Download Impedance, Balance, and Output/Input Connections for Digital Audio

CSC 191E
Digital Sound for Music and Theatre
Spring 2008
Burg
Impedance and Output/Input Connections for Digital Audio Recording
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.1
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 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)
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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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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.
Should the impedance of the output and input match?
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 FastTrack 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, as you’ll see below).
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.
Well, short of soldering the connection, 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.
What else should you know about impedance?
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.
How do you connect an electric guitar to a digital audio recording input?
We have three M-Audio FastTack 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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