Output impedance

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I was set to move onward in our explanation of the output analog stage of a DAC or preamp but one of my readers asked me to explain output impedance which I mentioned a few days ago. This is a subject that folks should understand at least at some level because so many things in high end audio are affected by it.

Output impedance refers to a device's ability to deliver unrestricted current or power when passing a musical signal - it measures the amount of restriction or hold back of that signal. It is important to understand output impedance only as it is relative to the input impedance of whatever the amp is driving. In other words, the output impedance of a DAC is only meaningful when you are considering what that output is going to feed into (the preamp or amp it's connected to).

The easiest way to imagine what output impedance means is to picture a resistor in series with the output of the amp. This resistor is always present regardless of how low the output impedance is because there's no such thing as a perfect source or output. A perfect output would have zero output impedance - meaning the value of this mythical resistor is zero and therefore would have no affect on the musical signal passing through it.

As there are no perfect amplifiers with zero output impedance we have to assign a number to this resistor in the signal output of the amp. The easiest way to calculate this value is to drive music through the amp into another resistor connected to ground and measure how much of the musical signal is lost at the junction of the two resistors (remembering that when we pass music through a resistor we convert some of that musical energy to heat and it is lost). As the resistor to ground gets lower and lower in resistance (less resistance) the level at the junction will continue to decrease and, at one point, that level will be 1/2 of what you started with. It is at this point we can then say what the output impedance of the amp is - which would be exactly the same as the value of the load resistor going to ground.

Here's what's important: whatever you are trying to ask the output amplifier to drive must be at least 10 times higher in impedance and preferably 100 times or more. Why? Because you don't want to lose any of the musical energy being sent to the receiving device and you don't want to stress out the amplifier that's sending the music in the first place.

So here are some practical examples. If the input impedance of your power amplifier is 10k then the output impedance of your DAC or preamp feeding it must be at least 1k and better if it's 100 Ohms or less. If it's 100 Ohms you'll only lose a tiny amount of signal at the junction between the preamp and the amp - 100th of what you are sending, just for understanding sake (not entirely accurate but you get the idea).

Here's another example: a loudspeaker. Let's say your loudspeaker is an 8 Ohm speaker whose impedance dips as low as 3 Ohms at its lowest point (speakers don't have flat impedance). That means the output impedance of your power amplifier should be at least 0.3 Ohms and probably better at 0.03 Ohms to really have very little affect.

Tubes generally have higher output impedance than solid state products. One piece of evidence you see in most tube power amplifiers is the output transformer of the tube amp. This is there to more closely match the high output impedance of the amplifier with the low input impedance of a loudspeaker.

Bottom line: output impedance is always better lower.

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Paul McGowan

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