without the absurd size, weight, and heat. (If you've ever heard crossover notch distortion you know why Class A amps sound better than A/B ones, it's incredibly audible and annoying in small amounts.) And the low weight, tiny form factor, and efficiency of the AHB are really welcome. It's a save-the-planet kind of amplifier, without the sonic compromises of Class D.

But perfect amplifier? No. It excels in steady state measurements, but it isn't as dynamic sounding as other good amplifiers with which I've compared it. Again, this isn't a question of steady state measurements or amplifier power -- I'm not clipping it. But listen to a piano on the Benchmark and another good amp and you'll hear what I mean: it rounds off the attack.

So the Benchmark isn't a perfect amplifier, but anyone who has never heard a really good amplifier or thinks that all amps sound the same is going to be blown away by the sound. It's a bargain, too, by the standards of high end amplifiers, and with its sophisticated protection circuitry, quite bulletproof!

Thank you for your response. What I understand about your post is that there is no perfect amp, but the AHB2 is the nearest an amp can be to perfection, technologically speaking. And even then, as recordings, rooms, DACs and speakers are not perfect, it doesn't matter because subjectivity will always prevail. Even if you have the perfect amp, the perfect recording and the perfect speakers, it won't necessarily mean that you would like the end result, because human hearing is not perfect.

Based on what I've seen wrote here about psycho-acoustics and how the human wants to hear what they want to hear, my guess is that if all your equipment is perfect, your hearing would force you to like what you are hearing, because otherwise that would mean that you are crazy or deaf. Good thing is that you didn't have to spend more than $100,000 to get to realize this.

Thanks to ASR, now I know that you can get an almost perfect amp for $3,000, an almost perfect DAC for $1,000 and now my homework is to try to get a perfect room, speakers and recordings on other sites. Or I can get a good enough amp for $1,000 and a good enough DAC for $100. I didn't know this before I joined the forum. I really appreciate all the input from all the experts and enthusiasts, and how welcoming the members are.

 

[RayDunzl]

Sample Frequency Response adjustment of my system taken at preamp output with (red and blue) and without (light blue) Room Correction applied for two different types of speakers:

Red - Martin Logan reQuest
Blue - JBL LSR 308

Maybe the contribution of the wire becomes moot.

 

[maty]

Feed forward (control)

https://en.wikipedia.org/wiki/Feed_forward_(control)

The Three types of Control System (a) Open Loop (b) Feed-forward (c) Feedback (Closed Loop) Based on Hopgood (2002)

 

[maty]

...Another issue is, paradoxically, lack of distortion masking. Lower order harmonic distortion is known to render higher order harmonic distortion euphonic. This is because of the mechanism by which we detect the timbre of sounds. And higher order distortion is unfortunately quite common on poorly made recordings and badly-designed equipment. What this means in practice is that the AHB2 will sound great on clean recordings, but will pass the harshness through on more distorted ones, while some more colored amplifiers with lower order harmonics will tend to mask that harshness.

Better explained:

* H2 and H4… are good armonics.

* H3 and H5… are BAD armonics. Predominant H3 vade retro satana.

https://audiophilestyle.com/forums/topic/56397-questions-for-interview-with-bruno-putzeys-lars-risbo-about-purifi-audio/?tab=comments#comment-958956

 

[anmpr1]

Thanks to ASR, now I know that you can get an almost perfect amp for $3,000, an almost perfect DAC for $1,000 and now my homework is to try to get a perfect room, speakers and recordings on other sites.

What we need is a comparable speaker to go with SOA electronics. Loudspeakers (and the speaker-room interface) are the biggest problem in accurate sound, today (not including the recording itself). The listening room can not be changed very well. Speakers can be changed, but are a huge hassle to change and/or integrate into a listening environment. Can anyone imagine loudspeakers that match the performance of our best electronics, (FR and distortion-wise)? I don't think anything like that will happen in my lifetime. You just have to find something that sort of fits your criteria, and go with it. Personally I was always looking for a cross between the original Quad electostat (delicate finesse) and the JBL L100 (dynamics), all with a coherence and wave launch like the Harold Beveridge line source electrostatic, and sensitivity of Paul Klipsch's speaker. Fat chance of that ever happening. Well...in the meantime at least we can have SOA electronics.

 

[daftcombo]

Yes they are both audibly transparent.

Why? Our very best transducers have H2 or H3 that is 60dB down (0.1% THD)

I think some people will disagree & I would like to know the science justifying those disagreements.

 

[John_Siau]

View attachment 26613


*scratches head, wonders "Is that a lot? It doesn't look like a lot. Could be more."

If your wires are only 1.5 m long, there is not issue. At this length, you can get away with just about anything. The inductance will be too low to matter.

Nevertheless, check your numbers: I doubt that the conductors are just 8mm apart given the fact that the conductor diameter is 7.35 mm. This implies that the insulation is 0.35mm thick (which is very thin). Your calculation also assumes that the wires are tightly joined together over their entire length. Try increasing the spacing to a more realistic number and then look at what would happen with a 25 foot cable.