Volt, watt, amp conversion question.


Old 01-11-14, 04:49 PM
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Volt, watt, amp conversion question.

Say you have a 100 watt 120 volt A/C AUDIO system/speaker and a 100 watt 12 volt D/C AUDIO system/speaker. Is the DC system pulling 8.33 amps while the A/C 120 volt is pulling only .833 amps?

How is that possible? I'm using volts * amps = watts.

Also, I want to hook a home theater audio system up in a camper and have a 1,600 [constant] watt generator. Will the 1,000 watt home theater sound system actually be pulling 1,000 watts of power from the generator?

Say the volume goes to 30 and I run it on 15, will it only be pulling 500 watts?

The 1,000 watts is peak, which I would assume that the constant/RMS is 500 then?

So, at 15 out of 30 will it actually only be pulling 250 watts?

I wonder if I'm watching a movie and there is an explosion (in the movie) or some other loud noise, if my generator will have to rev up each time.

I very kindly thank you in advance for any help.

Last edited by PJmax; 01-11-14 at 08:58 PM. Reason: Removed unnecessary link.
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Old 01-11-14, 06:20 PM
Join Date: Oct 2008
Location: New England
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Volts times amps is correct for power, however an amplifier has other parts to power other than speakers. How much the draw will increase or decrease as you vary the volume or as the movie varies the sound is difficult to determine.

I built a 100W stereo way back in 66 using KT88 output tubes, and yes it could light a pair of light bulbs. Problem was, the human ear can only take about 1/2 watt, that may be high. No one at college had speakers that could handle it.

Let's see if one of the younger techs has a better handle on the power needs.

Old 01-12-14, 12:48 AM
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Audio output watts do not have a direct correlation to power input watts. Look at the labeling on the amplifier and it should state the power input requirements.
Old 01-12-14, 05:47 AM
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... Look at the labeling on the amplifier and it should state the power input requirements. ...
For power needs use the above.

Speakers are "inefficient". Typical high fidelity speakers are less than 2% efficient, i.e. 100 watts of amplifier power in for 2 or fewer acoustic watts out. (One acoustic watt at mid-frequencies such as middle C to 2 octaves higher is deafening). Outdoor PA speakers with horn assemblies can be something like ten percent efficient.

Oh, by the way, when a "speaker system" (box) is rated for so many watts, the tweeter can handle far less than that. The system can handle the wattage when playing sounds normally encountered in life, including orchestral music. If you overload an amp, the resulting distorted sound often is much richer in high frequencies compared with natural sound and you can blow the tweeter long before reaching half of the speaker system power rating.

As a point of information, small to moderate power vacuum tube amplifiers (technically called class A) usually "consumed" approximately the same amount of (AC) power (the loud sound maximum) whether the sound was loud or soft. Modern solid state amps and high powered tube amps consumed more power the louder the output (sound) (as class B) although tube amps had a fairly high "minimum" draw (as class AB)

In the oversimplified diagrams, the amount of power consumed is represented by the amount of color. The "program material" is the same for each class as shown.
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Last edited by AllanJ; 01-12-14 at 06:56 AM.
Old 01-12-14, 07:25 AM
Join Date: Apr 2007
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Suffice to say that your 1,000 watt stereo will survive nicely on a 1,600 watt generator.

If you read the specs on a typical home theater receiver you'll see that its rated output is, say, 70 watts RMS per channel with .09% Total Harmonic Distortion (THD).

However, the input power required is 120VAC, 60Hz, 200 watts. That makes no sense, because it would mean that the receiver is actually generating 350 watts -- 100 watts more than it is using!

It's not magic -- it is trickery in how they report the numbers. Read the specs carefully and the only conclusion you can reach is that their claims are based on only one channel operating.

They then list the rated output with a 1% THD, which at 145 watts into 8 ohms is about right for a 200 watt draw. People usually can't listen for long to a stereo with 1% or more THD. The ears become fatigued.

In reality, when your system is drawing the full 200 from the wall it will put 150 or so total watts to the 5 speakers, which is divided out depending on the requirements of the program material and the settings of the receiver. Each of the front speakers might get 40, and the rear 15. (The sub is either powered or fed from an outboard amp.)

40 watts may not seem like much, but in a typical living room it can be too much. To understand why, you have to look at speaker efficiency.

Forget acoustic watts. Unless you are a major system designer you don't have to deal with it at all. A far more common category of measurement is Sound Pressure Level (SPL), which uses the decibel (dB) as its unit.

A speaker efficiency spec will list the dB measured with 1 watt at its input at a distance of 1 meter. A typical home speaker is 90dB efficient with one watt at one meter. Each doubling of power results in an increase of 3dB, so:
-- 2 watts = 93dB
-- 4 watts = 96dB
-- 8 watts = 99dB
-- 16 watts = 102dB
-- 32 watts = 105dB
-- 40 watts = 106dB (rounded)

The 200 watt draw is with everything all the way up, which would be so distorted it could damage the speakers. Turn it down to half as loud. "Half as loud" is generally recognized by average listeners to be a drop of -10dB. At "half as loud" the speakers are putting out 96dB SPL, but the amp's power has dropped to 1/10th of its previous output, or 4 watts per channel. 96dB is quite loud -- certainly enough to fill a living room. Oh yeah, there are five speakers. Each doubling of speakers increases the SPL by 3dB. We're back up to about 103dB, accounting for the lower wattage to the two rear speakers.

Using the earlier input power to output power ratio, the amp is now drawing roughly 20 watts from the wall. On paper. In reality, there are other factors involved that increase the amp's consumption to cover the requirements of the program material, but it is nowhere near rated power.

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