Determine Output Power

[BluesDude]

I have a stock weber 5e3 kit. I could not find any specs on the rated output power (watts RMS) of the amplifier.

Does anyone know the common power output of these amps? How could I take some measurements to determine the output?

Thanks.

[Jazz P Bass]

Power Output

Power output of an amplifier is determined by measuring the RMS voltage at the speaker output. Square this voltage & divide it by the load.
Ideally you would want to have a sinewave generator to drive the amp.
Although strumming real hard on a guitar will do.
If you do not have a true RMS volt meter, mutiply the output voltage by 0.707 = VRMS.
An oscilloscope is ideal for this test, as it will show when the amp starts to clip.
There is no real value in measuring the clipped signal power.
Here is a good link:Audio power - Wikipedia, the free encyclopedia

[fyl]

You can expect 12 to 15 watts.

[Vortexion]

(EDIT: I've been writing this for a while, so I missed the above answers)

To measure the actual output of an amplifier you need a few pieces of test equipment:

1) An audio signal generator capable of producing stable, clean sine waves - or failing that, a CD of test tones and a CD player that you can put on repeat.

2) A good, stable, properly calibrated oscilloscope.

3) A dummy load of the correct resistance to match your amp's output tap. This dummy load must be capable of handling at least the full power of the amp, and preferably a good deal more. This is essential. You can't get away with using a speaker - partly because by the end of the testing process you will end up deaf and your neighbours will hate you (even more than they do already, hehe ), but also because a speaker is a reactive load with a lumpy impedance curve, so it will give a false result.

So, let's assume that you've got all these items. Here's what you do:

1) Connect the dummy load to the amp.

2) Using a coaxial lead and insulated crocodile clips, connect the output of your signal generator to the input of the amp's phase invertor valve - i.e. the signal generator's Negative connection should be clipped securely to ground at a point as close to the PI valve as possible, and the Positive connection should be clipped to whichever valve pin corresponds to the grid of the PI - in the case of your amp this is Pin 7 of V2B.

We're injecting the signal here because this is the last stage of the amp just before the signal gets split in two to feed the power amp itself. This ensures that our signal goes in clean, unaffected by the settings/behaviour of the earlier pre-amp stages, EQ stacks etc.

3) Connect the oscilloscope across the terminals of the dummy load. Set it to a suitable voltage range (10 Volts per division is a good starting point) and sweep rate (0.5mS per division would do). Ensure that the scope's input amplifier is set to its default calibration point (otherwise your measurements won't be accurate).

4) Switch on the signal generator, allow it a good 10-15 minutes to stabilise, then set it to produce a 1kHz sine wave. To begin with, leave its output level very low.

5) Switch on the oscilloscope, allow it 10-15mins to stabilise, then adjust the position of the sweep trace until it is sitting on the centre line of the graticule.

6) Switch on the amplifier and allow it to warm up. Ensure that all of the input channel volume controls are turned down to zero. Switch it out of standby.

7) Slowly start to increase the level of the signal generator output, and watch the sine wave growing in height on the 'scope screen. If necessary, re-adjust the scope's voltage range to keep the trace visible within the height of the screen.

8) Keep on increasing the output level of the signal generator until you see the top and bottom peaks of the sine wave start to flatten out. The amplifier has now hit its threshold of clipping.

9) Back off the signal generator's output level until the sine wave looks the proper shape again. The amp is now working at its full undistorted power.

10) Using the scope's screen graticule, measure the number of divisions between the positive and negative peaks of the waveform and multiply this by the volts-per-division setting of the scope. The figure you end up with is the peak-to-peak amplitude of the amplifier's output signal, in volts. Write it down.

11) Switch off the amplifier - having been run flat out for several minutes, it will appreciate having a rest! Note: don't touch the dummy load: it will be HOT.

Now we just need to do some simple calculations in order to turn that peak-to-peak voltage figure into something more useful. So grab a calculator and...

1) Divide the peak-to-peak figure by 2.82. This gives you the RMS voltage.

2) Multiply that figure by itself (i.e. square it)

3) Take the resulting figure and divide it by the resistance in Ohms of your dummy load.

The number you finish up with will be the amplifier's power output in Watts.

Example:

Let's say that our peak-to-peak measurement was 55V and that we have an 8Ω dummy load.

55 divided by 2.82 = 19.5 (I've rounded it down to the nearest whole number)

19.5 multiplied by itself = 380.25

380.25 divided by 8 = 47.53 Watts

Job done.

P.S.: Another, more accurate way of measuring the signal is to get hold of a DMM that can do "true RMS" measurement of sine waves. Connect it across the dummy load along with the oscilloscope, and instead of counting the divisions on the scope screen, you just take the reading from the meter. This will be the RMS voltage figure, so you won't need to do the "divide by 2.82" part of the calculations. Just multiply it by itself and divide the result by the dummy load's resistance.

[MWJB]

"To measure the actual output of an amplifier you need a few pieces of test equipment:" - Yes, a guitar, a cable, the amp and an RMS Voltmeter. Hook the voltmeter accross the speaker terminals, play, determine an average voltage (reading will fluctuate), square the average voltage the divide by speaker load, you might see around 18W+/- 1 or 2W.

That is your "actual" output. Vortexions instructions are for measuring "RMS clean output". Clean RMS output is, as Fyl says, 12-15W (check out the "57 Deluxe" schem at Fender.com for datuim voltages & amp settings), but most people don't play a 5E3 in its clean range.

[txstrat]

It's always very interresting to follow those explanations. Unfortunately I don't (and I believe many others as well) have an o-scope and neither have a signal generator.
I found the attached file on the net. What would you say how accurate or usable it is? Or can I just tear it apart (well, maybe I should print it out before )

I measured the voltage at the speaker of my DR when I hit a chord on the guitar. It was around 16 volts. According to the chart that would be around 30W@8 ohms. I know this would not be RMS since a guitar amp is able to put out more than it's RMS rating only thing it wouln'd be CLEAN power.

[MWJB]

If you have an oscilloscope, then it makes sense to use it (I don't own an oscilloscope either, never found a use for it yet)...but if you don't, then you can always use your ears...you'll still get a ballpark figure.

The chart is fine, anomalies in accuracy will be down to to how well power tests are executed (whatever the equipment used).

When you strum a chord hard, it is likely that you are briefly exceeding the RMS clean rating of your amp, which will for short periods make more than 25W. That's why you should play normally & try and determine the average voltage at the speaker, at a clean setting if you want to determine RMS power. Your "real world" power, in normal gig situations may exceed the RMS rating...but the RMS rating can make it easier to spot if the amp is distorting too early/lacking in proper headroom.

The thing with measuring things either way, is that measurements are really only useful when you have some kind of datum to compare to. If you have tolerant neighbours/ear defenders then you can download a constant tone from a PC signal generator (a constant tone just makes it easier to read the voltage developed at the speaker, without the reading jumping about). Always actually measure AC voltage applied at the input with a voltmeter. Again, as for the 5E3, Fender have AC voltages & settings listed for the DR at Fender.com.

[Vortexion]

"To measure the actual output of an amplifier you need a few pieces of test equipment:" - Yes, a guitar, a cable, the amp and an RMS Voltmeter. Hook the voltmeter across the speaker terminals, play, determine an average voltage (reading will fluctuate), square the average voltage the divide by speaker load, you might see around 18W+/- 1 or 2W.

I'm sorry, but doing this would be a complete waste of time, as any 'measurement' it produced would be meaningless. You're asking him to undertake an unrepeatable test with no constant parameters. "Determine an average voltage (reading will fluctuate)" - in other words, lick yer finger and guess! And why take the care to use an RMS voltmeter, when such devices can only deliver an RMS measurement from a SIne Wave?

Vortexion's instructions are for measuring "RMS clean output".

Yes. That's how amplifier output is measured. Whether or not the amp will be run into distortion when actually used is irrelevant. That's just creative use; that's music. It has nothing to do with the rated output power of the amplifier, which is what BluesDude wanted to find out.

[MWJB]

Measurements are not "meaningless", they are what they are...if you have a datum to work from they than become meaningful.

Your test is fine (apart from injecting the signal at the PI - what if you have an issue prior to that point & why disassemble the amp, just to make a power reading - your into chargeable time there already, before you even know if there's a problem?) but assumes the OP has access to an oscilliscope - if he did & knew how to use it, he wouldn't need to have posted.

"Lick your finger & guess"? - not exactly. Try it. You'll find that if you are careful & use common sense (a requirement for any such tests) that you'll get ball park figures, enough to point to a malfunction - even RMS figures can vary from amp to amp & how the test is performed, unless you have recorded data from a range of amps, of the same model, there's still a degree of "leap of faith".

Some amps are never run at their RMS rating...if they make that & no more, they are just as broken as an amp that doesn't make proper RMS W (feasibly the case with an untested build). So it's a good idea to measure both RMS W & power under typical load.

All amps, at the end of the day are evaluated by people's ears, if a reasonable RMS reading was all it took to ensure a great sounding amp, they would all sound great & none would be better than any other...put a few amps of the same type in a room & play them and you'll see that the reality isn't that simple.

[BluesDude]

Everyone... Thanks for the comments. Very useful information.

[Jazz P Bass]

RMS voltage

Seems to be some confusion on the meaning of RMS.
All it is , is a proper way to measure a sine wave.
Link:The RMS value of a set of values (or a continuous-time waveform) is the square root of the arithmetic mean (average) of the squares of the original values (or the square of the function that defines the continuous waveform).