Review and Measurements of Benchmark AHB2 Amp

[peng]

There is a lot of discussion on ASR about damping factor (DF), that is worth a search, so recommended.

Your observations aside, for the moment, align with a DF, confirmed by Stereo.de.

Stereo.de AHB2 damping factor measurements is an outlier.


Here are the Benchmark AHB2 for damping factor:


Here are the DF measurements from SoundStage:

View attachment 231747

Damping factor is most important in bass frequencies but with these we can compare 1 kHz.
Stereo.de 48, Benchmark 254, and SoundStage 225.

I have no idea what Stereo.de is doing, and perhaps they don't either.

Concerning your observations, the perhaps 48 seems low but is still higher than necessary.
I observe no issues with the AHB2 and tight bass driving the Salon2s full range.

I have disconnected the upper section and listened to the bass 150 Hz and below on the Salon2s which that are -3 dB 23 Hz and this cannot be described as tight with any amplifier. I have driven there with Parasound amps with DF spec'ed at > 1100 and I find no audible difference in bass. If anything, the AHB2 is better, perhaps due to its tightly regulated power supply and minimal phase shift.

- Rich

Great job on this but I bet no one can change Pogo's mind on this. He seems fixated on this DF thing and did not seem to want to trust the supporting math that had been presented to him before by various members.

 

[DonH56]

As others have said, output impedance is a complex function of operating and environmental conditions, including bias, temperature, supply voltage, output swing (voltage and current), etc. -- just as are many other metrics. But in practice, particularly for amplifiers with reasonably high feedback, it is primarily controlled by feedback and thus changes over PVT (process, voltage, temperature) and other things like power output are (or should be) minimal until feedback breaks down (e.g clipping begins).

Introducing damping factor, a metric I am not particularly found of, adds load impedance as a variable. For a fixed load on a test bench, DF is not unreasonable to measure and assess, but use a more realistic load like a speaker's complex impedance over frequency (and power) and DF begins to lose meaning, or at least become difficult to interpret since it is likely dominated by the load and not the amplifier. I would prefer a plot of output impedance, but that's just me.

It also does not help (me) that "damping factor" has a completely different meaning in control theory, related to pole/zero placement in a transfer function and resultant stability...

Onwards - Don

 

[617]

I sold mine. The damping factor made them unlistenable but ymmv.

 

[GXAlan]

I sold mine. The damping factor made them unlistenable but ymmv.

What do you mean by that? What speakers and what test track?

 

[617]

What do you mean by that? What speakers and what test track?

The sound was veiled. Brothers in arms. Nils Lofgren live. Diana Krall.

The new owner is an older gentleman with slightly diminished hearing. He is very happy.

Even my wife could hear it from the kitchen.

 

[MRC01]

So we have that humans are detecting 0.2 dB in volume difference and 5 cents in pitch, known as JND and meaning just a mere 0.08 Hz when attending the 27.5 Hz range, 1.27 Hz when in the 440 Hz range and 10 Hz in the 3520 Hz range. ...

If two piccolos are out of tune by that amount, it might make my head explode. That's several beats per second, not only obvious to hear but strongly dissonant. Now cue the usual piccolo intonation jokes...

Now if they weren't playing simultaneously,first one then the other, then all bets are off. What was obvious becomes subtle. You ...might... still hear it.

PS: here's a fun example and test why all the intonation jokes are about high pitched instruments, and to shed a bit of light on pitch perception.

 

[Laserjock]

Sarcasm is getting deep?

 

[restorer-john]

I would prefer a plot of output impedance, but that's just me.

There is an AP utility which @amirm has used in the past. It just looks a bit clunky IIRC.

The output impedance vs frequency plots on old amplifiers are way more interesting as they get a decent deviation going on.

These high feedback Class Ds are going to be mostly textbook perfect and therefore uninteresting.

 

[pogo]

The sound was veiled.

Was this in normal stereo mode or bridged?

 

[HarmonicTHD]

Was this in normal stereo mode or bridged?

Dude. Ever heard of sarcasm?

 

[Kal Rubinson]

Dude. Ever heard of sarcasm?

Yeah but some will need an emoji for it.

 

[617]

Sorry everyone

 

[peng]

Sorry everyone

Not your fault, you did a great job adding the wife could hear from kitchen part was better than an Emoji anyway. But maybe you are just doing it again..

 

[F1308]

You ...might... still hear it.

Then comes JND...5 cents.

The Use of Cents for Expressing Musical Intervals

It is of public knowledge that famous composers hesitated between OCTAVES !!!!

 

[pogo]

Sorry everyone

That was a good one and so close to reality

After the stereo team has confirmed that the DF was measured properly at a 4ohms load and not bridged ((63Hz/1kHz/14kHz): 48/45/19), I still wonder how the reduced control of the chassis and especially in bridge mode should not have an effect?
Where would the values be with 3ohms, ...?

 

[SIY]

That was a good one and so close to reality

After the stereo team has confirmed that the DF was measured properly at a 4ohms load and not bridged ((63Hz/1kHz/14kHz): 48/45/19), I still wonder how the reduced control of the chassis and especially in bridge mode should not have an effect?
Where would the values be with 3ohms, ...?

For someone as obsessed with DF as you are, your understanding of it and its effects is remarkably sketchy. Can you please try to study up on this a bit more before blasting out even more nonsense? That means actual textbooks and journals, not magazine marketing pieces.

 

[RichB]

After the stereo team has confirmed that the DF was measured properly at a 4ohms load and not bridged ((63Hz/1kHz/14kHz): 48/45/19), I still wonder how the reduced control of the chassis and especially in bridge mode should not have an effect?
Where would the values be with 3ohms, ...?

Perhaps the EE's can correct me if I am wrong, at 4 Ohms the DF is 1/2 of the 8 Ohms specification.
So, the AHB2 drops from 370 to 185 for 4 Ohms.


Your prayers have been answered. Benchmark provides an article and downloadable spreadsheet where you can plug in your speaker cable length, gauge, and minimum load.

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

- Rich

 

[GXAlan]

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

- Rich

What is missing is that Kenwood in the 1980s solved the problem of the speaker wire with Sigma Drive where negative feedback was applied at the speaker terminal. This gave you 15,000 damping factor!

Kenwood L-08M High Speed Power Amplifier Manual | HiFi Engine

www.hifiengine.com

For the record, I have one that was resto-upgraded by QuirkAudio. Sounds incredible for classical music, but I don’t like the sound of vocals — it’s too clean. Formal E1DA Cosmos based measurements to come soon.

 

[pogo]

Perhaps the EE's can correct me if I am wrong, at 4 Ohms the DF is 1/2 of the 8 Ohms specification.

It doesn't seem to be that simple in reality and is probably also design-dependent. What has also been observed, is that at constant load, for example, 8ohms but different volumes, the DF can change!

 

[RichB]

It doesn't seem to be that simple in reality and is probably also design-dependent. What has also been observed, is that at constant load, for example, 8ohms but different volumes, the DF can change!

True. Amplifiers are not typically measured for impulse response.
Stereo.de has a measurement that is not typical for DF.

Damping factor is traditionally measured at 1W (2.83V@8R).

Whatever, they are doing, it is not comparable to manufacturer DF specifications, though we cannot be sure of those either, especially, when they do not provide frequency. Benchmark provides this data in detail.

Stereophile, choses to measure their amps into a standard simulated load. Unfortunately, they apply inconsistent scales, making it difficult to compare.
Here is an example that compares the AHB2 to the Gryphon Apex Stereo because it is a recent review of a stratospheric price point.

Benchmark ($2999): https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements

The output impedance, including a 6' speaker cable, was a low 0.09 ohm at 20Hz and 1kHz, rising slightly to 0.22 ohm at 20kHz. As a result, the modification of the Benchmark amplifier's frequency response due to the interaction between this impedance and that of our standard simulated loudspeaker was just ±0.1dB

Gryphon Apex Stereo ($99,000): https://www.stereophile.com/content/gryphon-apex-stereo-power-amplifier-measurements

Gryphon specifies the Apex's output impedance as 0.015 ohms. My measurement was slightly higher in the right channel, at close to 0.05 ohms (including the series resistance of 6' of speaker cable), and significantly higher in the left channel, at 0.17 ohms.

The AHB2 stated output impedance is 0.09 at 20Hz and 0.22 at 20kHz.
The Apex stated output impedance is 0.015.
Unfortunately, Stereophile is inconsistent with graph scale.

Speakers also vary in load, including effect load, eyeballing the graphs.
Into 2 Ohms, the AHB2 is down 0.25 dB at 20Hz and 0.75 at 20Khz.
Into 2 Ohms, the Apex is down 0.4 dB at 20Hz and 0.6 dB at 20kHz.

Here is the AHB2 speaker load measurement squished in paint to be approximately the same scale:

If you want to look for indicators for bass performance, you might want to look into linearity into load.

Stereophile has measured the NAD M33 integrated amp ($4999).

The M33's output impedance at the speaker terminals was a low 0.08 ohm at 20Hz and 1kHz, rising to 0.1 ohm at 20kHz. (These values include the series impedance of 6' of spaced-pair loudspeaker cable.) Fig.1 shows the frequency response at the speaker outputs, with the M33's A/D converter set to its maximum sample rate of 192kHz. Unlike the M32, where a peak developed at the top of the audioband, especially into higher load impedances (footnote 1), the M33's audioband output is flat and rolls off above 20kHz, reaching –3dB at 55kHz. However, note the imbalance between the channels, with the left channel (blue and cyan traces) 0.6dB higher in level than the right (red and magenta).

The AHB2 stated output impedance is 0.09 at 20Hz and 0.22 at 20kHz.
The M33 stated output impedance is 0.08 at 20Hz and 0.1 at 20kHz.

Into 2 Ohms, the AHB2 is down 0.25 dB at 20Hz and 0.75 at 20Khz.
Into 2 Ohms, the M33 is down 0.65 dB at 20Hz and 0.85 at 20kHz.

If you have difficult effect load, clearly the AHB2 will perform better despite the AHB2 maximum DF is 350 and the M22 spec is >800.
Given the Stereophile measurements, you are more likely to have reduced bass in a difficult to drive speaker in the M33 than the AHB2. Even the $99,000 Apex is bested, in linearity into load...

- Rich