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Author Topic: Measuring amplifier voltage output with a multimeter  (Read 48551 times)

jack smith

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Re: Measuring amplifier voltage output with a multimeter-A better explanation
« Reply #20 on: October 01, 2009, 01:53:10 pm »

Ivan Beaver wrote on Thu, 01 October 2009 11:52

Let's try a different approach at power handling-ie sub damage.

Let's talk about your body and what it can take.

What is the highest temp your body/skin can take without pain-discomfort or damage?

120
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Art Welter

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Re: Measuring amplifier voltage output with a multimeter-A better explanation
« Reply #21 on: October 01, 2009, 02:31:08 pm »

Jack,

Ivan explained the voice coil heat analogy to running your finger through a candle flame nicely.

Your Celestion speakers got 600 watt PEAKS through them for 5 hours continuous.
That would only be a small fraction of the average power of  a continuous 400 watts RMS of a sine wave, which would be like leaving your finger in the fire rather than passing it through quickly at each kick/snare hit.

Any speaker, low, mid or high will burn up when too much average signal is put through them over a long enough time period.

In the case of HF diaphragms, the voice coils have very little thermal mass, so it does not take long to burn them with a continuous tone, like feedback- a second could be enough.

If you look at your  console meters while music is playing and see 10 dB peaks, your speakers are seeing roughly 1/10th of that power on  average.

If there is only 3 dB peak to average ratio, the speakers are seeing roughly half the peak level.

Big difference, with a  600 watt amp which is just driven below clip in the first case, the speaker sees only 60 watts of average power.
Second case, 300 watts average power. Ram the 3 dB dynamic range music into a limiter, cutting the peak to average range down further, and the average power could approach 600 watts with no clipping, which could burn  a 400 watts rms (AES 2 hours specs) speaker.

Art Welter
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Jeff Babcock

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Re: Measuring amplifier voltage output with a multimeter-A better explanation
« Reply #22 on: October 01, 2009, 02:33:50 pm »

jack smith wrote on Thu, 01 October 2009 13:53


Thank yopu Ivan, thats how I wanted you to talk to me, nicely, and explain it in lamens terms. Not everyone has a NASA engineering degree like you. Now I understand.

By the way, is it the same for subs than mid/high cabs? Or is it only generally subs that are under threat from damage when too much power is put through them?

By that, I mean a couple of hundreds watts more power than they're rated continuous rating. I was just wondering, because I had some Celestion speakers a while back, and they were rated at 400 watts rms (AES 2 hours specs) and I put 600 watts through them for 5 hours continuous and they worked time after time after time flawlessly. Obviously I kept the sgnal at 0db maximum so didn't overrun the system at all.


Jack,
I think Ivan has been extremely patient.  Try to keep in mind that this subject keeps coming up over and over again on this forum.  

You say you understand, but I think you are still not seeing the whole picture.

Look at the question you asked above about tops vs subs.  Why do you think that tops would be immune to the same sort of damage?

Your celestion speaker example, how do you know you put 600 watts into those 400W speakers.  Just because the amp says it is capable of that wattage does not mean you are sending that wattage to the speakers if you are sending music signal to them.

EDIT: Art beat me to it....

jack smith

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Re: Measuring amplifier voltage output with a multimeter-A better explanation
« Reply #23 on: October 01, 2009, 02:38:55 pm »

So the vu meters on a mixer are peak meters are they, not rms meters? Is that why I never ever blown a single speaker then? Because even though I thought with the system running upto 0db after setting gain structure, the peaks were only hitting 0db.
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Ian Hunt

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Re: Measuring amplifier voltage output with a ruler
« Reply #24 on: October 01, 2009, 02:45:19 pm »

Connect condensor mic to input
Engage phantom
Advance all knobs fully clockwise
Adjust all faders fully up
Switch the amplifier on
Advance amplifier gains to just below 11
Stand clear of the speaker
Unplug mic
If the distance between the voice coil and the speaker cabinet can be measured on the ruler
It's too loud.
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Andy Peters

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Re: Measuring amplifier voltage output with a multimeter-A better explanation
« Reply #25 on: October 01, 2009, 06:42:44 pm »

jack smith wrote on Thu, 01 October 2009 11:38

So the vu meters on a mixer are peak meters are they, not rms meters?


An analog VU meter will read -- get this -- volume units. There is a particular time constant associated with this (google can tell you exactly what), but essentially it is some sort of average. But if you put a 1 kHz tone through the system, when your main output VU meter reads 0 dB, the output voltage at the jacks will be +4 dBu, or about 1.23 Vrms.

Now if your mixer has an LED ladder meter, RTFM to determine whether it is a true VU meter, a peak-reading meter, or some random-response thing. Having said that, if you put a 1 kHz tone into the mixer and set it up so that the meter reads 0 dB, then the output voltage at the jacks should still read +4 dBm or 1.23 Vrms.

THE EXCEPTION, of course, are the Mackies that have the meter calibrated such that 0 dB on the meter is actually 0 dBu at the output.

The point here being that the meters are well-behaved for continuous tones and less so for music signals.

Quote:

 Is that why I never ever blown a single speaker then? Because even though I thought with the system running upto 0db after setting gain structure, the peaks were only hitting 0db.


You've never blown a speaker because you are running everything very conservatively. If you set up your console so that its meters read 0 dB on peaks, then its peak output is about 1.23 Vrms (ugh, using rms when talking about peaks) and that voltage is a typical amplifier's input sensitivity (the voltage at which it puts out Full Tilt Boogie).

So in other words you don't run the thing into the red.

-a
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jack smith

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Re: Measuring amplifier voltage output with a multimeter-A better explanation
« Reply #26 on: October 02, 2009, 06:25:15 am »

I try and make sure that I dont ever overrun the system because I dont pay hundreds for a speaker, just to shag the hell out of it it like a red headed step child. I take care og my equipment.
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