## Impedance measurement using the soundcard headphone jack

On: 15 Nop, 2018

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The easiest way to perform an impedance measurement with minimal additional equipment is to use the headphone output of the sound card. All you need is a reference resistor Rref and a small amount of cable.

The measurement setup is s...

The easiest way to perform an impedance measurement with minimal additional equipment is to use the headphone output of the sound card. All you need is a reference resistor Rref and a small amount of cable.

The measurement setup is shown in Figure 2.1.2. According to the LIMP user manual, the reference resistor Rref should have a value of 100 ohms, but this can vary between 33 and 100 ohms, depending on the sound card.

The high value of the reference resistor is dictated by the fact that soundcard headphone outputs are usually not designed for connection to speakers. Figure 2.1.3 compares the impedance curves of a standard headphone and a speaker. The much reduced relative impedance of the speaker shows clearly how the headphone output could suddenly be asked to deliver far more power than it was ever designed for!

Figure 2.1.4 shows the specifications for the headphone outputs of two common soundcards.
- The ESI UGM96 delivers a maximum output voltage of +4 dBV = 1.0 * 10^(4.20) = 1.584V;
- The RME Fireface UC provides +19 dBu = 0.775 * 10^(19/20) = 6.907V.

Figure 2.1.4 Soundcard headphone output specs (in German), ESI UGM96 (Klinkenbuchse = jack; Ausgangspegel = output level; A-gewichtet = A-weighted; Ausgangsleistung = output; Impedanz = impedance) .

Figure 2.1.5 shows a practical implementation of the circuit illustrated in Figure 2.1.2. Such a set up is not necessarily essential, but it does help to prevent errors that might result in damage to the soundcard.

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