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Old 1st July 2008, 08:20 PM   #1
pjpoes is offline pjpoes  United States
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Default Measuring an amplifier with my humble devices

Ok I'm wondering what the best way to measure an amplifier would be given the testing equipment I have. A Tektronix 2430A digital storage scope, digital multimeter (Not an RMS meter), 4x eight ohm power resistors rated at 500 watts each from ENAPROS (Called a Dynamic Braking Resistor), and a computer/stereo for test tones (No actual function generator as of yet). I'm wondering what I can can test with this equipment, and the best way to do so.

My understanding is that at a bare minimum I should be able to measure an amplifiers output power up to clipping. I could see the clipping by the change in the output waveform on the scope (right?). To do this, I hook up the load, input a test tone, and increase the level until clipping is seen. I then back off to just before clipping, and look at the voltage. I measure the resistance of the load, and input the resistance and voltage into the formula E squared / R. Is this all correct? Will this then give me the correct maximum watt rating before clipping? Does the Oscilloscope give an RMS voltage reading, or no? I know very little about this oscilloscope, it was a "Gift" from the particle physics department of Purdue.

I've been told you can measure SINAD using one of these, but I'm assuming that requires a filter, correct. Without a notch filter to remove the fundamental, I would not have the ability to take distortion measurements, correct? What about S/N? It has the ability to display in db's instead of volts, but I was assuming this really just gave me the total noise. Again, without a filter to only look at the audioband, I'm assuming I don't have the ability to measure S/N correctly. I think I can filter it to 100khz, which I think I have seen used for audio measurements before.

Now to ensure I don't break my O-scope, while using it with this setup, lets make sure I understand how to hook it up. I connect the ground lead to the negative, but only for non-bridged amplifiers, correct? I assume this is for any amplifier using a bridged or "balanced" topology, correct? In that case, I just connect the ground to chassis, and the other lead to the positive? With most amps however, I'm assuming I connect the main test lead to the positive, and the ground to negative (i.e. Black, ground, etc.). These leads are connected in parallel, essentially, with the load, correct?

Anything else I can do with this setup to get useful measurements of my amplifiers? Thanks for any and all help, while I sometimes think I know what I'm doing, it still helps to ask these very simplistic questions.
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Old 1st July 2008, 11:59 PM   #2
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Nice scope. You can measure power with the multimeter, but be
aware that most multimeters are accurate around 60 Hz or so,
and often not very at high frequencies. Of course you can read
the voltage off the scope - for a sine wave, the rms value is
the peak to peak divided by 2.83.

Assuming that you like building things, you can assemble a good
enough set of tools to measure the things you want. Maybe you
would like to start with a function generator....
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Old 2nd July 2008, 01:07 AM   #3
jcx is offline jcx  United States
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all but the cheapest built in sound card in a PC should manage >80 dB S/N, <0.1% distortion so you have a good start right there

you can check sound card performance with free version of RMAA
http://audio.rightmark.org/products/rmaa.shtml

you need attenuation for the ADC input if you want to measure power amp levels

a very high performance/price pci sound card is the ESI Juli@ for ~$130 if you want more than your motherboard chipset delivers
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Old 2nd July 2008, 01:17 AM   #4
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Not all amps have their negative outs connected to ground so be careful! Sometimes the positive is ground and signal is on the negative output but inverted. Sometimes one channel has positive as ground and the other negative as ground.

Connecting the ground lead to chassis is safer.
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Old 2nd July 2008, 01:37 AM   #5
Leolabs is offline Leolabs  Malaysia
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If you have a true rms multimeter,a good quality soundcard,signal generation software,plus all those instrument you are having right now,then it's enough.
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Old 2nd July 2008, 03:00 PM   #6
pjpoes is offline pjpoes  United States
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Yeah the scope turned into an awfully nice gift. I wish I knew how to use half of its functions. I'm still figuring things out. Many of the scientists here at Purdue have a preference to HP scopes, and this bias makes them think that everything Tektronix is "Broken." When they gave it to me they said it was broken, when I asked what, they said, well its not an HP. It's better than anything I could have afforded, so I can't complain.

I'm definitely not afraid to make things myself, and I think you are right, a function generator would be a good start. I have a pile of opamps, perf board, caps, and resistors just waiting to be put to good use. I've also see some web sites offering instructions for notch filters, that I could use for Distortion measurements.

As for this computer business, was this all about using the computer as a Function generator? I have that ability now, I guess I should have added that, as its what I currently use for a Function generator. Just because of location issues, I prefer to have separate tools if I can, its a bit inconvenient to have to bring everything into the room with my computer. None the less, I intend on purchasing some measurement equipment that require the computer, and so this may become my measurement room.

My understanding about using the computer for S/N and Distortion Measurements is that it will be limited by the quality of the ADC's and noise there in, as it will have to use digital internal notch filters. What I was told was that, even with the very best ADC's available, I would be very limited in my ability to measure very low distortion and noise.
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Old 3rd July 2008, 05:47 AM   #7
fotios is offline fotios  Greece
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Quote:
Originally posted by Nelson Pass
Nice scope. You can measure power with the multimeter, but be
aware that most multimeters are accurate around 60 Hz or so,
and often not very at high frequencies. Of course you can read
the voltage off the scope - for a sine wave, the rms value is
the peak to peak divided by 2.83.

Assuming that you like building things, you can assemble a good
enough set of tools to measure the things you want. Maybe you
would like to start with a function generator....
I would like also me to build a passive notch filter, let us to say in 1KHz, because i can't trust any of those PC sound card based multi-instruments. Of what order - at least - must be this filter to reduce the fundamental frequency in a sufficient level so as we can take a reliable measurement of THD with our trusty DSO?

Regards
Fotios
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Old 4th July 2008, 02:20 AM   #8
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take the oscope, set it up for ch1/ch2 add mode, and set ch2 to invert. this subtracts ch2 from ch1. your sound card output goes to the amp input and scope ch1. amp output goes to an 8 ohm dummy load and scope ch2. null the scope display by adjusting the ch1 and 2 gains for a signal null. the residual on the screen is the distortion. a little bit of math will give you the distortion. you must know the amp voltage gain.

not exactly a real distortion analyzer, but you don't need filters.
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