Calculating And Measuring Power Audio Amplifier

Many people are asking how to estimate power amplifer audio and how to measure it. Power cannot be explained without understanding what that voltage, current, and resistance. Ohm's law States that V = I x R, where V is the voltage, I is the current, and R is the resistance. The unit of voltage is expressed in Volt, the unit of current in Ampere, and a unit of resistance in Ohm. Ohm's law was discovered by Georg Ohm in 1827.

Power itself has a sense of energy that brought about or consumed divided time. This power is expressed in units of Watt. Electrical power is expressed as P = (V x Q)/t. P is the power in Watts of, V is voltage, Q is the electric charge (electric charge) in coulomb and t is time in seconds. The electric charge is split time is the electrical current so that power can be expressed P = V x I. In relation to Ohm's law, power can be expressed as P = V x V/R or P = I x I x r.

Audio signal is the signal back and forth or AC (alternating current ) then the power calculations become more complex. The signal back and forth have a voltage that is constantly changing at any time so that its power is also changing all the time. In order to facilitate the analysis, then we use sine signals.

Average Value

The average value of the signal back and forth is the average value in half a period. For sine signal, the average value is equal to the maximum value is divided (2/π).

Average Power is the energy of the resulting average in one period. The average power is used to calculate the required transistor or cooling estimate an increase in temperature (power dissipation) on electronic components. The average power is expressed as follows: (I ² _Peak x R)/2

The Value Of The Effective

If direct current 1A resulted 100-degree temperatures in a resistance, then alternating current sine-shaped of 1A maximum (peak value) will generate temperatures 70.7 degrees on the same resistances. 70.7/100 or 0.707 is called the value of effective or root mean square (RMS).

The rms voltage of the sine signal = 1/√ 2 of voltage peaks. Current sine signal rms = 1/√ 2 of the current peaks. Power rms = V rms x I rms

Audio Power Amplifier

Specifications of the power amplifier using power RMS audio with sine signal of 1 kHz, 8 Ohm load or at 4 ohms, and Total Harmonic Distortion (THD) maximum of 1%.

To the amplifier is not Bridge Tie Load (BTL), maximum voltage is limited by its power supply voltage and voltage of transistor saturation finale. Whereas the maximum current is limited by the maximum current of the transformer. Of course it should be ascertained transistors work on area Safe Operating Area (SOA).

On the power supply that is not regulated, the greater the currents drawn amplifier, voltage power supply getting down. This is because the power supply has a non-zero output impedance. Generally a good transformer is not experiencing a decrease in voltage is more than 5% when the maximum current encumbered. Not to mention the ripple voltage of power supply the magnitude depends on the magnitude of the capacitance of rectifier diodes after elco. Diode rectifier also reduces the voltage of power supply approximately 1V.

Whereas the saturation voltage transistors can reach large collector current dependent 3V.

Calculation example

For example there is a transformer-32V 5A CT will be used as an audio amplifier. How much is the maximum power estimated at 4 Ohm?

Power supply voltage on the load at the maximum = (peak voltage x 95%) – voltage diodes – voltage ripple.

Peak voltage = 32 x √2 = 45 V (rounded for easy).

Voltage diodes = 1 V Voltage ripple = 3 V Power supply voltage on the load at the maximum = 38,75 V

Maximum signal voltage (peak) = Voltage of power supply at maximum load – transistor saturation voltage = 38.75 – 3 = 35.75 V

RMS power at 4 Ohms = (Signal voltage RMS)² / R = ( 35.75 / √2 )² / 4 = 160 W RMS (rounded).

To load the speaker are slightly different from the above calculations, because the speaker impedance is not purely resistive in nature, but rather have inductance as well. So cos φ or power factor needs to be taken into account. However, this will not be discussed here because it is too complicated for beginners.

The Way Of Measurement

The way of the power measurement of audio amplifiers can be described as follows:

In the input amplifier 1 kHz sine signal being given from the signal generator. The output of the amplifier is connected to the dummy load a power resistor whose value is approx. 4 Ohm or 8 Ohm. Make sure that the maximum power of the dummy load greater than maximum power amplifiers. If the voltage peak of the output amplifier is very high, it needs to be lowered with attenuator or voltage divider. Then the signal is inserted into the Distortion Analyzer. Output Distortion Analyzer input to oscilloscope to see the visualization.

1. Adjust the frequency signal genarator 1 kHz sinus.

2. Adjust the level of the signal voltage generator until THD Analyzer shows a 1% THD.

3. Measure the voltage seen on oscilloscope.

If the measured peak voltage of 20V and on attenuator the voltage is divided 5, then the real peak voltage 20 x 5 or 100V.

RMS Voltage = 1/√2 x 100V = 70,7V rms.

Suppose a dummy load resistance of 4 Ohm, then RMS Power = (70,7 x 70,7) / 4 = 1250 Watt rms.