Learning Supplement for
“Linking Science, Art, and Practice Through Digital Audio1”:
Impedance and Output/Input Connections
for Digital Audio Recording
Assignment Type:
Goal:
Concepts:
Software needed
for this learning
supplement:
Reading
To explain impedance and its significance in linking microphones, input
devices, and settings
Impedance, source (i.e., output), load (i.e., input), dynamic mics
none
1 Definition
Impedance (abbreviated Z) is the measure of the total opposition to current flow in an alternating
current circuit. Measured in ohms, impedance is the sum of two components, resistance (R) and
reactance (X). Ohms is abbreviated , 1,000 ohms is abbreviated k, and 1,000,000 ohms is
abbreviated M.
The source of an audio signal – like a microphone or electric guitar – is called the source or
output.
The place where you plug this source into a sound card, external sound interface, or mixer is
called the load or input.
The significance of impedance is that it affects what types of microphones and instruments
should be plugged into what kinds of sound cards and mixers.
A microphone up to about 600  is low impedance, between 600 and 10,000  is medium
impedance, and 10,000  or more is high impedance. Most good mics are low Z, particularly
those with XLR connectors.
An electric guitar generally is a high-impedance source.2
You can check the impedance of your outputs and inputs by reading the device specifications.
A high impedance mic or instrument will generally output a higher amplitude signal, measured
in voltage, than a lower impedance one. However, don’t be fooled into thinking that high
impedance is necessarily better than low impedance. In the case of microphones, most good
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This material is based on work supported by the National Science Foundation under Grant No. DUE-0717743.
This worksheet was written by Jennifer Burg.
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Some guitars have a built-in preamp circuit, sometimes called active electronics. If your guitar requires batteries,
then it probably contains active electronics. Guitars with active electronics usually provide a low impedance output.
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mics are low impedance, like the Shure SM58, an excellent and durable dynamic mic for voice
recording.
Figure 1 Shure SM58 microphone (actual size about 6 ½ “ long)
So how do you know what to plug into to what when you want to hook up a microphone and/or
an instrument to a sound card, external sound interface, or mixer in order to make a digital
recording? To some extent, it helps to know the impedance of the output and input. To some
extent, you can simply be guided by the shapes of the input and output jacks.
2 Matching Impedenace for Output and Input
The impedance of output and input don’t have to match exactly.
In general, the audio output (e.g., the mic) should have lower impedance than the input (the place
you plug the mic in). Connecting a lower impedance output to a higher input is called bridging.
Consider the Shure SM58 mic, which you’ll use in the lab. It has male XLR connectors at the
end. You plug these into a cable that has female XLR connectors at one end and male at the
other. The male end of the cable can be plugged into the XLR connectors of a sound interface,
like the M-Audio Firewire 410 or M-Audio Fast Track USB we have in the lab. The M-Audio
input has an impedance of about 1500 . The SM58 mic has an impedance of about 150 to 300
. The output is lower impedance than the input by a good amount (but not too much).
It’s not good to connect a higher impedance output to a lower impedance input because the
frequencies that are recorded may be distorted. For example, you should not connect a high
impedance electric guitar to a low impedance input jack.
What if you want to connect the Shure SM-58, which has XLR connectors, to the internal
SoundBlaster sound card of one of the computers in the digital media lab – a sound card which
doesn’t provide XLR type input but has ¼” TS type inputs instead. You can see such an input
pictured below. The ¼” input is indicated with a red arrow.
TS inputs are generally high impedance inputs, even higher than XLR inputs. The inputs shown
in the picture below are line level, with very high impedance – between 10 k and 1 M.
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Figure 2 SoundBlaster X-Fi Platinum Sound Card
If you were to connect a low-Z mic into a very high-Z input like this line level input, you would
get a weak signal. This is because the high-Z input is designed to receive a relatively high
voltage from a high-Z mic, and so the input signal is not amplified much.
You wouldn’t be able to plug the SM58 microphone into the ¼” jack anyway, because the
connections don’t fit together. That clues you in right away that you need an adapter.
The solution is to put a matching adapter between the mic cable and the input jack.
Figure 3 XLR to TS adapter (actual size about 4 ¼” long)
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The adapter transforms the input from low-Z balanced to hi-Z unbalanced. It has female XLR
connectors on one end and a TS connection on the other end. You connect the mic cable to the
XLR side and plug the TS connection (also called a phone jack) into the sound card.
3 Effects of Cable Length
Another disadvantage of a high impedance output that it isn’t good when you have to connect the
output to the input by means of a long cable. With a long cable, there’s more chance that high
frequencies will be lost in the audio signal. This is because high frequencies are affected more
by reactance. Also, a longer cable provides more chance for the high impedance output to attract
electrical interference on its way to the input. This can add noise to the recording in the form of
a hum.
A low-Z mic can be used with hundreds of feet of cable without picking up hum or losing high
frequencies. A medium-Z mic cable is limited to about 40 feet, and a high-Z mic is limited to
about 10 feet. The extent to which high frequencies are lost depends also on the capacitance of
the cable.
4 Electric Guitars
We have three M-Audio Fast Tack USB external sound interfaces that have ¼” TRS inputs for
guitars. You can plug an electric guitar directly into this input.
The one M-Audio Firewire 410 that we have allows you has a pre-amp at the mic/inst input.
You can plug an electric guitar directly into this input.
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