Review and Measurements of Benchmark AHB2 Amp

[Coach_Kaarlo]

I love the Benchmark effect. Accurate, transparent reproduction of what is recorded without colouration or alteration. I like it so much that I just ordered a second AHB2.

But it's not for that slightly obvious reason. After some old school analog blind testing it seems there is something to the thoughts others have voiced regarding grunt and attack (lack of power). Let me (try to) explain what I experienced and then pose a question for those perhaps able to answer it slightly more scientifically.


System 1 = An old Accuphase P300 with 180W per channel measured (claimed 150W), and a slew rate over 20v/ms, driving a pair of Yamaha NS-2000 speakers.

http://www.accuphase.com/cat/pcten.pdf

https://www.hifiengine.com/manual_library/yamaha/ns-2000.shtml


System 2 = DAC2-HGC and AHB2 driving the same Yamaha NS-2K speakers.


Many many areas of sound/ speaker performance were better. Transparency, clarity, accuracy. The Benchmark gear exceeds expectations and delivers on the the hype etc. Then we started being more analytical and trying to isolate various categories of accuracy - vocal realism, instrument separation in classical, etc etc.

After some very enjoyable listening the Benchmark missed a step. The selection was a Roger Waters track (@ 6 min 10 sec into It's a Miracle from Amused to Death) with real kick drum "kick". Listening at 70dB and then again at 90dB with each system showed up a big difference. Unmistakable, clear, and a confusing difference. The understanding I had was the AHB2 can drive any speaker, and the Accuphase is quite similar in power (slightly more). What I heard was a lack of authenticity in the reproduction of the kick drum attack / leading edge (of the sound). The Accuphase was able to move the drivers sufficiently to deliver both the real sound and physical force of the kick drum in a life-like way, and the low frequency driver movement / stroke was also visible. The Benchmark not only failed to reproduce the sound but was unable to move the driver visibly at all. Changing the Benchmark into mono and repeating the test with one speaker solved the physical movement and improved the sound greatly, but it was still just a little off the reference sound of a pair of HD-650 headphones plugged into the DAC produced, and still behind the old Accuphase.

Yes, there are many gaps and holes in my both my knowledge and my experiment (if you could call it that). However it seems to me that the grunt needed to move those big old 33cm (13 inch) cones and magnets might be beyond a single AHB2 in stereo.

What have I missed? What have others experienced? And before you respond at least try and reproduce the sound I am talking about for yourself.

Cheers.

 

[trl]

Hi and welcome to the ASR forum!

It would be awesome if you could please borrow or purchase a calibration mic (even a cheap one should do) and record with REW the two sounds of your amplifiers coming from the same speaker/speakers, from the same position. The SPL can be done with REW as well, as long as you use the same mic and same mic's position, if REW SPL readings are identical then it should be fine.


http://www.whatsbestforum.com/showthread.php?17765-Room-Measurement-Tutorial-for-Dummies-Part-1
https://www.audiosciencereview.com/...om-measurement-tutorial-for-dummies-part-2.5/

 

[Willem]

My hypothesis would be that what you are observing is the effect of power on dynamic peaks. How to measure that is beyond me.

 

[trl]

Coach_Kaarlo stated that his tests were done at 70dB SPL and 90dB SPL, probably average SPL and not peaks. The speakers are having a rating of 90dB SPL @1KHz, but probably due to they're not brand new, the sensitivity is couple of dBs lower. However, I estimate that an average SPL of 90dB at listening position (2-3 meters) will be reached with 10-50W RMS/channel across audible bandwidth, so the dynamic power should not be a determining factor here.

 

[SIY]

What have I missed?

Basic controls. Careful level-matching between amps. Well-controlled blinding procedures.

 

[Coach_Kaarlo]

Hi and welcome to the ASR forum!

It would be awesome if you could please borrow or purchase a calibration mic (even a cheap one should do) and record with REW the two sounds of your amplifiers coming from the same speaker/speakers, from the same position. The SPL can be done with REW as well, as long as you use the same mic and same mic's position, if REW SPL readings are identical then it should be fine.


http://www.whatsbestforum.com/showthread.php?17765-Room-Measurement-Tutorial-for-Dummies-Part-1
https://www.audiosciencereview.com/...om-measurement-tutorial-for-dummies-part-2.5/

Thanks very much for the links.

I am a week or two from being back in the studio / listening room. I will indeed try and source a mic and also get REW up and running. However I will also have said second AHB2 in hand so I may be able to at least solve the issue in one sense. If 2 x AHB2 in mono fail to drive the speaker to similar output force or grunt (compared to the 150W Accuphase) - then power is not the problem! Something else going on, dynamics perhaps, rather than steady state?

Will report back, while also following this discussion.

 

[Coach_Kaarlo]

Coach_Kaarlo stated that his tests were done at 70dB SPL and 90dB SPL, probably average SPL and not peaks. The speakers are having a rating of 90dB SPL @1KHz, but probably due to they're not brand new, the sensitivity is couple of dBs lower. However, I estimate that an average SPL of 90dB at listening position (2-3 meters) will be reached with 10-50W RMS/channel across audible bandwidth, so the dynamic power should not be a determining factor here.

Was using an app called Decibel X pro - yes measuring spl with an iphone is not ideal, but I'm only interested in getting the magnitude of spl difference below the obvious discrepancy in the observed and heard differences. Used a 1kHz test tone with phone 1m away from speaker for each test. Similar levels for both tests, but very dissimilar results.

Meaning, if I hit my thumb with a 1kg hammer or a 3kg hammer, it doesn't change the fact it hurts.

 

[SIY]

Was using an app called Decibel X pro - yes measuring spl with an iphone is not ideal, but I'm only interested in getting the magnitude of spl difference below the obvious discrepancy in the observed and heard differences. Used a 1kHz test tone with phone 1m away from speaker for each test. Similar levels for both tests, but very dissimilar results.

Meaning, if I hit my thumb with a 1kg hammer or a 3kg hammer, it doesn't change the fact it hurts.

Level match the electrical signal at the speakers. That's much more accurate. The blinding part then becomes the next gate, and it may involve an attenuator and an assistant.

 

[TLEDDY]

.What have I missed? What have others experienced? And before you respond at least try and reproduce the sound I am talking about for yourself.
Cheers.[/QUOTE]

There was a cartoon featuring a character named “Max Headroom”.

I like Head (room). For no good reason, I, on rare occasion, like to turn the gain up to “11” just to clean out ear wax and move gonads up into the abdomen.

90 dB is @#$%ing loud! It takes amp power with gonads mentioned above to control the relatively mass one is attempting to move. Technical gurus, please step in-I do not have the math/engineering knowledge to expound scientifically.

 

[anmpr1]

The Benchmark not only failed to reproduce the sound but was unable to move the driver visibly at all. Changing the Benchmark into mono and repeating the test with one speaker solved the physical movement and improved the sound greatly...Cheers.

I presume the clipping light never went on in stereo mode? In mono you've doubled the output power, that's for sure. With woofer pumping, is it a damping factor issue? I'm just guessing. The Yamaha doesn't seem to be a particularly difficult load for a good amp to drive. 6 ohms nominal, 90dB SPL, it says here. My speakers are a bit more sensitive (103 dB/watt) and with a single Benchmark I've been startled at the 'attack' transients of percussion instruments even played at moderate levels.

 

[Panelhead]

AHB2 fanboy here.
I wonder if damping factor is what you are hearing. The bridged AHB2 has twice the output impedance of the same amplifier in stereo.
Also try swapping the monitors from one room to the other. The speakers age.
Also the Accuphase may have some aging effects. Usually starts with filter caps. This will allow more mains frequency leakage.
I want to believe the AHB2 is more accurate. True fanboy.

 

[scott wurcer]

The Accuphase was able to move the drivers sufficiently to deliver both the real sound and physical force of the kick drum in a life-like way, and the low frequency driver movement / stroke was also visible. The Benchmark not only failed to reproduce the sound but was unable to move the driver visibly at all. .

I'm most intrigued by this. The last time I saw this someone was trying to demonstrate that two bit identical files had different bass response depending on where they were on the SD card. This behavior should be easy to verify several ways, it might demonstrate how easily our senses are fooled. OTOH the driver actually not moving would probably indicate activation of some protection circuitry.

 

[John_Siau]

I love the Benchmark effect. Accurate, transparent reproduction of what is recorded without colouration or alteration. I like it so much that I just ordered a second AHB2.

But it's not for that slightly obvious reason. After some old school analog blind testing it seems there is something to the thoughts others have voiced regarding grunt and attack (lack of power). Let me (try to) explain what I experienced and then pose a question for those perhaps able to answer it slightly more scientifically.


System 1 = An old Accuphase P300 with 180W per channel measured (claimed 150W), and a slew rate over 20v/ms, driving a pair of Yamaha NS-2000 speakers.

http://www.accuphase.com/cat/pcten.pdf

https://www.hifiengine.com/manual_library/yamaha/ns-2000.shtml


System 2 = DAC2-HGC and AHB2 driving the same Yamaha NS-2K speakers.


Many many areas of sound/ speaker performance were better. Transparency, clarity, accuracy. The Benchmark gear exceeds expectations and delivers on the the hype etc. Then we started being more analytical and trying to isolate various categories of accuracy - vocal realism, instrument separation in classical, etc etc.

After some very enjoyable listening the Benchmark missed a step. The selection was a Roger Waters track (@ 6 min 10 sec into It's a Miracle from Amused to Death) with real kick drum "kick". Listening at 70dB and then again at 90dB with each system showed up a big difference. Unmistakable, clear, and a confusing difference. The understanding I had was the AHB2 can drive any speaker, and the Accuphase is quite similar in power (slightly more). What I heard was a lack of authenticity in the reproduction of the kick drum attack / leading edge (of the sound). The Accuphase was able to move the drivers sufficiently to deliver both the real sound and physical force of the kick drum in a life-like way, and the low frequency driver movement / stroke was also visible. The Benchmark not only failed to reproduce the sound but was unable to move the driver visibly at all. Changing the Benchmark into mono and repeating the test with one speaker solved the physical movement and improved the sound greatly, but it was still just a little off the reference sound of a pair of HD-650 headphones plugged into the DAC produced, and still behind the old Accuphase.

Yes, there are many gaps and holes in my both my knowledge and my experiment (if you could call it that). However it seems to me that the grunt needed to move those big old 33cm (13 inch) cones and magnets might be beyond a single AHB2 in stereo.

What have I missed? What have others experienced? And before you respond at least try and reproduce the sound I am talking about for yourself.

Cheers.

There are several important differences between these two amplifiers that can change the way the low frequencies are reproduced. The Accuphase response is -3 dB at 5 Hz while the AHB2 extends all the way down to 0.1 Hz (-3 dB). The effect of this is that Accuphase delivers frequencies near 20 Hz slightly late (due to the low-frequency phase response errors which are a direct result 5 Hz lower limit of the Accuphase amplifier). A second issue is that the damping factor is much higher in the AHB2. The Accuphase cannot damp the drivers as quickly. The third issue is that the Accuphase may have output current limitations that are contributing additional distortions. In contrast, the AHB2 output stage is drawing current from tightly regulated power supply rails and there is no current limiting circuit in series with the output stage. The output current of the AHB2 is only limited by an overload protection system that measures the output current and then shuts the amplifier down is an unsafe load (or short circuit) is causing excessive output current. Until this shutdown threshold is reached, there is no limit to the output current delivered by the AHB2. Before this threshold is reached, the high-current warning lights will flash, but this is not an indication that current is being limited, it is just a warning that you are near the shut-down threshold.

With the Accuphase, the combination of late delivery and less damping will emphasize the bass on some recordings. You seem to have found one recording where this added emphasis was desirable in your room with your speakers, but it is quite likely that the bass in a different recording would sound uncontrolled.

You can always find one or two recordings that will be enhanced by a defect in the reproduction chain. The problem is that this same defect will have a detrimental effect on far more recordings than the few that it may "improve". It is also not possible to rule out a difference in playback level, given the fact that the levels were not accurately measured when making these comparisons. I like to play a test tone at low levels while connecting an AC volt meter across the speaker terminals.

The red clip lights on the AHB2 are triggered by a difference between the input signal at the XLR inputs and the output signal at the speaker terminals. If the difference is more than 1% the light will flash. In other words if the output sagged by 1 % or if the THD exceeded 1% this light would light. The fact that the lights are not coming on is iron-clad proof that the AHB2 is having no difficulty delivering the voltage and current necessary to move and damp your drivers. The light is triggered by a full input-to-output comparison, not simply by output-stage clipping.

If you are not lighting any warning lights on the AHB2, the voltage at the speaker terminals is precisely tracking the voltage at the input terminals (adjusted for the gain of the amplifier). The AHB2 is having no difficulty driving your speakers. The same may not be true of the other amplifier.

 

[digitalfrost]

 

[mocenigo]

I'm most intrigued by this. The last time I saw this someone was trying to demonstrate that two bit identical files had different bass response depending on where they were on the SD card.

WAT

 

[RichB]

I can say with certainty, that the AHB2 (not bridged) had no trouble moving the Salon2's 3 eight inch woofers.
The but the Kraken scene in Aquaman should get those woofers moving (with bass management off of course).

Also, most of the time, what is perceived as a bass attack is mid-bass or high frequencies.
I have listened to the Salons bass section only by disconnecting the straps.
The bass section is crossed at 150 Hz. It's truly unlistenable and nothing produced would lend one to use the term "speed"

- Rich

 

[mhardy6647]

With the Accuphase, the combination of late delivery and less damping will emphasize the bass on some recordings. You seem to have found one recording where this added emphasis was desirable in your room with your speakers, but it is quite likely that the bass in a different recording would sound uncontrolled.

I would opine (and this is just an opinion, albeit not an entirely uninformed one) that this is the most likely explanation -- Occam's Razor and all that.

EDIT: In case the image above is too abstruse (e.g., if one doesn't have kids the age of ours)

 

[anmpr1]

I appreciate John's explanation, and the time he spends here discussing these matters. And I love my AHB2 (and DAC3). If only he could make it look as nice as an Accuphase, for Benchmark dollars...

 

[GXAlan]

There are several important differences between these two amplifiers that can change the way the low frequencies are reproduced.

Very helpful post!

The Accuphase response is -3 dB at 5 Hz while the AHB2 extends all the way down to 0.1 Hz (-3 dB).

Actually the P300 is +0dB at 5Hz, -3dB at 90 kHz.
http://www.accuphase.com/cat/pcten.pdf

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Accuphase-P-300-TEST-1975-01.pdf

The effect of this is that Accuphase delivers frequencies near 20 Hz slightly late (due to the low-frequency phase response errors which are a direct result 5 Hz lower limit of the Accuphase amplifier).

Would this mean that DC stable amps like the old Citation 7.1 or the Kenwood Laboratory series or when the 80s era Sony’s have less phase response error?

A second issue is that the damping factor is much higher in the AHB2. The Accuphase cannot damp the drivers as quickly.

Agreed. In that Era, Accuphase has the E202 and P20 which allowed users to select damping factors of 1 or 5 for “older speakers” which could enhance subjective performance by allowing the woofers to move more.

A lot is said about damping factor being meaningless once you get beyond 50. Subjectively, I think high damping factor amps have better control of bass, but it may be a true true unrelated phenomenon. That said, what about the Kenwood L-02a and L-08m with damping factors of 15,000 ?

It is also not possible to rule out a difference in playback level, given the fact that the levels were not accurately measured when making these comparisons.

Yes, Input sensitivities are different. Accuphase is 1V to reach 150W into 8. AHB2 is 2V. So this may be the big one.

Edit: in this review of the very different P-600, it notes that Telarc’s Stravinsky Firebird Suite has a burst where it went from 2 watts to 840 watts.
https://www.vintageshifi.com/repert...e.php?pdf=Accuphase-P-600-TEST-1984-08-us.pdf

Since the P300 can do 400w into 4 ohms transiently, what can the AHB2 do short term?

 

[mocenigo]

I appreciate John's explanation, and the time he spends here discussing these matters. And I love my AHB2 (and DAC3). If only he could make it look as nice as an Accuphase, for Benchmark dollars...

Matter of taste. I prefer the cleaner look of the Benchmark. And I do not like vumeters.