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How many watts does an amp use?

W

Whippet

Aug 30, 2014
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#1
Sorry to post such a stupid question but I need to run a transformer from the wall before I plug some of my amps into, in some countries I visit often.

If an amp is rated at 400 watts at 4 ohms and 800 watts at 8 ohms, how big should my transformer be? is 400 and 800 the actual usage of watts or is the actual usage smaller?

I do know that when you turn any electrical appliance on, there is spike in the wattage and I need to get a transformer that can handle 1.3-2X the actual wattage that is given on the spec sheet.

Thank you for all information in advance.
 
#2
Usage is typically a good amount greater than output. The numbers you stated are output. So if you're cranking 800 watts out, you could be using 1,500!! Just check the back of the amp - they usually say.

That said, if you are running an 800 watt amp at a low volume, you're not pushing 800 watts in to your cab, nor consuming 1,500 watts from the wall.

(note - 1,500 just a number I pulled out from nowhere. Could be way less, or possibly more. I dunno. I recall old guitar amps that were 25 watts but consumed 80 watts and people would try to list them as 80 watt guitar amps.)
 
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#3
ashtray said:
Usage is typically a good amount greater than output. The numbers you stated are output. So if you're cranking 800 watts out, you could be using 1,500!! Just check the back of the amp - they usually say.

That said, if you are running an 800 watt amp at a low volume, you're not pushing 800 watts in to your cab, nor consuming 1,500 watts from the wall.

(note - 1,500 just a number I pulled out from nowhere. Could be way less, or possibly more. I dunno. I recall old guitar amps that were 25 watts but consumed 80 watts and people would try to list them as 80 watt guitar amps.)
Click to expand...

Thanks for the reply. I had a feeling it was going to be as you said, consumption and output are totally different. For now I am using the Tec Amp Puma 900 and this thing as a voltage selector so it's no big deal but I am thinking of getting the Mark Bass Multi Amp which doesn't have the voltage selector. I might have to scrap this idea knowing that the step up transformer (115 to 230 volts) at 1500 watts usually weighs anywhere from 30 to 40 pounds and 1500 watts is probably not going to be enough.

Anyway thanks for the quick reply. Much appreciated.
 
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#4
The amp design really dictates what the draw will be. As an example a good SMPS class D amp wastes little power (as heat) so its requirements would be MUCH less than a line frequency power supply class A/B output amp for the same output power.

If the amp has a lot of power, but is pretty light you can be pretty sure it uses a SMPS. The output amp operating "class" is much harder to determine if not listed.
 
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#5
Where to start. 1) The maximum that the amp will use is listed on the date plate (normally on the rear near you output connections.) 2) Using less volume, uses less wattage. 3) Due to the variable efficientcies of different amplifier designs, there is no way for anyone to reliably tell you how many watts an amplifier will consume.
 
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#7
The only reliable way to tell is to plug it in and measure it. As others stated this will be difficult because the amount of power you pull from the wall is based on the amount of power the amp needs to replicate the input signal. Since the amp settings and playing affect this greatly, taking measurements is the only way to know for sure.

That being said I plugged a meter into an outlet to check idle power in a couple of my Tone Hammers. It's something I've been curious about for a while but just haven't done. Here are the figures I found:

TH350: 7.5W (mute), 12W (operate)
TH500: 10.0W (mute), 16.5W (operate)

These are real-time figures pulled from the wall as the unit runs. I'm not able to test playing with a cab hooked up due to being in a small apartment. But this at least shows how low the power figures are at idle.
 
#8
ashtray said:
I recall old guitar amps that were 25 watts but consumed 80 watts and people would try to list them as 80 watt guitar amps.)
Click to expand...

LOL, but if they list 'm as "80 watts guitar amp" they actually aren't saying anything wrong, it's the same as listing a vacuum cleaner as 2000watts but it doesn't tell you how hard it will suck (no pun intended) ;)
 
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#9
Whatever you determine your consumption is, add at least another 50% capacity when specifying the transformer. This is an engineering safety factor.

Despite what the labels say on the chassis, as you mentioned there is an initial surge when you turn on the amp when all the capacitors have been fully discharged. The amount of this surge will vary depending on the design. When the amp is running the draw is less.

There is an inexpensive meter called a kill-a-watt (see amazon or some hardware stores) that can measure power consumption. Handy to have. Kill A Watt Meter - Electricity Usage Monitor | P3
 
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#11
Thanks! This is also good info if thinking about running a small combo on a battery pack. I was pondering this yesterday trying to figure out what size pack will power my Hartke A25 if we get a garden gig. I can always use one of those in hurricane country but the last one I had only did 12v plus it died. I checked the manual but it only showed amps needed. I will check the back label.
 
#13
The manuals for my old Hafler DH-200 stereo power amps have the good taste to include not only a very detailed output power spec, but also list quiescent power use of 100 watts and maximum power use of 370 watts (both channels delivering 100 watts RMS into 8 ohms.) Short of that, the power rating on the back of the appliance is the best figure.

Otto
 
#15
Typically speaking, the lower your impedance is, the higher your wattage output should be, so your numbers are backwards.

Generally speaking, at 120V AC, 100 watts = 1 amp. So at a 4ohm load, an amp that will put out 800 watts should have a power supply that can provide those 800 (+) watts at 8 (+) amps.
 
#16
Whippet said:
Sorry to post such a stupid question but I need to run a transformer from the wall before I plug some of my amps into, in some countries I visit often.

If an amp is rated at 400 watts at 4 ohms and 800 watts at 8 ohms, how big should my transformer be? is 400 and 800 the actual usage of watts or is the actual usage smaller?

I do know that when you turn any electrical appliance on, there is spike in the wattage and I need to get a transformer that can handle 1.3-2X the actual wattage that is given on the spec sheet.

Thank you for all information in advance.
Click to expand...
A good rule of thumb is ampls consume DOUBLE the RMS output plus a little. But you can get a reasonably accurate number by reading the back panel - there is usually an indication of input power stated in watts.

But DO NOT USE the inverters they sell for travelling. They will not work properly - anything over 50 watts is for use with heating elements ONLY (like curling irons, etc). I used to sell that stuff and I don't know how many people burned out their equipment or their inverters by using them improperly (often after I counselled them against doing what they did - more often after buying them from a less informed sales-person). DON'T DO IT!
The lower wattage ones are fine for battery chargers etc. but won't have the power for an amplifier.

If you are travelling overseas regularly:
Get an amp with a switchable power supply in the back (like Eden's older stuff) or a switching power supply (Class D).
If you don't travel overseas a lot - RENT.
 
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#18
nashvillebill said:
Watts is volts multiplied by amps, so if all you have in the manual is amps, multiply that by the line voltage. Or if the manual doesn't say watts or amps, use the value of the fuse if the amp has a fuse. Example: amp has a 5 watt fuse and you're running it on a 120 volt circuit. 120 volts times 5 amps equals 600 watts.
Click to expand...
That's a great tip - as long as the original fuse is in there.
 

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