Review and Measurements of Benchmark AHB2 Amp

[Thomas savage]

I guess nothing changes all the verbiage and wiggling to not actually do a blind no peeking test.

Well anal probing by space aliens is a new factor in the equation, louder verbiage and rather urgent wiggling I'd guess.

 

[Willem]

I fully agree with Frank about evaluation methodology. Technical improvement has indeed been slow, particularly at the upper end of the market. My current Quad 2805 main speakers had developed a fault and for a while I had to fall back on my older Quad ELS57s. The family resemblance is obvious, and even though the 2805s are clearly better, the ELS 57s were very good as well. In the process, I also had to use my old Quad 303 power amp instead of my 606-2, and could not detect any sonic differences. Methodologically, these are of course invalid observations, but I cannot be bothered to do a proper test.
The only major sonic improvements I have experienced over the last few decades were the introduction of the CD, and more recently room equalization.

 

[Frank Dernie]

I find it fascinating that people are seemingly terrified of blind listening. It's like they know they will fail.........

This is the nub of the debate.
For years people have been propounding audibility of things where there is no mechanism by which a change could have taken place.
There is a huge load of spurious and often ridiculous discussion about how DBT and level matched type of comparison is perhaps inconclusive.
Harbeth had a long term offer of free speakers iirc for anybody who could tell the difference between 2 properly engineered and suitable amplifiers. Nobody took them up on it, despite extensive discussion on several forums.
I think, at the back of their minds, many enthusiasts know that they would not hear a difference so invent some new way of evaluating that they can “hang their hat on” to continue believing the bollox.
I decided over 10 years ago that If I could not hear a difference on level matched comparisons, including sighted, there was no point changing.
Now it is ergonomics, styling and other biases in general which dictate my choice of everything bar speakers and power amps.

 

[maty]

...I actually have no idea why, could be a particularly good sounding recording I am listening to (IME the recording quality is massively more important for SQ than the hardware we use to play it on which makes a mockery of Hi-Fi in a way anyway), it could be my mood, some furniture I have moved or something, but the one thing it can not be is a change in my Hi-Fi, since there hasn’t been one.
.

I agree. That is why I continue using the second-hand AVR amplifier (€ 115 at home) that I bought to experiment after seven years. The current sound is so good with the excellent recordings that I usually enjoy that I have doubts if someone else would do better.

The only problem, which is relative because I usually hear in the near field, is that its sound is 2-D (now much less than the original). Some diyers/builders say that with a global NFB < 29 dB the sound is more 3-D. And then there is the harmonic, monotonically decreasing profile. But then I have reached the limit of improvement and I need a new one with those premises. I found it in class A but I need more power and less heat, that is, class AB at least. Without certainty I stay as I am.

 

[maty]

Off topic

Yesterday, vzaichenko and ODFN feedback.

https://www.diyaudio.com/forums/solid-state/338814-lateral-cfa-120w-bsa-post5831905.html

https://www.diyaudio.com/forums/solid-state/338814-lateral-cfa-120w-bsa-post5832131.html

https://www.diyaudio.com/forums/solid-state/338814-lateral-cfa-120w-bsa-post5832165.html

- End off topic -

 

[SIY]

If orangejello, in the same audio system, with only the amplifier -with very good measurements- as a differential element, with equal levels and well recorded acoustic music with great dynamics, he is able to appreciate different depth so we will have a verification.

"If." But he hasn't, so again, not a single example.

 

[DonH56]

Is there ANY way to keep technical threads focused on the performance without wandering off into pissing contests about what people think they hear? I was trying to follow the max power test results but it's impossible in this mess...

 

[SIY]

https://www.audiosciencereview.com/...ements-of-benchmark-ahb2-amp.7628/post-188535
https://www.audiosciencereview.com/...ements-of-benchmark-ahb2-amp.7628/post-190052

@restorer-john is unhappy because these are toneburst tests.
I am unhappy because I don't have that Power Cube.

I think that's a decent summation.

 

[svart-hvitt]

https://www.audiosciencereview.com/...ements-of-benchmark-ahb2-amp.7628/post-188535
https://www.audiosciencereview.com/...ements-of-benchmark-ahb2-amp.7628/post-190052

@restorer-john is unhappy because these are toneburst tests.
I am unhappy because I don't have that Power Cube.

I think that's a decent summation.

I previously asked @John_Siau if Benchmark will introduce kW versions of their amplifier. Would a 5kW version put all these questions to rest?

 

[Sal1950]

Well anal probing by space aliens is a new factor in the equation, louder verbiage and rather urgent wiggling I'd guess.

Do you have first hand experience? Have you somehow documented the alien invasion?
BTW, Love the new avatar. LOL

 

[John_Siau]

I'd guess that the main reason for this somewhat complex power section of this amp is to make it as lightweight as possible - since manufacturer's strategy is direct sales, it's pretty important to keep the weight as low as possible - amps need to live through shipping process. In the end, it's the cost - packing cost and shipping cost. But manufacturing cost as well, since quality transformers are expensive. But I'm not convinced if classic transformer of a decent capacity would not be a yet better choice when it comes to sound quality.

There were three primary reasons for choosing a switch-mode power supply when designing the AHB2:

1) Lower noise: The switch-mode power supply delivers a huge reduction in magnetic fields due to the elimination of line-frequency magnetic components. This eliminates the strong line-frequency (and line harmonic) magnetic interference which is always the largest contributor to hum and buzz in the output of an amplifier. Filter capacitors and electrostatic shielding have absolutely no effect on magnetic fields. It is also very expensive to build line-frequency magnetic shielding. Power amplifiers require high-power transformers and at line frequencies these transformers have to be very large. These large line frequency transformers emit correspondingly large magnetic fields. In contrast the magnetics in the AHB2 operate at 500 kHz and above and emit much weaker magnetic fields due to their small size. Also, given the small size, we are able to encapsulate the magnetics in ferrite pot cores. This encapsulation reduces the stray magnetic fields. Furthermore, the magnetic fields are well above the audio band and any resulting interference can be removed will a low-pass filter without impacting the audio. At 500 kHz and above, it is also much easier to build effective magnetic shields. We provide 2 in the AHB2. There is one above and below the power supply. Sensitive electronic devices such as preamplifier and D/A converters can be placed directly on top or underneath the AHB2 without the usual risk of magnetic interference.

2) Regulated DC outputs: Switch-mode power supplies can be regulated without a power penalty. The AHB2 has regulated DC voltage rails which means that the rails do not sag when driving low impedances. This is one of the reasons that we get nearly a 2: 1 increase in power when driving 4 Ohms instead of 8 Ohms and it is why we get nearly a 4:1 increase in power when operating in bridged mono. It also explains why THD does not increase when the load impedance decreases. Virtually every other power amplifier has an unregulated supply.

3) Efficiency: The switch-mode power supply is much more efficient than a linear power supply. Consequently, the AHB2 amplifier runs much cooler than it would with a linear supply. It was a high priority to build a passively cooled amplifier. Fans are not acceptable in a high-performance listening space.

Designing a high-power switch mode power supply (as we did) is a non-trivial task. It also means that the product will be subjected to a long series of expensive safety test to meet requirements in each region where the product will be sold. It would have been much cheaper and easier to build the AHB2 with a linear power supply. But, we would have had to put the power supply in a separate chassis to achieve the same noise performance. There would also have been a mandatory separation distance on the order of about two feet between the linear power supply and the audio stages to prevent magnetic interference.

Linear supplies are much noisier than well-designed switch-mode power supplies. The noise difference is due to the stray magnetic fields and not the voltage ripple. Voltage ripple is easily removed with filters but magnetic fields are problematic. The design with the lowest stray magnetic fields wins. This eliminates linear power supplies.

 

[John_Siau]

The complexity of the power supply is to support class-G operation. That is, it is fundamentally a class-AB amplifier with power supply rails that track the incoming signal level.

Actually this is incorrect. The supply does not produce the tracking rails. It only provides DC voltages. It is actually easier to do this with a linear supply. The main difference is that we can achieve a much better SNR with a switch-mode power supply due to the much weaker stray magnetic fields (which also happen to be at frequencies that are well above the audio band).

@DonH56, that was still a good guess! And we did discuss this in the design process. It would have been possible to generate tracking rails with a class-D amplifier, but we chose to keep the entire amplifier analog. We wanted to keep IMD and THD low and we wanted to avoid producing out-of-band noise.

 

[FrantzM]

The more I read about this marvel, the more I want one. I hate you people!

 

[Sal1950]

The more I read about this marvel, the more I want one. I hate you people!

@John_Siau John, how about having a lotto for 1 free AHB2 for us poor folk?

 

[John_Siau]

@orangejello :

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? ).

Not possible. The amplifier stays clean and mutes silently when an unsafe condition is detected. The amplifier includes a tweeter protection function that will shut the amplifier down if high-frequency signals are present at full output for an extended period of time. This cannot be triggered by music and the circuit is not in the audio path.

 

[John_Siau]

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.

The AHB2 does not limit transients. The protection circuit is not in the audio path. We monitor output current, output voltage, THD, and a number of other parameters with an FPGA. If the FPGA detects a fault condition that could damage tweeters, it mutes the amplifier. Sweep tests generated by an AP test station can generate relay switching transients that can trigger the tweeter protection. When the auto-ranging circuits in the Audio Precision switch, high-level transients can be produced. These can be enough to trigger the protection if the amplifier is already delivering a continuous high-amplitude sinusoidal output. You would not wan't to have a speaker connected to an amplifier when it is driven by an AP test station. The auto-ranging relay switching transients would take out your tweeters. The protection network in the AHB2 does its job, but it make life more difficult for the guy doing bench testing.

 

[John_Siau]

If every professional sound reinforcement amplifier shut-down every time someone bumped a mic or unplugged a guitar, they'd be a lot of unhappy concert goers around the world. A transient input overload is hardly an uncommon situation and most amplifiers can handle that without feeling the need to shut-down.

The tweeter protection, as implemented, would not be appropriate in a sound reinforcement environment, but the AHB2 is not intended to be used in that application. It is intended to be used in studio control rooms and in high-end hi-fi systems. In both of these intended applications, the amplifier will often be driving very expensive speakers that could easily be destroyed by the available power.

 

[RichB]

Above is why if you "hate" a device, you can still fall in love with it in testing. You listen more attentively, you hear more detail, and you think it is a better piece of audio.

Some reviewers listen first and measure after because bias goes both ways.
Some like the look of the hardware, some like the look of the measurements.
Both lead to bias.

I once recommended the Pioneer SC-07 receiver (140 WPC ice amps) to drive his B&W CM10s.
When I listened to this system, it did not sound quite right to me. Although he seem pleased.

Here is a review by Michael Fremer (golden ear'ed reviewer) for Sound and Vision:

Price: $2,200 At A Glance: Ultra-sophisticated setup and calibration system • Punchy-sounding ICEPower amplification • Versatile LAN-based Home Media Gallery with iPod connectivity
Read more at https://www.soundandvision.com/receivers/pioneer_elite_sc-07_av_receiver#FeAQzWYk4VUv1I5F.99

These measurements look good but they are at 1kHz into resistive loads.

The CM10 impedance dips below 4 ohms and has over 45 degree phase angles., not an easy load.

Then, there was this review by Audioholics that measured the SC-97 at 4 ohms from 20 to 20kHz:

https://www.audioholics.com/av-receiver-reviews/pioneer-sc-07/sc-07-measurements-and-analysis

Driving 4-ohm loads was an entirely different story. The SC-07 simply fell apart when running full bandwidth (20Hz to 20kHz) continuous power measurements. As I tested at frequencies above 5kHz with only 1 channel driven, the internal cooling fan would instantly come on right before the receiver would go into gross distortion and shut down at levels above 100 watts.

So in this case, there seems to be an underlying measurements that could account for audible differences.
Observation led to research.

Whenever possible, I perform single blind-tests at home after level matching using a multi-meter at the speakers playing 50Hz, 400Hz, 1kHz, and 2kHz tones. The have been small voltage differences that were frequency dependent but not significant.

Toole and Olive (Harman) performed extensive testing to develop a statistically significant model for what makes a good sounding and preferred loud-speaker.

I am not aware of the DBTs with a "representative" speaker load by representative listeners that statistically prove that the AHB2, NCore N400, and a "representative" linear power supply "classic" A/B amplifier.

Without extensive tests with statistical significance, SBT and DBT are those amps, that speaker, that room, with those listeners.

The best I can do at home is properly level matched SBT's.
FWIW, it is my speakers, my rooms, my listeners, and our opinion.

[U]orangejello[/U], please consider level matching, as suggested using test-tones, and try to setup a single blind test.
It won't make any difference many here, but it may increase your confidence in your preference and amplifier selection.
I think that is what you are looking for.

- Rich

 

[Kal Rubinson]

Some reviewers listen first and measure after because bias goes both ways.
Some like the look of the hardware, some like the look of the measurements.
Both lead to bias.

Simply choosing an item for review is an indication of bias.

 

[RichB]

Simply choosing an item for review is an indication of bias.

Absolutely, but a good reviewer does point out observed differences in a manner that is also not unfair.

- Rich