The Physics
An encyclopedia of scientific essays

Power of a Home Stereo

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Cutnell, John D. and Johnson, Kenneth W. Physics. New York: Wiley, 1995: 632. "[T]he AC voltage across the speakers is 6.00 V. The main speaker has a resistance of 8.00 Ω, and the remote speakerhas a resistance of 4.00 Ω …. Since the speakers are in parallel, the equivalent resistance is … 2.67 Ω …. [T]he total current is … 2.25 A …. [T]he total power can be obtained directly using the equivalent resistance
RP = 2.67 Ω and the current …
P = I2rmsRP = (2.25)2(2.67 Ω) = 13.5 W"
13.5 W
"Sound Recording and Reproduction." Microsoft Encarta 97 Encyclopedia. 1996. "Depending on the requirements of the speaker system, an amplifier may deliver from 10-125 watts of electrical power." 10-125 W
Davis, Gary and Jones, Ralph. Sound Reinforcement Handbook. Milwaukee: Hal Leonard, 1990: 199. "As a rule of thumb, low power amplifiers (up to 100 watts continuous per channel) should have a slew rate of at least 10 volts per microsecond. High power amplifiers (over 200 watts) should have a slew rate of at least 30 volts per microsecond." < 100 W
(low power)
> 200 W
(high power)
Denon Electronics. Personal Audio System D-C35. Korea: Nippon Columbia: 16. "General: Power supply: 120 V, 60 Hz Power Consumption: 100 watts Speakers: Impedance: 6 ohms Maximum input power: 50 watts Amplifiers: Input impedance: 4 kohms (MIC) 47 kohms (AUX) Output impedance: 6-16 ohms Audio output: 30 watts + 30 watts" 100 W

Stereophonic sound systems, more commonly known as stereos, are a popular form of home entertainment. The technology of these devices involves the conveyance of sound through at least two different channels, delivering different sound signals through each channel to give the sound greater depth and dimension.

Power is the measure of energy consumed by a device per unit of time. The unit of power is the watt, which is the derived quantity of joules/second. The power of a device using electrical energy is measured by the formula P = IV, which is the product of the current and the voltage through the device. Ohm's law, (V = IR), provides ways to obtain several variations of the power formula, manifesting that the resistance of a device is also a key factor in determining its power capacity.

The power values of home stereos extend over a wide range. This is due to the varying resistances in the different components of the system, and some stereo models have more features than others. The key feature of a stereo that really decides its power requirements is the amplifier. The purpose of the amplifier is to boost the power delivered to the stereo; initial power is known as input power, and the result following amplification is called output power. Input power is calculated through the power formula using a derivation of Ohm's law, P = V2/R, and the value of the resistance is determined by the input impedance of the equipment. Output power is calculated in the same way, except that the resistance value used in this case is the load resistance. The power gain of the amplifier is derived from the ratio of power output to input; this value is maximized when the input impedance and load resistance are equal. With all constituents of the stereo taken into account, a single system can have power requirements from 10 to over 200 watts.

Alexandra Kougentakis -- 2001