Question re: watt usage while amp is silent but not sleeping

How many watts does a class A/B 2-channel amp use while it is not receiving an audio signal, but is not in sleep mode? The amp is silent, no fans, and barely warm. (I don't have an watt meter.)

Sorry, I think there was a thread about this not so long ago, but I tried several searches to no avail. Thanks!

 

"Kill A Watt" is your friend.

I use this one:

P3 P4400 Kill A Watt Electricity Usage Monitor https://www.amazon.com/dp/B00009MDBU/ref=cm_sw_r_cp_apa_i_XKimEbNVAFPXF

 

Usually this is listed in the specs/manual. For instance for my Luxman amp -

Power consumption 380W
150W (under no signal),
0.5W (at standby)

So basically 0.5W is "off but plugged in", 150W is "on but no sound", and presumably the full 380W is "Guns and Roses concert in your living room".

 

A typical moderately-biased class AB amp consumes between 1/4 to 1/3 of its maximum rated power consumption at idle.

 

Thank you for a recommendation. Now in my Amazon cart!

You are correct, sir. I checked the specs in back of the manual, and sure enough it was there!
Power Consumption
15oW - max
20W - no sound
.4W - sleep

Thanks - Yep, amp is rated at 45W per side into 8 Ohms, so I guess it would be 45*2=90W, and 20W is about 25% of 90W.

 

A less rigorous thought experiment...

We know that essentially all of the "wasted" power (from the amp at idle) will be in the form of heat energy (per unit of time - for the technically-minded.)

The amount of heat being given off in this case has been characterized as "barely warm."

Now take a desk lamp. Put a 15W (incandescent) bulb in it. Turn it on. Wait a minute or three and then place your hand over the lamp - about the same height as you place your hand above the amp (say 6"). Roughly speaking does it feel like there's more or less (convective) heat rising from the lamp, or the amp? If less or equal from the amp then you're in the ballpark and you have your (approximate) answer. If more from the amp, then repeat with a 25W, then 40W, then 60W, then 75W, then 100 W bulb until things are roughly the same. Because of the relative geometries heat will be felt somewhat differently from both, but given the "barely warm" measurement, my bet is that the lamp with a 15W or at most 25W bulb will feel about the same as the amp.

Jeff

 

I checked my Cambridge Audio 651A some time ago and I recall it was around 50 watts.

 

10 to 20 watts when off, 15 to 45 in sleep mode, 20 to 250 in stand by.

 

Does it really matter?

 

When my Pioneer SX 1250 is off, I can unplug it and the street lamp outside brightens
I watched a video of a tech restoring one and he checked the draw it had while off and plugged up. Anyway mine stays unplugged

 

You're joking right? If not, you better have your electric company check the service lines around you.

 

Because I'm an anal retentive, I looked up the service manual and schematic for the SX 1250. When biassed correctly, there is 100mV across the emitter resistors of the power transistors, which means they are running at 200mA. The rails are +/-70V, so the standing dissipation in each channel is 28W, which is 17% of the full 160W output.

Anyway the total standing dissipation in the power transistors of both power amp channels is 56W, add a bit for all the other gubbins in this monster late 1970's receiver and get about 70W at idle, turned on with no signal. Now there are three power outlets on the rear panel, one switched by the on/off switch and two unswitched.

But if you have nothing else connected to those unswitched rear panel sockets, there is zero power draw with the unit switched off. No surprise there then.

 

FWIW here's some actual measurements taken with my kill-a-watt on my amps. Tested were 1200 wpc monoblocks on 4ohm 91db sensitive speakers. With no signal each amp draws 66 watts so a total of 132 watts for stereo. At -21.5 db level (background level) they use 80 watts each for a total of 160 for stereo. At -13.5 db (which is loud) they use 86 watts for a total of 172 watts for stereo. Now these measurements were taken before I had swapped all 28 of the amps incandescent lights over to led's and that conversion dropped each amp down 11 watts at an idle or 22 watts for stereo. I did not take any measurements at volume but I would assume those measurements would all be 11 watts less for each amp.
For comparison sake the fridge in the same room uses 170 watts. Imo power consumption of most stereo equipment isn't as big a deal as we might like to think it is. No doubt there are exceptions such as big class A amps or driving difficult loads at extreme volumes.

 

Power consumption depends a lot on the power supply design and whether they are considered Consumer or Professional use amps. Vintage amp/ receivers with linear power supplies may always draw 10- 20 watts or more power when in idle or standby mode while modern amps that have switching power supplies may draw just a few watts of power in standby. Also as mentioned by motorstereo the incandescent bulbs for displays in vintage audio consume a lot of power as well.

There are now International Energy Efficiency Regulations which require Consumer audio products to reduce power consumption to just a few Watts after a specified time when sitting at idle and require they go into standby mode. This is not my area of expertise but we have an Engineer at the Audio company I work for who specializes in making sure our product comply with these regulations.

 

Haha! Of course I needed that

 

And why not"full 380W is "Van Halen concert in your living room"? All those harmonics Eddie does with his fingers and guitar must spend a lot of Watts, and Alex is no slouch on drums.