Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

Yes, bridged amps see half the impedance of the speakers! So the sound quality may suffer. And the amp may run too hot! I don't recommend doing this! Sound quality is always better driving a higher impedance! Better to run two mono amps - one per speaker. Or one channel of each amp into one speaker.

You would need 2 AHB2 amps, one for each speaker. They can be run in bridged mono to 4 ohm speakers. There is an application note page on Benchmarks site where they did this with PCM IB2S monitors. 

@djones51 thanks but the axes of the plot they have are not readable to me. Does the impedence of the speaker ever dip below 4 ohm. This is not clear to me.

https://benchmarkmedia.com/blogs/reviews/ahb2-driving-pmc-ib2s-studio-monitors

@geek, have you considered emailing Benchmark and asking them? I have done that with other companies. I admit that chart isn't easy to read maybe they can send you a better one. 

My impression is that these amps are run a lot in bridged mono mode, and that indeed Benchmark may recommend this.  I seem to remember a recent review of the DAC3 mentioning this.  They may not react to bridging the way conventional amps do.

@djones51 I am preparing to ask this question. Right now I am trying the speaker manufacturer / dealer to get me answer to the question "what the lowest impedance the speakers can dip to". 

Every speaker manufacturer should publish impedance vs frequency graph or just state the lowest impedance. Only very few do that, I don't get why.

I did get one response from John @ Benchmark regarding what the tested output @ 4 Ohm in bridged mode is. 

@twoleftears I am glad you mentioned this, I did also hear about this but no concrete info anywhere regarding this. I will try to find out. Thanks for point this out.

geek101
Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

I would never bridge an amp all you gain is watts, distortion goes up, damping factor is reduced, stability into low impedance’s is reduced, and current ability is reduced. "Basically" you turn a good amp into a high wattage P.A. amp.

If your speaker terminals allow it, and you have two "identical stereo amps" like 2 x AHB2’s, you should vertically bi-amp each stereo amp to each speaker.
http://www.av2day.com/wp-content/uploads/2014/05/biamp2.jpg

The reason these guys preferred vertical bi-amping over horizontal bi-amping, is that the powersupply in each amp can be totally accessed by just one bass driver channel when needed for the biggest dynamics.
http://av2day.com/2014/05/bi-amping-vertical-vs-horizontal/

Cheers George

Test results unbridged and bridged.

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

"But the power output doesn’t stop there. With the flick of a switch you can transform the AHB2 into a monoblock amplifier. Unlike so many other amps with a bridged mono option, the AHB2 does not limit you to just 8 Ohm nominal speakers. In bridged mono, the AHB2 is rated for 6 Ohms, but there’s a bit of an asterisk to that.

In bridged mono, the AHB2 will deliver 380 Watts into 8 Ohms, and 480 Watts into 6 Ohms. Technically speaking, you can even drive speakers whose variable impedance dips into 3 Ohm range. However, if you try to exceed 486 watts into 3 Ohms bridged mono, you will trip the 18 Amp over-current protection and the amplifier will shut down."

from https://www.pooraudiophile.com/2015/07/benchmark-ahb2-power-amplifier-review-the-first-commercial-power-amp-with-thx-aaa-technology.html

all amps very different, call Benchmark

@twoleftears I did read the article by pooraudiophile.com and did reach out to Benchmark a while ago and asked about possible tested numbers till 4 Ohm when bridged. Here is the response I have recieved:

OUTPUT VOLTAGE INTO VARIOUS LOAD IMPEDANCES

< 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

Use dBV to calculate the peak SPL from your speaker/amplifier combination. Use the following formula: Amplifier output voltage in dBV + speaker sensitivity at 2.83V - 9 dB. Example: (29.03 dBV at 8 Ohms) + (90 dB SPL @ 2.83V 1m) - 9 dB = 110 dB SPL at 1 meter

AHB2 OUTPUT VOLTAGE AND OUTPUT POWER

Mode, Impedance, Noise Voltage,   Maximum Output Voltage, Watts

                             dB relative to 2.83 Vrms, dB relative to 2.83 Vrms

Stereo 8-Ohms -112 dB 20 dB 100

Stereo 4-Ohms -112 dB 19.8 dB 190

Mono 8-Ohms -109.2 dB 25.8 dB 380

Mono 4-Ohms -109.2 dB 24.1 dB 518

OK, so Benchmark amp is not totally immune to the effects of bridging, but I still see them sending out pairs to reviewers so they can use them in bridged mode. So they must have a fairly strong level of confidence in their performance.

Is the raise of Noise floor from -112 dB to -109.2 dB really bad?, Seems like this is quite better than many implementations by wide margin at least 6 to 9 dB below, or did I get this wrong?.

Word has come through that the speaker can only dip to 3.8 Ohm. So given that Benchmark AHB2 is stable till 3 Ohm. 

Now the question I have is that the amp is rated at 18A per channel unbridged. Once bridged what does it put out?. I think there is a way to do the math from above numbers, when I plug it in I get 11A for 4 Ohm number and that does not sound right. 

Can anyone help?.

@georgehifi I cannot bi-amp since Legacy Aeris has the low frequency section already powered.

geek101 OP203 posts11-10-2018 11:56am@georgehifi I cannot bi-amp since Legacy Aeris has the low frequency section already powered.

That a pity, would have been perfect, if the upperbass unit had it's own speaker terminals, and the mid and highs their own.

Anyway I would not bridge the amps to get more watts, for the disadvantages I listed above. I suggest you get bigger amps, good value are the John Curl designed Halo A21+ 300w or the Halo JC5 400w.

Cheers George   

Read my review of them at Dagogo.com 

The AHB2 is far superior in Mono mode, no comparison. YMMV 

Work with the power cord on the Legacy Wavelet; it will make a difference. Also, the quality of the XLR cables used for the bass section will likewise make a difference. I have had many experiences with Legacy speakers in such matters (reviews), so you may trust my guidance. I am not interested in debating my recommendations.  

Read my review of them at Dagogo.com
 I am not interested in debating my recommendations.

That's odd because in your review, there's no mention of how it sounded in bridged mode (or as you call it, mono mode).
https://www.dagogo.com/benchmark-media-systems-ahb2-power-amplifier-review-2/
You  right, so how could there be any debate at all? 

Cheers George 

The reviews from Douglas, pooraudiophile.com and measurements from stereophile all encouraged me to try this amp out. I appreciate all their work and comments. 

Now I just have to decide if I need to buy two amps or just live with one. Does bridging bump up the 18A per channel limit or keep it the same. Non bridged amp can send 18A per channel so I am guessing bridged version can put more than 18A to a single channel. How do I calculate this number?.

Now I just have to decide if I need to buy two amps or just live with one.

Just Google and read the technical explanations from technical backgrounds of the "disadvantages of bridging stereo amplifiers" 

Cheers George

Simple answer is to buy one and see what you think. A second is just a click away at the same price. The top half of the Aries is not that hard to drive just like the top half of the Vandersteen powered models. My choice would be the BHK 250 which you can get for way under the retail price from selected on-line dealers. 

geek101 OP

Your prepared to spend $6k for 2 x benchmark's that in my opinion will degrade substantially once bridged.

Or get the right amp designed by John Curl, only equaled by Nelson Pass in my books, and get his
for $6k 
 https://www.audioadvisor.com/prodinfo.asp?number=PAHAJC5
or for $3.1K 
https://www.audioadvisor.com/prodinfo.asp?number=PAHA21P

Cheers George

 I have AHB2, few month old.  In instruction manual rev G  p.22 it states:  

Output Current: 29 A peak into 1 Ohm, both channels driven 

The same is on their website (it used to be 18A).  Perhaps they improved power supply (higher limit).

@georgehifi Benchmark has no peer in controlling distortion , zero cross over distortion but without class A hassle. I will get answers regarding damping factor and total current. I am buying them used btw.

@kijanki thank you so much pointing fhat out. Will make sure to newer revisions. 

From the review: 

"So, what about that comparison of cables between the Benchmark and Clarity sets? I set up the following components and rotated sets of cables between them: Salk Audio StreamPlayer III, DAC3 DX, AHB2 Amp (two in Mono mode), and PureAudioProject Trio15 PAP Horn 1 Speaker. "

and 

"Again, the four components in question were the DAC3 DX, AHB2 Amplifier (2)"  

I think the typical reader would understand that "(2)" meant used in Mono mode, as I had been doing earlier. 

It’s clear that you Benchmark bridging "experts" have no idea what happens to a stereo amp electronically when it’s bridged.

Guarantee whoever spends the bucks and does it here, will be back asking for other alternatives after they lived with their bridged amps.
When they’ve realized it was a backward step from using the same amp non bridged, even though it didn’t have the watts to go loud enough.
And they’ve realized all they gained was watts, and everything else sound quality wise took a hit.

Cheers George

@kijanki My amps manual says the same i.e 29A peak into 1 Ohm. Is this per channel I am assuming?. Most amp state this number per channel if I understand that right.

@kijanki never mind the website does mention 
"MAXIMUM AUDIO OUTPUT CURRENT

• 29 A peak, per channel, both channels driven"

@georgehifi look at the following from Benchmark AHB2 manual page 22.

Signal to Noise Ratio
132 dB A-Weighted, Stereo Mode
135 dB A-Weighted, Bridged Mono

Distortion
THD+N
1 kHz, 80 kHz LPF, at full rated output into any rated load 
   < -118 dB (< 0.00013%) - Stereo Mode
   < -118 dB (< 0.00013%) - Mono Mode
THD
1 kHz, 20 kHz LPF, at full rated output into any rated load 
   < -119 dB (< 0.00011%) - Stereo Mode
   < -120 dB (< 0.00010%) - Mono Mode

This only leaves damping factor. 

Any thoughts?.

geek101 OP
Any thoughts?.

Yes! believe me? it’s in the wording, you need to read like a tech to find the tricks, as this is very misleading to the novices, as the last two distortion figures are NOT I repeat NOT in bridged mode, but measured one mono channel only.

Then find the output "current rms" output for a single channel and then what it does with two channels in bridged mode. And you couls also do peak current for both.

Then find the damping factor single channel and compare to bridged.
Also the output impedance for both, which is in relation to damping factor.

Cheers George

Quote from instruction manual:

The AHB2 power amplifier can be run in two modes: 

 STEREO mode - Two output channels 

 MONO mode - One output channel in bridged mono 

The bridged MONO mode will deliver almost 4 times as much power into an 8-Ohm speaker, and will do so without any increase in distortion. The MONO mode is an excellent choice for high-power applications. In MONO mode, two amplifiers will be required for stereo operation. 

kijanki3,

we still don’t get the full Bridged spec picture.
There are no specs accompanying these copy/pasted excerpts you put up, again misleading, if you are referring it back to the other figures on the other page that geek101 put up.

Lets see both distortion graphs THD+N  and THD for both bridged and non bridged into 4ohms or less. This will be the true test of what the OP wants to do.


Cheers George

Here you go: Page 26 "performance plots" fig. 1, 2, 3

https://cdn.shopify.com/s/files/1/0321/7609/files/AHB2_Manual_-_Rev_G.pdf?6581480924542235084

These plots show almost identical THD at 4 ohm, 8 ohm and bridged 8 ohm max power. As for damping factor - it does not make any difference, since it is already very high.

In the application notes on calculating the performance of an amplifer/speaker combination under,  ' AHB2 THD+N and Power into 4 OHM BRIDGED MONO' it shows THD+N is .00083% at 518 W. It reaches 1% at about 590 W. 

@georgehifi This article https://benchmarkmedia.com/blogs/application_notes/ahb2-driving-pmc-mb2s-studio-monitors also talks about second iteration of using ABH2 as mono blocks. 

Seems like outstanding question is just the damping factor everything else looks excellent to me. I will ask this question to Benchmark and see what they say.

Benchmark does seem to perform quite well in bridged mode, in-fact lot of thought went into making it work in that mode. That is why they never made it put out a lot of watts in stereo mode. Even at $6k and putting out 500w per channel at non existent distortion and quite good voltage peak 64.4(beats the requirement I have which is 54) is a deal compared to what is in the market today. Please feel to correct me if I am wrong.

Only Bryston 4B3 comes close at this price range and it suffers from cross over distortion that benchmark does not, if I understand it right.

I’ve tried geek101 with links to pro and cons and every which way, the site and manual, and post you’ve presented are just too ambiguous.

Purchase another one I hope for your sake two bridged amps can do it as good as just one  bigger non bridged amp can, I seriously doubt it.

Cheers George

Are you speculating, or do you have actual experience with the amp in different configurations?

didnt thx design the amplification and power stage of this amp

@onhwy61 I only have experience in running it in stereo mode. I am looking and specs and reviews from others to consider the pros and cons.

@d2girls from what I heard the feed forward error correction is THX. I am not sure if they designed the amplification and power stage.

Are you speculating, or do you have actual experience with the amp in different configurations?

If you have a technical background and understand what happens when you bridge an amp, you wouldn’t be asking this question.

Cheers George

And of course all topologies of amps behave identically when bridged, and no amps are ever specifically designed to be bridged.

geek101 OP

BTW geek101 OP your Legacy Aeris I just noticed are 94db!! why do you need more than one AHB2, one is plenty and will do the 4ohm load on it’s ear, and not have any disadvantages of it bridged.

A bridged one beside all the cons I mentioned, will be looking at a 2ohm load and if that speaker has high -phase angles at 2ohm impedance dips, it could be even lower as seen by the amp.

Cheers George

@georgehifi Legacy Aeris is spec'd for 500w. I am just considering starting with a setup that does it all when dealer comes to calibrate. If I consider the cost of extra Benchmark it is 10% of my system and I can always sell it or re-purpose it.

The challenge is find 500w of cleanest power that does well in rest of the parameters (current, voltage and damping etc) and optimized for price. 

To my limited knowledge even after bridging Benchmark seems to out perform its peers. Correct me if I am wrong. I am happy to learn more.

@twoleftears Is that sarcasm or a fact. Sorry sometimes I don't get sarcasm.

geek101 OP
Legacy Aeris is spec’d for 500w.

Probably peak also.
Here’s your spl at 3mts in your room for a stereo AHB2 seeing a 4ohm load (210w) into your 94dB effcient speakers.

110dB spl! say good-by to your hearing.

And "if" they were 8ohm 94dB speakers and your amp would give then give 105w your still at an in room SPL of 107dB!! still ear shattering!

To my limited knowledge even after bridging Benchmark seems to out perform its peers.

If this were the case, and there are no downsides as I've explained, then why not just have bridged amps if they out perform in all areas. 

Cheers George

The latter...

@twoleftears  thanks :)

@geek101 

Now I just have to decide if I need to buy two amps or just live with one. Does bridging bump up the 18A per channel limit or keep it the same. Non bridged amp can send 18A per channel so I am guessing bridged version can put more than 18A to a single channel. How do I calculate this number?.

The output current is not increased in bridging, it stays the same, only the voltage doubles so the power quadruples. The amp now thinks its driving  half the resistance so a 4 ohm speaker looks like 2 ohms. 2 ohms x 18 amps will voltage clip at 39 volts and thats peak not RMS. This is now lower than you would clip had you NOT bridged the amplifier.   :(   An amplifier of this power would likely have about 60 volt rails and would provide those 60 volts to the point where 18 amps became the limit at around 3 ohms.

 Now you are paying more to get less. Seems no one has yet to point out this little problem. I expect some disagreemet with this calculation so lets chat about it.

I read the specs, the amp is not rated to drive a 4 ohm load bridged let alone when the 4 ohms dips. 

With 18 amps of current be happy. The amp is rated down to 3 ohms and will happily drive 2 ohms, but don't bridge it. If you want two amps then use them as bi amps. At least you have a chance of some sonic advantage. If you are smart, filter the inputs so the woofer channel doesnt see the highs and the tweeter channel doesnt see the lows. Just a simple RC at the input.

Hitting a few more posts lightly.

They promote bridging so they can sell you two amplifiers obviously. 

Why do we need all this power?

They promote bridging so they can sell you two amplifiers obviously.

geek101 OP seeing I gave many other cons and hardly any pros, glad someone else said this, I held off that one, good one Roger now I don’t have to bring it up.

Cheers George

According to Benchmark the amp in bridged mono  the maximum unclipped output voltage is 45.5 Vrms (64.4V peak) into a 4 Ohm load. 

georgehifi, my question was in the form of yes/no.  I can only interpret your non-response as "no, I have no direct experience with the amps in bridged mode".  This dosen't mean that your speculations are not correct.