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Review and Measurements of Benchmark AHB2 Amp

SIY

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By the way, this is what I was talking about regarding the lack of a zero crossing spike in a properly adjusted AB amplifier (stolen out of an old review of @Kal Rubinson of a very non-exotic Adcom home theater amp).

adcom.jpg
 

Sal1950

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The guy I was visiting last night is a fan of Wilson speakers, which are infamous for that. In my mind, that's a pathologically bad design, but if one is committed to them, yes, a more elaborate power amp is needed than for driving what I would consider better engineered speakers. The amps I use wouldn't hack it.

With speakers having a non-pathological impedance curve, Aczel's maxim remains true.
+1
 

GrimSurfer

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Maybe it is because in the Swedish LTS series amplifier testing only one amplifier has ever been straight wire transparent in both sighted and blind listening and that amplifier is a Bryston SST.

http://mjaudiolab.pl/images/stories/Bryston/swedish14bsstreview.pdf

I'm not questioning the excellence of Bryston's SST. Rather I'm trying to reconcile why an owner of such an amp would advance/repeat the facile assertion about amps of certain equivalences sounding the same.

If this is Daverz view (and I'm not saying it is because I get the sense he's just trying to be provocative), perhaps he could provide a contrasting series of views to those presented by John Siau on how micro acoustics (my expression, offered as a way of framing many of the issues carefully articulated by Mr. Siau) can be captured though curiosity-discovery when the characteristics fall outside the common metrics captured and presented by the industry?

I think Daverz understands this unless his choice of amp was purely by chance. Either way, he's a fortunate person...
 
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restorer-john

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y the way, this is what I was talking about regarding the lack of a zero crossing spike in a properly adjusted AB amplifier (stolen out of an old review of @Kal Rubinson of a very non-exotic Adcom home theater amp).

index.php

Here's a Pioneer brochure from July 1985 where they extol the virtues of their NSA (non-switching amp) type II. The Benchmark plot does look remarkably similar to this Class B plot...

1559270956426.jpeg


"I not biased against Benchmark!" yelled the competitor's amp... ;)
 
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Blumlein 88

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I'm not questioning the excellence of Bryston's SST. Rather I'm trying to reconcile why an owner of such an amp would advance/repeat the facile assertion about amps of certain equivalences sounding the same.

If this is Daverz view (and I'm not saying it is because I get the sense he's just trying to be provocative), perhaps he could provide a contrasting series of views to those presented by John Siau on how micro acoustics (my expression, offered as a way of framing many of the issues carefully articulated by Mr. Siau) can be captured though curiosity-discovery when the characteristics fall outside the common metrics captured and presented by the industry?

I think Daverz understands this unless his choice of amp was purely by chance. Either way, he's a fortunate person...

I wished some of these publications would step up and do some testing like the LTS did. I've done that myself before. Despite all your measures and such when connected to a tough load not all amps sound the same. In fact maybe almost no amps sound the same. I think we've got total transparency except for microphones and speakers. And amplifiers if they aren't well selected for the speaker. The amplifiers if we knew more could soon become transparent.
 

John_Siau

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@John_Siau Not sure if my question got lost in the morass. :D Am I looking at this incorrectly?
Take another look. The transients occurs about half way between the zero crossing and the peaks of the sine wave which is 2.83 Vrms which is 8 Vpp. The transients are occurring at about +/- 2 V.

Adjusting the bias could reduce these transients at a given operating temperature, but like most class-AB amplifiers, the bias drifts somewhat with temperature.
1559308480043.png
 

John_Siau

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Found the full quote:

As I have said, and written, innumerable times, any two amplifiers with high input impedance, low output impedance, flat frequency response, and sufficiently low distortion and noise will sound exactly the same at matched levels if not clipped.

http://www.biline.ca/audio_critic/audio_critic_web2.htm#ba

If "sufficiently low distortion" means as low as the ABH2, then in practical terms all amps will sound different.
I would add one more: The phase response needs to be linear. You can have a flat frequency response without having a linear phase response and the non-linear phase response will change the way transients sound. A non-linear phase response will have no impact on a continuous set of tones, but it will change the apparent frequency response of transients.
 

SIY

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Take another look. The transients occurs about half way between the zero crossing and the peaks of the sine wave which is 2.83 Vrms which is 8 Vpp. The transients are occurring at about +/- 2 V.

Adjusting the bias could reduce these transients at a given operating temperature, but like most class-AB amplifiers, the bias drifts somewhat with temperature.

Thanks for the response, John. To my eyes, the spikes look pretty close to zero crossing, which would be consistent with your claims about crossover distortion:
zero.png


But as you say, it's easy to adjust out, which is the case for the engineered AB amps I have on hand and have tested. This sort of graph is indeed traditional, but I think it's not exactly representative. Even with thermal drift (which for bipolars and most MOSFETs at the current ranges they're run will take them to HIGHER idle current), an engineered AB amp that isn't misadjusted doesn't show that phenomenon.
 

John_Siau

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JohnYang1997

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Thanks for the response, John. To my eyes, the spikes look pretty close to zero crossing, which would be consistent with your claims about crossover distortion:
View attachment 27010

But as you say, it's easy to adjust out, which is the case for the engineered AB amps I have on hand and have tested. This sort of graph is indeed traditional, but I think it's not exactly representative. Even with thermal drift (which for bipolars and most MOSFETs at the current ranges they're run will take them to HIGHER idle current), an engineered AB amp that isn't misadjusted doesn't show that phenomenon.
The distortion differences are amplified. It's not like they produce 10% or more distortion. It's not easy to adjust out. Or it's not able to be adjusted out completely. Most rely on better designed gain stage and feedback techniques to obtain higher ol gain at high frequency to reduce this. Unless it's class A it's always there at the class b/class ab output stage.
 

SIY

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Thanks for pointing this out. I will add that note to the feed forward application note. Not trying to be deceptive. Our goal is to present accurate and useful information.

And just to be clear, since I've been critical of this particular illustration, I absolutely believe that your product is excellent and that you're trying to be accurate and honest in your marketing.
 

SIY

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TIt's not easy to adjust out. Or it's not able to be adjusted out completely.

Perhaps a bit of reading of Self and Cordell's books, along with some bench time with power amps and trimmer adjustments, might convince you that this assertion is entirely incorrect.
 

RichB

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Perhaps a bit of reading of Self and Cordell's books, along with some bench time with power amps and trimmer adjustments, might convince you that this assertion is entirely incorrect.

As a layman, I am trying to understand this. Every crossover distortion graph for classic feedback Class A/B amps I have seen show crossover distortion in their notched out charts.

Staying contemporary, here are some of the Constellation Performance Centaur II 500 (500 WPC, $55,000) charts from Stereophile:

ConstellationFreq.jpg


This amp's chart got the low-res scale but it may still be down 0.2dB at 20kHz into 4 Ohms.

ConstellationCrossover.jpg


In terms of the AHB2 discussion, at measurement 1 watt would be a better indicator for audibility.
In any case, cross-over distortion is clearly not eliminated and this measurement at 50 WPC may be not be indicative.


- Rich
 

John_Siau

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Perhaps a bit of reading of Self and Cordell's books, along with some bench time with power amps and trimmer adjustments, might convince you that this assertion is entirely incorrect.
The theory is good, but a lot of things have to be done correctly. The reality is that most class-AB amplifiers don't live up to what is theoretically possible, nor do they stay perfectly trimmed over temperature, varying loads, and time. This is why it is so important to run low-power THD measurements, before and after heating up the amplifier. It is also a good idea to measure an amplifier that has been in service for a number of years.

The feed forward technique can be used to achieve excellent performance from a pure class-B output stage. The feed forward correction eliminates the need for precise bias control. At one point in the design process, THX suggested that we build our amplifier with zero bias (a class-B output stage), but we opted for class-AB to achieve the best possible performance. The AHB2 uses a slightly under biased class-AB output stage with class-H tracking rails. The tracking rails kick in just above 1/3 power. The feed-forward system also keeps the class-H transitions distortion-free.
 

SIY

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The theory is good, but a lot of things have to be done correctly.

And in engineered AB amps, they are. I have three commercial SS AB amps on hand (an Adcom, a Cambridge, and a Bryston), none of which show crossover distortion zero crossing spikes. That's not to say that feedforward schemes like yours don't have advantages- clearly they do, as the performance of your gear demonstrates- but I take issue with this one specific claim about competitive technologies.
 

SIY

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As a layman, I am trying to understand this. Every crossover distortion graph for classic feedback Class A/B amps I have seen show crossover distortion in their notched out charts.

Staying contemporary, here are some of the Constellation Performance Centaur II 500 (500 WPC, $55,000) charts from Stereophile:

The graph you show does NOT indicate crossover distortion. It's nearly entirely 2nd and 3rd harmonic with no spike at the signal zero crossing.
 

bravomail

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Late to party. Thx for review, Amir! We finally see a product which performs for its money. Pay-for-play. Since I don't have that much spare change, as many here, I can only dream, that somebody will come up with lower performance but at much lower price.
 

SIY

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And not to continually batter a deceased equine, here are more randomly grabbed residual measurements of well-known engineered AB amps (top to bottom: Parasound, Classe, Bryston, Pass Labs) that fail to show crossover distortion. It's almost as if the engineering of this sort of amplifier is actually understood!:D

Parasound3500fig5.jpg

classe.jpg

BrystonFIG4.jpg

Passfig08.jpg
 

edechamps

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@SIY: thanks for providing a contrary opinion to @John_Siau's article. John's test definitely had me wondering for a while there about the audibility of distortion in typical amplifiers, but the fact that he only tested against a single AB amplifier made me reluctant to take the results at face value. As always, skepticism pays off. What the results do show is that we should really take a close look at THD+N at low power levels (1 watt and less) when measuring amplifiers, and John's results are pretty convincing on that point.
 
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