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

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

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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