Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

@tobes 

With all due respect George, it's not a matter of believing what I want.
Implying higher distortion in bridged mode for the AHB2 is incorrect.
This has been independently verified.
Not sure why you keep insisting on this?

https://www.stereophile.com/content/benchmark-media-systems-ahb2-
power-amplifier-measurements 

Thanks for this link. JA knows how to measure things. However he did not precondition the amp bridged at 4 ohms. No biggie here, but the OP is asking about bridging at 4 ohms which I am certain would get very hot. 

My answer to the OP remains the same. Do not bridge this amp into a load that dips below 4 ohms. For that load, the amp will provide more power if not bridged. That analysis does depend on getting some of the facts straight, such as 18 amps vs 29 amps. 

Guys if you really want to do something. Bypass the crossover in your speaker, crossover at line level with simple RC or active crossover, get two amps and let each see their driver DIRECTLY. Put a monster SS on the bottom and some nice tube or class A SS amp on the top. Generally less poweron top because most tweeters are padded down in the crossover. The crossover in a speaker is its weakest link. 

Implying higher distortion in bridged mode for the AHB2 is incorrect.

It’s all in the measurements and how they’re presented, just look.

Distortion graphs

Here is the graph NOT BRIDGED 8ohms
https://www.stereophile.com/images/1115BAHB2fig04.jpg

Here is the graph NOT BRIDGED 4ohms
https://www.stereophile.com/images/1115BAHB2fig05.jpg

Here is theStereophile graph BRIDGED 8ohms
https://www.stereophile.com/images/1115BAHB2fig06.jpg

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.

Cheers George

@george

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. 

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

Now that I am sucked into this I want to study their white papers for veracity.  I understand there is some power supply wonderfullness going on. Feedforward is always amusing too. What a world we live in. 

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