AP1_MeasuringBox-Rev4Eng.pdf
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A R T A - A P P L I C A T I O N N O T E
No 1: The ARTA MeasuringBox
The ARTA MeasuringBox for easy setup of loudspeaker impedance and frequency
response measurements.
Typical, low cost measurement equipment for loudspeaker measurements with ARTA,
STEPS and LIMP should consist of:
1)
High quality soundcard or USB audio device (i.e. Terratec EWX 24/96 or M-Audio
USB Transit),
2)
Calibrated microphone (i.e. Behringer ECM 8000 or MBC 550),
3)
Microphone preamplifier with calibrated gain control (i.e. Monacor MP102),
4)
Power amplifier (any audio amplifier with volume control and output power 10-50W),
5)
A switch box and cables for easy connections of audio devices.
Figure 1 shows a simple switch box - the ARTA MeasuringBox. It is intended for loudspeaker
impedance and frequency response measurements.
Figure 1. Front view of the MeasuringBox.
On the front side of the MeasuringBox there are:
-
RCA jack for connection of a microphone preamplifier,
-
Binding posts are for loudspeaker connection,
-
Switch SW1 is for switching between impedance measurements and frequency
response measurements,
-
Switch SW2 is for impedance measurement calibration. Position
Imp Cal
. is for
calibration using
Limp
. After calibration the switch SW2 has to be returned to position
measurement
.
On the back side of the MeasuringBox there are two RCA jacks for connection to soundcard
left and right line input channels, and binding posts to power amplifier output. The red post is
signal line and the black post is a ground line. Figure 3 shows the picture of connections
inside the ARTA MeasuringBox.
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A R T A - A P P L I C A T I O N N O T E
No 1: The ARTA MeasuringBox
Figure 2. Back view of the MeasuringBox.
Figure 3. The picture of connections inside the MeasuringBox
Figure 4 shows a schematic diagram of the ARTA MeasuringBox. Table 1 shows the parts list
of the ARTA MeasuringBox. Figure 5 shows a connection plan of the ARTA MeasuringBox.
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A R T A - A P P L I C A T I O N N O T E
No 1: The ARTA MeasuringBox
Note 1
Power amplifier / loudspeaker ground and output low-level grounds are separated by a 1kOhm
resistance.
Note 2
Do not use bridged amplifiers with virtual ground!
Safety
The soundcard input is protected by Zener diodes. The power amplifier is protected as described in
the manufacturer's manual. It means that you have to take care on the nominal loudspeaker
impedance.
Figure 4. The schematic diagram of the MeasuringBox
Element
Value
Box
Plastic "Euro box"
Rref
Reference resistor 27 ohm/10W
R1, R3
8k2 (all resistors are 1%)
R2, R4
910
R5
715
R6
1k
D1,D2,D3, D4
Zener diode 4.1V / 0.5W
J1
RCA jack - red.
J2, J3
RCA jack - black
BP1-2, BP3-4
Dual binding posts (red and black)
SW1
DPDT 6A switch
SW2
SPDT switch
Table 1. Elements of the MeasuringBox
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A R T A - A P P L I C A T I O N N O T E
No 1: The ARTA MeasuringBox
Figure 5. Connection plan for Measuring box
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A R T A - A P P L I C A T I O N N O T E
No 1: The ARTA MeasuringBox
A little math for designers or what is necessary to adopt box elements for special needs.
In the default configuration the box has the following operational characteristics. Resistors
R1, R2 together with soundcard input impedance Zin, form voltage divider
r
that is equal to
r
= (R2 || Zin) / (R1 + R2 || Zin)
Maximal voltage that can be applied from power amplifier to soundcard line input reference
channel is equal to the
soundcard_voltage_rms_sensitivity
/
k
. Then,
maximal_power
that
can be used in measurement is equal to
maximal_power
= (
voltage_rms_sensitivity
/
k
)
2
/
nominal_loudspeaker_impedance
For usual values Z = 10k, R1= 8k2, R2 =910 and voltage sensitivity 1V we get that maximal
power is equal to 29W/4ohm or 14.5W/8ohm.
If we have a power amplifier that is not able to deliver this power the voltage divider must be
lowered accordingly, alternatively if we want to make testing with higher power we must
increase voltage divider.
The value of resistor R5 has to be determined from the following expression:
R5 = R1 || R2 - Zout
where Zout is microphone amplifier output impedance.
This equation came from the requirement that both soundcard input channels are driven from
generators that have same source impedances. In this configuration we assume Zout=100 ohm
(i.e. a value for MP102 preamplifier).
Using the ARTA MeasuringBox in Calibrated Measurements
For
measurement of impedance
with program LIMP you, have to put calibration switch in
position 'Imp Cal' and follow usual calibration procedure that is necessary for each impedance
measurement. After calibration is done you must put calibration switch in position for
'Impedance measurement'.
In
Dual channel frequency response measurement mode,
in ARTA and STEPS program,
we have to enter proper values for left and right preamplifier (probe) gain. In this mode
impedance calibration switch is not used. We assume that right soundcard channel is used as
reference channel and left soundcard input channel is used as response channel.
For right preamplifier you have to enter voltage probe divider
r
= (R2 || Zin) / (R1 + R2 || Zin) = (910 || 10000) / (820 + (910 || 10000)) = 0.0923
For left preamplifier you have to enter
l
=
mic_preamp_gain
* Zin / (Zout + R5 + Zin)
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