Guessing WPC from rated wall draw ?

[SEAWOLF97]

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I'm frequently presented with receivers/amps on a "buy or not - now" situation
without web access.

To guess the WPC , I generally look at the backside for max power consumption , then
divide that number by 3 . Figuring 1/3 for running the gear and 1/3 for output wattage per channel. IE: if the label says 300 watts , I ASSUME about 100wpc.

This method usually is somewhat close. This week I ran across a Realistic receiver (yeah, I know) that was marked "200 watts" , so using my guide I guessed that it output about 60WPC. Got home and checked , RS says 16 WPC (minimum).

Questions: Does that amp really squander 168 watts just to run itself ? Is my guideline for guessing what is generally used , or is there a better one ?

I do realize that the rated draw is not a constant number , but a maximum, as is the power output number a minimum (in this case)

anyone ?

[martin2395]

It really is guessing! Since you have amps that run in Class A/AB/B/D/G/H you can't reliably tell how much power it delivers just by looking at the power consumpion.

A Krell KSA-50 for example uses around 200W when it's sitting idle, I bet that it might hit 1kW when you push it in 2 ohm.

Technics often posted outrageous power consumpion on their amps, 2x100-150W had 1000W PC for 230V models.

[honkytonkwillie]

I believe the "Max 300 Watts" or whatever is printed by the power cord on the rear is a maximum (instantaneous?) that the appliance will draw during normal (i.e., not in a failure mode). Undoubtedly there's a standards body like Underwriters Laboratories, or IEEE or that spells out what it's required to say.

300 Watts draw at the wall for a 16W/ch rating seems a little gluttonous. But when "normal" listening might only average 1W/ch, I'd be surprised if the draw at the wall is greater than 20W.

Of course you could probably MAKE it pull 300W from the wall, playing sine waves or white noise. And class A/B amps have a nominal 50% efficiency? That would be 75W/ch to the speakers +150W of pure heat coming off the case. For a few monents hopefully.

[honkytonkwillie]

Oh yeah - just last week I got a Kill-A-Watt meter to measure this very thing. I'll report back when I get around to some real world measurements.

[Slare]

I think such a method works pretty well for standard solid state amplifiers/receivers/avr's... though 1/3 would seem pretty conservative, maybe closer to half or a little more to correlate modern power draw ratings to modern all channels driven sum of rms power. Having compared and knowing the tested outputs of a lot of modern AVR's, at least, your method has definite merit. You can definitely see a very big difference in the back panel rating between AVR's known to put out their rated power to 5-7 channels vs. those that only do it on 1-2 channels.

Things can get really hairy with digital or hybrid amps though.

Edit - I see you mean 1/3rd to estimate 1 channel in a 2 channel receiver/amp. So effectively 66% for summed RMS power output. I think 50-70% would cover a good bit of solid state gear pretty accurately. Low power AVR's maybe lower as there's a ground floor to run all the processing/networking stuff included nowadays.

[honkytonkwillie]

As promised, I'm back with some data. Some interesting findings. A few caveats however. I didn't set out to gather publication quality data, or even independantly repeatable measurements for that matter. I was just trying to get a rough idea of how much it costs to operate these amps in the scenarios which I employ them. I don't recall which of three sets of speakers I was using for the measurements I wrote down, and their sensitivities range from 87-93 dB/W @1m. I didn't take any SPL measurements as I was more or less aiming for the output level each amp spends most of it's time working. And the music selection was totally inconsistent. Whatever came up next in the iPod is what I listened to over the course of a couple days that I was goofing around.

I pretty quickly settled on two useful values to measure: power consumption at idle, that is, powered on but with no signal; and power consumption at listening volume.

The amps:

• FiiO A1 - a pretty little black chip amp, rated at 15W/ch @8 Ohms,
• Decware EL34 SET amp, rated 5.9 W/ch @8 Ohms,
• 10 year old Sony HT receiver, Shiny Decal rating of "110W x 7!" but the manual gives a proper 2-channel rating of 100 W/ch @8 Ohms,
• Mackie FR1400, 300 W/ch @8 Ohms,

The Fiio was a nice surprise. It works 24x7 running a Bose sub/sat thing in the kitchen. At idle it drew 1.5 watts, and playing it's typical low level background music it skyrocketed up to 1.9 watts. Wow. I didn't think it'd be THAT miserly.

The Decware EL34 was my main interest, as those beautiful tubes are hot 24x7 as well, normally playing low level background music through some NHT Super Zeros. I knew the power draw on this would be rather high compared to it's rated output, and for months I've wondered just how guilty I should feel for the pleasure she brings me. At idle she drew 90 watts. At typical background levels she also drew 90 watts. Full blast Deep Purple? Yup, 90 watts. That's Class A for ya. That's comes to $102/year at the rates I pay for electricity. Totally worth it.

Next came the Sony. At idle it was pulling 65 watts. At typical background levels it also draws 65 watts. I was able to get it up to a constant 71 watts playing music a bit louder, but not loud enough to disrupt a conversation with my wife. I just now looked at the rating on the rear panel by the power cord: "120V 60Hz 300W". So Seawolf - your divide-by-3 heuristic is pretty close. You could probably even safely divide by 4 or 5.

Last came the Mackie. It's a piece of gear I use to drive a sub and some mini monitors with my synthesizers. To me it can get scary-loud. It has the quaint habit when you turn it on of humming for a second while making the lights dim while only drawing at a meager 280 watts peak before settling down to 75 watts at idle. What a phony little show-off! If you didn't know better you'd think he was an 1800W hair dryer with that little act. Anyhow, for some reason I didn't write down what the power draw was when listening to music. I seem to recall I never got it past double digits.

Biggest surprise by far has got to be the FiiO chip amp that didn't get beyond the 2 watt mark. That's like, what an iPad charger might draw just being plugged into the wall without the iPad connected! Well, maybe Apple devices are actually smarter than that. But still - this is idle wall-wart territory!

That's all for now. I've got a few more amps that I haven't tested yet. An old Crown D-150a II, an even older Sony HT receiver, a couple of powered subwoofers, and a whole fleet of car audio amps I run once in a while from an inverter. I probably won't bother writing about them here, but if anyone is just dying to have me try something else, lemme know.

[SEAWOLF97]

As promised, I'm back with some data. .....

thanx willie for the research . sounds like my WPC guesstimates aren't grossly bad ... (generally) , tho it sounds like front panel stickers don't even agree with owners manual on the same product.

I get a good laugh from the car stereo op amps = 2000 WPC !!! (into 1 ohm ?)

and yes, I'm aware of many of the affecting factors.

PS: I checked out one of those "Kill A Watt" devices from our local library ... My big BGW amp had a large draw number on the rear , but at idle it was drawing about 15 watts. Even running at moderate gain, it didn't increase draw very much.

[toddalin]

My Crown PSA-2XH puts out 700 WPC into 2 ohms and consumes as much as 20 amps (2,400 watts) making it 58% efficient. It will pop my house circuit breaker (probably 15 amps) if cranked. Can draw 90 watts at idle. http://www.crownaudio.com/media/pdf/legacy/k1071-8.pdf

[badman]

My Crown PSA-2XH puts out 700 WPC into 2 ohms and consumes as much as 20 amps (2,400 watts) making it 58% efficient. It will pop my house circuit breaker (probably 15 amps) if cranked. Can draw 90 watts at idle. http://www.crownaudio.com/media/pdf/legacy/k1071-8.pdf

Awfully hard to use that much zoot though, in a home, even in a big space like yours. Here's a related thread at DIYaudio- how much voltage do your speakers need? Note that 37% use <2V- as in, a fraction of a what, and over 70% use a handful of watts or less.

http://www.diyaudio.com/forums/multi...s-need-20.html

I am in the <1V range the VAST majority of the time myself. I should hook my flea-powered amps back up and see how we do, the 200WPC I have into 8 is a little overkill with my horns.

[toddalin]

Awfully hard to use that much zoot though, in a home, even in a big space like yours. Here's a related thread at DIYaudio- how much voltage do your speakers need? Note that 37% use <2V- as in, a fraction of a what, and over 70% use a handful of watts or less.

http://www.diyaudio.com/forums/multi...s-need-20.html

I am in the <1V range the VAST majority of the time myself. I should hook my flea-powered amps back up and see how we do, the 200WPC I have into 8 is a little overkill with my horns.

In my case, the power is necessary and does not go to waste. I use mine bridged in mono to push the 12 ohm load (~800 WRMS) of the two voice coils in series on a JBL W15Gti for a sub.

[SEAWOLF97]

I see you mean 1/3rd to estimate 1 channel in a 2 channel receiver/amp. So effectively 66% for summed RMS power output. I think 50-70% would cover a good bit of solid state gear pretty accurately. Low power AVR's maybe lower as there's a ground floor to run all the processing/networking stuff included nowadays.

On the Technics integrated that runs my PC sound , the rear panel specifies 300watts. So ... I guesstimated it at 100 wpc. Was a happily surprised when I looked up the specs and it was rated 130wpc (New ClassA)

It replaced a Technics class H receiver that was rated 50wpc ... the ESS/Sonance speaker combo that I'm running was much happier with the power increase.

[1audiohack]

[QUOTE=badman;364768] Here's a related thread at DIYaudio- how much voltage do your speakers need? Note that 37% use <2V- as in, a fraction of a what, and over 70% use a handful of watts or less.

http://www.diyaudio.com/forums/multi...s-need-20.html
[QUOTE]

I did that test. My results are on page 87-88. I am in the 80 volt peak camp with my Arrays. That's nearly 1100 watt peaks. No little amps for me.

Barry.