Output Impedance

Why does Output Impedance matter?

We generally want our source output impedance to be significantly lower than the impedance of the headphones. This is due to 3 main reasons:

• A large output impedance can attenuate the signal reaching the headphones, resulting in not enough volume output.
• Headphones have different impedances at different frequencies. This is kind of an extrapolation of point 1 - we now have the signal attenuated in different amounts depending on frequency. This can result in dramatic changes in frequency response. Headphones with more varied impedance curves are affected more by this effect. This is because the headphone impedance and the output impedance of the amp form a voltage divider which varies by frequency. We will go into the math later. Note that planar magnetic headphones typically have very flat impedanc curves, and so don't suffer from frequency response shifts.
• Electric damping factor is reduced as output impedance increases. A common rule of thumb is that the output impedance should not exceed 1/8th of the nominal impedance of the headphones. So, for 300 Ohm headphones, you'd want no more than 37.5 Ohms output impedance; for 16 Ohm headphones, no more than 2. This is just a rough guideline, as they are exceptions to the rule. One such exception are planar magnetic drivers. These seem not to require electrical damping in the same way as dynamic drivers.