How to work out the current an amplifier uses

[No Longer A Member 220516]

Hi I need some help finding out how to work out the ammount of current an amplifier uses. As an example lets say you have a 2000watt amplifier but of course it wont always be sat at 2000watts output if at all. So if say you have two speakers and each one is rms at about 400watts each. How would you calculate the ammount of Current (Amps) it will be drawing from say a ring main.

Is this a possible thing to work out or does anyone have any round about figures. The main reason for my thinking behind this is having a small generator and wanting to know roughly what power/current I will need.

 

Thanks in advance

 

[Yorkie]

As far as I know, there's no way of calculating the mains power drawn unless you know the precise efficiency of the amplifier. Best thing I can think of is for you to get one of those plug in Power Meters (eg http://www.maplin.co.uk/Module.aspx?ModuleNo=223573) and try it out on mains. Alternatively, just read the amp's manual or rating plate which will allow you to have a conservative figure of the total power drawn, amp at full whack.

[mkfs9]

A 2000 watt amp always puts out 2000 watts. It is the input signal you increase when turning up the amp. So be carefull if your cabs can only handle 400 watts. The amp should have power consumption on the rear somewhere. go for the max it will use then you will have spare watts to plug something else in.

[No Longer A Member 220516]

hi guys thanks for reply these are things I do currently use to work out but I want to know in the case when this information is not listed.

Also the 2000watt amp was an example and 400 watts as an average output per speaker as I know and do is match like for like with power on amps and speakers. I work with this equipment all the time but just would like any easy way if any to calculate the current usage! Thanks for the suggestions so far though.

[David Duffy]

A 2000 watt amp always puts out 2000 watts. It is the input signal you increase when turning up the amp.

What the hell???? I'm hoping that's just poor wording. <_<

 

A 2000W amplifier will put out a maximum of 2000W. What I think mkfs9 was trying to say is that the gain of the amplifier is fixed.

 

Current draw of amplifiers is a tricky thing. Maybe best to either measure the actual draw, or use the manufacturer's spec as a guide.

[paulears]

A 2000W amplifier does NOT always put out 2000W, irrespective of the setting of the front panel attenuators. 2000W (or whatever the spec says) is the maximum power handling capability of the output devices, and the spec tells you essentially how powerful it is driven to maximum output by a specific maximum input.

 

Ohm's law will work to let you calculate the figures, if you have all the variables. What is certain is that the actual running current the amplifier will require is far below what you estimate. You cannot even start to work it out when playing your favourite test track. Playing a constant tone allows some sensible results, but who does this. Music that is bass heavy will increase the current, and music with less full range content will be less.

 

The advice to use one of those cheap Maplin meters is actually quite sensible, and in normal use the power consumption is actually much lower than expected, or calculated using estimated variables. Many amplifier manufacturers provide a figure in Amps for power consumption, but is that idling, or an average, or peak inrush current. This can be many times the running current, hence why amp racks that lose their power trip the breaker when power gets re-applied. For what it's worth, I watched one of our visiting bands use one of these Maplin meters - on just one in a rack of 4 Peavey 2600 amps, he was very puzzled that even though the red peak light was just 'tickling' on, it was drawing less than 3 amps. Perfectly normal.

[Simon Lewis]

Hi I need some help finding out how to work out the ammount of current an amplifier uses.

 

The answer is 'less than you think' but Sod's Law states that it my be more than the generator can give.

 

The peak to mean ratio of music means that the actual power being delivered may only be 1/8th to 1/3rd of the potential power output of the amplifier. However, depending upon the type of power supply in use, for the amp to deliver musically useful transients, it helps to have a "stiff" source of mains power. A small generator may not be good at this, and the voltage supplied may drop.

 

Also, depending upon the efficiency of your amp, some of the power drawn will provide useful audio output, and some will simply be turned into heat. This varies by output topology and actual design.

 

If I am sizing up a generator, I would want it to be able to deliver at least 1 to 1.5 times the total output power of the amps to give some headroom. There are other considerations too, such as the generator's ability to maintain a stable output voltage.

 

Simon

[Dmills]

Perfectly normal.

Indeed, I have 4 Vortex 6 which routinely get run off a single 16A socket, but there is something a little subtle to notice.

 

The over current protection when fed from the grid is normally quite slow for overloads of less then about 5 to 10 times the protective device rating, so as long as the voltage drop does not become excessive on a transient you can pull a LOT of power for a few tens or hundreds of milliseconds from a supply that is protected at a fairly low level (More so if class C breakers are in use). Combine this with the fact that peak/average ratio for music is usually reckoned to be about 8 or so, and you can see how my 24KW of amps might be quite happy on a 16A (RMS) supply, in spite of the odd few milliseconds when they draw several times that.

 

Now the critical point with generators is that (particularly with small ones), the source impedance may be much higher then that of the grid, and large transient current demands may both saturate the magnetics, confuse the voltage regulator and cause frequency changes, none of which are good. While working with RMS currents makes reasonable sense when grid connected, small generators (Like the amplifiers themselves) really need to be rated for nearer to the peak demand.

 

A good trick with small generators is to hang a KW or so of static load (site lighting) on there as well as the PA, it makes the rapid power demand variations smaller in percentage terms which helps the voltage and speed regulation.

 

Regards, Dan.

[Jivemaster]

Another vote for the plug in meter from Maplin and others, they are quite common now with people checking energy usage. Even remote reading ones are available.

 

As a rough guide an amplifier will draw about 1.8 - 1/4 it's rated power with live music, 1/4 - 1/2 it's rated power when used with compressed music. Only in industrial applications such as servo driving or the like will power draw approach the rated power, which some amps can't take for thermal dissipation reasons.

 

 

Measure the current draw under realistic levels while playing, the idling current may be tiny.

Edited May 8, 2010 by Jivemaster