How does VOLTS = WATTS???

Is this amp OK?
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SirNickity
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Re: How does VOLTS = WATTS???

#16 Post by SirNickity »

Grant Bunter wrote:You may find a sine wave into the amp input will not fluctuate as much as pink noise...
I've been led to believe using anything other than a sine wave is asking for trouble. I tend to overthink things in an effort to understand why the world works the way it does, so here's how I figure this works. Throw tomatoes if I botch it up...

When you measure AC, you're ideally measuring RMS. Whether your meter is capable of true RMS or merely simulated is a matter of cost, but for a sine wave source, it should be pretty close either way. The wave shape matters though... A sine wave average voltage is 0.7 times its peak. A square wave is ideally 1.0 times its peak.

Now, I haven't tested or researched this yet, but I've been pondering it lately. Here's how I put the pieces together in my head to where it makes sense, and is hopefully also accurate and correct. (Take with a grain of salt for now.)

The excursion of a driver is going to follow the wave shape, so it will move by an amount that is represented by the voltage, peak to peak. However, you can't measure peak voltage on most meters, only average. (On a scope, you could... but not everyone has a scope.) So, the voltage limits have to be expressed in RMS. Therefore, using a wave shape other than a sine will cause the average to shift, despite having the same peak. This would cause you to over-limit the amp if you calibrated with, e.g., a square wave.

OTOH, using a wave shape with less average power might cause you to under-limit the amp, such that peak voltage is much higher than you expected.

Tada! Now... did I get it right?

byacey
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Re: How does VOLTS = WATTS???

#17 Post by byacey »

Gregory East wrote:If you follow the maths gingerbread trail

Power = volts squared / impedance
This is the most direct route formula. Make sure it's RMS volts that you use in your calculations.
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byacey
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Re: How does VOLTS = WATTS???

#18 Post by byacey »

SirNickity wrote:
Grant Bunter wrote:You may find a sine wave into the amp input will not fluctuate as much as pink noise...
I've been led to believe using anything other than a sine wave is asking for trouble. I tend to overthink things in an effort to understand why the world works the way it does, so here's how I figure this works. Throw tomatoes if I botch it up...

When you measure AC, you're ideally measuring RMS. Whether your meter is capable of true RMS or merely simulated is a matter of cost, but for a sine wave source, it should be pretty close either way. The wave shape matters though... A sine wave average voltage is 0.7 times its peak. A square wave is ideally 1.0 times its peak.

Now, I haven't tested or researched this yet, but I've been pondering it lately. Here's how I put the pieces together in my head to where it makes sense, and is hopefully also accurate and correct. (Take with a grain of salt for now.)

The excursion of a driver is going to follow the wave shape, so it will move by an amount that is represented by the voltage, peak to peak. However, you can't measure peak voltage on most meters, only average. (On a scope, you could... but not everyone has a scope.) So, the voltage limits have to be expressed in RMS. Therefore, using a wave shape other than a sine will cause the average to shift, despite having the same peak. This would cause you to over-limit the amp if you calibrated with, e.g., a square wave.

OTOH, using a wave shape with less average power might cause you to under-limit the amp, such that peak voltage is much higher than you expected.

Tada! Now... did I get it right?
Correct. As I mentioned, it's important to ensure the real RMS value is used for calculations. How you arrive at that number is of equal importance to maintain accuracy. Best thing is to use a meter capable of true RMS, and use a sinewave signal to simplify things.

If you are using a scope, measure the peak voltage with respect to the zero crossing point of the sinewave on the graticule, and multiply by .707 to calculate the RMS value. Make sure your scope is calibrated, but that's a whole other thread.
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Grant Bunter
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Re: How does VOLTS = WATTS???

#19 Post by Grant Bunter »

Nick,
I've been thinking through your post.

I said sine wave rather than pink noise, because:
sine wave is at one frequency (ideally somewhere in the middle of a pass band for the box your testing) but for an amp that is kinda irrelevant.
With pink noise differing frequencies will tend to make your voltage reading fluctuate more than a single frequency was my thinking. That could be wrong as well...

As far as limit for various drivers, that was discussed at length in another thread.

For the purposes of the OP's original post, sidetracking the discussion is also kinda irrelevant, and for that, I apologise.
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SirNickity
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Re: How does VOLTS = WATTS???

#20 Post by SirNickity »

You're probably right. I haven't watched a meter with a noise source, but I wouldn't be surprised to see it vary. I might try that just for fun. I forgot we were talking about measuring amps, and not so much about setting limiters. Still, if you're doing so with the intention of judging suitability for a particular cab/driver, it's probably best to follow the limiting procedure anyway.

Hm, some things to think about, for sure. Might be a bit off topic, but I think the OP's original question is answered and this is probably relevant to why they were asking. If not, my apologies too. :-)

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mk5150
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Re: How does VOLTS = WATTS???

#21 Post by mk5150 »

actually I am just taking it all in, i hear you guys talking about sine waves and pink noise, where can i get a sine wave to play through my system? I guess you can get different frequencies? Im new to doing that and i've always adjusted accordingly with either a recorded track or live band situation, but the way you guys explain sounds like the correct way and like i said, im taking all this in and learning from you guys the correct ways, by the way, my first 30in wide titan 48 is just about complete!! The first of many more titans as long as they live up to their reputation, and also will be building the dr250's, thats another day and another thread

ketoet
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Re: How does VOLTS = WATTS???

#22 Post by ketoet »

the titans won't let you down. the dr250's won't dissapoint neither.
i previously had a mackie active system and thought it was pretty good.
the 4 titans 48 and 4 dr250 i have now make the mackies pee their pants.
and they're a lot cheaper :-)

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Gregory East
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Re: How does VOLTS = WATTS???

#23 Post by Gregory East »

audacity is a free program. Follow the instructions in our tips section for setting limiter with voltmeter.
Last edited by Gregory East on Sun Aug 12, 2012 7:22 pm, edited 1 time in total.
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gzc_stageTech
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Re: How does VOLTS = WATTS???

#24 Post by gzc_stageTech »

mk5150 wrote: . . . where can i get a sine wave to play through my system? I guess you can get different frequencies? . . .
another option is the TrueRTA. It is also a great analyzer 'for your toolbox'. I have the 24 dB version and its well worth the investment. As for an iPad version, I also have the StudioSix Digital analyzer package. It too has an integral signal generator. Plus, it has a sweet SPL meter.

But you do need a reliable DVM (volt meter) for voltage measurements.

Oh, and you'll love the kick and bass thru the Titans, especially a cluster of 4 V-stacked. It is different sound than you'll hear from a dual 18 direct radiator sub tho. The Titans are the way bass should be...clean, without cone break up distortion.

Welcome home....
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byacey
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Re: How does VOLTS = WATTS???

#25 Post by byacey »

In order to measure and determine the RMS power output of an amplifier, you need a steady, repetitive signal waveform to measure. Noise is random and not repetitive simply because it is a series of random events, and the level does indeed jump up and down in a random nature, as it should. It's also near impossible to determine at what point a random noise waveform begins to clip at the output; with a clean sinewave, this is very easy to view on a scope.

The way I test amps after servicing is with a large 4 Ohm dummy load resistor on the output, with a scope connected across the resistor. I feed the amp a 1KHz sinewave and advance the input level until I see the peak of the sinewave begin to wobble and flatten a little. The wobble is created by 120 Hz ripple on the power supply rails. Of course, in an amp with a switching supply, you wouldn't see this. At this point I measure the peak voltage on the scope, and multiply the peak voltage by .707 to arrive at an RMS value. Also, you can sweep the frequency across the audio bandwidth to see what the usable bandwidth of the amp is. Most amps will do at least 20 Hz to 20 Khz, some will work a little lower and higher. Don't do this for any longer than it takes to get a reading; it's hard on the output devices.

If you square the measured RMS voltage, and divide that number by the resistance value of the dummy load, this gives you the real world power output of the amp in watts. Theoretically, a 4 ohm load equates to exactly double the power output vs an 8 ohm load, but with many amps it doesn't work out exactly right. This is because the source resistance of the amp being something above 0 ohms, due to the output transistors being used, the value of the emitter resistors, etc.. A better quality amp will usually come closer to the theoretical value of double the power when the impedance is halved.
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