Benchmark ahb2 power amplifier

[Ralf11]

This unit has gotten rave reviews (despite a 'relatively' low price) and uses a feed forward technology to minimize crossover distortion.

 

I'm interested in any hearing impressions, listening tests, or comments on the theory of operation.

 

It is often rated with amps costing an order of magnitude more...

[arcman]

I'm thinking of one myself

[blue2]

18 minutes ago, arcman said:

I'm thinking of one myself

This review on TAS suggests speaker matching is essential (not so good with low sensitivity loads) therefore home demo required. Tested here with several speakers by Tone Audio, and more here by Stereophile who found a flaw with one recording.

[Kal Rubinson]

2 hours ago, blue2 said:

............................., and more here by Stereophile who found a flaw with one recording.

But the reviewer just bought three of them.  😎

[Ralf11]

6 channels?  I thought you were a 5 channel guy?

[phosphorein]

12 hours ago, blue2 said:

This review on TAS suggests speaker matching is essential (not so good with low sensitivity loads) therefore home demo required. Tested here with several speakers by Tone Audio, and more here by Stereophile who found a flaw with one recording.

 

The AHB2 is a wonderful partner for vintage Martin Logan Requests which are not the easiest speakers to drive. They are dead quiet and run cool even at high volumes.

[Kal Rubinson]

5 hours ago, Ralf11 said:

6 channels?  I thought you were a 5 channel guy?

Three channels, L/C/R, driven by the Benchmark monoblocs.

[Ralf11]

ok, are there other channels?  or is this a 2nd system, or am I wrong ini thinking you have ~~5 channels in your system...

[sandyk]

Kal

 Do you know how they manage to increase S/N to such a degree given limitations due to Johnson noise in resistors etc ?

 Signal/noise (A-weighted): 132dB (stereo) is much higher than most amplifiers are capable of .

Was this measured at the low gain setting ?

Regards

Alex

[Kal Rubinson]

44 minutes ago, Ralf11 said:

ok, are there other channels?  or is this a 2nd system, or am I wrong ini thinking you have ~~5 channels in your system...

Sure.  The two surround channel speaker are driven by a different stereo amp.  I may get another Benchmark or two for them but there are other changes afoot and that decision will wait until that dust settles.

[Kal Rubinson]

33 minutes ago, sandyk said:

Kal

 Do you know how they manage to increase S/N to such a degree given limitations due to Johnson noise in resistors etc ?

 Signal/noise (A-weighted): 132dB (stereo) is much higher than most amplifiers are capable of .

Was this measured at the low gain setting ?

I cannot directly answer that.  However, Benchmark is pretty demonstrative of their measurement philosophy and results.  You can also read John Atkinson's efforts to measure the amp as part of my subjective review.

[psjug]

The AHB2 looks like a great amp, and I certainly wouldn't mind owning one.  But I am a little bothered by something that Benchmark claims here: https://benchmarkmedia.com/blogs/application_notes/laboratory-use-of-the-benchmark-ahb2-power-amplifier

 

"The important difference between the AHB2 and class-D amplifiers is that the AHB2 does not produce switching noise. Class-D amplifiers are measured with brick-wall AES17 filters that ignore the switching noise above 20 kHz or 40 kHz. Tweeters can demodulate this ultrasonic noise and fold it into the audible band."

 

Has anyone shown tweeters having any response to typical class D switching frequencies of hundreds of KHz? 

 

edit:  they go on to talk about problems with Class D and measurement systems, which is known to be a problem.  But in the above quote regarding tweeters aren't they talking about the tweeter making audible sound when subject to Class D switching frequencies?

[sandyk]

14 minutes ago, psjug said:

Tweeters can demodulate this ultrasonic noise and fold it into the audible band."

 

That is the area that needs verification. If correct, it may be a problem.

[psjug]

46 minutes ago, sandyk said:

 

That is the area that needs verification. If correct, it may be a problem.

I think it is BS.  Here is more that they say:

 

"More importantly, this ultrasonic noise may become audible when it is demodulated by the non-linearities that occur in all loudspeakers. This potential source of distortion may detract from our musical enjoyment. Class-D amplifiers are compact and cheap, but are not necessarily a good choice for a high-end Hi-Fi system."

 

I really don't know why they see a need to take these swipes at Class D (maybe GUTB got a job at Benchmark).  It makes me think less of the company, unless they can back it up.

[sandyk]

34 minutes ago, psjug said:

really don't know why they see a need to take these swipes at Class D (maybe GUTB got a job at Benchmark).  It makes me think less of the company, unless they can back it up.

 

Class D fanatics are just like Vinyl fanatics, or Vacuum Tube fanatics .  (or dyed-in-the-wool Republicans ?  )

[Ralf11]

what about power supply fanatics?

[sandyk]

22 minutes ago, Ralf11 said:

what about power supply fanatics?

 ALL of those mentioned above need a good power supply to  get the best from them

 Even Oppo 203s,204s or whatever.

https://www.google.com.au/search?biw=1920&bih=893&tbm=isch&sa=1&ei=tSCsXNnqI5HnwQOpyauYDw&q=oppo+203+Linear+power+supply&oq=oppo+203+Linear+power+supply&gs_l=img.12..0i24l2.222827.233251..239336...0.0..0.220.3461.0j20j2......1....1..gws-wiz-img.5IUwrusLwAU#imgrc=Qv1IksWIM5_9eM:&spf=1554784671205

 

[arcman]

I'm thinking this would be a great, super quiet amp for my 101 db Zu Speakers. I can add "flavor" by tube rolling my Rogue preamp. Plus, the gain settings will allow me to ditch the passive pre between my RP-5 and amp. My current Nuforce STA200 has higher gain than normal thus making my preamp tube hiss a bit too loud. 

[psjug]

6 hours ago, sandyk said:

 

Class D fanatics are just like Vinyl fanatics, or Vacuum Tube fanatics .  (or dyed-in-the-wool Republicans ?  )

 

If you think that is me - I'm not really so much for or against Class D.  I guess you don't like manufacturers to speak the truth?

[Kal Rubinson]

2 hours ago, arcman said:

I'm thinking this would be a great, super quiet amp for my 101 db Zu Speakers. I can add "flavor" by tube rolling my Rogue preamp. Plus, the gain settings will allow me to ditch the passive pre between my RP-5 and amp. My current Nuforce STA200 has higher gain than normal thus making my preamp tube hiss a bit too loud. 

It should do this well.  I was originally attracted to the AHB2 by the low gain settings which complemented the very high output of my AR MP1 preamp. 

[kaka89]

14 hours ago, sandyk said:

Kal

 Do you know how they manage to increase S/N to such a degree given limitations due to Johnson noise in resistors etc ?

 Signal/noise (A-weighted): 132dB (stereo) is much higher than most amplifiers are capable of .

Was this measured at the low gain setting ?

Regards

Alex

 

By feed-forward "error correction". A lot of amp designer avoid adding negative feedback because it doesn't sound good tho, kinda opposite to what Benchmark is doing.

[kaka89]

I'm currently using Nuprime ST-10, and interested to know is it worth to upgrade to AHB2. Not sure how two compares.

[psjug]

8 minutes ago, kaka89 said:

 

By feed-forward "error correction". A lot of amp designer avoid adding negative feedback because it doesn't sound good tho, kinda opposite to what Benchmark is doing.

The TXH-AAA error correction corrects for distortion.  I think noise reduction is likely to have been achieved mostly with very clean power supply and careful attention overall in the electronics design, rather than any new technologies.  I like that the amp uses Class H power saving scheme.  It is cool to see what they have done to make a Class AB amp that can compete with Hypex in terms of efficiency and THD+N.

[sandyk]

3 hours ago, psjug said:

The TXH-AAA error correction corrects for distortion.  I think noise reduction is likely to have been achieved mostly with very clean power supply and careful attention overall in the electronics design, rather than any new technologies. 

 

I agree.

 It is a separate issue to feedback correction of distortion.

 

[barrows]

21 hours ago, sandyk said:

Kal

 Do you know how they manage to increase S/N to such a degree given limitations due to Johnson noise in resistors etc ?

 Signal/noise (A-weighted): 132dB (stereo) is much higher than most amplifiers are capable of .

Was this measured at the low gain setting ?

Regards

Alex

As i recall this is a low gain amp (I need to go back and look at the Stereophile measures), low gain is a sure way to reduce noise.  I suspect the rest of the engineering is good as well.  Of course nothing comes for free, with this low gain one will have to have higher gain in the source to reach the same levels in a given system context, higher gain in the source means higher noise there, so no free lunch.

This points out how there are benefits to be had by careful gain matching in a system.  With the right gain structure, one will have the lowest noise floor the components are capable of at the speakers.