Review and Measurements of Benchmark AHB2 Amp

[Coach_Kaarlo]

So on the topic of gain, I am trying to solve a clipping/ distortion problem with DSP, and balance that with an decent listening volume.

System elements;

1. Digital stream into a miniDSP SHD Studio using DIRAC Live 3.0 and PEQ filters derived from REW measurement
2. Digital (unbalanced) into DAC2-HGC, then long balanced XLR analog into L channel AHB2 / R channel AHB2, both obviously in mono
3. 3ft speaker cable into Yamaha NS-2000 three way reference speakers (an amp sits at foot of each speaker to recover some damping factor)

Gain Structure;

1. DSP set to -25dB to accomodate a reasonable dynamic range and the magnitude of DSP and EQ corrections
2. AHB2 set to Mid-Gain = 14.2 dBu (4 Vrms), I tried the lowest AHB2 gain setting but the DAC volume was maxed out to achieve 75 dB

This results in a 80 dB level with the DAC2-HGC volume positioned at roughly 12 o`clock depending on program/ content.

I remember reading something on here regarding the nominal gain setup between DAC2-HGC and AHB2 was to have the volume knob somewhere between 10 and 2 for nominal listening levels.

What I am thinking is that using the headroom requirement from the PEQ filter I generate in REW, then adding the 10 dB which DIRAC requires, will give me a gain setting which will prevent clipping. However, I am also mindful of over driving the speakers with too much correction as the AHB2 in mono at max output exceeds the speakers max power by 230W (speakers rated to 250W / 6 ohms nominal / 90dB/W/m)

It goes without saying that the DAC2 and AHB2 allow huge freedom with gain due to their low distortion, something I am grateful for at the moment. But looking at the images attached it is clear that the headroom required to get close to a semi linear response is significant.

How can I gauge the best balance between avoiding speaker damage through over extension of the woofer and not running out of dynamic headroom in the DSP?

Running the amp in the second highest gain setting seems like asking for trouble.

 

[Mikechw]

Mikechw,
I just noticed this post.

I would not recommend using different amplifier models in any biamp configuration. Amplifier delays must match exactly.

The delay through the AHB2 and the E-480 will not be the same. This means that you will have phase issues at the crossover frequency. Also, the 1 dB difference in gain is problematic.

Furthermore, there is no need to run amplifiers at a gain of 23 to 29 dB if you have a good preamplifier or DAC. In your case you would be forced to run the AHB2 at high gain just to match the excessively high gain of the E-480.

Operating at high gain will almost always increase the noise in your system because you will be amplifying the output noise of the preamplifier or DAC. With Benchmark DACs (DAC1, DAC2 or DAC3) or preamplifiers (LA4 or HPA4), you will want to run your AHB2 amplifiers at minimum gain (9.2 dB stereo or 15.2 dB mono). These benchmark products support the studio-level XLR connections where 0 dBFS is typically calibrated to 22 or 24 dBu.

Thanks for the info..
I only use my E-480 as a pre-amp now.
I have bought one more AHB2 and now I am running DUAL mono, one AHB drives left channel and other drives right channel.
I have also tried it on low gain and middle gain settings on my AHB2, eventually I settled it on middle gain.

Best Wishes.

 

[waynel]

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.

I think I finally understand the problem you were having. Were you using the 14dB PEQ boost centered between 45 and 60 Hz to compensate for your room null As you mentioned in the DAC3 thread? The kick drum is centered in this range and a 14dB boost to compensate for the null means you are demanding 25 times more power from your amplifier and 25 times more power handling from your speakers. Both amps are likely clipping and your speakers are likely compressing.

 

[Coach_Kaarlo]

I think I finally understand the problem you were having. Were you using the 14dB PEQ boost centered between 45 and 60 Hz to compensate for your room null As you mentioned in the DAC3 thread? The kick drum is centered in this range and a 14dB boost to compensate for the null means you are demanding 25 times more power from your amplifier and 25 times more power handling from your speakers. Both amps are likely clipping and your speakers are likely compressing.

No DSP used there - straight old fashioned digital to analog. And tests were done in another premise - actual studio with treatment. Hence the later conversations around damping factor etc. And also around the lack of any clipping.

 

[Coach_Kaarlo]

I think I finally understand the problem you were having. Were you using the 14dB PEQ boost centered between 45 and 60 Hz to compensate for your room null As you mentioned in the DAC3 thread? The kick drum is centered in this range and a 14dB boost to compensate for the null means you are demanding 25 times more power from your amplifier and 25 times more power handling from your speakers. Both amps are likely clipping and your speakers are likely compressing.

Think John covered your fears here;

Nice speakers!

The Harbeth 30.2 has a sensitivity of 87 dB at 2.828Vrms.

The maximum sinusoidal output of the AHB2 is 28.28 Vrms. This is exactly 20 dB higher than 2.828 Vrms. It is 100 W into 8 Ohms.

The maximum output of the AHB2 in bridged mono is 56 Vrms which is 26 dB higher than 2.828 Vrms. The AHB2 delivers 380 W at 0.0003% THD into 8 Ohms which is almost a 4x increase in power. 1% THD is over 400W.

This means that you can play 6 dB louder in bridged mono with the AHB2 and you will have almost exactly 4 x the power available to drive your speakers.

Let's do the math:

In stereo mode, the AHB2 will drive each speakers to a peak SPL of 87 + 20 = 107 dB SPL at full power at 1 m in an anechoic space.

With two AHB2 amplifiers, the power into each speaker will be almost exactly 4 times as high, and the peak SPL will be almost exactly 6 dB higher. 107 +6 = 113 dB SPL at full power at 1 m in an anechoic space.

As a general rule of thumb, the 1 m anechoic SPL will be close to the SPL that you will experience in your living room when listening to a pair at a normal listening distance. You are further away than 1 m, but listening to two speakers in a partially reflective room. I have found this rule of thumb gives reasonable results.

The crest factor of the music will reduce the average SPL to something that is 12 to 18 dB lower than the maximum sinusoidal level.

Given a crest factor of 8 (moderate mastering), the average level will be 18 dB - 3 dB = 15 dB lower than the maximum sinusoidal level. 107 - 15 = 92 dB SPL (average) for stereo and 113 - 15 = 98 dB SPL (average) for mono. The music power will be ((10^(-15 dB/20))^2)*100 W = 3.16 W stereo and 12.6 W mono.

Given a crest factor of 6 (more aggressive mastering), the average level will be 12 dB - 3 dB = 9 dB lower than the maximum sinusoidal level. 107 - 9 = 98 dB SPL (average) for stereo and 113 - 9 = 104 dB SPL (average) for mono. The music power will be ((10^(-9 dB/20))^2)*100 W = 12.5 W stereo and 50.3 W mono.

As you can see from the calculations, given the relatively high 87 dB sensitivity, you will probably find that stereo operation will give you enough output. Bridged mono will give you an average level of 104 dB SPL with 113 dB SPL peaks when playing pop music. This is very loud.

If you listen to recordings with very little compression (high crest factor), you may want two amplifiers.

The Harbeth 30.2 is rated for music power of up to 150 W, so you are safe, even at a crest factor of 6.

The AHB2 has a protection system that monitors the output of the AHB2. This monitoring system is not in the signal path. It simply shuts the amplifier down if an unsafe event occurs. There are several unique features of this system that protect speakers:

The AHB2 monitors THD and if it exceeds 1%, it shuts the amplifier down to protect the speakers. The AHB2 only produces 0.0003 % THD at full power. The 1% THD protection will only occur when the amplifier is driven into clipping. An over-driven amplifier is the leading cause of tweeter failure. This cannot happen with the AHB2. The THD detection will shut the amplifier down before you damage your tweeters.

The AHB2 monitors the DC output of the amplifier and shuts the output and power supplies down if DC is detected on the speaker outputs. DC can destroy a woofer. If the output stage of an amplifier is damaged, the most common result is DC on the speaker outputs. This can never happen with the AHB2.

The AHB2 electronically mutes on turn on and off. There is no relay in the signal path. There is no risk of damage due to a failed relay. The electronic mute system is also designed to be fail safe.

Other protections include monitoring of the output current and detection of short circuits. The AHB2 is bullet proof and it is designed to provide extraordinary protection for speakers.

If you play a high-amplitude sinusoidal test tone into your speakers, the AHB2 could burn them out. If you play music, you will not be able to cause damage (assuming the 150 W music rating is realistic).

 

[tuajay]

This is a review and detailed measurements of the Benchmark AHB2 Amplifier using THX technology to reduce distortion. I was going to request one for testing due to membership demand but to my pleasant surprise, the company volunteered to contact me and send not one, but two units! I requested the second one because of something I was seeing in the measurements that turned out to be my issue, not the amp's. The AHB2 retails for US $2,999 from the company direct.

The AHB2 is a very compact form factor, enabled by use of switching power supply to increase efficiency/density:

View attachment 26573​

There is just a power switch in the front. It might be but I constantly reached for the screw next to it to power it on! Having the power switch be slightly different color or shape may help with this.

There are a set of LEDs that show the status of the amplifier. They are driven by an FPGA (field programmable digital logic) that monitors all aspects of the amplifier and shuts down the unit if stressed. It did so very quickly and efficiently in my testing. Usually this is done with purely analog or maybe a small microprocessor. As such, the coverage of scenarios that could damage the amplifier is much less complete than using the approach Benchmark is using in AHB2.

Here is the back panel:

View attachment 26574​

This is a very minimalistic approach given the small amount of real estate available. Only input is XLR balanced which is just fine in my book. A gain switch allows the input level to be controlled so that you can just use an RCA to XLR adapter and still get full power (2 volt max in high gain). At the other extreme, the low gain setting is designed for professional pre-amps with lots of output, getting its max power at 9.8 volts. Using this scheme, the signal to noise ratio can be improved as you will see in measurements later.

For speaker terminals there are two sets: classic heavy duty banana jacks and SpeakOn. The SpeakOn are locking and provide the best performance. I lost about 3 dB of performance using the banana jacks due to less secure connect there. So the measurements you see are with SpeakOn jacks.

Another minor nit on that note: I had a rather cheap SpeakOn cable and its plug was just large enough to hit the silver screws that the sockets are mounted with. Typical round shaped SpeakOn ones are not an issue. Flush mount screws would solve this problem.

There is a switch for selecting stereo or mono bridged output. The latter quadruples the amount of power available and unlike typical bridging, according to Benchmark comes at no penalty in distortion! I plan to test this later as it has a minimum impedance of 6 ohm so I could not use the 4 ohm setup I used for this testing.

There is a very beefy AC mains cable with really nice locking tabs. Insert it and it stays put unless you push the two red tabs on each side. Nothing is more aggravating than the AC cable coming half-way lose from the IEC socket.

Being a proper company of course the AHB2 comes with full set of regulatory/safety/emissions certifications which is super important with power amplifiers given the high voltages and currents running around in them.

In use the AHB2 stays very cool for a power amplifier when it is idling, outputting little power which was a nice surprise.

As to THX technology, it merges a low-power but very low distortion amplifier with a high power but higher distortion amplifier. The distortion of the latter is not seen because it is producing so much power (so the ratio of distortion is lower). The general scheme is not new, dating back to 1980s but new implementation is. We have seen this in stellar performance of Massdrop THX AAA 789 amplifier which broke new ground in level of distortion and noise. Will the Benchmark AHB2 manage the same? Let's see.

Power Amplifier Audio Measurements
As usual, I start with my 5 watt output test using 4 ohm load using SpeakOn terminals as noted:

View attachment 26575

I hope your jaw is on the floor just like mine was when I saw this picture emerge! 113 dB THD+N in a power amplifier? Are you kidding me?

Look at the harmonic distortion. The worst case spike is below -130 dB! This is insanely good. Of course this type of SINAD (signal and noise ratio) crowns the Benchmark AHB2 as the best I have ever tested:
View attachment 26576

The performance was so good I literally had to rebuild my dummy load to get there. Even the quality of the metal used in the connectors matters to get to this level of distortion. I replace all my dummy loads with higher precision ones that have much less VCR (voltage coefficient of resistance). Resistor values can become voltage dependent creating distortions of their own. Up to about 105 dB of THD+N, it doesn't matter but beyond that, the VCR was the dominant distortion, not the Benchmark AHB2 amplifier!

It was requested that I show the breakdown and distortion so here we are:
View attachment 26577

Our best case hearing threshold is -116 dB SPL so no question that this level of distortion is totally inaudible. Separating the noise from THD, we see that it is noise that we are measuring as THD+N, rather than distortion:
View attachment 26578

And this is with exceptional noise performance of Benchmark AHB2 as we see in the graph of THD+N versus power:

View attachment 26579

Even in high gain, the AHB2 easily outperforms the DIY Hypex NC400 I had tested before which used to be the best amp I had tested.

We have 185 watts of power at incredibly low distortion of 0.00016%, besting the company specifications.

Note that the FPGA protection mode kicks in and essentially shots the amplifier down past the limit. You get absolutely distortion-less and noiseless performance until there is no more.

EDIT: here is performance in bridged mode using same 4 ohm load:
View attachment 26592

You get 500 watts of stunningly clean power. Protection circuit shut the unit down after that so it is quite safe to try. With a THD essentially matching the non-bridged mode, there is no reason to be afraid of trying this.

Intermodulation distortion versus power level shows the same story of clean power:

View attachment 26580

If you want to know what you get for extra money over bargain amplifiers, you can see the difference in the above graph. We are not talking about 5 to 10 dB but whopping 40 dB better!

I have had requests for intermodulation distortion using dual 19 and 20 kHz tones. Here is that:
View attachment 26582

THD+N versus output level looks far cleaner than anything I have tested before:

View attachment 26583

Even at the limit of our hearing (20 kHz), we have vanishingly low amount of distortion. The graph is exaggerated so shows a rise there but in absolute levels, despite 90 kHz bandwidth of the test, we have incredibly low THD+N of just .004%.

The sharp spike at 45 Hz in green shows the amplifier going into protection mode. So don't pump that sine wave continuously into it at 133 watts.

Frequency response is exceptional too as expected:
View attachment 26581

Since this is not a switching amplifier, there is no filter there allowing the bandwidth to go to 200 kHz and beyond. Lowest band of AM radio is 450 kHz so likely you could use the AHB2 for an AM radio transmitter!

Usually when we test switching amplifiers we see all kind of "interesting" things in their outputs above hearing range. The AHB2 is a classic configuration albeit, with a switching power supply so all is well and clean here:

View attachment 26584

My reference graph for the Hypex NC400 used an AES filter (by accident) so I ran the AHB2 both ways, with or without that filter. Using the filter (in green) so the two are equal, we see much cleaner spectrum below 200 kHz and of course, no massive switching spikes. Worst case spike is below -115 dB. In other words, the Benchmark AHB2 is clean even in the areas you are not looking! It is like a restaurant scrubbing their parking lot with soap and pad as well as their dishes.

My loose wires on dummy loads is not the best setup for measuring crosstalk but here it is anyway:
View attachment 26585

Where our hearing is most sensitive (2 to 5 kHz), separation is around 100 dB which is way, way more than we need. Despite the small enclosure, the AHB2 manages exceptional numbers here.

EDIT: forgot to run the classic SNR test in the original review:
View attachment 26636

Wow, assuming you play at peak of 120 dBSPL, your noise floor will be at -10 dBSPL! That is absolute silence.

Conclusions
It goes without saying that the Benchmark AHB2 breaks new ground with respect to performance of power amplifiers. Using it, you can be assured that any distortion that you hear is from other sources (speaker, source, content, etc.). This is what I look for in high-end audio: absolutely the best performance so no second guessing is involved. You buy once, and you are happy forever!

All of this comes from a company that is a model of transparency with proper and accurate measurements of their products on their site. And importantly, volunteering to have that data shown to be correct by independent sources such as us. Support form the company for me at least has also been exceptional.

Yes, $3,000 is fair bit of money but is pocket change in high-end audio. Sales tax is higher than $3,000 for most of those products! I hear a lot of talk from audiophiles to have the equipment get out of the way of enjoying the music. Well the Benchmark AHB2 at levels that assuredly passes full transparency. So if that is your moto, you better stop buying boutique products with no specs and independent measurements and get an AHB2. Everything you hear then will be what is in your source, not the dirty dishes that your amp my serve your food on.

Needless to say, the Benchmark AHB2 gets my strongest recommendation for power amplifier. The pink panther agrees, having hit the ball out of the park after I took that shot!

------------
As always, questions, comments, recommendations, etc. are welcome.

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How long would the Bi-Polar Output Power Transistors be able to Sustain the Power Metric .. due to the Thermal Compression .. as these are Rated upto 25° C Only ....

 

[DonH56]

How long would the Bi-Polar Output Power Transistors be able to Sustain the Power Metric .. due to the Thermal Compression .. as these are Rated upto 25° C Only ....

I do not think I have ever seen a power transistor rated to only 25 degC; that is nominal room temperature. What is your source for that assertion? I suspect there are typical parameters measured at 25 degC but the device is rated much higher than that, probably 85 degC (industrial range).

 

[misteracng]

If you use the ah2b to run a pair of revel 226be towers, would any of you suggest using a pair of the amps and running bridges mono? Or maybe that would be overkill. I’m just wondering how much more volume I could expect to get using a pair instead of one.

Thanks

 

[DonH56]

If you use the ah2b to run a pair of revel 226be towers, would any of you suggest using a pair of the amps and running bridges mono? Or maybe that would be overkill. I’m just wondering how much more volume I could expect to get using a pair instead of one.

Thanks

I'd start with one and watch the clipping indicators. No clipping, you're fine. That said, bridging yields almost 4x the power (6 dB) so provides lots of headroom if you really need it. I suspect you don't...


FWIWFM - Don

 

[waynel]

If you use the ah2b to run a pair of revel 226be towers, would any of you suggest using a pair of the amps and running bridges mono? Or maybe that would be overkill. I’m just wondering how much more volume I could expect to get using a pair instead of one.

Thanks

You’ll probably be fine with one unless you listen loud in a large room with no subs.

 

[RichB]

If you use the ah2b to run a pair of revel 226be towers, would any of you suggest using a pair of the amps and running bridges mono? Or maybe that would be overkill. I’m just wondering how much more volume I could expect to get using a pair instead of one.

I'd start with one and watch the clipping indicators. No clipping, you're fine. That said, bridging yields almost 4x the power (6 dB) so provides lots of headroom if you really need it. I suspect you don't...

FWIWFM - Don

I use AHB2s to bi-amp the Salon2s and that provides about 1 dB more headroom so not much more power. I don't do it for power.

I have never clipped the AHB2s running the Salon2s full range for movies or music at any level I find comfortable.
Even at very high volume, the Aquaman 4k UHD kraken scene was really moving the woofers and there were no clip (no indicators).

Using voltage measurements and and SPL meter, the AHB2 can product 104 dB in stereo at my 11 foot listening position in the 4500 cubic foot room.
This is driving the Salon2s, the 226bes are more efficient.

The clipping indicators on the AHB2s are accurate, so if they are not lighting up, your fine.
Here are a couple of posts by @John_Siau that address clipping and performance:

https://www.audiosciencereview.com/...f-benchmark-ahb2-amp.7628/page-43#post-190301
https://www.audiosciencereview.com/...f-benchmark-ahb2-amp.7628/page-18#post-180791

- Rich

 

[neutralguy]

Also, my latest application note explains why damping factor can produce audible changes in the frequency response of the system:

https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor

The application note says the AHB2 keeps response variation under 0.1db because it has a damping factor of 370. However, the damping factor is specified as frequency dependent in the AHB2 manual, dropping to as low as 34 at 20khz. It may still be at 370 at 119Hz where the example Focal speaker has a minimum impedance of 3ohms, but the speaker dips again to 6ohms at 10khz. Assuming a damping factor of 50 at 10khz, this results in a frequency response dip of -0.18db there (using the note's formulas), which is above the claimed 0.1db variation.

Stereophile's measurements into a simulated load that dips down into 4ohms also shows a variation of about 0.2db (+/-0.1db).

 

[RichB]

The application note says the AHB2 keeps response variation under 0.1db because it has a damping factor of 370. However, the damping factor is specified as frequency dependent in the AHB2 manual, dropping to as low as 34 at 20khz. It may still be at 370 at 119Hz where the example Focal speaker has a minimum impedance of 3ohms, but the speaker dips again to 6ohms at 10khz. Assuming a damping factor of 50 at 10khz, this results in a frequency response dip of -0.18db there (using the note's formulas), which is above the claimed 0.1db variation.

Stereophile's measurements into a simulated load that dips down into 4ohms also shows a variation of about 0.2db (+/-0.1db).

Here is a sampling of the latest Stereophile reviews with measurements into simulated load and at 2 Ohms.
2 Ohms is interesting because it sometimes produces more non-linearity than the simulated mode.

Classé Delta Mono ($10,999): +/- 0.15 dB (-0.5 dB into 2 Ohms)
Parasound Halo JC 1+ mono ($8,495): +/- 0.1 dB (-0.2 dB into 2 Ohms)
Bel Canto e1X ($6000): +/- 0.1 dB (-0.2 dB into 2 Ohms)
Primare A35.2 ($3495): +/- 0.1 dB (-0.2 dB into 2 Ohms estimated from crappy chart)
Parasound A21+ ($2995 ): +/- 0.1 dB (-0.75 dB into 2 Ohms)
Benchmark AHB2 ($2995): +/- 0.1 dB (-0.25 dB up to 1.5 kHz dropping to 0.8 at 20 kHz into 2 Ohms)

So the AHB2 has more variation into 2 Ohms at 20 kHz but is in the middle of the park.
I doubt the AHBE is significant for most speakers that have higher impedance at higher frequencies.
Many speakers have low impedance drops (and possibly more difficult phase angles, so effective impedance) in the bass range.
This would have a greater effect on the Parasound A21+ and, the most expensive in the group, Classé Delta.

None of these measurements include distortion and there are huge differences between the above amps.

- Rich

 

[Omar Cumming]

An 0.8 db dip at a frequency no adult can hear into a 2ohm load is a candidate for the most meaningless measurement ever.

Cheers

 

[RichB]

An 0.8 db dip at a frequency no adult can hear into a 2ohm load is a candidate for the most meaningless measurement ever.

Cheers

Measurement into 2 Ohm loads is still useful, even when the results prove to be minor.
The different performance between amps is interesting though.

- Rich

 

[TomJ]

How is the Xilinx Spartan-6 FPGA used in the AHB2? Is the protection circuitry based on DSP? I don't see anything about that online or in the user manual.

 

[anmpr1]

How is the Xilinx Spartan-6 FPGA used in the AHB2? Is the protection circuitry based on DSP? I don't see anything about that online or in the user manual.

According to Gary Galo writing in Audio Xpress (April 2015):

"Benchmark has also incorporated digital protection systems into the AHB2, which monitor the amplifier’s critical functions, protecting the amp from faults and overload. The digital circuitry detects temperature, current, voltage, distortion, DC offset (very important!), short circuits, and signal loss. The
circuitry is based on a Xilinx Spartan 6-series Field Programmable Gate Array (FPGA) chip..."
...

 

[John_Siau]

According to Gary Galo writing in Audio Xpress (April 2015):

"Benchmark has also incorporated digital protection systems into the AHB2, which monitor the amplifier’s critical functions, protecting the amp from faults and overload. The digital circuitry detects temperature, current, voltage, distortion, DC offset (very important!), short circuits, and signal loss. The
circuitry is based on a Xilinx Spartan 6-series Field Programmable Gate Array (FPGA) chip..."
...

This is correct.

One unique feature of the protection system is that it is entirely outside of the audio path. As you approach the limits, the protection circuits have no impact on the audio. There is no limiting of current until the safe limits are reached. In most amplifiers, the protection circuits will begin limiting current and begin increasing THD before the limits are reached.

The FPGA provides a very fast and sophisticated protection system. As a result, the AHB2 amplifiers have proven to be absolutely bullet proof. We pride ourselves in building reliable products, but the AHB2 probably has the best track record of any product that we have ever built.

 

[jtwrace]

@John_Siau so where does Benchmark go with this design from here? What is next to improve it?

 

[anmpr1]

An 0.8 db dip at a frequency no adult can hear into a 2ohm load is a candidate for the most meaningless measurement ever. Cheers

Today, one of the most meaningless measurements is how much an amp weighs and how big it is. I remember Peter Aczel once stating that any amp he could lift without straining his back was basically a 'flimsy' amp. And back then he had a point. How times have changed.