Originally Posted By: Mac
The ways in which Input Impedance affects audio is widely misunderstood.

If I have a headphone amplifier that is capable of developing 10 Watts of power into an 8 ohm load when the load is headphones, that same amplifier output, when fed into a typical Line Level input that has a much higher impedance, Ohm's law can tell us what amount of power that headphone amplifier is able to develop into that amplifier input, which is actually a load.

Ohm's law is E = IR where E is the Voltage, I is the Current and R is the Resistance.

Watts, in the DC arena to keep things as simple as possible here, is the Product of Voltage and Current developed across that Resistance.

Therefore we can extrapolate an algebraic that will allow us to use the two available knowns regarding that headphone amplifier.

If W = V x I then W = I*2 x R or W = V^2 / R

We know the Watts, we know the Impedance where that wattage is developed.

To find the Voltage that would be needed to develop those 10W then, we can use the formula V = (W x R)sub2

V = (10 X R)sub2

or

8.944 Volts

But the Impedance of our Line Level amplifier input is NOT 8 ohms!

A typical Line Level input can be as little as 2,000 ohms or as in the case with a lot of the highend sound devices built for recording, may be 10,000 ohms or sometimes as high as 1 million ohms.

Let's do the math for the 10,000 ohm (10K) amplifier input:

If we connect the output of the headphone amplifier, rated to deliver 10W into an 8 ohm load, directly into the 10K input of our monitor speaker amplifier, Ohm's law tells us that the same headphone amplifier can now only develop a much smaller Voltage and Current figure, which means it will develop much smaller amount of Wattage.

The headphone amplifier in this example is only capable of developing 0.008W, or 8 milliWatts of power into that 10K amplifier input.

Your amplifier input is then electrically safe from any sort of damage that the input could possibly cause.

Also, there will be no deterioration of the audio signal in this arrangement because the load, in this case very light load as compared to driving the very low impedance headphone or even speaker voice coil, cannot be "loaded down" by the amplifier input.

The mantra taught to young audio engineers is thus:

"In Audio circuits, you can always drive a higher impedance load with a lower impedance output with no problems whatsoever."


or

"Lower Impedance can always safely drive a Higher Impedance in Audio."


Note that the opposite scenario, attempting to drive a Higher Impedance output into a Lower Impedance input will indeed "load down" the output and thus create distortion, possibly clipping, and may hurt the driving amplifier circuitry if the load it is being required to drive is lower than the lowest value of load resistance that amp is designed to work safely into.


--Mac


Hi Mac,

As an introduction, my real "gig" is an Analog Integrated Circuit Design Engineer with 30+ years of design experience, including the design of Power Amplifier I.C.s so I fully understand the theory. Obviously you do too!

I was just addressing the statement that the impedances should be equal.

Wally G

Yamaha Montage 6 Roland J80, D50, Integra 7, BK-7m, FR-8X, 1967 Fender Jaguar, Fender Strat, Fender 1965 Twin Reverb reissue, Selmer Trumpet, Akai EWI, Studio One 4

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