Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

The misinformation in this thread from an individual who should know better is a little disturbing. The motive escapes me, but it appears to be deliberately malevolent.

From John Atkinson’s measurement comments:
"Commendably, the frequency response was not affected by the setting of the sensitivity switch, or by using the amplifier in bridged-mono mode. However, the ultrasonic response in bridged-mono mode shelved down faster than in stereo mode, and as the two channels’ output stages are in series in this mode, the output impedance, again including the 6’ of speaker cable, was higher than in stereo, at 0.14 ohm at 1kHz."

And his final comment:
"Benchmark Media Systems’ AHB2 is an extraordinary amplifier. Not only does its performance lie at the limits of what is possible for me to reliably test, it packs high power into a very small package, especially when used in bridged-mono mode. It is truly a high-resolution amplifier."
Read more at https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements#LdQ7lf...

I suggest people interested in the AHB2 seek out out tests/opinions from reputable sources that have actual hands on experience with the amp. Technical queries should then be directed to Benchmark themselves to avoid speculation and misinformation.

Perhaps 18A specification was for the early versions. R

Yes of course they were, just like when Sterophile does a mediocre review, the manufacturers comments always say they must have sent a faulty one in for the review.
Wouldn’t you think at they would send the best possible tweaked one they could!!!!
What a load of ****

Cheers George

I am amazed this has not been answered and laid to rest. What seems to be the problem? The 29 amps will only be supplied by the amplifier if the load resistance is low and the voltage is high enough to produce that current. Perhaps its the order of things. Voltage first, then resistance then figure the current. I think we have all come to agree that bridging does not allow the amplifier to provide more than 29 amps. Bridging just allows twice the voltage which increases the OPPORTUNITY for more current to flow, but its still limited to 29 amps. Which, BTW, is not enough current form some ESLs played loud.

I promise you that all amps that are claimed to do 100 watts into 8 ohms, 200 into 4 and 400 watts into 2 ohms have quite a bit more power at 8 ohms than stated. Thats how they do it, Its specmanship and nothing wrong with. They do it for the same reason that some here want to see those numbers keep doubling. If you truly have only 100 watts at 8 ohms you will not be able to double down. Not Possible.

Conventional amps have Emitter or Source resistors whose value becomes significant at low impedance loads, the resistance of wire and traces startes to come into play. Mainly the sag is in the power transformer. 

When you look in an amp and see nice heavy wire going to the speaker keep in mind that there are several hundred feet of rather thin wire in the power transformer. 

ramtubes319 posts12-07-2018 4:24am
"The 29 amps will only be supplied by the amplifier if the load resistance is low and the voltage is high enough to produce that current. Perhaps its the order of things. Voltage first, then resistance then figure the current. I think we have all come to agree that bridging does not allow the amplifier to provide more than 29 amps. Bridging just allows twice the voltage which increases the OPPORTUNITY for more current to flow, but its still limited to 29 amps."

Agreed, good post.

Ramtubes, AHB2 has load/line regulated SMPS, but as you mentioned, there are losses everywhere.  Exact doubling of power for half of the load means perfect regulation - an ideal voltage source, that is only possible with very deep negative feedback.  Such feedback might create instabilities or TIM distortions, but  feedback in AHB2 is quite different - it is non recursive, meaning that signal is not fed back to the input of the same amplifier but to another "error amplifier" that corrects output later.  Such arrangement eliminates instabilities (allowing for wide bandwidth of the amp) and TIM distortions.  I also noticed a lot of low bass added , in comparison to my previous class D amp (that had 5Hz low bandwidth limit).  My guess is that all DC servos have instability problems for very low frequencies, but AHB2, having no recursive feedback at all can go down to 0.4Hz.  Should 0.4Hz vs 5Hz difference be audible with 30Hz speakers? - I don't know, but it is in my setup (big surprise).  The same goes for the upper limit - more extension and airy treble.  Could this be phase delay at high frequencies with my class D amp?  Speaker itself has inductive character at higher frequencies accelerating phase.  In addition there are many amps, including one  of Lamm, that have lower (50kHz) bandwidth and are praised for wonderful airy highs.