how do you bench test amplifiers?

[nVRuckus 5,000+ posts]

What is the procedure to properly bench test an amplifier. What equipment is needed and how do you do it?

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[pwnt by pat 10+ year member]

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[iceteebone 5,000+ posts]

EQUIPMENT NEEDED TO MEASURE OUTPUT POWER

To accurately measure output power, you will need:

1.A regulated D.C. power supply capable of powering any amplifier that you need to test

2.A set of high power resistors. They must be rated to handle the power output of any amplifier which you need to test.

3.An oscilloscope to view the waveform.

4.Some sort of tone generator. A sine wave generator is a good choice but you may be able to get by with a CD player and a test tone disc.

5.A digital voltmeter capable of measuring A.C. voltage. A true RMS meter would be nice.

NOTE:This information is only to let you know how the test is done. If you have all of this equipment, you will probably already know how to make this test.

TEST EQUIPMENT SETUP

Connect the 12v power supply to the power wires on the amplifier.

Connect the power resistors (dummy loads) to all of the channels of the amplifier.

Set the gain controls for all channels of the amplifier to the same level. Either all of the way up or all of the way down will probably be the easiest. You can make fine gain adjustments at a later time.

Connect the tone (sine wave) generator to all of the amplifier channels. Turn the output level of the tone generator all of the way down.

Turn the power supply on. If it is adjustable, set it where you want it (12 volts, 13.8, 14.4 your choice).

Power the remote terminal of the amplifier.

Measure the output voltage of the D.C. power supply (it should be the same as before the amplifier was turned on).

Set all equalization to the off position and set all crossovers to full range.

Set the output frequency of the generator to any frequency that you want. I generally use 100hz. You should always use the same frequency or at least make note of the frequency used during the test.

Slowly increase the output of the tone generator until the output of the amplifier is approximately 2 volts A.C. Now set all of the gains to match the output of all of the channels.

Connect the oscilloscope to any channel (they should all be the same since you tweaked them).

Monitor the output voltage of the power supply, either with your volt meter or by the digital meters on the power supply. If the power supply doesn't have digital meters, use a digital multimeter.

Slowly increase the output of the tone generator while watching the oscilloscope. Increase the level until the top and/or bottom of the sine wave flattens out. Reduce the level until the wave is 'clean' again.

Now see if the power supply voltage has held to the preset value. Reduce the output level of the generator. If the D.C. voltage changed during the test, you must take this into account. A well regulated supply will have held the preset voltage.

Now connect the A.C. voltmeter to the output terminals of one channel. The channel connected to the scope would be a good choice.

Again Increase the level of the generator until just before the amplifier starts to clip. Make note of the voltage reading.

Turn the power supply off.

Quickly disconnect the dummy load of one channel and measure its exact resistance while it is still hot. The value of the resistor will change slightly as it heats up.

Now use the Ohm's law formula, P=E^2/R. If we found that the amplifier drove 35 volts A.C. across the resistor and the resistor had a D.C. resistance of 4.08 ohms, the amplifier produced 300.25 watts RMS.

P=35^2/4.08

P=1225/4.08

P=300.25 watts RMS

NOTE:

1.If the D.C. power supply does not hold its preset voltage, the power output will be lower than the amplifier is capable.

2.This test cannot be done using speakers in place of the dummy loads. The speakers will give you a false high reading at most frequencies.

Test Data:

The following data was taken in a test to show how calculating power when using the voltage across a speaker's terminals instead of the voltage across a non-reactive dummy load will result in false output data. The frequency is the test frequency. The voltage is the voltage that was measured across the terminals of the load (either speaker or dummy load). The power is the calculated power using the speaker's nominal impedance (4 ohms) and the voltage across the speaker's terminals at the various frequencies. The voltage across the dummy load will be virtually the same across the range of frequencies so only one voltage reading was necessary. The amplifier is capable of producing only 205 true watts (RMS power). When using a speaker, it appears to be able to produce as much as 376 watts (which it clearly is not capable of doing).

**edit**

courtesy of http://www.bcae1.com

 

[ramos 5,000+ posts]

You can get rough idea using a true rms clamp meter , and a dmm as well //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[nVRuckus 5,000+ posts]

good reading. When the wave on the oscilloscope flattens as you were describing is that considered clipping? Im actually considering ordering a true rms clamp. but an oscilloscope wil have to wait //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif dummy loads are just regular resistors?

 

[iceteebone 5,000+ posts]

good reading. When the wave on the oscilloscope flattens as you were describing is that considered clipping? Im actually considering ordering a true rms clamp. but an oscilloscope wil have to wait //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif dummy loads are just regular resistors?

yes.

 

[nVRuckus 5,000+ posts]

yes.

to both questions right?

 

[luvinthebass 5,000+ posts]

the wave is flattened at the top aka "clipped" //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

 

[nickk678 10+ year member]

You can get rough idea using a true rms clamp meter , and a dmm as well //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

I'd like to know how one goes about doing it with a clamp meter. I think the majority of people on the boards would have access to a clamp meter much more so than an oscilloscope.

 

[iceteebone 5,000+ posts]

i have access to an O-scope. i don't even know what one looks like or how to use it, but i have access to it //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[nVRuckus 5,000+ posts]

I'd like to know how one goes about doing it with a clamp meter. I think the majority of people on the boards would have access to a clamp meter much more so than an oscilloscope.

my guess would be just like with the oscope except you have to listen for audible clipping by playing tones instead of using sine waves.

 

[Johnny Drama 5,000+ posts]

Clamp method

Clamp the meter on AC amperage around your pos speaker wire leads. Place your DMM on the pos and neg speaker leads.

Play your tone and note highest voltage and highest amperage.

lets say it does 50v and 25a

v*a=wattage

v/a= resistance load in ohms.

50*25=1250 watts

50/25=2 ohm load

 

[iceteebone 5,000+ posts]

so for the tone couldn't you just download it to an ipod and hook it to the amp, instead of making some sort of contraption to play the tone?

 

[thch 10+ year member]

notes:

you may have better luck immersing some large resistors in water.

reactive loading, clipping, and nonlinear loading all present issues with the calculation. each one is a seperate case and should be viewed as such.

if the load is resistive, then either a clamp or DMM could be used, but clipping should be detetected. you can construct a twin-t bandstop for the frequency of interest. this will remove the test tone and leave only distortion, allowing a crude method of measuring distortion using a DMM.