IMP1 Impedance Fixture User’s Guide
The Audio Precision IMP1 Impedance Fixture provides
a convenient solution for measuring loudspeaker driver
impedance.
Overview
This document describes the use of the IMP1 impedance fixture with the APx Impedance/Thiele-Small or
Loudspeaker Production Test measurements, which
can produce a set of impedance curves and the derived
Thiele-Small parameters. Unless noted, references made
to settings and fields in the APx software refer to either
of these measurements.
When measuring impedance in one of the external configurations available in the APx500 software, an external
power amplifier and sense resistor are required. The
IMP1 provides a choice of two sense resistors, with the
values of 1.0 Ω and 0.1 Ω. XLR connectors on the front
of the IMP1 provide analyzer connections, and doublebanana connectors on the rear provide connections for
the driver and a power amplifier.
Driver impedance measurements require simultaneous measurement of the voltage across the driver voice
coil and the current through it. The voltage across the
coil can be directly measured by an analyzer input; the
current in the circuit is calculated from the measured
voltage across a precision resistor of known value, called
the sense resistor.
IMP1 rear panel
Impedance Testing with an APx analyzer
IMP1 front panel
In these tests, we are not measuring conventional audio
parameters; we are measuring voltage or current in
circuit components and making calculations with the
results. Consequently, one or two of the analyzer input
External Power
Amplifier
+
–
+
–
“From Amplifier”
APx Generator
APx
Balanced
Input
–
.1Ω
+
+
XLR
OUT 1 Ω
IN .1 Ω
+
+
–
+
–
GND
“Driver Voltage”
APx
Balanced
Input
1Ω
XLR
GND
–
OUT DRV
CURRENT
“To Driver 1 Ω”
IN AMP V
+
–
“Driver Current”
or
“Amplifier Voltage”
–
“To Driver .1 Ω”
APx Analyzer
IMP1 schematic
channels must have specific roles assigned, either to
Choosing a test configuration
measure the voltage across the driver, the voltage across
IMP1 and the APx impedance measurements support
the sense resistor, or the total voltage across the driver
two test configurations (with additional variations), deand the sense resistor. Please pay close attention to the
roles depicted in the illustrations below and identified on tailed below. Both configurations use a power amplifier
and sense resistor external to the analyzer.
the Channel control in the software. Refer to the Impedance/Thiele-Small and Loudspeaker Production Test
External/known gain
documentation in the APx500 User’s Manual and in the
In the APx software, this configuration is denoted as
online Help.
Ext./known gain.
In all cases, for best accuracy use short wires of very low
resistance and choose a power amplifier with low output The gain and phase response of the amplifier must be
known from a previous measurement to allow correction
impedance.
for amplifier characteristics. We recommend that you
Choosing the sense resistor value
perform the amplifier correction procedure available in
the Impedance/Thiele-Small and Loudspeaker ProducThe resistance of the sense resistor must be low comtion Test measurements in current versions of APx500.
pared to the nominal driver impedance to avoid affectSee the APx500 user documentation for more informaing the measured Q. The 0.1 Ω resistor is appropriate
tion.
for drivers of 2 Ω to 8 Ω. For higher impedance drivers
(such as headphones), the 1.0 Ω sense resistor is recommended.
Selecting a sense resistor requires two actions:
1. Set the IMP1 front panel switch to the selected resistance, with the button OUT for 1.0 Ω, or IN for 0.1 Ω.
2. Connect the driver under test to the appropriate set
of banana jacks, labeled for 1.0 Ω or 0.1 Ω on the
rear panel.
Both of these actions must be taken for correct measurements.
Once amplifier correction is in place, the selector switch
on the IMP1 front panel must be set to DRIVER CURRENT (OUT position). Only one connection is required,
from the XLR labeled DRIVER CURRENT to an analyzer
input. In the APx software, Channel (sense) must be set
to the input channel connected to DRIVER CURRENT.
Enter the amplifier gain and the value of the sense resistor into the data entry fields in the APx software. The
voltage across the sense resistor is measured. Since the
amplifier gain is known, the total voltage across the load
(driver plus sense resistor) is known. VDRV is calculated by
subtracting VSENSE from the total voltage across the load.
Amplifier
VDRV
(calculated)
RSENSE
VSENSE
to APx Input,
BAL or UNBAL
APx Generator
External/known gain test configuration
External/unknown gain
Advantage
Only one analyzer input is required for the impedance
measurement. Since the Loudspeaker Production Test
measurement requires an additional input for the acoustic test, this configuration is important for analyzers with
only two inputs.
Disadvantage
Amplifier gain and phase response must be carefully
measured and remain stable through out the testing
period. For amplifiers with level controls, there is the risk
that the gain may be accidentally changed.
In the APx software, this configuration is denoted as
Ext./unknown gain.
For this configuration, the resistance of the sense resistor must be precisely known, but the amplifier gain is
irrelevant. The analyzer inputs must be balanced.
The selector switch on the IMP1 front panel must be set
to DRIVER CURRENT (OUT position). Two connections are
required. For this configuration, the APx software assigns the roles of drvr (driver voltage) and sense (driver
current) to adjacent channel pairs. Be sure to connect
DRIVER VOLTAGE to the analyzer balanced input as-
Amplifier
VDRV
to APx Input
Ch n,
BAL only
RSENSE
APx Generator
External/unknown gain test configuration
VSENSE
to APx Input
Ch n+1,
BAL or UNBAL
signed the drvr role, and connect DRIVER CURRENT to
the analyzer input assigned the sense role.
Enter the value of the sense resistor into the data entry
fields in the APx software. The amplifier gain is measured, and the voltages across the driver and the sense
resistor are measured.
Advantage
This is our recommended configuration if your analyzer
has more than two inputs. Because VDRV and VSENSE are
measured simultaneously, no assumptions are made
regarding the amplifier response. Impedance results are
reported correctly across the frequency range.
Disadvantage
Requires two analog inputs for the impedance measurement. Since the Loudspeaker Production Test measurement also requires an input for the acoustic test,
analyzers with only two inputs cannot use Loudspeaker
Production Test in this configuration.
Nominal Driver
Impedance,
ohms
Maximum
Amplifier Output
Voltage, Vrms
(1 Ω sense
resistor)
Maximum
Amplifier Output
Voltage, Vrms
(0.1 Ω sense
resistor)
0
3.16
1.00
1
6.32
11.00
2
9.49
21.00
3
12.65
31.00
4
15.81
41.00
5
18.97
51.00
6
22.14
61.00
7
25.30
71.00
8
28.46
80.00
16
53.17
80.00
Four-wire configurations
There are unavoidable resistances in connections,
switch contacts and lengths of wire that will affect the
overall accuracy of measurements made with impedance
fixtures. In a four-wire configuration, use two additional
wires to connect the analyzer drvr connection directly
across the driver voice coil to improve overall accuracy.
Specifications
Sense Resistors
1.0 Ω, 1 %
0.1 Ω, 1 %
Power Dissipation
Power dissipation, continuous
The IMP1 has a maximum continuous power dissipation
of 10 watts. The IMP1 will typically become warm to the
touch with continuous testing at 10 watts.
Typical overall accuracy
The table in the next column lists examples of the amplifier output voltages that will attain 10 watts dissipation
in the sense resistor, for a range of driver impedances
and for both sense resistance values. Do not exceed the
recommended Amplifier Output Voltage for a given combination of Driver Impedance and sense resistor value.
10 W
Known gain, two wire: typically ±3 %
Unknown gain, two wire: typically ±2 %
Unknown gain, four wire: typically ±1 %
Caution: Continuous power dissipation of more than 10
watts in the IMP1 sense resistors can result in high case
temperatures and damage to the IMP1.
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