A-V receiver power ratings

I feel it's important to remind the forum (as a refresher) that many power ratings of A-V receivers can be deceiving. When an A-V receiver states for example that it can produce 100 watts x 7, it doesn't necessarily mean it can deliver 100 watts to all 7 channels simultaneously. It must state "All Channels Driven" into 8 ohms or the like. If it doesn't, then it means that 100 watts can be delivered to the two main channels but when the receiver is in either 5.1, 6.1, or 7.1 mode, the power delivered is reduced. Marantz for example guarantees a min of 70% of its rated power when in surround sound mode. They claim this 70% is conservative though. They don't print that anywhere but my Hi-Fi store sales person explained this to me recently. NAD gives a "simultaneous power output" rating. So just something to be aware of if you're basing an A-V receiver purchase decision on power output in surround sound. Discussion is welcomed !!
you guys got that? :)
And driving the speakers is not just about wattage but the amount of current behind those watts !
You're really looking for RMS watts throughout the power band, e.g. 20-20KHz, all channels driven simultaneously into a stated load, usually 8 ohms. In many good amps, that number doubles into 4 ohms. You should look at THM and IM distortion as well; these usually rise rapidly above the rated power, but that is of little concern if the power is adequate to your needs and the distortion base is low. You do need to read the fine print.


Could you elaborate or further explain what you mean by "the amount of current behind those watts"? That would help me tremendously. Do you mean in voltage? Thanks.
Pdn ; You are really testing this ole sole !
I will try but encourage corrections where needed !

Current is another name for amps . Watts are figured as the amount of volts times the amount of amps . This will give you the peak watts .
To get the 'working' wattage , or what is left after the losses from the theoretical peak through the circuitry , you multiply the peak wattage times a number which I believe is @ .686 . This is the RMS wattage refered to above .
As you can see a high voltage and low amperage will give you the same wattage as if you reversed the numbers to a low voltage and a high amperage using the same values . Say 100volts X 5amps = 500watts so 5volts X 100 amps = 500 watts also.

The current (amperage) is the driving force for the voltage . It is easier and cheaper to produce high voltage than it is to produce high amperage . The high amperage units will need bigger (heavier) transformers as opposed to a low amperage amplifier .
This is one area where the exspense comes in to play for A/V receivers . They will have small (lite weight)transformers that give high voltage but low amperage to achieve the stated wattage .
This is not necessarily a bad thing if you tailor your speaker choice accordingly .

These low curent amps will work much better if you choose a reasonably efficient speaker , say in the 90db. range and above . You will also want a rather benign or flat frequency response that does not have any wild swings and does not go very low . Here you may want to look for an impedence of say 8 ohms and a nominal impedence of 6 ohms for the low amperage amps.

The doubling down , mentioned above , is a way to tell if an amp has a higher current amount used in figuring the wattage . Here you want an output of something like 100 watts into 8 ohms and 200 watts into 4 ohms . Rather than 100 watts into 8 ohms and 125 watts into 4 ohms . Where the wattage into 4 ohms is double the wattage into 8 ohms denotes a higher current (amperage) amplifier that will drive more difficult speakers such as 8 ohms with a nominal impedence of 3 ohms . The higher current will be able to drive the voltage into a more difficult load that is presented by the speaker .

I hope that this is reasonbly correct and of some help to you .

Thanks very much for the in-depth explanation above. Wow, there's much to digest here. So what you're saying is that the higher the amperage and lower the voltage, the better the receiver will be in power, its ability to drive speakers, and sound. How would you know what the amperage is in a specific A-V receiver? Is that a spec we should be looking for? What would be an example (brand) of a high amperage/low voltage receiver over one that is not? I noticed that the better more powerful receivers and even source equipment will use a Toroidal transformer. Is this a good example? Good discussion. Many thanks.
Pdn ;
The examples , that I used above , were theory only .
While it is desireable to find high-er amperage amps you do not necessarily need to have lower voltage to go with it . It will probably be difficult to find these specs in any brochure that you may find available . That is why it was suggested to look at the wattage output into 8 ohms and compare it to the output into 4 ohms , the doubling down thing .

As far as the transformer types , I don't know . Maybe someone more knowledable than I could answer that question .

I am also not knowledeable as to what manufacturers would offer an A/V receiver that would doubledown the wattages .
Keep in mind that it costs more money to build an amp that will do this . How much do you want to spend ?

Most audiophiles seeking this type of power will go with seperates , an A/V processor called a pre/pro and a seperate 5,6, or 7 channel amp or a combination of amps to achieve the necessary amount of channels . Again , the cost can increase rapidly ! I doubt that you will find an A/V receiver that will put out this kind of power at any of the BB , CC or discount warehouses .

All is not lost though . Remember the other way to achieve workability , or synergy , is to select speakers that will work with the kind of power that the less exspensive A/V receivers will give you . This is usually a cheaper approach to take .

Unless you want to build a system for multi-channel music , the important speakers will be the front ones ,left , right and center . The rear or sound effects speakers won't need to be as efficient or of as high of quality . Just match the tonality of the fronts so that the voices will sound the same all around . You can use the level adjustment , in the processor , to compensate for less efficient rears and make the loudness equal to the fronts .

Good luck .
Volts x Amps = Watts, and from a purely mathematical standpoint, 100 volts at 5 amps is 500 watts - as is 5 volts at 100 amps as mentioned by Saki70.

The problem is that you can't just arbitrarily mix the volts and the amps and end up with the same results from your amp. There is the factor of the speaker impedance that dictates how much current will flow in the circuit. An amplifer does not push current to the speaker, the speaker draws current from the amp. This all works through the basic principles of Ohm's Law (E=IxR).

An amplifer tries to operate as a voltage source/voltage amp. This means that the amplifer will try to deliver the same voltage output (volume position notwithstanding) as the speaker impedance decreases (lower impedance = higher load = more current necessary).

In other words, if an amp is designed to deliver 100wpc @ 8 ohms, it will need to put out roughly 28 volts. This 28 volts into an impedance of 8 ohms will cause roughly 3.5 amps of current to flow. If the voltage and impedance variables do not change, then 3.5A is all that will every flow in this circuit. The amplifier cannot and will not force more current to be in the circuit than Ohm's Law dictates.

Since output power at 8 ohms is the typical basic rating, the amplifier mfg has designed the power supply voltage rails to support 100w at 8 ohms (28V and at least 3.5A) At 8 ohms, the power output is typically limited by the available voltage from the power supply rails, not the current.

When a 4 ohm speaker is substitued for the 8 ohm speaker, the "100w" amplifer in our example still tries to deliver 28V to the 4 ohm load. If the power supply is robust enough to fully deliver the higher current (about 7 amps) dictated by the lower 4 ohm load, the power delivered would be 200w - or double the 8 ohm rating. This is the "doubling down" effect often mentioned in amplifer discussions.

However, if the amplifer's power supply cannot fully satisfy the current demand of the lower impedance load, it's output voltage will sag. The inability of the power supply to deliver enough current and the resultant sag of the output voltage under load is why most amps and receivers don't double down as the impedance drops.

Also, it is my opinion that having all channels driven for a HT receiver is a bit of a red herring. Sure, it's likely to indicate a more robust power supply and amplifier, however it's not a guarantee of superior sound quality.

Further, it would be a VERY rare, if ever, that any HT source material will require maximum output on all channels at the same time. Of course, if you play continuous test tones at excruciating levels through your system, that's a different story.
Thanks everyone. Very good explanations and information. Appreciate it.