Power Output measurement with a Scope and Generator

[K O'Connor]

Hi Guys

Performing an output power measurement on an amp is simple if you have an oscilloscope and a sine wave generator. You also need  bench load, which is just a power resistor of suitable resistance and wattage rating.

Connect the generator to the amp input; connect the load to the output; connect the scope to the output. Set gain or volume to zero.

If the generator has a SYNC output, which can be called a TRIGGER, tie this to the EXT TRIG (external trigger) input of the scope. Set the scope to EXT triggering. This connection will keep the wave displayed on the scope from drifting from side to side.

Turn on the generator, scope and amp. Dial volume up until you see the sine wave get clipped - just a small amount of flattening is all you need to see. Then back off just enough for the peaks to be round again. measure the peak-to-peak voltage then dial volume to zero.

Divide the voltage reading in half for a peak value, then calculate peak power, then divide that by half for continuous power.

Why use the peak-to-peak voltage? It reduces a bit of error reading the value off the scope screen. of course, you might have a scope with built-in metering that can tell you the RMS voltage, in which case you can calculate the continuous power directly.

[soundmasterg]

Hi Kevin,

I have a scope and a signal generator, and a true RMS DMM, and a suitable bench load. What I have done is to put the signal generator signal into my amp once I have verified the AC voltage level from the generator. If I remember correctly I had it set at 120mV to simulate a typical humbucker guitar and I used 1kHz instead of the 400 Hz that you suggested. I have the amp output plugged into an 8 ohm resistive load, which is probably 100-200 watts. These are huge resistors. Then I bring the volume of the amp up and play with the controls until I get the max signal before clipping, watching the sine wave on the scope. I back it off just below the clipping threshold and measure the output with my DMM as an AC voltage. I then square that voltage reading on the DMM and divide it by the resistance to see what the power output level is.

Is this an accurate way to find out what the RMS power output of the amp is?

Greg

[K O'Connor]

Hi Greg

Yes, what you do is use the scope for the visual indication of the signal purity and then take the reading more conveniently from the meter. It's always good to make full use of the tools you have at hand

Have fun

[jmcd]

I finally got my hands on a signal generator. Immediately, I used it along with my scope to test the output power of some of my amps, including one with the Kevin's 10W power transformer. Sure enough, that amp was pumping out 10W of continuous power.

Another amp with a 20W power transformer set was pumping out... 15W!

Thanks KOC - for the testing guide and for the great PT!