# Testing the output of amplifier using a multimeter

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#### Simon16v

How much truth is in this video on testing the output wattage of amps?

It all seems to make sence in my head but i've tryed this methood on a few of my amps and they are putting out 25watt RMS when they are rated at 300Watt RMS.

I know this is a car amp but surley its same principal on the power amps which im testing.

If this is true im going a buy a new amp

Cheers,

#### Mooly

Paid Member
The method stated in the video is totally wrong. You can not use music to determine RMS levels. The meter has shown and recorded a transient peak in the signal, held on the display by the peak sample and hold facility on that meter. The meter may also only be accurate over a limited frequency range... perhaps only up to 440hz.

To test an amp the conditions must be correct. The amp must be terminated into a resistive load... why... because a speaker is not a pure resistance and it's loading on an amp varies with frequency.

The amp must be driven with a sine wave and the output increased to the point of clipping. Only then can any meaningful measurement be taken.

#### AndrewT

be careful to not confuse your measurement units.
The You Tube presentation talks about measuring the max AC voltage at the speaker terminals. He tells you to select the max AC button option.

This is actually holding the Vpk, not the Vac max.

He uses the formula correctly:- Power (average) = Vrms^2 / Rload = Vpk^2 / Rload / 2

His Vpk = 44.9Vpk = 31.75Vrms (or Vac).
and maximum power into a 2r0 load is ~500W.

But, the amp did not have a load on the speaker terminals. Attach a Dummy Load to the speaker terminals and repeat the test. The measurement will be different. A good amp might be 1 to 2dBV less output 2r0 cf 100k loading
A poor amp will be >3dBv down, a bad amplifier will be >6dBV down.
if that test voltage was 44.9Vpk with a 100k dummy load and fell to 25Vpk when driving a 2r0 Dummy Load then that amp would be -6dBV into 2r0. = rubbish, put it in the bin.

The other problem is that the meter you use must be reasonably accurate at the test frequency you choose to use.
Most DMM are OK @ 50Hz to 200Hz.
A normal test frequency is 1kHz. You would need to get your meter checked for accuracy over a range of test frequencies.

#### richie00boy

Utter stupidity. You have no idea if the amp is clipping or not.

#### AndrewT

when measuring Vpk and the output unloaded, you are effectively at the peak output with a fully charged PSU.

If the amp is clipping slightly or just below clipping the Vpk will be quite similar.

If the amp is well short of clipping then the measurement will be well short of the maximum unloaded output voltage.

If the output is severely clipping, then the Vpk is likely to be the half rail voltage. Once again into a high impedance load this will be little different from a Vpk that just below clipping.

The two problems, as I see it, are meter inaccuracy @ test frequency and the lack of a Dummy Load.

#### wakibaki

Your multimeter probably works OK @ 50Hz (mains frequency). You can drive the amp @ 50Hz using a soundcard, or make a 50Hz CD. You can observe the waveform using the soundcard too, although you'll need a resistive divider.

A dummy load... the cheapest 100W 2R resistor in Farnell is £4.72p. You'd be better with 4 of those, or at least 2 to do 4R.

w

#### AndrewT

................if that test voltage was 44.9Vpk with a 100k dummy load and fell to 25Vpk when driving a 2r0 Dummy Load then that amp would be -6dBV into 2r0.
just noticed a typing error.
half voltage is 22.5Vpk, not 25Vpk for -6dBV.

#### Speedskater

You can do a quick test of the frequency response of your meter using a "Test Tone" CD or Wave file on the meter's 2 Volt Range.
I think the meter's frequency response will be the same on the higher voltage ranges.

#### MJL21193

But, the amp did not have a load on the speaker terminals. Attach a Dummy Load to the speaker terminals and repeat the test.

His meter was set to rms AC with the "peak hold" on - this is the max rms AC reading, not peak voltage.
It's a ballpark estimate, good enough for the car audio guys.

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#### pacificblue

You can drive the amp @ 50Hz using a soundcard, or make a 50Hz CD.

It is very likely that a decent woofer has high impedance at 50 Hz due to its resonant frequency. If you measure the voltage at 50 Hz and calculate the corresponding output power for the nominal impedance, most amps will appear to be monstrously powerful, while really only producing a few Watts.

#### Simon16v

Cheers for all the comments guys, looks like its not that easy to test the output of amps. I guess if you use the same song "a song that bumps" and test different amps (most of my amps have limiters so there not going to clip) then i can read the output to see which gives out more "real" power i guess.

#### AndrewT

don't guess.
You're more likely end up with the wrong answer.

Buy, borrow, or build a dummy load. A combination of four 8r0 200W resistors allow testing of all audio amps upto 1000W into 2r0.

Test and measure the amplifier properly if you want meaningful results.

#### wakibaki

It is very likely that a decent woofer has high impedance at 50 Hz due to its resonant frequency.