Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

@tobes 

Why are you amazed Roger?
Some of us own low efficiency 8 ohm nominal speakers - for which the bridged mono AHB2’s are perfect.
Honestly, Benchmark appears to be one of the least marketing oriented companies around. They don’t push BS, they comprehensively measure everything they make and are very much about accuracy and low coloration - not boutique sound tailoring.

I do fine with 35 watts so I am curious what is your listening level and speaker efficiency? Have you measured the peak voltage at your speaker? If not we can estimate it fairly accurately with just your efficiency and SPL at 1 meter. Can you do this for us please?

Since you brought up "extensive protection" what happens when this amplifier drives a 10 uF capacitor, typical of many ESLs.

What is straw man about informing people that damping not damping but actually a variation in frequency response due to impedance variation that occurrs with high output impedance?

I read JA's report. The Benchmark is about as perfect an amplifier as one can imagine. Yet we still dont know its current capability and some other things. This is an issue to a varying degree with many amplifiers, yours excepted, of course.

I find the "strawman" accusation most inapproptiate to this thread. We are trying to figure out something here. Does anyone have a schematic for me to peruse?

Ok, Im really getting into this amp now. Have a look at this white paper, its good, and the amplifier construction, ugh.

https://benchmarkmedia.com/blogs/application_notes/14680625-the-ahb2-a-radical-approach-to-audio-pow...

I fix rare amps like the Reference Line that one of my customers just loves. It is hard to work on. I like to think I can fix anything but I would not even bother to open up this Benchmark after seeing these photos. A blown up cap is no problem, but if the power supplies shut down or the amp circuits there is no chance in Hell to fix this at the component level at any reasonable cost. Unless these prove unusually relaible I would not want to own one past the warranty. Once the boards become unavailable you will have a nice boat anchor.

Has anyone any experence with having one of these repaired? Its all surface mount, perhaps board swapping is the only way. Im sure the factory doesnt mind fixing them because they have the boards to swap around. But try to find even a smart tech who would like to take this on.

This is John Siau, VP and Director of Engineering for Benchmark Media Systems.

I would like to clear up some erroneous information in this thread. The AHB2 is a unique design, so some of the usual assumptions do not apply.

1) The peak output current of the AHB2 is 29 A not 18 A. Many of the posts above are based upon the 18 A number which is incorrect.

2) The SNR of the AHB2 improves by 3 dB in Mono Mode (bridged mono). The signal is 6 dB higher but the noise is only 3 dB higher, producing a 3 dB improvement in SNR when running bridged mono. The low-level noise produced by the two channels of the amplifier is uncorrelated white noise and this is why the noise only increases by 3 dB when using the two outputs differentially.

3) The THD is virtually identical when comparing stereo mode to bridged mono mode. This is achieved through the use of the feed-forward error correction.  Every other power amplifier will show a substantial increase in distortion when bridged.

4) All references to "Mono Mode" or "Bridged Mono" on our website or in the AHB2 manual refer to the bridged mono mode of the amplifier.

5) The THD produced by the AHB2 does not increase as the impedance decreases. The THD into 2 Ohms, 4 Ohms, 8 Ohms and no load are virtually identical.  Again this is a unique characteristic that is achieved through the use of feed-forward error correction.

6) The AHB2 is not just stable into low-impedance loads, it stays clean when driving low impedances. The THD does not change with loading.

7) We do not specify the output power at 1% THD because, unlike other amplifiers, the THD does not gradually rise as the output level increases. The amplifier maintains 0.0003% THD+N, into any rated load, until the clip point is reached.

8) The rated "continuous average output power" of the AHB2 is not specified below 3 Ohms stereo or 6 Ohms mono because of the FTC regulations (16 CFR Part 432) that require continuous operation at 1/3 rated power for one hour. This is a long-term thermal limitation when playing sinusoidal test signals and has little significance when playing music. It should not be taken as indication that lower impedances cannot be driven cleanly.

9) The AHB2 has a high damping factor and this allows excellent performance in bridged mono. The damping factor is 350 into 8 Ohms stereo and 175 into 8 Ohms bridged mono.

The AHB2 will deliver THD+N < 0.0003% at full output voltage into various loads is shown in the following table:

< 0.0003 % THD+N at the following output voltages and load impedances, 20 Hz to 20 kHz

• 29.03 dBV, 31.25 dBu, 28.28 Vrms into 8 Ohms, both channels driven
• 28.92 dBV, 31.14 dBu, 27.93 Vrms into 6 Ohms, both channels driven
• 28.81 dBV, 31.03 dBu, 27.57 Vrms into 4 Ohms, both channels driven
• 28.57 dBV, 30.79 dBu, 26.83 Vrms into 3 Ohms, both channels driven
• 27.14 dBV, 29.36 dBu, 22.76 Vrms into 2 Ohms, both channels driven
• 35.05 dBV, 37.27 dBu, 56.57 Vrms into 16 Ohms, bridged mono
• 34.83 dBV, 37.05 dBu, 55.14 Vrms into 8 Ohms, bridged mono
• 34.59 dBV, 36.81 dBu, 53.67 Vrms into 6 Ohms, bridged mono
• 33.16 dBV, 35.38 dBu, 45.52 Vrms into 4 Ohms, bridged mono

Bottom line, the AHB2 is well suited for bridged mono operation into 4-Ohm nominal impedances and the performance is virtually identical to stereo mode except that the power is nearly 4 X higher. Dips in the speaker impedance curve are not a problem and the AHB2 drives these cleanly.

The SNR actually improves by 3 dB when running bridged mono, and the THD is virtually unchanged. We almost always demonstrate the AHB2 in bridged mono mode at trade shows and have done so with many different hi-fi and pro loudspeakers. In most cases, these demonstrations have used speakers with a 4-Ohm nominal input impedance.

The following link will take you to a series of application notes that discuss the performance of the AHB2 in more detail. They include a plot of THD vs output level under various load conditions:

https://benchmarkmedia.com/blogs/application_notes/tagged/ahb2

Thanks, great response gearbuilder 

Benchmark have been making ground breaking products for years. They are an embarrassment to most of the old school industry. Out performing incumbent market leaders technically (measurements) by considerable amounts at reasonable cost.

This is healthy and exciting for us as hi-fi customers. Of course reliability is always a concern especially with radical new designs. Time will tell if these new products like the AHB2 and the HPA4 perform as reliably as their DACs.

Of course measurement based design targets measured performance and therefore only address a portion of the market. Flavoured designs like tube amps etc. will always have their important place and many audiophiles will continue to opt for flavour over pure tech specs.

That said. You could consider the AHB2 as a transparent way to drive speakers (if you like the sound of your speakers) and then look to a tube preamplifier to add some sweetness or sizzle to taste.