THE ALL CHANNELS DRIVEN FALLACY

[PENG]

And the real burning question that started this whole soap box rant for me is this...... the two channel amp has a meeter on the front admittedly not laboratory accurate but at 1 watt it is louder than my 100 Sony that even the reviews say 80 to both channels in stereo mode. WHY??????
Sorry to vent, but I don't think I am the only one that wants a real answer
Joe

Placebo effect (a high power amp must be better than a Sony) may have something to do with that, or the meters were telling truth. Your 100W Sony should have no trouble dealing with a 1W requirement, with plenty of headroom, at least in terms of how loud it is, sound quality may be a different story.

 

[PENG]

please correct this "theory" if right or wrong ...

Given:
Yamaha has 130wpc (2600) 130 for two
HK has about 75-90wpc (HK635) 75 for ALL, 90 for two.

Conclusion:
Yamaha is capable of higher peaks than HK.

yes? no?

If tested to be true, most likely yes. H.T. mag tested 630 and 3805 the same way and the 3805 has much better 1,2 and even 5 channel driven power, especially in 4 ohms. Buck may not believe those results because the 3805 weighs a few pounds less. Mind you the newer HK models are a few pounds lighter with the same specified output, except for the 745.

 

[skrivis]

please correct this "theory" if right or wrong ...

Given:
Yamaha has 130wpc (2600) 130 for two
HK has about 75-90wpc (HK635) 75 for ALL, 90 for two.

Conclusion:
Yamaha is capable of higher peaks than HK.

yes? no?

It depends.

If the wpc figures you give are the peak figures, then the Yamaha can provide more peak power.

If the wpc figures are closer to the normal rated FTC wpc figures like are given for 2 ch amps, then we can't tell what the peak ratings are from the info given.

You could very well have a "130 wpc" amp that has a tightly regulated power supply, so that 130 w is the absolute maximum it can output.

Then you could have a "90 wpc" amp that will actually output peaks of 180 w because there is more dynamic headroom.

The way I see it, since full power with all channels driven is not possible in the real world, we have to come up with some sort of typical load and show how well the amps perform under those conditions.

With a typical AC mains feed, a typical listening room, typical program content, and typical SPL at the listening position from typical speakers... performance is thus and so for each channel. So you'd be working back from typical conditions to figure out how much power would be used, and then describing how well a given amp delivers that much power.

It isn't going to be easy, and there isn't any one number that will describe the performance. But this is true of everything in the universe.

 

[hqqns]

Ok I think it might be an idea to put some numbers to the hypothesis of all channels driven at the same time. That is if my drivers have have low sensitifity, say 88 db how loud will it be if driving all of them (7,6,5,or 2) lets say 16W RMS etc

WRMS
1 +0dB
2 +3dB
4 +6db
8 +9dB
16 +12dB
32 +15dB
64 +18db

Speakers #
1 +0dB
2 +3dB
4 +6dB
5 +7dB
7 +8.5dB

Then for our example of 16W into 7 channels with each speaker having a sensitivity of 88 dB (1W@1m or 2.83V@1m) we get

88 + 12 + 8.5 = 108dB

ouch!

now if the amps could deliver on thier promises of 130W RMS then we get

88 + 21 + 8.5 = 117dB

Hearing loss anyone?

Of course this is ingnoring room gain because we are talking not 4 pi space, so add a few more more db on top of that unless you have a HUGE room.

MAke up your own mind!

 

[MACCA350]

Remember those sensitivity figures are taken at 1 meter from the speaker. So at 4 meters away your down, at a guess, maybe 6db to start with, but that may equal out the room gain.

cheers

 

[hqqns]

Too true,
These figures i felt needed to be put up to get some perspective on the discussion. I agree that manufacturers should not be ambiguous about their figures, that is, they should define exactly what they are measuring. But I can understand also why they sometimes do this. It happens in all industries, the engineers/scientists are not the marketing people and vice versa. These figure show that thes amps are quite capable of delivering plenty of umpff once we introduce the fact that music/movie soundtracks are are not as demanding as a continuous sine wave (RMS)

If we think about it, there are significant passages , especially in movies, of low power dialog/low volume passages, followed by loud passages. Here the power capacitors come into their own and become significantly more important and the power figure at the back of the amp.

Which leads me to one of my pet hates and that is using power figures and not the more import figure of the voltage rail which supplies the power transitor/plastic fantastic. So what am i saying here?

I'm trying to say that if the amp had a power consumption of 500W and if it were a stereo power amp, you would say, that that's not bad! But it seems that people want to multiply this figure for every channel as more are added. To me this seems a little silly considering that if it enough to blow your ears out with two speakers, the same amount should be enoguh to blow your ears out with more drivers (loudspeakers)

This brings me back to the voltage rail which supplies all of the channels with voltage that are tapped to drive the speakers. This is were the figures come from where the manufactures claim that each channel is capable of driving 130WRMS for each channel (so long as the total power capability is not exceeded) That is, the plastic fantastic, is the same for each channel. It just happens that the power supply (transformer+power capacitors) supplying the voltage rail (70+ Volts is good) runs out of juice when all the plasic fantastics ask for their fair share !

I hope I've made some some sense and not brought up too many contentious arguments

 

[westcott]

I think there is something missing from your calculations. I would have to dig up the information but if I remember correctly, for every 3dB increase in volume, requires a doubling of amplifier power for most speakers.

Quote from Audio Revolution

"The RF-7s have an impressive sensitivity rating of 102dB. This means that it takes very little power to drive them to ear-piercing levels. Klipsch claims that the mains will reach 120 dB with only 64 watts of amplifier power. In comparison, a speaker with a sensitivity rating of 88dB requires over 1000 watts of amplifier power to reach this same level. This may not sound important, but the THX specifications require that a theater bring the system to a sound pressure level of 117 dB during the loudest passages in a movie soundtrack. With this system, most typical receivers will have adequate power to achieve this goal."

Now you can see why power ratings, sensitivity, and Ohm rating are so important.

Here is a link to the article with the above information.

http://www.avrev.com/equip/klipsch5point1/index.html

I remember seing the math being done. I think it was in Dr. Tooles white paper. It was very enlightening and if I can find it, I will try to post it for you.

 

[hqqns]

I think there is something missing from your calculations. I would have to dig up the information but if I remember correctly, for every 3dB increase in volume, requires a doubling of amplifier power for most speakers.

This is true, and is exactly what i have shown. Please notice as i go from 1W to 2W there is a 3dB rise and from 2W ro 4W there is another 3dB rise (totalling 6dB)

I'm a great fan of sensitive speakers, unfortunatley to keep sensitifity up for lower frequencies you need more surface area (an over simpification i know but basically true) for the same motor for any given driver with a corresponding increase in speaker box volume (not what your gf/wife woud appreciate for 7 speakers in the living room ) though easier to achive for higher frequencies.

 

[westcott]

This is true, and is exactly what i have shown. Please notice as i go from 1W to 2W there is a 3dB rise and from 2W ro 4W there is another 3dB rise (totalling 6dB)

I'm a great fan of sensitive speakers, unfortunatley to keep sensitifity up for lower frequencies you need more surface area (an over simpification i know but basically true) for the same motor for any given driver with a corresponding increase in speaker box volume (not what your gf/wife woud appreciate for 7 speakers in the living room ) though easier to achive for higher frequencies.

Try this calculator. You will find that it would taker over 1000 watts to drive an 88db sensitive speaker to 117dB (THX reference levels). I did include 3dB of headroom to make the calculation closer to reality even though I used one meter distance to keep it simple.

http://www.crownaudio.com/apps_htm/designtools/elect-pwr-req.htm

Something is wrong with your calculations.

 

[hqqns]

There is nothing wrong with my calculations, my calculations as seen above is for 7 speakers not 1. I think you would probably need 1000W for 1 speaker to produce 117 dB but thats not the case fobr 7 working in parallel ie the total of all. None of the 7 would be anywhere near 117 dB

Edit: and in fact if you multiply 7*130W you get 910W ie almost 1000W to get 117dB from one speaker. i stand by these calcuations, which actually agree with the 1000W theory you present

 

[gene]

hggns;

Your estimates for 7 speakers at 88dB sensitivity driven with 17 watts seems pretty accurate (not factoring in room gain which can be much higher than you anticipate)

Use this calculator as a swag:
http://www.myhometheater.homestead.com/splcalculator.html

If you place the speakers near walls, you can expect SPL levels at around 112dB and this doesn't factor in ceiling reflections which can make that level even higher!

Also remember for every doubling of distance from the speaker you lose about 3dB of SPL (6dB as per the inverse square law in a 4pi space).

In reality however, most of the power consumption for home theater is in the subs. You speakers are normally being powered with just a couple of watts with peaks ranging up to 20dB to reach THX levels in a normal theater room meaning the amp can see peaks of 100-250wpc to each channel when called upon. This is why you want an amp that has plenty of headroom.

 

[westcott]

There is nothing wrong with my calculations, my calculations as seen above is for 7 speakers not 1. I think you would probably need 1000W for 1 speaker to produce 117 dB but thats not the case fobr 7 working in parallel ie the total of all. None of the 7 would be anywhere near 117 dB

Edit: and in fact if you multiply 7*130W you get 910W ie almost 1000W to get 117dB from one speaker. i stand by these calcuations, which actually agree with the 1000W theory you present

I agree with your calculations for seven channels driven.

I stand corrected although I think using seven channels for the calculations is a little unrealistic in a typical home theater setup or listening room. It is rare all seven channels are asked to produce the same sonic information at the same time. I think two or three would be more realisitic.

 

[hqqns]

hggns;

No offence but it's "hqqns" , upside down for "subby"

Your estimates for 7 speakers at 88dB sensitivity driven with 17 watts seems pretty accurate (not factoring in room gain which can be much higher than you anticipate)

Use this calculator as a swag:
http://www.myhometheater.homestead.com/splcalculator.html



If you place the speakers near walls, you can expect SPL levels at around 112dB and this doesn't factor in ceiling reflections which can make that level even higher!

Also remember for every doubling of distance from the speaker you lose about 3dB of SPL (6dB as per the inverse square law in a 4pi space).

In reality however, most of the power consumption for home theater is in the subs. You speakers are normally being powered with just a couple of watts with peaks ranging up to 20dB to reach THX levels in a normal theater room meaning the amp can see peaks of 100-250wpc to each channel when called upon. This is why you want an amp that has plenty of headroom.

Cheers, I think you are spot on. The reason I didn't incude room gains was that I didn't want to over complicate the numbers (it was only meant to be a general guide) Perhaps I should have ???

In the case of whats being called upon ie peaks as you say and hence the capacitors are so important in these situations and not necessarily the RMS value. Once again this is where votage rail comes into play again.

That is can the voltage stay at 70+ volts (the great thing about volts is that it's independent of speaker impedance- W= v^2/R) What Im saying is that when you say 100-250wpc i I dont know if you mean 4ohm or 8ohm etc

70 volt rail, 8 ohm speaker(s) -> 70^2/8 = 610W
70 volt rail, 4 ohm speakerr -> 1220W

in real terms this never happens due to these voltage rails not staying at 70V due to the load being applied. .... umm I might be getting off track here

All I wanted to do is get some percpective on overall power level and hence sound intensity levels.

I hope I have done that :/

Edit When I reread this I'm not sure if im making any sense... oh I do like a good drop of red :/

 

[hqqns]

I agree with your calculations for seven channels driven.

I stand corrected although I think using seven channels for the calculations is a little unrealistic in a typical home theater setup or listening room. It is rare all seven channels are asked to produce the same sonic information at the same time. I think two or three would be more realisitic.

np, Once again you are right, and this is exactly why i presented those numbers and introduced the voltage rail and the total output. It turns out that it doesnt matter which channel produces more output, they all draw from the same source. The fronts obviously draw more leaving less for the rear which don't use it any way. Whch brings us back to the orignal problem of maunfactures overstating power ( by saying 7 channels by 130W for example) and not stating what the total power output can be!

And don't expect them to ever give us the info we really need, to do this would be silly for them because 99.999% of people would not know what to make of how much charge their capacitors can hold blah blah...

 

[gene]

That is can the voltage stay at 70+ volts (the great thing about volts is that it's independent of speaker impedance- W= v^2/R) What Im saying is that when you say 100-250wpc i I dont know if you mean 4ohm or 8ohm etc

Amps typically don't run up to their rail voltages b/c of drop off characteristics of the transistors, though you can get close if you use good enough ones. When you see the Cap V rating, its usually good design practice to keep your rails 10-15% lower than their rated values. Hence an 80VV cap should never see a voltage higher than 68-72V.

My example was referring to an 8 ohm speaker. In order to deliver 200 watts continuous power into an 8 ohm load you would need rail voltages of around +-60V or a single 60V rail if you bridge the current amplifier output stage.

 

[hqqns]

Nice explanation.

This is why my subwoofer (completley designed by me) runs 2x 12" subs in a 4th order bandpass enclosure (flat (<1 dB) from 21Hz and 80Hz and -10dB at 10Hz) using a 350W RMS into 4ohm mono power amp. http://www.jaycar.com.au/productView.asp?ID=KC5372&CATID=25&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=557

Very sweet sound from organs to explosions.

The only problem is that the enclosure is bloody huge (340L) and I'm in the process of moving it into another room (due to the gf/wife factor) and just piping (litrally) the sound through the wall

Anyway this is off topic now may need to be split????

 

[hydro]

Hi
Isn't that funny I said a 100 watt per channel amp made 17 watts and was told I just didn't understand Sooo how will we ever find out if anyone really makes a 25 or even a 50 watt the doesn't cost a fortune. I guess we will just have to suck it up.
Joe

 

[gene]

<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><A href="http://www.audioholics.com/techtips/audioprinciples/amplifiers/ACDTEST.php"><IMG style="WIDTH: 125px; HEIGHT: 77px" alt=[clip] hspace=10 src="http://www.audioholics.com/news/thumbs/clip_th.jpg" align=left border=0></A>More often than not we see folks on our forums and other forum websites debating how powerful a particular amplifier or receiver is based upon a single test popularized by many print magazines and some online publications called the All Channels Driven (ACD) test.<SPAN style="mso-spacerun: yes">&nbsp; </SPAN>What consumers fail to realize is almost NO multi channel amplifier that is rated beyond 100wpc can meet the ACD specification for continuous power delivery on real world AC power lines. This article discusses the impracticality of the ACD test and how much power you can expect an amplifier to deliver on real world AC lines.<SPAN style="mso-spacerun: yes">&nbsp; The question remains, "<EM>Can your amplifier really deliver its claimed power</EM>"?&nbsp; Read the article to find out.</SPAN></SPAN></P>
<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">[Read the ACD Test Article]</SPAN></P>

 

[Dan Banquer]

The ACD Test

"The PF for most consumer amplifiers is 0.65 – 0.72. This means only 65%-72% of the available branch rated current can be turned into REAL power. The larger the amplifier load, the worst the PF becomes. A light bulb however, does not have any reactive components – nothing changes the phase angle between the current and voltage. It is a called a resistive or linear load. It’s PF=1. A light bulb that consumes 1A @120V is a 120W light bulb. (120W = 1A x 120V x 1)."

I have tested a few power amps under various load conditions and have found that most appear to have a PF of the numbers quoted above at idle and low power conditions. However, when the load increases the PF gets gets higher (up to approximately 0.8 to 0.85). This is due to the rectifier diodes being "on" for a longer period of time. The article appears to differ with the measurements I have taken.
Why?
d.b.

 

[ggunnell]

I have two questions after reading the ACD article (great work, everyone who contributed!):

Do you have any specific comments on the Power Bandwidth test -- as currently performed, as perhaps it should be performed?

This quote piqued my curiosity:
"According to Bruno Putzeys of Hypex, it turns out that the output choke is usually the dominant factor in determining amplifier output impedance, because it's outside the loop. Before the output choke, any reasonably designed amp has negligible output impedance. In addition, boat-sized amplifiers typically have such powerful (and hence slow) output stages that loop gain needs to be scaled back a bit, increasing HF distortion. Funnily enough, it is precisely HF distortion that fools the human ear into thinking the bass is tightly controlled."
How does HF distortion affect perception of LF?

Thanks!