IMPEDANCE FAQ
by Bruce Bartlett
Impedance is one of audio's more confusing concepts. To
clarify this topic, I'll present a few questions and answers
about impedance.
WHAT IS IMPEDANCE?
Impedance (Z) is the resistance of a circuit to alternating
current, such as an audio signal. Technically, impedance is the
total opposition (including resistance and reactance) that a
circuit has to passing alternating current.
A high impedance circuit tends to have high voltage and low
current. A low impedance circuit tends to have relatively low
voltage and high current.
I'M CONNECTING TWO AUDIO DEVICES. IS IT IMPORTANT TO MATCH THEIR IMPEDANCES? WHAT HAPPENS IF I DON'T?
First some definitions. When you connect two devices, one is
the source and one is the load. The source is the device that
puts out a signal. The load is the device you are feeding the
signal into. The source has a certain output impedance, and the
load has a certain input impedance.
A few decades ago in the vacuum tube era, it was important to
match the output impedance of the source to the input impedance
of the load. Usually the source and load impedances were both
600 ohms. If the source impedance equals the load impedance,
this is called "matching" impedances. It results in maximum
POWER transfer from the source to the load.
In contrast, suppose the source is low Z and the load is high
Z. If the load impedance is 10 times or more the source
impedance, it is called a "bridging" impedance. Bridging results
in maximum VOLTAGE transfer from the source to the load.
Nowadays, nearly all devices are connected bridging -- low-Z out
to high-Z in -- because we want the most voltage transferred
between components.
If you connect a low-Z source to a high-Z load, there is no
distortion or frequency-response change caused by this
connection. But if you connect a high-Z source to a low-Z load,
you might get distortion or altered response. For example,
suppose you connect an electric bass guitar (a high-Z device)
into an XLR-type mic input (a low-Z load). The low frequencies
in the signal will roll off, so the bass will sound thin.
We want the bass guitar to be loaded by a high impedance, and
we want the mic input to be fed by a low-impedance signal. A
direct box or impedance-matching adapter does this. Such
adapters are available from Radio Shack.
The adapter is a tube with a phone jack on one end and a male
XLR connector on the other. Inside the tube is a transformer.
Its primary winding is high Z, wired to the phone jack. The
transformer's secondary winding is low Z, wired to the XLR. You
plug the guitar cord into the phone jack, and plug the XLR into a
mic input in a snake or mixer. Use it with a bass guitar,
electric guitar, or synth.
This impedance-matching adapter works, but is not ideal. The
load it presents to the bass guitar might be 12 kilohms, which
will slighly load down the high-Z guitar pickup, causing thin
bass.
An active direct box solves this problem. In place of a
transformer, the active DI usually has an FET (Field Effect
Transistor). The FET has a very high input impedance that does
not load down the bass guitar.
HOW DO I KNOW WHAT A MICROPHONE'S IMPEDANCE IS?
Check the impedance spec on the mic's data sheet. A microphone
rated at 150 ohms to 300 ohms is considered low impedance. About
600 to 2000 ohms is medium impedance, and 10,000 ohms or more is
high impedance. Most mics are low Z, and all mics with XLR
(3-pin) connectors are low Z.
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 without losses. A high-Z mic will
lose highs and pick up hum if the cable exceeds about 10 feet.
This high-frequency loss depends on the capacitance of the cable
as well as the mic impedance.
I'M CONNECTING A MIC TO A MIXER. IS IMPEDANCE A CONSIDERATION?
Yes. If your mixer has phone-jack inputs, they are probably
high Z. But most mics are low Z. When you plug a low-Z mic into
a high-Z input you get a weak signal. That's because a high-Z
mic input is designed to receive a relatively high voltage from a
high-Z mic, and so the input is designed to have low gain. So
you don't get much signal amplification.
Solution: Between the mic cable and the input jack, connect an
impedance matching adapter. It steps up the voltage of the mic,
giving it a stronger signal.
The adapter is a tube with a female XLR input and a phone-plug
output. Inside the tube is a transformer. Its primary winding
is low Z, wired to the XLR. It secondary winding is high Z,
wired to the phone plug. Connect the mic to the XLR; connect the
phone plug to the mixer's phone jack. Then the mixer will
receive a strong signal from the mic.
If your mixer has XLR inputs, they are low-Z balanced. In this
case, simply connect the mic to the mixer using a mic cable with
a female XLR on the mic end and a male XLR on the mixer end. A
low-Z mic input is typically about 1500 ohms, so it provides a
bridging load to a mic that is 150 to 300 ohms.
HOW ABOUT SPEAKER IMPEDANCE?
Most loudspeakers are 4 or 8 ohms nominal. Actually the
impedance varies with frequency. A speaker rated at 8 ohms
impedance might range between 4 and 50 ohms, depending on the
frequency of the signal.
If you connect two identical speakers in parallel, the total
impedance is half. For example, two 8-ohm speakers in parallel
present a 4-ohm load to a power amplifier. If you connect two
identical speakers in series, the total impedance is doubled.
Two 8-ohm speakers in series present a 16-ohm load to a power
amplifier.
Suppose you have a speaker connected to a power amplifier.
Generally, the lower the speaker impedance, the more power you
get from the power amp. Some amps can handle a 2-ohm load, but
many will overheat. Try to keep the load 4 ohms or higher.
IS A 70-VOLT SYSTEM A HIGH-IMPEDANCE SYSTEM?
Yes. In a 70V system, a power amp is connected to a step-up
transformer which raises the amplifier output voltage while
decreasing its current. This high-voltage, low-current signal is
sent on long cables to a distributed system of many loudspeakers
throughout a venue. Since the power is distributed with low
current, there is little power loss through the long cables.
At each speaker, another transformer steps down the voltage and
raises the current. That is, it brings the impedance down to 4
or 8 ohms to match the speaker.
SHOULD I CONSIDER IMPEDANCE WHEN I CONNECT TWO LINE-LEVEL DEVICES?
This is seldom a problem. In most audio devices, the impedance
of the line output is low -- about 100 to 1000 ohms. The
impedance of the line input is high -- about 10K to 1 Meg ohms.
So every connection is bridging, and you get maximum voltage
transfer.
Some audio devices, such as passive equalizers, require a
terminating resistor at the input or output for best performance.
CAN I CONNECT ONE SOURCE TO TWO OR MORE LOADS?
Usually yes. You can connect several devices in parallel
across one line output. Suppose you connect a mixer output
simultaneously to a recorder input, an amplifier input, and
another mixer's input in parallel. The combined input impedance
of those three loads might be 4000 ohms, which still presents a
bridging load to the mixer's 100-ohm output impedance.
Mics are a different story. If you connect one mic to two or
more mixers with a Y cable, the combined input impedance will be
about 700 ohms or less. This can load down some microphones,
reducing the bass in dynamic mics or causing distortion in
condenser mics. One solution is to use a transformer mic
splitter.
CAN I CONNECT TWO OR MORE SOURCES TO ONE INPUT?
Not recommended. If you combine two or more sources into a
single load, the low output impedance of one source will load
down the output of the other source, and vice versa. This can
cause level loss and distortion.
If you want to combine the signals from two devices into one
input, you need to put a series resistor in line with each device
before combining them. That prevents each device from loading
down the other. A minimum resistor value might be 470 ohms per
source. If the source is balanced, use one resistor on pin 2 and
one on pin3 -- two resistors per source.
SUMMARY
* Impedance (Z) is the opposition to alternating current, measured
in ohms.
* Microphones and line outputs are usually low Z.
* Electric guitars, synthesizers, and line inputs are usually high Z.
* XLR mic inputs are low impedance; phone jack mic inputs are high
impedance.
* Speakers are usually 4 to 8 ohms.
* Equal impedances in parallel result in half the impedance.
* Equal impedances in series result in twice the impedance.
* Connect low-Z sources to low-Z inputs. (A low-Z input is
usually 7 to 10 times the source impedance, but it's still called
a low-Z input).
* Connect high-Z sources to high-Z inputs.
* Connect a low-Z source to a high-Z input through a step-up
transformer (impedance matching adapter).
* Connect a high-Z source to a low-Z input through a step-down
transformer (impedance matching adapter, or direct box).
APPENDIX FOR TECH HEADS
Impedance includes both resistance and reactance. The
reactance part is inductive reactance and capacitive reactance.
Both vary with frequency. The resistive part is constant with
frequency.
Impedance is measured in magnitude (ohms) and phase (degrees)
at a certain frequency. Usually the magnitude part is all we are
concerned with. The magnitude part of impedance is:
Z = SQR (R^2 + (XL - XC)^2))
where SQR means "square root", R = resistance, XL = inductive
reactance, and XC = capacitive reactance, all in ohms.
Many devices don't have much reactance, so we often consider
impedance to be the same as resistance.
Bruce Bartlett is the author of "Practical Recording Techniques 2nd Ed." published by Focal Press.
Copyrighted 2000 by Cassette House. May not be reproduced in whole or part without permission.
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