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Old 25th July 2009, 04:29 PM   #1
doug20 is offline doug20  United States
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Default How do I measure amps?

I have many amps and I would like to compare them using other methods.

I currently measure the loudspeaker response with each amp and I do listening but I would like to connect the outputs to something that will allow to capture the frequency response without room interferance.

I can capture the pre outs of my AVR and I want to see the difference if any.
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Old 25th July 2009, 08:58 PM   #2
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Default Re: How do I measure amps?

Quote:
Originally posted by doug20
I have many amps and I would like to compare them using other methods.

I currently measure the loudspeaker response with each amp and I do listening but I would like to connect the outputs to something that will allow to capture the frequency response without room interferance.

I can capture the pre outs of my AVR and I want to see the difference if any.
Hi Doug,

What measuring equipment do you currently have already?

jd
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Old 25th July 2009, 10:47 PM   #3
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Measuring amps can be tricky (and even dangerous to the equipment!). Right Mark Audio Analyzer (RMAA) is a great free software application that can be used to determine distortion and other parameters, but it's not trivial to measure a power amp (or receiver etc) using a PC soundcard. There are many issues such as:

The power amp can put out up to 200 volts peak to peak and most sound cards can only handle about 10 volts peak to peak or even less. So you need a resistor divider to drop the voltage to something the sound card can safely handle without overload, clipping or damage. You really need to know what you're doing as one wrong connection, or the wrong value resistors, etc. and you can easily damage your soundcard/motherboard/etc.

Some amps use bridged outputs or otherwise don't like either of the output terminals grounded. If you connect the output of this type of amp to ground (i.e. your PC soundcard input) you might cause big problems--you could damage the amp and/or the soundcard (and possibly even the PC).

Even for amps that have an output at ground potential, some of them don't like that same ground being common with their input ground. Again, this can create a problem with RMAA and using a souncard to test an amp. The resulting ground loop can easily introduce enough hum and noise to badly corrupt your measurements. And a few amps like to oscillate under these conditions and might self destruct in the process.

Those who test amps often have equipment that allows them to carefully monitor the amp under test including its power consumption from the AC line, the outputs on a scope, etc. This allows the amp to be immediately shut down if anything bad (like oscillation) might happen to hopefully prevent it from failing.

You need "dummy loads" if you want to test the power output capability. These are ideally non-inductive large power resistors rated at 8 ohms with a power rating that exceeds your largest amp. You can't pump full power test signals into a pair of speakers and expect the speakers to survive. Music has a much lower peak to average power and different frequency spectrum than typical test signals. If you want to also test into 4 ohms, multiple channels at once, etc. you need multiple dummy loads.

An oscilloscope is very useful for observing the output waveform, voltage levels, clipping, etc. Even a relatively cheap small handheld digital scope (they start around $150 or so) will work for basic measurements. You can play test signals from a wave file or CD and observe the output on the scope with the amp driving a dummy load. In this way, you can determine power at clipping, etc. Battery powered scopes have the advantage of being isolated from AC ground solving the ground problems mentioned above. AC powered scopes, and PC (USB) scopes usually have grounded inputs.

A DMM can be used to measure RMS voltage levels and, with a good one, you can get more accurate power and noise readings. But most cheap DMM's are only specified for accuracy at (or near) 60hz. Some are horribly inaccurate at higher audio frequencies (even 1000hz). So you need to know the specs before trusting any DMM measurements. The high-end Fluke DMM's, for example are accurate to beyond 20 Khz but their cheaper models are not.

At least a DMM that works at audio frequencies or a basic scope are nearly essential to set levels when using any kind of analyzer like RMAA. Otherwise you have no idea what the levels are when using any PC software.

So if you just want basic power outputs at different frequencies and impedances, signal to noise ratios, etc. you can get there with a few basic instruments. If you want detailed measurements including THD, exact frequency response, noise floor, etc. then you need something like RMAA but have to deal with all the amp-to-soundcard issues above. RMAA's abilities are limited by quality of the (internal or external) soundcard. You likely have to spend at least around $100 for an external soundcard that will exceed the specs of a high quality amp at 1 watt. Plus the dummy loads, divider networks, cabling, etc. And that will only work for amps that can handle having their output grounded.

If you want a plug-and-play solution to go beyond the above you're looking at something like an Audio Precision system but they start around $10,000 and go up from there. Another option is older used "analog" test equipment like AC voltmeters, distortion meters, analog scopes, etc. You can find examples on eBay. But few measure amplifiers that way anymore.

Just remember power amps can put out enough power to make your soundcard, PC, test equipment, etc. go up in smoke and the amp itself may well die in the process. I actually had a big power amp start a fire on my bench once. An externally mounted output transistor literally exploded and caught a nearby paper towel on fire from the flying molten metal. It left a small "crater" in the heatsink. I wish I had pictures to bring to Burning Amp this year
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Old 25th July 2009, 11:22 PM   #4
xaudiox is offline xaudiox  Philippines
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very good post RocketScientist. thank you
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Old 27th July 2009, 02:57 AM   #5
doug20 is offline doug20  United States
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Janneman, I have WT3 (woofer tester) and I have measuring mic and software

-profesionally calibrated ECM8000
-Arta, TrueRTA, REW
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