Adjusting an amp with an oscilloscope
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On The Tree
Joined: Apr 2007
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From: Boise, ID
Adjusting an amp with an oscilloscope I've read quite a bit on how to get an amp tuned right with a scope, but everything I've read talks about how to adjust an amp when the amp is capable of over-powering your speakers. The speakers I have coming are rated at 130W RMS and my amp is only 111W RMS/channel. Do I just tune the amp for 111W output? If I go above this, will the amp distort, clip, overheat, meltdown, or ?? I'd like to be able to see if the amp can safely go higher than what it's rated for, but I don't know if I should be looking for distortion or something more subtle.
You can adjust the amp for 111 Watts RMS and that would be safe. Some amps are underrated and if the amp has a birthsheet, go by that. For example, my MTX 6500D is rated at 500 Watts RMS and the birthsheet states 610 Watts RMS. You can use a oscope and look for clipping with a test tone CD. You do not want the amp to clip since that will kill the speakers.
The scope shows a graph of the power. You want to adjust the gain as high as it will go without the sin wave turning to crap. The peaks of the waves should be rounded. When they turn into a square top you have a clipped signal which will damage your speakers.
Everything posted is good info. Blacker mentioned this: You MUST USE SINGLE TONES when using the O-scope. They don't function like a spectrum analyzer where you get to look at an overall frequency spectrum. They are good for deep analysis of single point in time. So you'd set for something like 2K hz and adjust for size and capture area.
To be more specific, it's a function of measuring V/t.So you are actually looking at the voltage output at a specific time junction. 2K hz is 0.0005 sec. (1/Freq) Or the range would be .5 ms or 500 microsec.
Then you'd look at the shape of the wave form like 95bat said. Google "Sine Wave" and memorize the shape. That's what you want to see. It won't be absolutely perfect. No man-made electronic wave form is perfect nor is it perfect in nature. There will be minor deviations from a true line/curve. Tiny spikes along the curve is fine so long as the shape is generally smooth and uniform. Don't get too hung up on the shape if it looks like a normal sine-wave curve overall.
The scope doesn't measure power, just voltage. To get the power you'd have to view the peak voltage and the amprage. Then convert to watts. rms and avg are derived from the peak. ( 1.414 x Vrms= Peak Volts AC and avg being Vavg= .637 x Peak Volts AC) The amplifier works by either voltage gain or amperage gain to achieve a certain Watt output. You may have to use a high-voltage probe to measure some amp's output.
If you are using a Scope to set-up your amps all you are really looking to do is to set the gains so they don't clip. You start with the HU and measure where it starts to clip. (What volume setting). Then you move down stream and set each device up to the point where it won't clip. You are basically establishing your high baseline value.
After this is done, you can go downwards, but going upward will clip the signal depending on the music. It is possible to play your music louder then the measured clip since we all have different values of clipping that we don't hear. BUT, clipping is harder on the speakers and the electronics since a clip is a HIGH DC signal. It would be like moving very fast and then Dead stopping. Momentum and force is great here and can rip a speaker aprt literally.
To be more specific, it's a function of measuring V/t.So you are actually looking at the voltage output at a specific time junction. 2K hz is 0.0005 sec. (1/Freq) Or the range would be .5 ms or 500 microsec.
Then you'd look at the shape of the wave form like 95bat said. Google "Sine Wave" and memorize the shape. That's what you want to see. It won't be absolutely perfect. No man-made electronic wave form is perfect nor is it perfect in nature. There will be minor deviations from a true line/curve. Tiny spikes along the curve is fine so long as the shape is generally smooth and uniform. Don't get too hung up on the shape if it looks like a normal sine-wave curve overall.
The scope doesn't measure power, just voltage. To get the power you'd have to view the peak voltage and the amprage. Then convert to watts. rms and avg are derived from the peak. ( 1.414 x Vrms= Peak Volts AC and avg being Vavg= .637 x Peak Volts AC) The amplifier works by either voltage gain or amperage gain to achieve a certain Watt output. You may have to use a high-voltage probe to measure some amp's output.
If you are using a Scope to set-up your amps all you are really looking to do is to set the gains so they don't clip. You start with the HU and measure where it starts to clip. (What volume setting). Then you move down stream and set each device up to the point where it won't clip. You are basically establishing your high baseline value.
After this is done, you can go downwards, but going upward will clip the signal depending on the music. It is possible to play your music louder then the measured clip since we all have different values of clipping that we don't hear. BUT, clipping is harder on the speakers and the electronics since a clip is a HIGH DC signal. It would be like moving very fast and then Dead stopping. Momentum and force is great here and can rip a speaker aprt literally.
