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Is there any way I can measure the power output through the headphone out of my vintage speaker amp?

post #1 of 2
Thread Starter 

I have a marantz pm325 amp that power a pair of floorstanding speakers I have

 

It has a headphone jack and I was wondering if there was any instrument or anything I could do to find out how much power (in watts?) this amp puts into headphones

 

 

looked all over the place online and couldn't find anything

post #2 of 2

Short Answer:

Measure the voltage between ground and the negative side on the jack.  Do the math below.

 

From Joule's law we know:

 

#1     Power = Voltage Squared over Resistance

P = V^2 / R

or

 

#2     Power = Current Squared times Resistance

P = I^2*R

 

Since most amps are constant voltage sources use formula #1  ( volume knob controls the constant voltage )

 

Power = V^2 / R

 

Where V is voltage measured by a volt meter on the jack from ground to rail and R is the resistance of the load ( your headphones )

 

Let's say your headphones have a 60 Ohm nominal impedance.  You measure .25 volts out of the jack at 75% volume.

 

Seeing that P = V^2/ R

P = .25 volts ^2 / 60 Ohms

P = .0625 / 60 Watts

P = .001 Watts or 1mW of power

 

See how the power changes depending on the impedance of your headphones?  That is one reason it is important to have a basic understanding of Joule's law.

 

One can work backwards using Ohm's law to find the current draw.

 

We have P = .001 Watts

P can also be equal to I^2 * R

.001 = I^2 * R

We know R is 60 Ohms so we can easily calculate I.

I^2 = P / R

I^2 = .001 / 60

I = ( .001 / 60 ) ^.5

I = .+/- .004 Amps or 4 mA

 

Conclusions:

With a 60 Ohm headphone at 75% volume on our amp we measured a power of 1 mW and a current of 4 mA.

 

From that knowledge you can calculate the estimated volume using the sensitivity of a headphone.

 

The sensitivity of our 60 Ohm headphone is listed to be 106 dB / mW.  Simply multiple 106dB * our 1 mW value found before and we get 106dB of loudness ( pretty darn loud ).

 

Hope that helps.

 

Disclaimer:  There may be mistakes so if you find one please feel free to correct it below.

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