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Quality Power - Page 3

post #61 of 637
Quote:
Originally Posted by goneten View Post

Well, if you're using bass management then my post wouldn't really apply since I was discussing full range use. I think we're all in agreement that if you used a higher crossover frequency it would somewhat alleviate your power requirements in the low bass. If Audyssey set the trim levels to a negative value then that probably means the speakers being used are quite sensitive.

Well, here is the rub and additional information--The Klipsch RF-7s are rated at 101 dB/1 meter and are "8 Ohm Compatible" (as per Klipsch website) but reportedly dip down to 2.8 Ohms a couple of times above the crossover point used in Bass Mgmt...Does this change yours or anyone else's opinion even though Audyssey set the trims that low?

I was also curious if on an AVR that isn't rated to drive 4 Ohm speakers and Audyssey also sets trim levels in the negative territory on could that still be justification for not needing an external amp listening at or near Reference Level?
post #62 of 637
Quote:
Originally Posted by Wayne Highwood View Post

It would. But some consumers look at the sensitivity rating alone and think they can get away with the lower end AVR.

I still think a speaker that requires robust amplification is a more questionable than criticizing AVR's for not handling the rare oddball speaker.

Questionable or not it is the speakers I own.... I also wasn't criticizing any AVR but asking a question about power requirement to drive those speakers efficiently.

I also totally agree about what you say about what "most consumers" do, but I am finding more and more folks are getting their information on line and purchasing speakers and amps while relying on majority opinion instead of actually auditioning.
post #63 of 637
Quote:
Originally Posted by Zen Traveler 
Well, here is the rub and additional information--The Klipsch RF-7s are rated at 101 dB/1 meter and are "8 Ohm Compatible" (as per Klipsch website) but reportedly dip down to 2.8 Ohms a couple of times above the crossover point used in Bass Mgmt...Does this change yours or anyone else's opinion even though Audyssey set the trims that low?

It would be interesting to see how much voltage is actually required for the RF-7's to achieve 105 dB at a 3-4 meter distance assuming full range use. I think if you assume that a robust amplifier is needed even with such sensitive speakers then perhaps you need to question the assumption. One way to test the assumption would be to measure to see if a lesser amplifier was clipping at a particular SPL. Did you measure to see if a lesser amp was clipping at reference?

Most people's musical tastes don't include continuous sine waves below 4 ohms, but musical signals. Musical signals have a crest factor. So right off the bat things are not nearly as difficult as they may appear on paper, on a test bench. The other thing is that sometimes when people compare amplifiers they ignore the gain levels between amplifiers, so a dedicated power amplifier may sound stronger in the bass section, have more authority etc due to a stronger gain.
post #64 of 637
Quote:
Originally Posted by goneten View Post

It would be interesting to see how much voltage is actually required for the RF-7's to achieve 105 dB at a 3-4 meter distance assuming full range use. I think if you assume that a robust amplifier is needed even with such sensitive speakers then perhaps you need to question the assumption. Did you measure to see if a lesser amp was clipping at reference?

.

Here is my post on the Audyssey Thread and I am arguing against most people needing an external amp with these speakers, but do wonder if a power supply being rated to drive 4 Ohm speakers is what is important to consider:
Quote:
Originally Posted by Zen Traveler View Post

Thanks SoM. I have a 9.2 Klipsch Home Theater that on paper are between 97 dB to 101 dB and are "8 Ohm compatible" but my RF-7 towers have impedance dips that go as low as 2.8 Ohms and I suspect the others aren't as efficient as their Sensitivity numbers claim. That said, they sounded great at loud volume with the Denon THX Ultra II AVRs 4802R and 4806 but then when I had to run this system with the AVR-3805 noticed that I couldn't play it as loud (about 6 dB lower). I realize that the upgraded Audyssey helps, but was curious what others thought about the power requirments.

I now have the AVR-4311ci which I picked up recently and it sounds great at -6 to 0 (depending on what I'm listening to). The reason I asked my question is that Audyssey sets my trim levels between -6 to -8 for all of my speakers and I see the same for others using Klipsch speakers and then they seem to feel they also need a separate amp to get to Reference levels and I actually thought this was an argument against that reasoning.
post #65 of 637
Quote:
Originally Posted by Zen Traveler 
That said, they sounded great at loud volume with the Denon THX Ultra II AVRs 4802R and 4806 but then when I had to run this system with the AVR-3805 noticed that I couldn't play it as loud (about 6 dB lower). I realize that the upgraded Audyssey helps, but was curious what others thought about the power requirments.

It's very unlikely that the 3805 would not have sufficient power - the difference in power between those receivers is like splitting into the wind. A 6 dB improvement would mean the 4806 had quadruple the power of the 3805. Not happening. Perhaps the effect you describe is your imagination running wild? Not saying you're necessarily wrong, but perhaps there are other factors at play besides amplifier power that may be responsible for the performance you describe using the lower spec receiver.
post #66 of 637
Quote:
Originally Posted by goneten View Post

It's very unlikely that the 3805 would not have sufficient power - the difference in power between those receivers is like splitting into the wind. A 6 dB improvement would mean the 4806 had quadruple the power of the 3805. Not happening. Perhaps the effect you describe is your imagination running wild? Not saying you're necessarily wrong, but perhaps there are other factors at play besides amplifier power that may be responsible for the performance you describe using the lower spec receiver.

The Denon AVR 3805 and the 4802R are comparable because they don't have Audyssey, but the former is not rated to drive speakers in a multichannel configuration that have a 4 Ohm load and the latter is certified down to 3.2 Ohms...

Insofar as my imagination, I will admit to having an active one but my comparisons were done using an SPL meter and listening to familiar material over a long period of time is where I got that 6 dB number....That said, the AVR-4806 is also THX ultra II rated and has a more primitive form of Audyssey than my AVR 4311ci which I now own and I listen to material closer to "0" and that may very well be attributed to the better EQ program and not power requirements. For the record and using the SPL meter, I listened to material on the AVR-4802R, 4806 and 4311ci louder without it sounding bright and on the AVR-3805 was when I couldn't raise the volume to the same level without it doing so.

You didn't mention anything about impedance factor or Frequency Response Curves in your post above and wonder if that is the point you may be missing in my question about Power needs and Current supplied/available at louder volume.
post #67 of 637
Quote:
Originally Posted by Zen Traveler 
You didn't mention anything about impedance factor or Frequency Response Curves in your post above and wonder if that is the point you may be missing in my question about Power needs and Current supplied at louder volume.

The frequency response curve of all three amplifiers would be ruler flat. The impedance is certainly a factor but I am not convinced that it was responsible for the brightness you experienced. There are many variables that could account for added brightness that may have nothing to do with amplifier power and since we know nothing about your listening test and how it was conducted it's difficult to really comment.

Perhaps you should ask Arnyk what he thinks.
post #68 of 637
Quote:
Originally Posted by Zen Traveler View Post

Quote:
Originally Posted by goneten View Post


The Denon AVR 3805 and the 4802R are comparable because they don't have Audyssey, but the former is not rated to drive speakers in a multichannel configuration that have a 4 Ohm load and the latter is certified down to 3.2 Ohms...

Based on what?

I find the following in the user's manual (the usual most complete source}

3805 Audio section
• Power amplifier
Rated output: Front: 120 W + 120 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
160 W + 160 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Center: 120 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
160 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Surround: 120 W + 120 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
160 W + 160 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Surround Back: 120 W + 120 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
160 W + 160 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Dynamic power:
140 W x 2 ch (8 Ω/ohms)
210 W x 2 ch (4 Ω/ohms)
240 W x 2 ch (2 Ω/ohms)

4802R •Audio Section
Power amplifier
Rated output: Front: 125 W + 125 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
150 W + 150 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Center: 125 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
150 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Surround: 125 W + 125 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
150 W + 150 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Surround Back: 125 W + 125 W (8 Ω/ohms, 20 Hz ~ 20 kHz with 0.05% T.H.D.)
150 W + 150 W (6 Ω/ohms, 1 kHz with 0.7% T.H.D.)
Dynamic power:
170 W x 2 ch (8 Ω/ohms)
270 W x 2 ch (4 Ω/ohms)
350 W x 2 ch (2 Ω/ohms)
post #69 of 637
Quote:
Originally Posted by goneten View Post

The frequency response curve of all three amplifiers would be ruler flat. The impedance is certainly a factor but I am not convinced that it was responsible for the brightness you experienced. There are many variables that could account for added brightness that may have nothing to do with amplifier power and since we know nothing about your listening test and how it was conducted it's difficult to really comment.

Perhaps you should ask Arnyk what he thinks.

I agree, but seem to be in the minority in thinking that adding an external amp is as beneficial as others seem to claim with the same set of speakers...That said, I totally understand there could be more variables but I listen to the same multichannel music disks over the years MANY times on the various AVRs and have done so nightly for almost 12 years...I will check back and see what others think and appreciated your response.
post #70 of 637
Quote:
Originally Posted by arnyk View Post

Based on what?

The THX Ultra II certification on the upper-end units that requires they be able to drive multiple speakers (7 in this case) down to 3.2 Ohms and I was under the impression that others don't necessarily fit that requirement and in fact thought lower end AVRs didn't. Again, I am not saying that this is the case but trying to find out why others seem to claim so much improvement using external amplification.
post #71 of 637
Quote:
Originally Posted by arnyk View Post

...I find the following in the user's manual (the usual most complete source}

I just looked at the manual for the AVR-3805 and on page 15 it states "Speakers with an impedance of 6 to 16 Ohms can be connected to the Front Speaker terminals." It also states the same for surround but adds "Be careful when using both A+B at the same time because using speakers with an impedance less than 8 Ohms will lead to damage."
post #72 of 637
Quote:
Originally Posted by Zen Traveler" 
The THX Ultra II certification on the upper-end units that requires they be able to drive multiple speakers (7 in this case) down to 3.2 Ohms and I was under the impression that others don't necessarily fit that requirement and in fact thought lower end AVRs didn't. Again, I am not saying that this is the case but trying to find out why others seem to claim so much improvement using external amplification.

Not Arny, but the reason why some wax lyrical over external amps is due to the method of comparison. Two different amplifiers will usually have different gain structures. One may sound quite a bit louder than the other due to the change in gain level - if you readjust for this, the audible improvements tend to vanish into the ether.

The other thing is that people are generally self-affirming, so of course they would expect an improvement using a new piece of gear. Of course, there is a possibility that the additional power from an external amp did, in fact, have an audible impact on the sound assuming the additional power was required.

If your speakers have an impedance minimum of 3.2 ohms at some frequency then you'll find that the impedance will be higher at other frequencies and I'm guessing, having never used the Klipsch speakers, that the vast majority of the frequency range would be well above 3.2 ohms. Musical signals are fluctuating all the time, so the amp isn't going to "see" 3.2 ohms constantly. The actual impedance is going to be all over the map.

If you have ever had time to browse the Emotiva forums you'll find plenty of threads where people bought a new XPA-2 or XPA-5 only to learn that it made no actual improvement to sound quality. What they later realised is that their existing amplifiers were more than sufficient for the task at hand but made the assumption based off memes, whether it be from magazines, or from the general public, that more robust amplifiers improve sound by default.

I don't want to get into a perception/imagination argument, but the method of comparison is key - if you do not account for all the variables that may affect perception in a comparison, then you can't really be certain of what you experienced.
post #73 of 637
Quote:
Originally Posted by goneten View Post

Not Arny, but the reason why some wax lyrical over external amps is due to the method of comparison. Two different amplifiers will usually have different gain structures. One may sound quite a bit louder than the other due to the change in gain level - if you readjust for this, the audible improvements tend to vanish into the ether...


I don't want to get into a perception/imagination argument, but the method of comparison is key - if you do not account for all the variables that may affect perception in a comparison, then you can't really be certain of what you experienced.

I agree completely with the above, but what I experienced with my SPL meter in hand is that I could not get the AVR-3805 to run my system as loud as I wanted and could with the other units...I lived with it for almost a year. That said, what I posted above about what the AVR-3805s Owner's manual states seems pertinent to this discussion and my observations.
post #74 of 637
Thread Starter 
Quote:
Originally Posted by arnyk 
There are various stages of clipping, depending on how much excess signal is involved.

I spoke to someone else on this topic and he told me that amplifiers employing anti-clipping circuitry could be limiting dynamics of the signal, in which case if the amplifier is not clipping it's not a guarantee that you don't have enough power. If the amp is compressing the sound in a non-offensive way, I might not even be aware of it. It does sound interesting.
post #75 of 637
Quote:
Originally Posted by Zen Traveler View Post

Quote:
Originally Posted by arnyk View Post

Based on what?

The THX Ultra II certification on the upper-end units that requires they be able to drive multiple speakers (7 in this case) down to 3.2 Ohms and I was under the impression that others don't necessarily fit that requirement and in fact thought lower end AVRs didn't. Again, I am not saying that this is the case but trying to find out why others seem to claim so much improvement using external amplification.
Quote:
Originally Posted by Zen Traveler View Post

Quote:
Originally Posted by arnyk View Post

Based on what?

The THX Ultra II certification on the upper-end units that requires they be able to drive multiple speakers (7 in this case) down to 3.2 Ohms and I was under the impression that others don't necessarily fit that requirement and in fact thought lower end AVRs didn't. Again, I am not saying that this is the case but trying to find out why others seem to claim so much improvement using external amplification.

I did some scouting around and found some Denon AVRs that had the THX Ultra II certification and lacked the 4 ohm power rating. I don't think that the two are necessarily linked.

The claims of night-and-day improvement in sound quality with external amplification have so far been based on sighted evaluations, so personal bias may have a lot to do with these proponent's perceptions.
post #76 of 637
Quote:
Originally Posted by OllieS View Post

Quote:
Originally Posted by arnyk 
There are various stages of clipping, depending on how much excess signal is involved.

I spoke to someone else on this topic and he told me that amplifiers employing anti-clipping circuitry could be limiting dynamics of the signal, in which case if the amplifier is not clipping it's not a guarantee that you don't have enough power. If the amp is compressing the sound in a non-offensive way, I might not even be aware of it. It does sound interesting.

A tiny minority of all audio gear have these features. If it is relevant to a particular comparison, then the issue is fair game. For most audio gear, IME it isn't.
post #77 of 637
Quote:
Originally Posted by arnyk View Post


I did some scouting around and found some Denon AVRs that had the THX Ultra II certification and lacked the 4 ohm power rating. I don't think that the two are necessarily linked.

The claims of night-and-day improvement in sound quality with external amplification have so far been based on sighted evaluations, so personal bias may have a lot to do with these proponent's perceptions.

Thanks for checking, but to get the THX Ultra II certification it states that all channels must be able to go down to 3.2 Ohms with all channels driven...I looked at the online manual for the AVR-4802R and it states on page 12 that "the protector circuit may be activated if the set is played at long period of time when speakers with an impedance lower than the impedance specified (for example speakers with a lower impedance of 4 Ohms) are connected..." I also know that the AVR-4806 is able to drive 4 Ohm loads as well so I guess at the very least this hasn't disproved my assumption that the impedance dips in the RF-7s are a factor in needing a power supply that can hang with them.

I also agree with what you say about "night and day improvements," using an external amp especially when paired with an AVR that does provide enough current to drive more difficult speakers.
Edited by Zen Traveler - 8/17/13 at 3:57pm
post #78 of 637
Understand that amplifier impedance ratings are based on full rated power. Understand also that it means that time is required to overheat the power stage of the amp. Understand also that low impedance dips in the curve occur only at some frequencies. In a nutshell, we don't encounter those dips long enough to hurt anything and we don't operate our amps at anywhere near full rated power at all, let alone for long enough to overheat the amp. Your receiver is overkill for driving your speakers unless you are putting them in an auditorium sized room. If you want a nice, shiny new amp, then by all means buy one. But don't buy it for the wrong reasons. Thinking you will get better sound is not a good reason.
post #79 of 637
Quote:
Originally Posted by FMW View Post

Understand that amplifier impedance ratings are based on full rated power. ...

...Your receiver is overkill for driving your speakers unless you are putting them in an auditorium sized room. If you want a nice, shiny new amp, then by all means buy one. But don't buy it for the wrong reasons. Thinking you will get better sound is not a good reason.

I don't want or need "a shiny new amp" and think the AVR-4311ci drives my Home Theater nicely...I can't comment if it's overkill or not and reported what I and others were experiencing with lower end AVRs driving their system and postulated a theory on why it sounded extremely, unlistenably, bright at the higher SPL levels I was accustomed.

Fwiw, the actual dips in the RF-7s go down to 2.8 Ohms a couple of times and in my 9.2 Home theater have 5 other large speakers (and 2 smaller ones) that I assume have similar Response Curves, but may not dip as low. I primarily listen to Multichannel music and that is more demanding than movies...That said, do people with 4 Ohm speakers and less than 90 dB Sensitivity benefit from an AVR that is rated for 4 Ohm loads or does it really not matter? I don't mean this facetiously and am asking to get a better understanding...

EDIT: This is what I experienced at the louder levels on the AVR-3805--At lower levels it was fine:
Quote:
Originally Posted by arnyk View Post

Small amounts of clipping can make some music pick up a little brightness.

Medium amounts of clipping can make most music sound harsh, sour or a little muddy.

.

Edited by Zen Traveler - 8/18/13 at 8:31am
post #80 of 637
Quote:
Originally Posted by Zen Traveler View Post

That said, do people with 4 Ohm speakers and less than 90 dB Sensitivity benefit from an AVR that is rated for 4 Ohm loads or does it really not matter?

It's case specific. In the majority of cases, it doesn't matter.

For those cases involving huge rooms and high listening levels, then yes, it matters. In the example I mentioned, the room was big enough for a basketball court, just freakin' huge. Your gear is even more powerful than what that guy used in that room, and he was happy as a clam. At least for a while. I think he has Khorns now. Unless your room is even bigger than his, you should have more than ample power.
post #81 of 637
Quote:
Originally Posted by Wayne Highwood View Post

It's case specific. In the majority of cases, it doesn't matter.

For those cases involving huge rooms and high listening levels, then yes, it matters. In the example I mentioned, the room was big enough for a basketball court, just freakin' huge. Your gear is even more powerful than what that guy used in that room, and he was happy as a clam. At least for a while. I think he has Khorns now. Unless your room is even bigger than his, you should have more than ample power.

I agree that I have more than ample power given my results with the AVR-4311ci...That said, this description below makes me think that the reason I didn't have the same results with the AVR-3805 at louder volume is because of what you said when y'all weren't happy about the sound:
Quote:
Originally Posted by Wayne Highwood View Post

Be careful with this one. ..., I've personally heard the Klipsch RF7II make decent AVRs hiccup where an equivalently rated power amp sailed right through. Sound was distinctively strident with anemic bass when using the AVR, and distinctly effortless and clean with the amp, reflecting exactly the sort of thing described in Arny's post upthread. Those particular speakers are on the more sensitive side, but have low impedance dips ....the result is they present more of a load than some amps are designed to deliver. They really do suck lesser amps dry, which is kind of a glaring design flaw if you think about it. I suppose their market is composed of folks who would use more capable amps anyway.

My question is possibly in a larger room a person would benefit from a power amp, but in smaller rooms and even using Bass Management doesn't the RF-7s. given they dip down to 2.8 Ohms above the crossover point (100hz and slightly over 1,000 Hz if I am remembering correctly), still "suck lesser amps dry" at/near Reference Level? The real question I am trying to find an answer to (and if it pertains to my Home Theater) is do the impedance ratings on an AVR matter?
post #82 of 637
Quote:
Originally Posted by Zen Traveler View Post


I agree that I have more than ample power given my results with the AVR-4311ci...That said, this description below makes me think that the reason I didn't have the same results with the AVR-3805 at louder volume is because of what you said when y'all weren't happy about the sound:


My question is possibly in a larger room a person would benefit from a power amp, but in smaller rooms and even using Bass Management doesn't the RF-7s. given they dip down to 2.8 Ohms above the crossover point (100hz and slightly over 1,000 Hz if I am remembering correctly), still "suck lesser amps dry" at/near Reference Level? The real question I am trying to find an answer to (and if it pertains to my Home Theater) is do the impedance ratings on an AVR matter?

The load they present is still more taxing than some speakers, but it doesn't matter so long as reaching the desired level is within the AVR's capacity. The smaller room and using bass management will reduce the power required considerably. Also consider FMW's pertinent point from just a few posts up, that your music and soundtracks won't be sending constant, high level signals to those potentially power hungry spots.
post #83 of 637
Thanks for responding, Wayne and I am still under the assumption that the reason my speakers sounded bright at louder volume on the AVR-3805 is because I was taxing it's power supplies, that I am not/was not on the other AVRs at the SPL I want to listen.
post #84 of 637
I want to ask the more knowledgeable folks, could the load of the RF7II with all it's dips and phase angles cause an amplifier to clip below it's rated output?

There is a spot about 110hz that looks like a 5 ohm load topped with a -45 deg phase angle. That's potentially ugly in the heat generating, current sucking manner. That's also well above where they would be high passed, as Zen pointed out. If, and this is a big and unrealistic "if", but if content at that frequency is sustained enough to tax the amplifier, wouldn't it mess up across the whole spectrum on any simultaneous content, at power levels lower (considerably lower?) than the amp's rated power?
Edited by Wayne Highwood - 8/18/13 at 6:14pm
post #85 of 637
Quote:
Originally Posted by arnyk View Post

IME Class D is still an evolving technology with a tendency towards having more distortion than conventional Class AB amplifiers.

The two generic problems appear to be nonlinear distortion due to output device speed issues (slide 12 below) and high source impedance issues due to the required output low pass filter (slide 53, below):

http://www.irf.com/product-info/audio/classdtutorial2.pdf

While the above article dates back to 2009 and can be questioned because of its age in this rapidly-moving technology, it appears to be some of the most recent information that I can find online. News of any updates would be appreciated.

BTW I think that sonically transparent Class D amplifiers are possible, particularly in the context of modern AVR automated system optimization facilities such as Audyssey, MCACC and YPAO. The nonlinear distortion described above appears to be low enough to not be an audible problem. It's just that it may be higher than we see in common AB amplifiers.


That's a very comprehensive pdf. Thanks, lots of info there.

Quite a lot of people look to NuForce for leading the way in "audiophile grade" class D amps as they started doing them some time ago when everybody else was just laughing at them.

I only have a few bits of blurb from the manufacturer embedded in some reviews but maybe you could decipher any technical merit in what they are doing in regards to other class D amps...

From http://www.6moons.com/audioreviews/nuforce15/3.html

"The technology difference between NuForce and other class D amps can be summarized as having a higher switching frequency, featuring instantaneous feedback and error correction and very low phase shift due to our unique self-oscillation design which does not utilize a pulse-width modulation clock. NuForce owns its patented technology and controls the design and production of every generation of products where many other class D amplifiers utilize off-the-shelf modules from ICEpower, TI, Hypex and others. This allows us to make improvements at any level and stage of the design."

And from http://www.6moons.com/audioreviews/nuforce8/duo.html

"'Normal' switching amps run a fixed high-frequency saw-tooth carrier wave to modulate the audio signal such as to produce a square wave of the same frequency as the saw tooth wave but with squares of various widths (class D) which encode the frequency and amplitude data of the music to switch the output transistors accordingly. The fact that the transistors are switched fully on or off eliminates the permanent quiescent current throughout to generate class D's high operational efficiency - about 85% in NuForce's case. Conventional class A ovens throw this away as heat. Of course with class D, the HF garbage of the carrier wave has to be subtracted from the audio signal at the outputs via a filter. NuForce posits that's where compromises enter.
They claim class-leading performance in switching matters in four discrete ways. There's higher bandwidth without phase errors. Both are compromised by common class D solutions due to the interactions of the output filter's LC network and the speaker's fluctuating impedance. NuForce claims to have implemented a negative feedback loop which taps directly off the speaker terminals (hence post filter) and a new form of modulation (not fixed sawtooth wave but self oscillation). The core argument is that the feedback loop includes the (error-producing) output filter.
A common issue with class D designs is that the phase-inverted feedback signal can generate amplifier instabilities since the output filter creates its own phase errors which the NFB loop doesn't account for. This can create full 360-degree phase shifts to invert feedback phase back to signal phase to cause oscillation. The patented NuForce solution is claimed to disable such behavior while linearizing the behavior of the output filter. Then there's a < 10 milliohm output impedance for a high damping factor regardless of frequency, the latter claimed to enable very low distortion specs mostly irrespective of output voltage (0.05% THD+N during power peaks). While on power peaks, this small box claims to make short-term 288, 576 and 1125 watts into 8, 4 and 2 ohms respectively. For RMS, there's a permanent 160, 200 and 200 watts. The 'ideal' power doubling into halved impedances thus exists only momentarily."
post #86 of 637
Quote:
Originally Posted by kiwi2 View Post


From http://www.6moons.com/audioreviews/nuforce15/3.html

"The technology difference between NuForce and other class D amps can be summarized as having a higher switching frequency, featuring instantaneous feedback and error correction and very low phase shift due to our unique self-oscillation design which does not utilize a pulse-width modulation clock. NuForce owns its patented technology and controls the design and production of every generation of products where many other class D amplifiers utilize off-the-shelf modules from ICEpower, TI, Hypex and others. This allows us to make improvements at any level and stage of the design."

Seems fair enough. They have their own technology which may give them a technological advantage.

I found this review of one of their products:
http://www.stereophile.com/content/nuforce-icon-usb-input-integrated-amplifier-measurements

That includes this:



and this:





The frequency response curve shows very good resistance to response changes in the audible range due to different speaker loads.

The nonlinear distortion performance seems relatively poor compared to modern Class AB and even other Class D products. It might even be audible under some conditions.
Quote:
And from http://www.6moons.com/audioreviews/nuforce8/duo.html

"'Normal' switching amps run a fixed high-frequency saw-tooth carrier wave to modulate the audio signal such as to produce a square wave of the same frequency as the saw tooth wave but with squares of various widths (class D) which encode the frequency and amplitude data of the music to switch the output transistors accordingly. The fact that the transistors are switched fully on or off eliminates the permanent quiescent current throughout to generate class D's high operational efficiency - about 85% in NuForce's case. Conventional class A ovens throw this away as heat. Of course with class D, the HF garbage of the carrier wave has to be subtracted from the audio signal at the outputs via a filter. NuForce posits that's where compromises enter.

I agree that in this area - the handling of removing the carrier wave, they do an exceptionally good job.
Quote:
They claim class-leading performance in switching matters in four discrete ways. There's higher bandwidth without phase errors.

Phase errors are probably not an area of audible benefits in this case.
Quote:
Both are compromised by common class D solutions due to the interactions of the output filter's LC network and the speaker's fluctuating impedance. NuForce claims to have implemented a negative feedback loop which taps directly off the speaker terminals (hence post filter) and a new form of modulation (not fixed sawtooth wave but self oscillation). The core argument is that the feedback loop includes the (error-producing) output filter.

Repeating the claims about their improved output filtering which appear to improve actual performance.

A common issue with class D designs is that the phase-inverted feedback signal can generate amplifier instabilities since the output filter creates its own phase errors which the NFB loop doesn't account for. This can create full 360-degree phase shifts to invert feedback phase back to signal phase to cause oscillation. The patented NuForce solution is claimed to disable such behavior while linearizing the behavior of the output filter. Then there's a < 10 milliohm output impedance for a high damping factor regardless of frequency, the latter claimed to enable very low distortion specs mostly irrespective of output voltage (0.05% THD+N during power peaks). While on power peaks, this small box claims to make short-term 288, 576 and 1125 watts into 8, 4 and 2 ohms respectively. For RMS, there's a permanent 160, 200 and 200 watts. The 'ideal' power doubling into halved impedances thus exists only momentarily."[/quote]

The claims about < 0.05% THD appear to disagree with published tests such as those above that show > 0.10% THD.
post #87 of 637
Quote:
Originally Posted by Wayne Highwood View Post

I want to ask the more knowledgeable folks, could the load of the RF7II with all it's dips and phase angles cause an amplifier to clip below it's rated output?

There is a spot about 110hz that looks like a 5 ohm load topped with a -45 deg phase angle. That's potentially ugly in the heat generating, current sucking manner. That's also well above where they would be high passed, as Zen pointed out. If, and this is a big and unrealistic "if", but if content at that frequency is sustained enough to tax the amplifier, wouldn't it mess up across the whole spectrum on any simultaneous content, at power levels lower (considerably lower?) than the amp's rated power?

Thanks, Wayne--I am curious as well. smile.gif
post #88 of 637
Zen, have you seen this article? I had thought they used the RF7II as an example, and published the actual voltages and currents required...they've since changed it to a hypothetical THX speaker, going with THX limits for their calculations. (I wonder if Klipsch requested they take reference to their product out of the piece? More likely my faulty memory making stuff up.) Anyway, I don't think it changes the money shot, as it was the same in the original version as I remember it, which is this:

Surprisingly, in spite of the rather significant advantage in voltage sensitivity, the 96dB sensitive loudspeaker demands more current to reach a given SPL than the 90dB sensitive loudspeaker. Of course, as mentioned previously, which one is a better fit for your amplifier ultimately depends upon its capabilities...

Edited by Wayne Highwood - 8/19/13 at 8:20am
post #89 of 637
Quote:
Originally Posted by arnyk View Post

The claims about < 0.05% THD appear to disagree with published tests such as those above that show > 0.10% THD.

Thanks. Looking around at other class D amp reviews show some mediocre measurement results. It will be interesting to see measurement results from NuForce's new STA-100 amp once more reviewers start getting their hands on them.

One thing I had noticed from reading other class D amp reviews was criticism of poor S/N Ratio with no source input.

I have noticed with my STA-100 amp that when it is on but the AVR pre is off there is a small amount of audible hiss coming from the speakers. But the moment I turn the AVR pre on, the hiss disappears to almost non-existent.
post #90 of 637
Quote:
Originally Posted by Wayne Highwood View Post

Zen, have you seen this article? I had thought they used the RF7II as an example, and published the actual voltages and currents required...they've since changed it to a hypothetical THX speaker, going with THX limits for their calculations. (I wonder if Klipsch requested they take reference to their product out of the piece? More likely my faulty memory making stuff up.) Anyway, I don't think it changes the money shot, as it was the same in the original version as I remember it, which is this:
Surprisingly, in spite of the rather significant advantage in voltage sensitivity, the 96dB sensitive loudspeaker demands more current to reach a given SPL than the 90dB sensitive loudspeaker. Of course, as mentioned previously, which one is a better fit for your amplifier ultimately depends upon its capabilities...

Klipsch has been IMO playing games with speaker efficiency claims for a long time. I seem to recall a speaker of theirs that dipped down to 2 ohms. Sounds dire, but it may or may not be as dire as it looks.

From a consumer viewpoint, efficiency is defined by how loud the speaker can get while playing music and/or dialog before his amplifier generates audible distortion. The amplifier may not even be the major source of audible distortion in his system because speakers generate copious amounts of distortion. Amplifier clipping with music is not just one number, but potentially a family of numbers that varies with program material and amplifier. Remarkably little intelligence about this issue is revealed by steady pure sine wave bench tests involving resistive loads.
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