Input and Output Impedances

What does High/Low Impedance Mean?

We often hear people speak about audio devices as being either high or low impedance. What does this mean? Why is it a problem when you connect a device with a high output impedance to another device that has a low input impedance?

In this section, we hope to de-mystify this concept, and build a conceptual understanding that will help you in your use of audio equipment.

Circuit Example: Voltage Divider

To understand input and output impedances of audio devices, we need to first examine a simple electrical circuit knows as the age divider.

In the above example, we would like to determine $V_a$. First off, as noted in the diagram above, when resistors are connected in series ( connected end to end ) - their total resistance is determined by adding their individual resistances. In this case:

$Total Resistance = 1 + 9 = 10 \Omega$

We are interested in determining the voltage at point A. This can be determined easily by the following reasoning. First off, since we know the total resistance, we can determine the current through the circuit by applying Ohms Law.

$I = {V \over R} = {10 \over 10} = 1 Amp$

Now by apply

Solving for the current through the circuit, by applying Ohms law to the total resistance.

Determining the voltage at point A, by using Ohms law.

Using the voltage divide concept to understand Input and Output impedances.

A simple re-arrangement of the diagram used in the preceeding example results in the following:

In this diagram, the voltage source ( the battery ) has been grouped with $9 \Omega$ resistor. It turns out that all devices can be simplified in a similar manner - specifically into a voltage source and an impedance ( in this case a resistance ) which is known as its output impedance.

In this example, the $1 \Omega$ resistor is known as the electrical load, and similarly all devices with input connections can be characterized by an equivalent impedance ( in this case a simple resistance ) which is known as its input impedance.

In the example above, take notice of the fact that the battery was a 10 Volt battery, but only 1 Volt was delivered to the device which was connected to it. This was due to the fact that when the device with the $1 \Omega$ input impedance was connected, a current of 1 Amp resulted, causing a voltage drop across the internal resistor of $9 \Ohms * 1 Amp = 9V$

In this next example, we examine what would happen if we hooked up a device with a much higher input impedance, namely $10,000 \Omega$. There are a couple of ways to think of what would happen in this case.

Looking at it using the voltage divider concept. In this case, the total Resistance would be given by $R_t = 10,000 + 9 = 10,009 \Omega$. The voltage at the output of the first device would be given by the following ratio:

Output Voltage $= 10 * { 10,000 \over 10,009} \approx 10 Volts$

Hence we see that with a relatively high input impedance ( 10,000 Ohms ) the voltage delivered is approximately equal to the voltage at the battery

Summary

Summarizing we see that we get a decrease in voltage when we connect an audio device to another unit which has a low input impedance. This can be significant when the output voltage of the preceeding unit is relatively high. In order to avoid a significant loss in voltage we would like the output impedance of a unit to be as low as possible, and the input impedance of the device it is connected to, to be as high as possible.

In terms of voltage and current, high-output impedance devices typically deliver low currents while low-output impedance devices are capable of delivering higher currents. Alternatively, a device with a high-input impedance draws low currents, thus not placing much of a load or a demand on the device to which it is connected. Devices with low input impedances can draw significant currents thus can only be driven effectively by devices with low output impedances.

Device High/Low Typical Impedance
Microphone- Low Impedance Low $150 \Omega - 300 \Omega$
Microphone- Medium Impedance Low $600 \Omega - 3,000 \Omega$
Microphone- High Impedance High $\ge 10,000 \Omega$
Guitar Pickup High $\ge 10,000 \Omega$
Old audio equipment Low $600 \Omega$
Line Level Ouput Low $100 \Omega$ - 1,000 \Omega$Typical Output Impedances Device High/Low Typical Impedance Headphones Low$75 \Omega - 150 \Omega$Speaker Low$8 \Omega$Line Input High$10,000 \Omega - 1M \Omega$Typical Input Impedances Some Examples Universal Audio Solo/610  Microphone Input Selectable$500 \Omega$/$2K \Omega$Direct Input (for instruments) Selectable$47K \Omega$/$2.2M \Omega$Output Impedance$600 \Omega$Radial Engineering Passive DI  Input Impedance$10K \Omega$Output Impedance$600 \Omega\$
page revision: 51, last edited: 26 Sep 2009 21:29