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Amplifiers - rated watts vs. watts per channel

Thumpy

Thumpy

Well-Known Member
I thought I had an understanding of rated watts (the maximum amount of power an amplifier might consume) vs watts per channel that an amplifier can produce. In fact, I noted a trend that historically has served me well. The rated watts value is almost always printed on the back of the amp with the rest of the manufacturer's info. If one takes that rated watts amount and divides it by 4, that amount is roughly the watts per channel that the amp produces. I said "roughly". For example, my Fisher CA-880 integrated is supposed to be able to produce 100 wpc and whaddya know, it's rated to consume 400 watts of power out of the wall. Where does the rest of the power go? Some goes to lights and meters, sure, but most of the rest is lost as heat, or so I understand. My Realistic STA-2300 is rated at 600 watts and it can produce 120 wpc. Yes, it's a receiver, not just an amp, and some power has to go to the tuning section, etc.

My point is that in my experience, the wpc an amp produces is ALWAYS substantially less than the rated watts that the amp draws. This makes complete sense to me because of losses, particularly to heat. Well, welcome the Pioneer SA-1040. This amp draws a maximum of 170 watts, yet it is rated to produce 100 wpc. It's printed right in the owner's manual. As a stereo amp, that means two channels, each producing 100 watts, for a total of 200 watts. How can an amp produce 200 watts when it only draws 170 watts? Doesn't this violate the law of conservation of energy?
 
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Thumpy said:
... Doesn't this violate the law of conservation of energy?
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Nope.

The back panel draw isn't always, in fact often not, listed at full rated output of the amplifier.

Additionally, the same model of amplifier/receiver (whatever) can have different consumption ratings on the back due to different regulatory requirements of different market regions. There can be substantial difference.

For example, the Yamaha M-80 can have four (or is it five?) different consumption ratings based on the intended market region. The lowest one is, from memory, 600W while the highest one is 1700W, again for the same amplifier.

Another good example is the Crown XLS1000 amp (215wpc @ 8 ohms) I was playing with a while back. I believe the back panel lists 175W, yet I maxed out the capability of a Kill-A-Watt meter and caused it to beep overload when I was doing rated power tests on the amp.

You can see some measurements of the XLS1000 here https://audiokarma.org/forums/index...your-dummy-load-s.757605/page-5#post-10328083 and in post #99.
 
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The reason doesn't require a rocket scientist to figure it out. Amplifiers are not 100% efficient. Some power is always wasted in producing heat.
 
Perhaps that will give you a ballpark figure for vintage amps and receivers.

Just out of curiosity I compared the input vs output of my new 5 channel AVR. Power consumption is stated at 260w. While it's rated power output is 80w x 5 = 400w. Either today's class AB amps are incredibly efficient, or somebody isn't being entirely truthful.

Consider this. I had a 200w stereo power amp that weighed 35 lbs. It had a VA rating of 600w. Whatever VA is. This new 400w AVR only weighs 17.
 
charles 1973 said:
Perhaps that will give you a ballpark figure for vintage amps and receivers.

Just out of curiosity I compared the input vs output of my new 5 channel AVR. Power consumption is stated at 260w. While it's rated power output is 80w x 5 = 400w. Either today's class AB amps are incredibly efficient, or somebody isn't being entirely truthful.
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It's neither.

As said earlier, back panel consumption typically is NOT listed at maximum/full rated audio output.
 
Stereodiver said:
Ok than,,,, what are the parameters for the ratings of the “Back panel consumption”?
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You'd have to get from the manufacturer what were the conditions that it was tested under. UL has some rating methodology that applies to UL-listed gear (you can find out the details if you buy the standard) but that's still contingent on how the manufacturer rates the gear.

Pro audio gear is usually more clear in the ratings. The back panel rating for QSC amps, for example, is at 1/8 average rated output into 4 ohms, using a pink noise signal. That is roughly equivalent to playing the amp at the verge of clipping with typical program/music.
 
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Usually the wattage rating listed on the back is max wattage consumed from the power outlet. Nothing to do with power output in watts. Its how many watts is drawn to power the unit. I see listing all the time where the seller lists the max wattage on the back pannel and assumes it means wpc. Some manufacturers do list total watts produced from both channels for total watts produced Realistic for one did this. So, a 20 watt amp would acually be 10wpc. Others list wpc. You just have to read the specs to know which rating is being used.
 
If there are receptacles on the back panel those are factored into the rating.

For sure though the output of the unit factors into the back panel rated draw. How it factors in isn't necessarily clear but absolutely it factors in.

What it is usually not is the max draw.
 
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The watt rating on the back of a receiver is about as well related as a watt output rating in the owners manual to reality.
5 channel amplifiers are tested a channel at a time. The reasoning is that seldom will all channels be at full output. As a result, the power supplies are grossly undersized in most surround receivers. Even most entry level stereo receivers aren't going to get close to rated output into a real pair of speakers. It's a long story
 
Thumpy said:
I noted a trend that historically has served me well. The rated watts value is almost always printed on the back of the amp with the rest of the manufacturer's info. If one takes that rated watts amount and divides it by 4, that amount is roughly the watts per channel that the amp produces.
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I get what you are saying on most vintage stuff, receivers I found that to be true as well, I find it closer to a 1/3 though.

If one looks at this all the time like we apparently do, it used to play out more often than not.
 
whoaru99 said:
If there are receptacles on the back panel those are factored into the rating.

For sure though the output of the unit factors into the back panel rated draw. How it factors in isn't necessarily clear but absolutely it factors in.

What it is usually not is the max draw.
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If all circuits were the same it would be easy to calculate wattage drawn from the wall to power output. But here are so many circuit designs and topologies that could be difficult I would think.
 
Lavane said:
If all circuits were the same it would be easy to calculate wattage drawn from the wall to power output. But here are so many circuit designs and topologies that could be difficult I would think.
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The circuits are not the main factor, IMO. Oh, sure, Class A, Class AB, Class D, etc., all have different efficiencies but typical ranges are understood. The main unknown is the specific test condition when establishing the rating.
 
NH-MAN said:
I get what you are saying on most vintage stuff, receivers I found that to be true as well, I find it closer to a 1/3 though.
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On my Sansui 9090, it is .407 (110 wpc/270 rated). On my Sansui G-971 it is .33 (200 wpc/600 rated). On my Pioneer SX-550, it is .25 (20 wpc/80 rated). On my Realistic STA-2300 it is .20 (120 wpc/600 rated). On my Kenwood KR-9600 it is .13 (160 wpc/1200 rated). What's it all mean? Very little, apparently.
 
Thumpy said:
I have to disagree. I have a McIntosh MR80 that has a receptacle marked 300 watts on the back, but the unit has a 25 watt rating
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That is actually what it will draw from the wall plug when using an accessory. The Mc power cord should be able to withstand the load.
 
It is true that a large amount of power coming from the AC outlet to the amplifier is "wasted" in heat, because of the way the output section of the amplifier is made and the fact that it isn't very efficient. The large heatsinks in high power amplifiers and sometimes fans, make a point about that.
But one detail that is often overlooked is the relationship between output power and heat generated (i.e. efficiency).
A typical class B - AB amplifier will generate most heat (i.e be the least efficient) at about 1/3 it's rated power output. So, a 100WRMS per channel amplifier will waste most of the heat at about 33WRMS output power. At that power output, you can do some rough estimates that 33Watts are coming out of the amplifier as audio signal and 60-80 Watts are consumed in heat.
At higher power levels, the efficiency goes up and the least heat is generated at max output level.
The same applies to class A amplifiers. They will always take in the same amount of power. Without a signal, all power is wasted. With a loud signal they cool down.

Of course, all that can be measured but are valid for pure tones that can be set to a specific level to correspond to an output power. Music is not like that.

Listening to music that has a high crest factor (low average level with high dynamic range and peaks) will have a 100WRMS amplifier operating perhaps at 1-5WRMS on average with 50-90 brief, sub second, peaks. This means that the consumption profile of the amplifier is much more complex, varies widely against time and depends on the profile of the music content (or test tones) it is fed with.
 
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