Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

I imagine in the 4 ohm bridged mode the current limiting would become apparent. JA is often kind by omission. I think the amp would shut down with a 4 ohm bridged load. Thats like 2 ohms per channel as you well know.

Yes and before it shut down, the distortion would be higher than the 8ohm they did give.

I am just amazed at how well Benchmark marketing has pulled off selling 2x amplifiers.

Great marketing, all in the wording and the omission of certain measurement graphs.

I understand there is some power supply wonderfullness going on.

Yes, like Krells Plateau biasing except done on the voltage rails instead,
Quad 405 I think used something similar, and maybe Devialet Class-D amps do something similar also.
" Output voltage is variable by a factor of 8 from +/- 10V to +/-80V and constantly changes to minimize overall thermal dissipation."

Cheers George

40 amps of current is needed to produce 40 volts peak (100 watts RMS) across a 1 ohm dip in impedance. If this impedance is reactive the amplifier will be really unhappy.

I don't understand it.  40V across 1 ohm (or 40amps thru 1 ohm) would make 1600W. 

Note: Watts RMS means something different and has no relevance.  Power, obtained by multiplying  RMS current and RMS voltage is AVERAGE power (equal half of peak power for sinewave).  It represents DC power that would produce the same amount of heat.  RMS Power does not represent anything useful.

AHB2 power supply is highly regulated SMPS capable of producing large currents.  As for amp being less stable in bridged mode - AHB2 is designed to be stable since it does not have feedback in traditional sense.  Of course in order to reduce distortion that low it has to have negative feedback, but this feedback in not recursive, meaning it is not zapped back to the front of the amp.  Instead it drives another amp  (error amplifier), with another set of power transistors, that at the very end corrects output signal.

Also, low output impedance, doubled in bridged mode, might be important for driving complex loads, but it is already very low, about 0.03ohm @1kHz.  For the purpose of damping the speaker membrane it does not matter, since most of the speaker resistive impedance is in series anyway, reducing highest obtainable effective DF to 1.5

I don't understand it. 40V across 1 ohm (or 40amps thru 1 ohm) would make 1600W.  

 Kijanki,

Thank you. I am glad someone is paying attention.. You are correct it would make 1600 watts. My bad. 

I appreciate your bringing up the speaker resistive issue which indeed lowers the effective damping factor. Please keep reminding people and good luck getting that point across. From what I read it appears people think damping somehow "controls" the speaker. We know the series resistance limits that control. 

If we put aside the term "damping" , because thats not what really matters, we need to inform people that an amplifier with high output impedance may cause a marked change in the frequency reponse of their speaker. Thats what really matters.
 
I invite you to the task at hand.

If we put aside the term "damping" , because thats not what really matters, we need to inform people that an amplifier with high output impedance may cause a marked change in the frequency reponse of their speaker. Thats what really matters.

Yes add that to the other cons, the amp starts to behave like a tone control.
(ie: not flat from 20hz to 20khz)

Cheers George

They are very similar yes? what do you think will happen if another measurement was done at 4ohm in the bridged mode????? It will be far worse than the non bridged into 4ohms or 8ohms, that’s why they didn’t give the graph even though they probably did measure it.

George, this is speculative at best.
The AHB2 has very comprehensive protection. I suspect the bridged AHB2 will maintain its ultra low distortion into low impedances until protection kicks in.

If you recall I said this in my initial response above:

George I agree with you that with efficient low impedance speakers the OP may likely be better off with a single AHB2.

What I took exception to was your incorrect blanket assumption that bridging the AHB2 would result in higher distortion. This is NOT true for the specified 6 and 8 ohm loads.

Despite what is specified, Benchmark demos at shows with 4ohm nominal speakers (see link above). Do you think they would do this if the sound quality was compromised?

I am just amazed at how well Benchmark marketing has pulled off selling 2x amplifiers.

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.