Benchmark AHB2 - To 'mono' or not to 'mono'

I own a single Benchmark AHB2 amp and have been considering another in order to run both in bridged mono mode, which will provide significantly more power to my speakers and presumably, greater dynamics. I've read in other threads where other owners (and perhaps others with opinions) had implied both positive and negative impressions concerning this approach. Assuming I'm not considering purchasing other amps at this time, does anyone have experience with both approaches and will you please share your impressions?
I do not have this amp, but I am looking to upgrade and have looked into benchmark.  What speakers do you have and what other amps have you compared it with. Thanks. 
Benchmark AHB2 - To ’mono’ or not to ’mono’

I’ve said it before in other threads here it is again

"Nice amp, if you want to hear them at their best, don’t bridge "if there’s no need to", as all you’ll gain is watts, everything else takes a hit when you bridge amps.

More watts.

Worse damping factor
Higher output impedance (has relevance to damping factor)
Lower stability (especially into low impedance’s)
Current ability is reduced (especially into low impedance’s)
Higher distortion.

And if you have two of them better to run them in stereo mode and vertically bi-amped, instead of bridging (mono’ing)."

Cheers George
@pcc67 - The only objective comparisons (head to head) have been with a Bryston 60W integrated amp and with a Rega Osiris. The comparison with the Bryston and the superior mid-bass resolution of the AHB2 during my in-home trial of the latter was the basis for my decision to hold on to the AHB2.

The comparison with the Osiris came later. The owner of the Osiris (my brother) left the duel with the feeling that he preferred his amp hands down. He's all about the dynamics. I thought the AHB2 portrayed everything more cleanly; more definition and air between the individual voices. The AHB2 was slightly less dynamic, which I attributed to its lower wattage delivered to the speakers. (Hence my curiosity about bridging, because I did appreciate the dynamics; it just isn't a deal-killer for me like image is.) 

Speakers used for these comparisons were Joseph Audio Pulsars.
Vertically and passively bi-amp the Pulsars with a pair of AHB2 amps, rather than bridging. Plus, what George said.
@georgehifi - Well, George, your post on the other thread was what made me start to question whether I should go the bridged mono route! I've seen posts by others that stated that going bridged mono was awesome, or words to that effect.

(Now, I read these discussions regularly and mostly lurk, lacking anything worthwhile to add - and I avoid the flame wars about cables and such, but... ) I know there's a significant amount of experience represented by quite a few of the people who post here. So I started the thread hoping for some good advice, and really appreciate yours. No point in spending money (or spending it wrongly) on improvements that really aren't.
What George said, bridging doesn't improve the sound quality, it just gives you more power for higher SPL, but it reduces the amp ability to drive low impedances.

Do you see the clipping leds on the AHB2 constantly flashing? If not, there's no need for more power.
Actually you might perceive an increase in dynamics with 2 amps bridged, not due to more power but due to the decreased damping factor of the amp.

A lower damping factor will make the amp sound less tight and it may give you a slight bass bump at the speaker resonance frequency.

Since the Pulsars are an easy load there's no harm in trying.
wwoodrum OP

What I will also add, if one amp has enough power to easily drive said speaker, you can gain extra dynamics and more bass grunt without losing quality by what I said before. To vertically passive bi-amp two stereo amps (linked).
What happens then is all the bass power from the power supply from one amp, has it available to just that one bass driver, of the channel it's hooked up to. The other has the same. 
So you can get better sound with two amps, (just not in bridged mode.)

Cheers George
I have seen the amp clip at what I thought should have been a reasonable volume, but only on a very demanding recording - Steely Dan's 'Gaslighting Abby' from "Two Against Nature." But just that once and so I'm not too concerned in that respect. 

In general, I'm extremely happy with the performance of the amp and the Pulsars together, and I don't want for the ability to create more loudness in my listening room with most of what I listen to. Maybe I should just leave well enough alone until I'm ready to invest in some nice neutral-sounding mono-block amps with more horsepower. If I had the second amp, I'd go with George's advice and try bridging too, to see what my ears tell me. Since I don't, it seems like the money I'd spend on the second amp would be better spent in other ways.

Thank you all.
A lower damping factor will make the amp sound less tight and it may give you a slight bass bump at the speaker resonance frequency.
For the damping ot the membrane movement - speaker's own impedance is in the series with the amp's output.  Most of this impedance is resistive.  Assuming 6 ohm (for 8ohm speaker)  and 0.05ohm output impedance, we might have 6.1ohm vs 6.05ohm  - not audible IMHO. 
My first question was going to be whether you really need the extra power for your situation. It seems you may not.

JA at Stereophile measured the Pulsar’s sensitivity at 83.5dB(B)/2.83V/m, which is quite low, so certainly some situations/program may benefit from more power than the 100W/ch of a single AHB2.

There would be absolutely no issues running the AHB2 bridged with your Pulsars - their impedance is mostly above 8ohms and only drops as low as 6.5ohms.
The distortion specs of the AHB2 in bridged mode will be just as vanishingly low as in stereo mode (see Audio Precision measurements conducted by JA and elsewhere).

Regardless of the measurements, I can say that in my case (ATC SCM-19 - low sensitivity 8ohm rated speakers like yours) I got an audible improvement from bridged AHB2’s.
The sound was essentially the same (no surprises) but more ease and better sound staging with the bridged amps. I also took the opportunity afforded by mono blocks to change to short speaker cable runs (and longer XLR interconnects). This probably contributed to the improved sound I noted. Certainly, being able to run very short speaker cables is an advantage of monoblocks.

Bottom line.
If you don’t want/need the extra power or the cabling flexibility afforded by mono blocks, just stick with the stereo AHB2 - but rest assured if you decide to go to bridged mono AHB2’s you won’t encounter a sonic penalty with your speakers.
For the damping of the membrane movement - speaker's own impedance is in the series with the amp's output. Most of this impedance is resistive.

A speaker relies both on mechanical damping and electrical damping, its Qms and Qes parameters; Raidho for example expressly recommended high damping factor amplifiers for their speakers as their woofers had high Qms and relied on the electrical damping of the amplifier. There are threads on trying to find a suitable amplifier for them as their bass would sound loose and bloated with low and normal damping factor amplifiers. Just compare a Pass and an Audionet amp and you will see how their bass is very different; the Audionet's bass is much more damped and dry with better transient response, but it can sound dynamically constricted with low Q overdamped speakers

Since the OP's Pulsar Seas woofers have more mechanical damping they may not need too much electrical damping from the amplifier; bridging the amp may offer a more dynamic bass if the stereo amp/speaker pair is overdamped.   

Electrical damping occurs because moving coil generates voltage that produces current in direction that causes motion of the membrane in opposite direction hence stopping the membrane.  This current is equal back EMF (voltage on the speaker) divided by impedance in the circuit that consists of amplifiers output impedance, cable impedance, and the driver itself.  At the end it comes to 6.1ohm vs 6.05ohm difference (about 1%).  Also for the same reason the highest effective damping that can be achieved is equal about 1.5 (nominal speaker's impedance divided by resistive part of this impedance).  It might explain why some tube amps, that have very low DF (as low as 1.5), still sound great.

There might be other reasons why some amplifiers sound worse when bridged. It might be unregulated (in most cases) power supply voltage that is sagging when twice the current is demanded.  AHB2 power supply is line and load regulated and should not sag.   From all the reviews, I've read, AHB2 sounded even better in bridged mode.
You lost me.  What do you mean by bridged.  Why would you ever want mono.  Wouldn’t you lose stero?  Wouldn’t this be like listening to an AM radio.  I still don’t understand why people would ever buy a $20,000 turn table because don’t you looses lot of detail compared to digital by way of CD player.  The first thing I noticed when hearing vinyl is the crisp highs disappeared such as cymbals.  The tightness of sound disappears and sounds muddy.  I have read articles about this and many think listening to music played on vinyl sounds inferior to digital.  Vinyl sounds warmer but the detail disappears.  I like to hear the detail.  Kind of compares to listening to compressed music as compared to listening to 16 and 24 bit.
I love this amp! Absolutely, definitely, without any doubt what so ever get another one!
Not sure what planet we are living on but when you mono strap this amp you drop the impedance of the output stage and increase the damping factor! Not only that but power is everything in an amp. The ability to handle dynamic peaks without clipping just makes a system sound so much more alive and this amp does the mono strap routine as well or better than any other amp I have tried it with. I do not think you could make a bigger, better improvement for $3k in any otherwise well set up system. And, these amps do a very creditable job of driving my speakers (with the addition of a 3/4 ohm series resistor in the primary) which means they will drive anything. The build quality is absolutely top notch. You will never have a reason to buy another amplifier. 
@tobes - I find that I tend to stop following discussions after the trolls find them and do what they do. I'd rather politely ignore them than provide them with the attention they so obviously lack and need. I don't know all of the names yet, but they do tend to self-identify fairly quickly and in a pretty pathetic way.
@tobes - Regarding your former post, it is true that I don't know how much I need the extra power. I have a 14'x20' listening room to which I've been gradually applying acoustic treatments to get rid of slap echo and eliminate standing waves (with some professional advice). I have found that treating the direct reflection points on the side walls and ceiling in the room has improved the system image drastically, but also allows me to play my system more loudly and to gain a better appreciation of the dynamics in a recording, and now those dynamics are present everywhere there is image, not just tending towards the center (if that makes any sense at all) - as was the case before I started treating this room. I was thinking that the extra power would improve almost everything about this, particularly in more dynamic recordings.
So far it sounds as if those who've actually used a pair of AHB2s in bridged-mono mode have no regrets....
(with the addition of a 3/4 ohm series resistor in the primary) which means they will drive anything.

You have successfully just made the damping factor that was already diminished because of bridging, even worse by installing even more series resistance, good work.

I was thinking that the extra power would improve almost everything about this, particularly in more dynamic recordings.
If you don’t use all the dynamic headroom power with a single stereo amp, you will gain nothing by bridging having even more. Headroom is head room, if you haven’t reached it there’s no reason to make it higher by bridging and taking a hit on everything else.

You just take that was a good stereo amp, send it down the path of becoming a mono P.A. amp because of the reduction in all the other parameters it will take a hit on by bridging.

You guys need to see the forest for the trees, if you want/need more power and want to have two of these stereo amps, just vertically bi-amp them, then you gain the power in the bass and dynamic headroom, without taking a hit on everything else if you bridge them.

Cheers George
talk to Benchmark, then borrow one and find out which you prefer...real listening in your own system, not theoretically why you should or shouldn't...
@jl35 - That’s the final step, to be sure. I won’t do that until I’m reasonably sure that’s the way I am going. I don’t want to create ‘distressed merchandise’ for them if I can help it. 
@wwoodrum - only you can say whether you need more headroom with your room/speakers.
My room size is similar to yours and my speaker sensitivity around the same - and truthfully a single AHB2 drove my ATC’s to high levels without issue. Nonetheless adding a second amp improved the sound in terms of dynamic ease and soundstaging.
My motivation was not only the extra headroom but, as previously mentioned, the placement flexibility that mono blocks offered - allowing me to tidy up my setup using short speaker cables.

As you probably realise from the specifications, unlike conventional amplifiers, there is no downside to bridging the AHB2. The error correction circuitry ensures the Benchmark’s distortion remains vanishingly low as impedance drops/varies. This may well be another reason for the AHB2’s perceived neutrality vs other amplifiers - i.e. distortion in other amplifiers typically varies with impedance (whether bridged or not).
Of course the output impedance does double when bridged which means damping factor halves - in the AHB2’s case dropping from from 350 to 175 (at 20Hz/8ohms). This is still 10x higher than a typical transformer coupled tube amp and is unlikely be of much/any audible consequence. Certainly I heard none in my setup.

If you get a second amp you have the option to try both bridged mono and vertical bi-amping - so why not? For bi-amping you will need an XLR splitter of some sort if your preamp doesn’t have dual sets of XLR outputs.
Both options have the advantage of dedicating a single amp per channel and using short speaker cables. The bi-amp option further allows a seperate amp for woofer/tweeter - though most of the work is still being done by the amp on the woofer, which still has only 100W/8ohms.
You will gain a slight increase in headroom with the vertical bi-amp option while bridged mono will give a significant 6dB more headroom.
With more sensitive speakers, where extra headroom is redundant, the bi-amp option may well be preferable.
I use bridged mono and bi-wire to my ATC speakers (allowing removal of the link at the speakers).

I’m not going to gush over the improvements offered by the second amp as in overall terms they are not massive. Nonetheless, in my setup it does sound better.
Only you can decide if it’s value for money.
Some people spend more on speaker cables than the cost of an AHB2 amp so, if you’re into expensive cables, you could potentially save money moving to mono blocks and short cable lengths ;-)
I’m not going to gush over the improvements offered by the second amp as in overall terms they are not massive.

But isn't it the incremental improvements that sometimes make the biggest difference? Take speaker cables for instance - a person who doesn't do this for a hobby might not notice much of a difference between a $400 cable and one three times its price, or even 10x the price. But for you and I, that might make a huge difference in the way we experience the music. That concentration and depth of image that improves the overall presence of the music makes the difference worth spending, to us.

Anyway, I'm encouraged by your experience with two of the amps. (And have always wanted to listen to ATCs but have not yet done so.)
Yes exactly - personally I wouldn't go back to one amp.
I just didn't want to create unreasonable expectation about the addition. I do think the improvements extend beyond merely providing more headroom. 
As long as you understand this is an incremental improvement, rather that some sort of paradigm shift.    
Whoops, my error. Got my bridged and parallel modes mixed up. Right voltage doubles, damping halves. 
Troll, as in trolling for business? I hope not. The audio business sucks.
Better off selling automobiles. 
Whatever AHB2s bridged acquitted themselves beautifully comparing them to Pass XA 200.8s driving single transformer electrostatic panels from 130 Hz up. I just had to use resistors in series with the primaries to prevent ultrasonic ringing. 
An unbelievable savings in cost, space, heat and efficiency.   
Sorry @mijostyn when you mentioned damping factor doubling and adding a resistor in the primary I thought you were taking the piss.
Trolls as in the mythical beings that live in caves and assail unwary travelers; the modern context is here:

[off topic] Not that I’ve never appreciated really nice work by the very best, but most of them aren’t even close. To wit: this review of Audioquest K2 speaker cables...

tobes, at my age you have to piss all the time. woodrum, that definition of troll adequately fits audiophiles in general as it seems there is as much mythology as there is reality in the practice. Trolls are most dangerous when they are trying to sell you stuff.
Recently I added a second AHB2 to my system to run it in mono.

I was running MacBook Pro (JRIVER) > Benchmark DAC3 > AHB2 > ATC SCM19v2 (as of yesterday – there are two ahb2, running in mono and providing 380watts into 8ohms). My speakers are ATC SCM19v2 (85db / suggested power from manufacturer is 75-300 watts), so I was hoping to get better performance with more wattage.

 After reading posts from different fellow members on this site and other forums, I was not expecting any big changes in the sound quality. But boy, I was so wrong. I guess different ear hear differently.

 I was very happy with single AHB2 (100watts) pumping music into my speakers, with great sound-staging, tonal balance, accuracy, clarity, bass punch, excellent mid-range etc etc. But sometimes, with some tracks, I did wish for fuller/warmer sound, and was doing google research into some tube preamps to use with my setup for the occasions when I really wanted that extra warmth. That problem is now solved with two units.

With two AHB2 (in mono configuration), the sound quality has changed considerably. Tracks which sounded thin earlier, were fuller with more ‘body’ and warmth. The sound staging is definitely much better now. It has completely changed my listening experience to say the least.

 I am expecting to spend more time with my system over the coming weekend, and will share more soon.

Hopefully, I will like this change.

With two AHB2 (in mono configuration), the sound quality has changed considerably. Tracks which sounded thin earlier, were fuller with more ‘body’ and warmth.
That to be expected with half the damping factor and double the output impedance, that will always make the bass and lower mids sound richer.

Now try vertical passive Bi-Amping with the both amps back in stereo mode, and prepare to be amazed!!!

Cheers George
georgehifi in order to make that work right you need a rather expensive digtal crossover to do it right. Then you bypass the in speaker analog network.
ssnk you really should look into Channel D's Pure Music. You do have to buy it but you get a free trial period. It will use your iTunes as a library and does neat things like network stream and up sample. It was designed to work specifically with Apple computers.
georgehifi in order to make that work right you need a rather expensive digtal crossover to do it right.
No sorry your wrong, forget digital xovers your still using the speakers internal xovers.
Just vertical bi-amp as per the link, if you have 4 speaker terminals on each speaker (remove the links between them).
The sound will be far better than bridging the amps into mono.

Cheers George

How do I split the output from the pre-amp/DAC? Does someone make a decent splitter cable with XLR terminations? Or am I missing something?
Post removed 
For a long time now, on more than one thread, my opinion is that George continues to be wrong. The amps are better in Mono. I reviewed them for, and I believe I am correct that George has never used them.  
I might consider an Audioquest product, but since the AHB2 only accepts XLR, those would be an issue. Call me crazy, but a guy who invests in Nordost cables has no business buying a Chinese splitter for $10.99 on eBay without some nice references from others. I suspect the improvement from vertical bi-amping might be obscured by the crappy cable in the middle.

 I am going with the opinions of those who say they’ve done it.
And yes, my speaker cables (Frey 2) were more than the amp and worth every penny. 
Post removed 
wwoodrum, do not chintz out and get the AQ splitters. If you have considerable funds in your system, get a superior product. Having compared directly several times in several systems the AQ splitters (RCA) to various Y cables (I have also compared the AQ RCA Y cables, and they are average), I suggest you get a superior product, namely the Audio Sensibility Y Cables. 

I reviewed the Audio Sensibility Y cables, both XLR and RCA for in connection with my Schroeder Method of Interconnect Placement (which I have done with the Mono Benchmark amps- revolutionizes their performance), and continue to use them. They are superb and worth the money. You will have superior sound quality using the Audio Sensibility Y cables. Yes, they are much more expensive, but are much better. I would not skimp on that.

I am not attempting to make this a bash George thread, as I have seen him give good advice. But in this particular detail I believe my advice is better, obviously because I have extensive experience.

You can elect to have AQ manufacture some custom XLR Y cables for you from the cable of your choice. That is not cheap at all, possibly far more expensive than the Audio Sensibility Y cables. They might outperform, but that is not a given. The AQ Y cables I had made are far more stiff and that creates issues in some setups where you have to bend the Y cable. Given these options you would be wise to avoid the hard splitters. I have used other splitters as well, and prefer Y cables holistically.  

In conclusion, you have several more significant steps to yield higher performance. You have not yet come close to maximizing the potential of the AHB2. You will also want to pay attention to power cords on the amps, even though -as you might suspect - some declare that it makes no difference. It does.

Anyway, it's on now to the all important listening session!   :) 
but since the AHB2 only accepts XLR

Plenty like these around, if you look at all differing prices.

I am going with the opinions of those who say they’ve done it.

Pitty, no I’ll rephrase that, sad, you’ll never know then, just how much better your system can get, by vertical bi-amping, instead of bridging (mono’ing) your amps.

Cheers George
@George - I’m not questioning the theory behind what you say. But I believe in empirical observation. Tell me you’ve done it  both ways with this amp and your way is better; I will gladly spend what I have to in order to try it. (That said, TWO pairs of Nordost cables is an investment not for the faint of heart, regardless of length!)
@douglas_schroeder -I have the Frey 2 on the power side from a Qb8 that is fed by a Valhalla2 20-amp cable. The results are sublime.
Tell me you’ve done it both ways with this amp and your way is better; I will gladly spend what I have to in order to try it.

Yes with many amps I have, not with this amp. There is no valid reason to believe it won’t be the same as EE engineering principals would prove it to be the same also.

There is no "magic voodoo" circuit in the AHB2 that can turn the EE engineering principals around 180 degrees and make it better spec’d bridged than in non bridged mode.
It will always have better specs in "stereo mode" than in "bridged mode", (save for extra wattage in bridged that all.)
Believe what you want, it sad though.

Cheers George

Thanks for sharing your thoughts on the potential root cause for change in SQ for mono application.

I would like to try ‘vertical passive bi-amping’, as per your suggestion. But since, I will have to spend some more time/effort/$$ to get appropriate speaker cables and other connectors, I guess it will have to wait for some time. Also, it will give me only 200watts per channel, and not 380watts, which I expected to get from the setup. Let me know, what you think.

I waited for the delivery of this second ahb2 for 2.5 months, but now I am truly shocked/surprised/disappointed by the mono performance of this otherwise excellent amplifier, to say the least. There is no return-policy in the country where I am located. So, I am pretty much stuck with this, or I can try selling it. Or, maybe I need to give it a bit more time to ‘settle-in’ (I don’t even know how it will settle-in any more than what it is right now though. I was told there should be no break-in period for his amp).

This new sound reminds me of my short time (except the noise floor) with ‘Cambridge Audio 851W’ (Power Amplifier, 200Watts @8ohms) – costing just 22% of what I spent on two AHB2.

I hope I am doing something wrong with the setup and it will get fixed. I had high hopes from this combo.


Thanks for your suggestion. I will try reading-up on ‘Channel D’s Pure Music’, and educate myself on the technology/equipment.

I will spend more time with the system over the weekend and will update again!

Thanks all for your suggestions
ssnkssnk, my vote is that the amp needs break in or that you are doing something wrong that once corrected you will be fine. I have not heard or used the amps, but most if not all of the articles and reviews I have read indicated that if you need more power, mono block them.  
Also, it will give me only 200watts per channel, and not 380watts, which I expected to get from the setup. Let me know, what you think.

As I showed someone else, your ATC SCM-19’s have quite an easy 5-6ohm impedance curve (green) and a -phase angle that’s fine also (red) and are 85db efficient, so to have 200w or 380w won’t matter, as you will not clip the amp/s either way, before bottoming out the speaker first.
So go with the better sounding configuration, and that’s vertical bi-amping of the 2 x stereo amps, you will be very pleased. 
And leave the bridging (mono’ing) up to guys that "think" they need the extra watts even though they are oblivious to the "fact" it effects the quality.

Cheers George