Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

@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?

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.

I would expect it to be higher. If the rails are 60 volts then the peak volts should approach 120, say 100 Volts with losses,  but the current won't support that as 25 amps are needed. BTW 18 amps is not an impressive amount of current for modern amp of that size. Should be more like 40 amps. The Mitsubishi Docking Amp of 1978 delivered 60 amps, I measured it!

Something is a little off here and the more I here the more I wouldnt want to bridge this amplifier. I have nothing against bridging in general and have made bridged amps.

The reason we bridge amplifiers is when WE DO NOT have a low impedance load. We bridge to get voltage not current. We bridge to get some juice into 8 ohms. When we have 4 ohms we dont bridge. 

Where are the italics around here? All caps is my best substitute. :)

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.

@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 presume you mean current compared to power output not physical size?
The AHB2 is very small and lightweight. IMO the power/current output - not to mention almost non-existent distortion, super high SNR, efficiency, cool running and (most importantly) resulting purity of sound - are very impressive.
Other than bragging rights, I doubt 40A of current has any advantage in driving practical loudspeaker loads.

I meant of that size power wise, not physically. Now we have a report of 29 amps. This is like the stock market. Or are we just playing poker and raising?

I will bet you that this amp bridged into a 4 ohm load played at 1/3 average power will probably overheat. Where is John Atkinson when we need him? :) 

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.

ps. I did read the review by JA. It's worth a look but doesnt answer all the questions. It's clear to me what the OP should do.  One amp or get busy and truly BiAmp. When are we going to get out of the sandbox?

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

Is this sarcasm or what you believe? Can we at least get a little more real, we might just figure out something.

@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. 

As usual, some of us try to help the OP and some of us go off on a tangent to defend their own stuff. Then everyone gets mad, I'm not mad though I am surprised at some of the responses. :(

I was surprised to see that the ATC 19 is actually 19 liters which is only 0.67 cu ft. I can't imagine why someone wants to connect this little guy to a 370 watt bridged amplifier. The speaker only goes down to 55 Hz. The resonance can be seen on the impedance graph.

I have no doubt that the Benchmark performs as stated. I am surprised that there is such fuss about whether distortion at the 0.0002 level doubles or not. Or whether the very high damping factor is cut in half. Or whether the imperceptable noise is doubled or not. Most likely your preamp will create more noise than this amplifier.

Its a bit like Woody Allen said...  "Boy, the food at this place is really terrible." The other one says, "Yeah, I know; and such small portions."

@tobes 

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.

I do fine with 35 watts so I am curious what is your listening level and speaker efficiency? Have you measured the peak voltage at your speaker? If not we can estimate it fairly accurately with just your efficiency and SPL at 1 meter. Can you do this for us please?

Since you brought up "extensive protection" what happens when this amplifier drives a 10 uF capacitor, typical of many ESLs.

What is straw man about informing people that damping not damping but actually a variation in frequency response due to impedance variation that occurrs with high output impedance?

I read JA's report. The Benchmark is about as perfect an amplifier as one can imagine. Yet we still dont know its current capability and some other things. This is an issue to a varying degree with many amplifiers, yours excepted, of course.

I find the "strawman" accusation most inapproptiate to this thread. We are trying to figure out something here. Does anyone have a schematic for me to peruse?

Ok, Im really getting into this amp now. Have a look at this white paper, its good, and the amplifier construction, ugh.

https://benchmarkmedia.com/blogs/application_notes/14680625-the-ahb2-a-radical-approach-to-audio-pow...

I fix rare amps like the Reference Line that one of my customers just loves. It is hard to work on. I like to think I can fix anything but I would not even bother to open up this Benchmark after seeing these photos. A blown up cap is no problem, but if the power supplies shut down or the amp circuits there is no chance in Hell to fix this at the component level at any reasonable cost. Unless these prove unusually relaible I would not want to own one past the warranty. Once the boards become unavailable you will have a nice boat anchor.

Has anyone any experence with having one of these repaired? Its all surface mount, perhaps board swapping is the only way. Im sure the factory doesnt mind fixing them because they have the boards to swap around. But try to find even a smart tech who would like to take this on.

@tobes

It’s a sealed cabinet design and bass rolls off slowly providing pleasing extension into the mid 40’s in room.
Obviously 380W is not required (or used by me) but 150-200W is useful with the low sensitivity SCM19’s if one wishes to explore the considerable max spl of which they are capable (108dB). ATC recommends 75-300W, though to be fair a single AHB2 drives the SCM19’s very well.
Since my equipment resides in a room corner I also wanted to exploit the placement flexibility of mono blocks which allows positioning behind the speakers and short speaker cables (which should also mitigate loss in damping factor from bridging - not that DF is an issue ;-)
In regard to your other snipe at me, go back and read the first line of my first response in this thread. My subsequent posts were then attempts to correct (apparently deliberate) misinformation. Something you apparently have no problem with.

Im not interested in getting into a fracus with anyone on here however you have stood fast and been a little in everyones face about your Bridged Benchmarks. You did not answer my questions about your listening level or power level. How do you know you need those watts?

There is no evidence that "It’s a sealed cabinet design and bass rolls off slowly providing pleasing extension into the mid 40’s in room" Granted it is sealed, and thus rolls off at 12 dB/octave.

Here are the specs.

Drivers: HF ATC 25mm Neodymium soft dome, Mid/LF ATC 150mm SL
Matched Response: ±0.5dB
Frequency Response (-6dB): 54Hz-22kHz
Dispersion: ±80° Coherent Horizontal, ±10° Coherent Vertical
Sensitivity: 85dB @ 1W @ 1metre
Max SPL: 108dB
Recommended Power Amplifier: 75 to 300 Watts
Nominal Impedance: 8 Ohm
Crossover Frequency: 2.5kHz
Connectors: Binding Posts/4mm Plugs, bi-wire
Cabinet Dimensions (HxWxD): 438x265x300mm (grill adds 28mm to depth)
Weight: 17.8kg



If the speaker is already down 6dB at 54 Hz. (which agrees with the impedance curve) it is going to be down another 6 db or so at 40. The rolloff of sealed box speakers is 12 dB per octave in all cases. Its physics man. When bass is more than 6 dB down you are hearing more 2nd harmonic than the fudamental because our hearing slopes off rapidly down there.

Im an information junkie, Ive been correcting things here the whole time while you keep fussing with George about whether this tiny distortion changes or not. No one is going to hear this low level of distortion!

The entire discussion about this miniscule amount of distortion is a tempest in a teapot. THE OP JUST WANTED TO KNOW IF HIS IMPEDANCE DIP WAS GONNA BE A PROBLEM. Many of us advised him it would and further he might get more power with one amplifier in stereo and a lot safer.

Besides the "strawman" snipe, you haven’t been very nice to George. He is in the industry.While he and I don’t agree on everything, we disagree on many things, yet our public conversation here has remained civil and I thank him for that.

I am certain ATC makes a very fine speaker. I think their maximum recommended amplifier playing heavy music would but an end to the speaker. The powered version only has 35 watts on the tweeter, you have 350 watts which is an accident waiting to happen. Voice coils do melt and you have no protection against that at all. One full level HF or ultrasonic event and bye bye tweeters.

If 29 amps of current ever goes through your woofer voice coils for more than fraction of a second ohms law will become quite apparent. With the amplifiers unbridged you have a 4 times greater margin of safety.

And now a little story..... I like to brag sometimes too.

I was engaged to fix a Bryston amplifier that had been back to the factory twice under warranty. It had already destroyed 2 x B&W woofers at $500 each (plus service calls). When it came back it blew up 2 more because the problem was intermittant. On my bench it did not show up either but I added external fuse holders and left a handfull of spares. There were no speaker fuses before. I said, lets not blowup any more expensive woofers, 2 amp fuses are cheap, the 20 cent ones will do the job and protect the woofers. Only DC would be doing this and you can play quite loud with 2 amps fuses. I asked his kids how loud dad played, they said not loud :)

I told the customer to call me when he had blown up ALL the fuses. The day I returned to get the amp again the house was cold (Santa Barara cold is not very cold, like 60) The amp finally showed it was putting out 50 volts of DC. Now we had a problem that could be verified at last..

Because the Bryston was old and not easy to get to the part that was the problem we sent it back to Bryston in Canada with a full page letter telling the tech to simply cool the amp and he would finally see it act up where it had not before. It was in the last few months of its 20 year warranty and free is cheaper than me. He cooled the amp and ended up replacing the whole mother board.

I will close with this. Elaine LaVasseur, besides being a fine artist, is co-founder of HI FI Club. Their company installs complete home systems and home theaters for wealthy Montecito filmstars and directors. They did both for Robert Zemeckis.

She comes up with clever things to say and told the frustrated Bryston customer, who had now lost $2000 of woofers with no end in sight, "Having Roger come to fix your stereo is like having Picasso come to paint your house" :)



https://santamariatimes.com/calendar/community/elaine-levasseur-collaborative-printmaking-and-home-s...

@geek101 

Thanks, its been fun. My well considered advice:

ONE AMP PER SPEAKER, BI WIRED, BI AMPED. 

Bridged, no, no, no

The OP has Legacy Aeris loudspeakers (a design that requires bi-amping) which according to at least one Legacy dealer really likes high power amps. The OP wants to know if the Benchmarks in bridged/mono mode will have trouble driving the Aeris.  

I looked at the manual and I dont see any requirement for bi-amping. I dont know of any speaker that "requires" bi amping though I am all for it if there is an active crossover before the power amps. 

This is a $21,000 speaker, Why use the Benchmarks on that? There are better choices and bridging the Benchmarks is not a good idea given the low impedance

@ramtubes, Impedance minimum for top section of Aeris is only 3.8 Ohm (this information is from the source). So it seems AHB2 is fine with that.

Which amplifier would you think can be a better option than Benchmark AHB2 given that I have gotten myself one amplifier for $2100?. What do you think in technical terms another amplifier can be better than Benchmark and at what price and size?. I am looking for 150 watts with no distortion per channel easy, which means no crossover distortion or anything else. Just because I have spent some money on a speaker does not mean I am going to weight that and buy an expensive amplifier. If I could find a $2k Legacy Aeris I would have purchased that instead. I want a great amplifier optimized for cost.

Please let me know I have been looking for a while. I just cannot audition 10 amplifiers to compare and select which sounds good in my room and with the speakers I have. I have to look at the specs, build quality, technical innovation and what generally people think. Anything that comes close to this in my opinion is Hypex NC500 based modules which again can be obtained for $2k approximately. I will consider if you can point me few of them.

General rule of thumb is fine but at the end I am constrained by my experience and capacity to audition amps and I like to re-verify past ideas. Technology changes and things change. If someone says X amp sounds good then I have no context, I will go read the specs, read the forums and reviews and move on.

Thanks for your questions.Right now you have an amplifier and speaker that are working for you. It would be wise to find out some things about how you listen.

Do you have and SPL meter, the RadioShack one is quite good? The cellphone aps accuracy is unknown but better than guessing, Guessing never works.

Do you have a meter that can read AC or Peak volts? An old analog meter would work also. We need to find your power level unless you already know it.

The sensitivity appears to be 95 dB. Is this at 2 or 2.8 volts? Try to find out, makes a 2 to 1 difference in power,

No one needs an amplifier with specs this good. If the distortion was a four timer higher you would not hear it. I hope the amp has good sound but I would not own it due its complexity. Distortion measurements are done steady state, music may not enjoy those same specs. They worked hard to get distortion this low yet I am unimpressed at how they did it. Look inside that amp and then a Bryston or something traditional. Look at a Hafler, or TransNova, perhaps even a tube amp. This amp has too many parts and the failure rate if chip capacitors is alarming. I pulled a noisy one out of a Mic pre. Last thing I would have expected.

What happens in Electronics is someone gets on a tear to make an amp that excells in some area. If they do this by creating some thing very complex they can make a monster.

Read the Atkinson review. His test equipment is the best and latest test gear yet he can hardly measure the distortion. No one needs this low distortion. How will you know how long it will stay that way?

The coments in this thread about the small increase in distortion was like children arguing over pennies. Same for the damping factor as I have pointed out.

Bottom line, go get a real amplifier. (My shields are UP) :)

Is this amplifier made completely in China, or boards or parts from China? I cant imagine a surface-mount board that large being populated in USA.

Ramtubes, could be my terminology is off, but if a loudspeaker has a built in power amp for the bass drivers and separate inputs for an external amp to drive the midrange/treble, then it requires bi-amping.

 We really shouldnt have to be doing all this guessing. I wish the OP would tell us more about how this speaker is set up in his system. 

I think he has the preamp/processor that goes with this speaker. If one looks at the back panel there is an XLR jack to drive the plate amp. There is one one pair of speaker terminals for the rest of the speaker so it cannot be biamped without going inside. What I do find curious is:  If you do not have their preamp, which is featured on the website, then where are you supposed to get the signal to drive the XLR?

If a speaker has a built in power amp  for the sub, no other amp is required there.  It wants a line level signal. So there is no biamping in this case. One could s

Many speaker makers, Vandersteen and others provide two pair of inputs that can be biwired, biamped or simply strapped for one amp. That is when biamping is possible. Some choose different amps in that situation as the upper speaker often needs less power than the lower (regardless of the sub). So a big SS amp on the bottom and a nice tube amp on the top. 

There are many of us who feel small amplifiers sound better. The sins that are committed in making a large amp will cause the lower power range to suffer. As an amplifier designer I happen to agree.

That is why the FTC stepped changed the rules in the 70's. Before distortion could be specified at full power where it is often lowest (in SS amps). At low levels there was often large amounts of crossover distortion. So they changed the rule to quote maximum distortion from 0.25 watts to full power. I would further reduce it to 0.000000025 watts to full power. We dont want distortion to climb at the bottom of the range.

It bothers me there is no sub control. Makes me think you have to buy their pre.

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. 

@kijanki 

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

Lets just take this first part. Feedback is not involved in doubling of power, deep or not. The losses I am speaking of are mostly in emitter resistors and turn on resistances of the output devices. Once an amp clips feedback cannot make up for these losses. 

I had mentioned in my review that the AHB2 is not able to drive the Kingsound King III electrostatic to an unlimited level of listening. However, there is no degradation of sound as the amp is pushed to its limit, and there is no degradation associated with mono use.

How did one stereo amp sound to a level it was comfortable at, and then the bridged at the same level? To gauge which sounded better. Not which could go louder!

George is correct and one amp might sound better. Im curious what the owner of these speakers considers unlimited level?  Lets watch the words.

BTW, ESLs exhibit clipping very similar to amplifier clipping. This occurrs when the air gap saturates, and I assure you it does. One can often smell the ozone close up to the speaker.... im not kidding.

@kijanki  Feedback lowers output impedance. You cannot make exact doubling of the power if DF=10.

Of course feedback lowers output impedance. However that has nothing do to with output power at clipping. Its not like there is some resistor in there.

I assume you are adjusting the input level as you explore these power output levels. 

What is this EXACT doubling of power all about.. Why does it have to be exact? 

@kijanki  Yes, drop across resistance will double, but reserves in power supply combined with the feedback will make up for that.

I dont see why it is a virtue especially if it is just a spec and not what is really going on. Has Stereophile tested any of these amps. Please provide a link.

I never said it is a virtue - just the opposite.

I, prehaps misinterpreted this statement as something you agreed with.. 

 

Exact doubling of the power is treated on this forum as a virtue.

 Under RMS testing (which I have responded to in your other thread) there is no power suppy reserve, its flat out. 

Please tell me how feedback is going to find power where there is no more. Feedback does not create power. Rather than keep telling me it does tell me HOW it does.

@kijanki  I agree, but amp that doubles power exactly has to have adequate power supply AND deep negative feedback. Exact doubling implies perfect regulation (ideal voltage source), that is not possible without deep feedback.

What part do you agree with? If you do agree that feedback does not make up for these losses why do you keep bringing up deep feedback. (also a term I have never heard)

1. What about the emitter resistors? You cannot ignore them. 

2. How good is the supply regulation? 1% ?

3. What does the deep feedback have to do with it? If the damping is even 10 thats enough. Its only going to affect the shape of the clipping and make it worse not better.

If for some silly reason you want the power to exactly double then you make a power supply that puts out more voltage as more current is drawn. Kinda upside down idea. 

If you can site an amplifier that exactly doubles bring it on. Show me.

@georgehifi  Got a feeling there’s a couple of latent shills helping out too

Likely indeed. The marketing guy did step in at one point. We really gave it a good shot.

Check out this thread claiming there is no RMS power.

https://forum.audiogon.com/discussions/rms-power#1665330

 Read the banner at the top of the Wicki page and check out the references given below. especially #8. Then read all the compliants on the talk page. Im writing someting up.

https://en.wikipedia.org/wiki/Audio_power  

@kijanki 

to me

From Wikipedia "Audio Power"

As described above, the term average power refers to the average value of the instantaneous power waveform over time. As this is typically derived from the root mean square (RMS) of the sine wave voltage,[6] it is often referred to as "RMS power" or "watts RMS", but this is incorrect: it is not the RMS value of the power waveform (which would be a larger, but meaningless, number).[7][8][9][10] (The erroneous term "watts RMS" is actually used in CE regulations.[11]) This is also referred to as the nominal value, there being a regulatory requirement to use it.

Al, leave it (he is not going to get it). . 

@kijanki  Ramtubes, I explained exactly what I agreed with by quoting your statement - read again.

Why are you bringing emitter resistors again, yes we know that they exist.  

I'm not sure why you're asking me what Benchmark voltage regulation is.  Perhaps you should direct this question to John Siau.

What does the deep feedback have to do with it?

Feedback lowers output impedance. You cannot make exact doubling of the power if DF=10.

If you can site an amplifier that exactly doubles bring it on. Show me.

There are many of them (I'm surprised you don't know that). For instance Rowland model 535 is specified for 250W@8ohm and 500W@4ohm.  Krell Evolution 302e specifies 300W@8ohm 600W@4ohm and 1200W@2ohm (both channels driven). Pass Labs XS150 states 150W@8ohm 300W@4ohm  etc.

You keep asking "what feedback has to do with it" while I'm trying to show you, that exact doubling of power cannot be achieved without deep feedback. Exact doubling of the power is treated on this forum as a virtue, while I don't think it is necessary a good thing and might be even harmful when deep feedback is used to spike-up amplifier's specifications while creating TIM distortions at the same time.

I know of all those amps and those specs. But those are marketing numbers. Can you tell me the actual measured clipping power at those various loads. 

I keep asking about the emitter resistors because when you drop the load impedance and more current is drawn the drop across those resistors (and any other resistances) doubles and it appears you dont want to take that into account. 

I have not been asking or referring to the Benchmark in this discussion. 

Exact doubling of the power is treated on this forum as a virtue

,

I dont see why it is a virtue especially if it is just a spec and not what is really going on. Has Stereophile tested any of these amps. Please provide a link. 

@kijanki 

Heres a Rowland that specs 60 into 8ohms,  120 into 4 ohms just the way you like it. The actual measured powers are "  Fig.7 indicates the THD vs power for 8, 4, and 2 ohms at 1kHz, one channel driven (bottom, middle, and top curves respectively; line voltage of 113V). The maximum output power at 1% THD was 84W (19.2dBW) into 8 ohms, 140W (18.5dBW) into 4 ohms"
Read more at https://www.stereophile.com/content/jeff-rowland-design-group-model-1-power-amplifier-measurements#D...

The spec is twice the power into 4/8 ohms is just as you like 120/60 watts. The measured is 140/84 watts, not double. Note as I predicted the 8 ohm power is understated as it must be. But 120/60 watts are just minimum specs, not what the amp CAN do. Good manufacturers always underspec to make sure they make spec. 

Here is a Krell speced very close to 2/1 power into 4/8.  However the actual ratio is 910/625 watts. hardly 2/1. 


 Although the Krell's specified maximum power into these loads is very high, at a respective 575W (27.6dBW) and 900W (26.5dBW), the amplifier exceeded those powers, clipping at 625W into 8 ohms (28dBW) and 910W into 4 ohms
Read more at https://www.stereophile.com/content/krell-solo-575-monoblock-power-amplifier-measurements#17OE2KQdwm...

Cant you see that the doubling down is just a spec and to make that spec they have to go way over on the 8 ohm to get the 4 ohm to work out. The Krell barely makes spec with only 10 watts to spare on 4 ohms but exceeds 8 ohm power by 50 watts. 

I have a friend that loves to see amps double down but none actually do. Its nice when they get close but its just impossible to be exact or just close. 

Where does this religion of doubling down get its followers?

@douglas_schroeder    When I refer to "level unlimited" I mean that the amp is at maximum output of the preamplifier turned full on and the speakers not hitting past perhaps 87-88 dB. I would call an amp "level unlimited" where in my use I would not turn the level up more, but the amp could certainly provide a higher listening level. It may not be a technically correct description, but it serves to indicate the situation. I do not attempt to listen to systems at "live", i.e. above 90 dB levels, and I do not recommend others to do so

It appears you may be confusing gain with power as many do.

I mean that the amp is at maximum output of the preamplifier turned full on and the speakers not hitting past perhaps 87-88 dB.

This is the statement I do not understand. The amp is doing what?

The top setting on a preamp does not conform to any percentage of the power amplifier's potential output. Some systems max out on power at 9 on the clock. some at 12 some not even at full up, which is a good situation where the 9 on the clock is very bad, though I hear some like it as they think there headroom above that setting, where there is not. 

90 db is a nice listening level, however do you sometimes find a CD might not get to 90 at full preamp level? And of course some will be too loud.

@georgehifi 

I think Benchmark has done an excellent job of convincing everyone to buy two. Hail to marketing. All those who follow can do is quote the party line. We will never get them to understand. 

@kijanki I think, we are talking about two different things. I have never mentioned actual amplifier’s power at clipping but rather manufacturer’s specifications (that are quite different), that calls for power doubling (or close to it). I have also noticed personal comments from you, placing my math ability in question and calling me to learn basic electronics. I believe that I have some comprehension of basic electronics designing it for 40 years and therefore I view your comments as harmful, therefore refusing to comment on anything you have to say from this point on. Perhaps one day, when you change your attitude we can start talking again

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If we are talking about two different things what are they? Im just talking about simple RMS power. I try to make my comments as non personal as possible. I will go back and have a look at them and apologize appropriately. Im not here to pick a fight. Lets just get the information right. :)

I still dont see why a manfacturer calling for power doubing on a piece of paper has anything do to with what the amp does. What spec makes it different than clippiing. Most power amps have very low distortion till they clip.

Declining to answer simple questions or support statements does us no good in a forum. I am here trying to correct things. If I am incorrect then bring me the proof. This is really simple stuff.

Im sure you math skills are fine. This is about a concept. Starting a post with "There is no such thing as RMS power." Is rather bold. We measure it all the time. If one wants to be contrary to accepted terms he had better have some good material to back it up. This I have not seen.

I was fine having a discussion until you quoted a very poor Wickipedia article and.. did you have to add to AL ???

Al, leave it (he is not going to get it). .

I politely objected to that. Not going to get it? I also offered to apologize if I was incorrect.

Why not build a few amplifiers to see if your theories are correct and dont forget the emitter resistors..  :)