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Old 31st May 2017, 11:06 AM   #1
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Default amplifier output measurements

hi I have a b&o icepower 50asx2-se (50w into 4R) what I want to do is measure the watts output
I have a 4R dummy load, multimeter and oscilloscope and audio signal generator (on my phone) could someone walk me through this please
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Old 31st May 2017, 11:56 AM   #2
AndrewT is offline AndrewT  Scotland
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connect the load.
connect the scope probe ground lead to the grounded output of the amplifier. You must NOT connect the ground lead to an ungrounded output. That would likely blow up the amplifier.
Connect the hot probe to the hot output.
Attach the leads of the voltmeter to the output of the amplifier. If you have a second voltmeter , then connect that to the input of the amplifier.
Attach your test signal at zero volts output (use a vol pot with a big knob) to the input of the power amplifier.
Slowly turn up the test signal and see the waveform on the scope. You should see a sinewave.
If you don't see a sinewave, then stop and find out why the signal is not getting though properly.
Now disconnect the dummy load and increase the test signal until you just detect a tiny bit of clipping of the output signal. Go back slightly until the slight clipping disappears. Measure the AC voltage. That is the maximum output voltage into an infinite load impedance.
Reduce the test signal to about 70% (~-3dB), momentarily attach the 4r0 load. Is the signal clipped or clean. Disconnect the load.
Increase the signal a couple of percent (+0.2dB) and check with the load to see a clean or slightly clipped. disconnect.
Repeat this +0.2dB increase remembering to disconnect the load as soon as you see/measure the signal on the scope. When you see a slightly clipped signal, then go back 0.1dB and check again. Now you have the maximum unclipped AC voltage into a known almost cold 4r0 dummy load. Expect AC voltage into the 4r0 to be 70% to 95% of the infinite loading.

But a speaker is NOT a resistive load.
Reduce your load to 2r0 (a very reasonable test for a 4ohms capable amplifier) and repeat the whole test/measurement.
Expect the output voltage into the 2r0 load to be somewhere between 60% and 90% of the 4r0 loading.


A good 4ohms amplifier will show a fall of -0.6dBV into a 2r0 resistive load.
A good 4ohms amplifier that has very tolerant IV limiting allowing severe reactance speakers should be able to drive significant voltage into a 1r3 dummy test load.

An alternative to avoid having to repeatedly disconnect the dummy load.
Attach a -20dB switch to the input signal.
You adjust - switch to +0dB - read scope - switch to -20dB - allow to cool - switch to +0db - read voltmeter - switch to -20dB - adjust allow to cool and repeat.

The switching signal, or disconnecting the load, does two important jobs. It stops the load overheating and possibly changing resistance value (and setting fire to the workbench) and it stops the heatsink getting too hot. The amplifier should remain at normal operating temperature during this test. You are NOT testing the quality of the aluminium used to make the heatsink.

There is a separate test usuallly at 30% to 40% of maximum output power that tests the effectiveness of the thermal design. But this comes AFTER you have found the maximum output power. And the amplifier gets very hot.
There is a further test. Begin with 1000Hz test signal, then repeat for 100Hz and compare, then repeat for 10kHz and compare.

This is the DIY version.

The manufacturers look for the maximum output power with the THD meter used to limit the test signal so that maximum THD never exceeds 0.1% (this is their way of specifying no clipping). We do it visually if we don't have a THD meter.
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Last edited by AndrewT; 31st May 2017 at 12:15 PM.
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Old 18th June 2017, 08:27 PM   #3
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Thanks for sharing the information, i was about measuring my new design, and it don't work because i forgot to solder one CMS resistor on the mute pin
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