General question about power

[Unregistered]

Can somebody tell me why a single power receiver can only generate a set amount of power while only driving two of its seven channels?
In other words, take the RXV-2400 for example. It can output 120 Watts to the two front channels. So that's 240 Watts that the power supply can generate. It can also then drive the rear and center channels at the same time. I know, the spec does not say "all channels driven." I read somewhere in this website (the 2400 review I believe) that the RXV-2400 could be expected to output about 80 Watts with all channels driven. For a 5 speaker setup that would be 400 W total. Why can't the power supply drive two channels with 400 W then?

 

[annunaki]

For one that is not how the amplifier is configured to deliver power. Two, the power supply is not big enough to drive all five channels simultaneously to even 80 watts rms per channel. Yahmaha rates most of their receivers with only two channels driven simultaneously to rated distortion 20hz-20khz at 8 ohms. Subsequently, to save on manufacturing costs, that is all the bigger they go in reguards to a power supply. This is because their is no wattage rating standard for surround sound receivers. They are simply meeting the bare minimum FTC standard for power ratings. When you see reviews of a Yahmaha receiver they almost always fall on their face when all channels are driven simultaneously. Typically, in the 26-40 watt per channel range. Not very impressive for a supposed 120 watt per channel receiver.

Some may argue that the over current protection was kicking in. I ask you this, would you want a receiver who's over current protection kicked in when all channels were driven simultaneously 20hz-20khz at 8 ohms to .1% thd? I know I wouldn't. When listening to a very demanding movie soundtrack near reference level or music in five or seven channel stereo mode for a party, I would bet those channels could reach those levels fairly easily.

I am in no way saying that Yahmaha receivers are bad equipment. They sound quite good, as many reviews have attested and are reliable. They just leave much to be desired in the amplifier department in terms of brute rms and dynamic power.

 

[av_phile]

Can somebody tell me why a single power receiver can only generate a set amount of power while only driving two of its seven channels?
In other words, take the RXV-2400 for example. It can output 120 Watts to the two front channels. So that's 240 Watts that the power supply can generate. It can also then drive the rear and center channels at the same time. I know, the spec does not say "all channels driven." I read somewhere in this website (the 2400 review I believe) that the RXV-2400 could be expected to output about 80 Watts with all channels driven. For a 5 speaker setup that would be 400 W total. Why can't the power supply drive two channels with 400 W then?

Pls take a peek at the back panel of your receiver and check to see the electrical power consumption that is indicated near the power inlet. That is an impecable barometer on the ability of your amp to deliver output power. There's this immutable law in physics that, simply stated, says you cannot give more than what you took in.

Let me give my comments on your querry:

(1) Assuming your receiver can conservatively output 80wpc from all 5 channels, thus totaling 400watts, going into stereo mode will not automatically mean you can expect 200wpc. The other amps are simply idle but still consume power though in lesser quantity. And they dissipate this power in the form of heat even when not in use. And depending on the design, these idle amps can still eat 10% to 20% of the available power. You should be thankful almost all commerical recievers and amps are in class AB topology. Otherwise, if class A, your receiver's amps will consume the same amount of electricity when idle.

(2) Remember that no power amplification gear is 100% efficient. That means you can expect the amp to have losses in the form of heat dissipation that I bet you must have observed. Hence, not all the 400watt available power from the receiver's transformer will be there for converstion into output power. About 25% of them are lost through heat. Could be more depending on design.

 

[Ross]

Some huge factors that come into play when dishing out the true needed power are source content, bass management, room size, speaker efficiency, and listening levels.

best,

 

[annunaki]

About 25% of them are lost through heat. Could be more depending on design.

Actually, with class A/B designs, about 40%-60% of available power is lost in heat at or near full output. Most Class A/B receivers would be in the 50% range in efficency at or near full power. At one watt of ouput, it is not uncommon to see Class A/B amplifiers in the 30% efficency range. Only full range class D amplifers will approach and/or exceed 75% efficency.

 

[Dan Banquer]

Amplifier Power

"And depending on the design, these idle amps can still eat 10% to 20% of the available power. You should be thankful almost all commerical recievers and amps are in class AB topology. Otherwise, if class A, your receiver's amps will consume the same amount of electricity when idle."

Given the quote above, I wonder if you have any hard data on your assertions. Most of the receivers that use a linear power amp design will probably be using an A/B topology with the minumum amount of watts for dissapation. Typically for this type of design the watts dissapated by each output strings are somewhere in the 5 to 10 watt range. This is calculated by taking the voltage across each output string and multiplying it by the current through it. When you crunch those numbers you quickly begin to realize that output transistor bias is very low compared to output power. In short; I think your estimate of 10% to 20% of available power is exaggerated.

(2) "Remember that no power amplification gear is 100% efficient. That means you can expect the amp to have losses in the form of heat dissipation that I bet you must have observed. Hence, not all the 400watt available power from the receiver's transformer will be there for converstion into output power. About 25% of them are lost through heat. Could be more depending on design."

How do we know that this a 400 watt transformer? If so, that is typically broken down into a VA rating. This means that if the transformer has a 50 Volt rms rating than the max current available is 8 amps. We also don't know if the transformer is used as a center tapped bridge configuration or not. If it is than the voltage doubles but the current divides by two. If your are sayimg that since the max amount of power out of the unit is a total of 400 watts than I can tell you right away that the transformer needs to be close to three times that due to the amplifier losses.
I am reading lots of assertions in your posts that leave me rather puzzled. If you would like to explain some of these further I would greatly appreciate it.
Dan Banquer
R.E. Designs

 

[Dan Banquer]

Amplifier Power for HT

I am going to take a few minutes here to explain to you home theater folks about power and how it relates to the bulk of HT, so please bear with me.
Typical music program material has the highest amplitudes in the bass region of music. In a typical home theater configuration this is sent to the powered subwoofer, and not to the receiver. Since the highest amplitudes and therefore the highest power consumption is now with the powered subwoofer the reciever has just been relieved of it's biggest "headache" so to speak. The receiver at this point will expend most of its power on the front three speakers and typically very little on the rears. In addition the great bulk of HT speakers are typically 90 db spl and 6 to 8 ohms in impedance. No great current pumping going on here, especially when you consider the average listening level is 80 db spl at the listening position. In conversations with GDS he has typically measured peaks at that level of less than 100 db spl for the main speakers. Given typical room size, anything more will not only be over pressurizing the room but possibly pissing off your neighbors. At the moment GDS and myself are estimating that a solid 50 watts rms into 8 ohms for each of the front three channels will probably do most folks just fine.
For you old stereo folks you have to remember that we are longer have just two speakers powering a room, but 5.1 speakers, and therefore the average power drops per speaker, with the exception of the subwoofer.
All for now;
Dan Banquer
R.E. Designs

 

[av_phile]

Given the quote above, I wonder if you have any hard data on your assertions. Most of the receivers that use a linear power amp design will probably be using an A/B topology with the minumum amount of watts for dissapation. Typically for this type of design the watts dissapated by each output strings are somewhere in the 5 to 10 watt range. This is calculated by taking the voltage across each output string and multiplying it by the current through it. When you crunch those numbers you quickly begin to realize that output transistor bias is very low compared to output power. In short; I think your estimate of 10% to 20% of available power is exaggerated.

Perhaps. But like i said, it depends on design. Shoud I have hard facts to prove my assertions? I already said that most commercial mid-fi gears out there, if not all, use the class AB topology. Sorry to dissappoint you, I've encourntered AB amps that can get overly warm when idle. Just for your info, my Acurus A250 consumes 75 watts when idle. It's also class AB. The Harman Kardon AVR7200 receiver consumes 120watts when idle from its 1040 watts transformer. Another class AB amp. You want more?

How do we know that this a 400 watt transformer? If so, that is typically broken down into a VA rating. This means that if the transformer has a 50 Volt rms rating than the max current available is 8 amps. We also don't know if the transformer is used as a center tapped bridge configuration or not. If it is than the voltage doubles but the current divides by two. If your are sayimg that since the max amount of power out of the unit is a total of 400 watts than I can tell you right away that the transformer needs to be close to three times that due to the amplifier losses.
I am reading lots of assertions in your posts that leave me rather puzzled. If you would like to explain some of these further I would greatly appreciate it.
Dan Banquer
R.E. Designs

I don't know if it was a 400 watt transformer. I don't own this receiver. My comments were based on the poster's inquiring conditions above if you would care to read. I was simply stating that IF the power available was 400 watts, based on the assumption all 5 channels are giving out 400watts altogether, it doesn't necessary follow that he can expect 400 watts output on 2 channel mode.

I totally agree with you that if "the max amount of power out of the unit is a total of 400 watts than I can tell you right away that the transformer needs to be close to three times that due to the amplifier losses." Pls tell that to your colleagues in this forum when they start to evaluate the Z9 receiver to check how a measly 1.2 KVA transformer can power 7 channels of a claimed 170watts RMS each plus another 2 channels of 50watts RMS each, not to mention the receiver's glorious preamp section, DSPs and tuners. Thanks.

 

[av_phile]

I am going to take a few minutes here to explain to you home theater folks about power and how it relates to the bulk of HT, so please bear with me.
Typical music program material has the highest amplitudes in the bass region of music. In a typical home theater configuration this is sent to the powered subwoofer, and not to the receiver. Since the highest amplitudes and therefore the highest power consumption is now with the powered subwoofer the reciever has just been relieved of it's biggest "headache" so to speak. The receiver at this point will expend most of its power on the front three speakers and typically very little on the rears. In addition the great bulk of HT speakers are typically 90 db spl and 6 to 8 ohms in impedance. No great current pumping going on here, especially when you consider the average listening level is 80 db spl at the listening position. In conversations with GDS he has typically measured peaks at that level of less than 100 db spl for the main speakers. Given typical room size, anything more will not only be over pressurizing the room but possibly pissing off your neighbors. At the moment GDS and myself are estimating that a solid 50 watts rms into 8 ohms for each of the front three channels will probably do most folks just fine.
For you old stereo folks you have to remember that we are longer have just two speakers powering a room, but 5.1 speakers, and therefore the average power drops per speaker, with the exception of the subwoofer.
All for now;
Dan Banquer
R.E. Designs

I couldn't agree more. That would be typical alright. But there are many exceptions to this. Not every audiophile you meet has a subwoofer. I know many whose multichannel set-up is not only for HT but for SACD and DVD-A that, because their players and receivers do not have the required bass management for such sources, use identical full-range speakers besides. Many SACDs and DVD-As do not have the .1 LFE information to drive the subs. In fact, there are some DVDs that do not have .1 LFE in their tracks at all.

But confining our discussion to 5.1 HT, your erudite explanation applies most convincingly to satellite-sub combinations and to set-ups with sub crossover point set at 80hz or more. I crossover my sub at 45hz. Hence, for all intents and purposes, my main front speakers and even the back are still dishing out the full audio spectra down to their lowest at 50Hz, -6db at 40hz. Depending on the DVD material, the rears can actually extract as much power as the fronts or contain as much bass. Not always, but it helps when the rear channels and speakers are up to the job.

And yes, 50 watts per channel in a 5 channel receiver is arguably the most one needs in a typical listening HT room to extract the most visceral aural realism one can get out of those mega blockbuster DVDs. The sub should take care of the power hungry low frequencies, indeed, relieving the receiver amps to deliver relatively lesser current for the mids and highs. Putting it another way, the receiver can now devote more power to the mids and highs when required. I hope your recommend 50 watts includes peak dynamic headroom abilities in the amp. Does it? Have you heard how a 50wpc amp with constrained or no headroom sound vs one with sufficient headroom? Thanks.

 

[PaulF]

Some valid points have been made about power supply dimentioning, but reading the original poster's question, it would seem that the rating of the output stage would be the major factor.

I'm going to use a water analogy.

Let's say we have a feeder pipe with a diameter of two inches (our power supply), supplying water to four outlet pipes with a diamteter one inch each, then the total amount of water coming in can flow to the output.

However, if three of the one inch output pipes are tuned off, you can't get the total volume of water coming in to flow out the single one inch pipe.

In order to allow each pipe (channel) to output the total amount of water (power) coming in, each ouput pipe would need to be rated to the maximum capacity of the input pipe, and therefore would run at 1/4 capacity when all pipes are on.

To dimention each channel of an amplifier under this unrealistic usage scenario would add significant cost to the design of the output stage.

 

[rgriffin25]

After reading all this I am left with this question.. How do the capacitors figure into the equation? If at all..

 

[Dan Banquer]

Power Amps

Capacitors when they are used in this application are for filtering and energy storage.
d.b.

 

[rgriffin25]

Thanks for the response Dan. I was hoping for a more detailed answer. I understand how a capacitor works. What I don't know is how HT receivers use them. There has been quite a bit of discussion about the validity of amplifier ratings. There isn't much talk about the power consumption of the receiver or the caps for that matter. Do these figures need to be thrown into the equation?

 

[Dan Banquer]

Power Amps & Supplies

It's not just one equation it's a more than a few. This is the kind of thing that I think would just take too long to even begin to explain in a format like this. If the mood strikes you I would recommend some reading on the basics of linear unregulated power supply, and maybe we can go from there. I wish I could be more help here.
d.b.

 

[av_phile]

Thanks for the response Dan. I was hoping for a more detailed answer. I understand how a capacitor works. What I don't know is how HT receivers use them. There has been quite a bit of discussion about the validity of amplifier ratings. There isn't much talk about the power consumption of the receiver or the caps for that matter. Do these figures need to be thrown into the equation?

Ht receivers use filter capacitors and its transformer circuits in much the same way as any mono or stereo integrated or power amp does - filter the residual AC ripples into as close to DC as possible and store enough DC energy to power the main transistors when the signal so requires.

Forget the technical details. Putting on a consumer hat, I just want to see that the power reserves of a receiver or amp is sufficient enough for the claimed output power. Many designs use discreet power cricuits for each channel - dedicated transformer, rectifying diodes and capacitors for each channel - the better to supply power when needed. More commonly, HT receivers and stereo gears share on just one set of such power circuitry. Regardless, the power available should be a match to the demands of the amplifying transistors when driving large signals CLEANLY.

Amplifer topology besides, providing CLEAN signals at instantaneous peaks or at high continuous levels can be a function of large and FAST power capacitors. Some designs using parallel multiple capacitors on the fact that capacitors with smallers ratings drain or respond much faster than those with high capacitance ratings. So in some circuits you could see 4 or 6 capacitors while some use just a couple of large coke can sized caps. Bottomline, regardless of the number of caps used, the capacitors should be sufficient in storage capacity and voltage as well as fast enough to respond to the power demands from the amp.

Just a word about electric power consumption. This is often a give-away about the true output power rating of an amp or receiver. I won't comment on this anymore as I have many times in my previous posts. Let me just give an example. A receiver claims an output of 250 wpc into 8-ohms on a 2- channel stereo with an electric power consumption rating of 300 watts. Another claims the same output but the electric power consumption is 1000 watts. I'd believe the latter anytime. The total electric power consumption should be at least twice that of the claimed output power of the combined channels rated conservatively. That should take care of instantanous peak requirements at high continuous levels as well as losses due to heat dissipation and the work done to amplify.