BRUEL & KJAER 2720 User manual

Technical
Documentation
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Power Amplifier Type 2720
English BE 1661 – 12
User Manual
ËBE-1661---ÂÎ
BE1661-12_Cover.fm Page 1 Friday, November 25, 2005 11:10 AM

BE 1661 – 12 May 2006
Power Amplifier Type 2720
User Manual

Trademarks
Neutrik and Speakon are registered trademarks of Neutrik AG
Copyright ©2002 – 2005, Brüel & Kjær Sound & Vibration Measurement A/S
All rights reserved. No part of this publication may be reproduced or distributed in any form,
or by any means, without prior written consent from Brüel & Kjær Sound & Vibration
Measurement A/S, Nærum, Denmark
Safety Considerations
This apparatus has been designed and tested in accordance with IEC 61010 – 1 and EN 61010 –
1 Safety Requirements for Electrical Equipment for Measurement, Control and Labo-
ratory Use. This manual contains information and warnings which must be followed to ensure
safe operation and to retain the apparatus in safe condition. Special note should be made of the
following:
Safety Symbols
The apparatus will be marked with this symbol when it is important that you refer to the
associated warning statements given in the manual.
Explosion Hazard
The equipment is not designed to be used in potentially explosive environments. It should not
be operated in the presence of flammable liquids or gases.
Warnings
• In order to avoid injury, it is recommended that two persons share the weight when lifting,
positioning and installing the power amplifier.
• Whenever it is likely that the correct function or operating safety of the apparatus has
been impaired, it must be made inoperative and be secured against unintended operation.
• Any adjustment, maintenance and repair of the open apparatus under voltage must be
avoided as far as possible and, if unavoidable, must be carried out only by trained service
personnel.
• Do not dispose of electronic equipment as unsorted municipal waste
• It is your responsibility to contribute to a clean and healthy environment
by using the appropriate local return and collection systems
• Hazardous substances in electronic equipment may have detrimental effects
on the environment and human health
• The symbol shown to the left indicates that separate collection systems
must be used for any discarded equipment marked with that symbol
• Waste electrical and electronic equipment may be returned to your local
Brüel & Kjær representative or to Brüel & Kjær Headquarters for disposal

Table of Contents
CHAPTER 1
Introduction......................................................................................................... 1
CHAPTER 2
Controls............................................................................................................... 3
Front Panel................................................................................................................................ 3
Back Panel ................................................................................................................................ 4
CHAPTER 3
Operation............................................................................................................. 5
Preliminary ................................................................................................................................ 5
System Checks and Connections ............................................................................................. 5
Operating Procedure ................................................................................................................. 8
CHAPTER 4
Characteristics.................................................................................................. 11
Signal Inputs............................................................................................................................ 11
Power Output .......................................................................................................................... 11
Frequency Response .............................................................................................................. 13
CHAPTER 5
Specifications ................................................................................................... 15
CHAPTER 6
Service and Repair ........................................................................................... 17

1
Chapter 1
Introduction
Power Amplifier Type 2720 has been especially designed to drive Modal Exciter Type 4825 but
may be used with any vibration or modal exciter requiring a 500 VA power amplifier. The rated
AC output is 500 VA into a 4 Ωexciter or resistive load. The power amplifier has a usable
frequency range from 40 Hz to 10 kHz (full capacity) or DC to 150 kHz (reduced capacity). The
maximum voltage gain is 18 V/V in low-impedance mode. Harmonic content of the output is
very small as heavy negative feedback is used.
The instrument can tolerate temperature and supply line variations while maintaining excellent
stability.
Type 2720 can be used as a voltage generator with low output impedance and a flat voltage
frequency response, or as a current generator with high output impedance and a flat current fre-
quency response. The RMS output-current limit is adjustable.

Power Amplifier Type 2720 – User Manual2

3
Chapter 2
Controls
Front Panel
Fig.2.1 Front panel of Type 2720
Display: The display shows output AC current and voltage on a LCD screen.
• The Temperature LED Indicator lights red if the Power MOS Output Transistors overheat.
The amplifier will then go into Reset state
• The Current LED Indicator lights red when output current to the load is exceeding the current
limit value set. The amplifier will then go into Reset state
• The State LED Indicator lights green when amplifier power is switched on
• The Voltage Mode or Current Mode LED Indicator lights green showing the output mode,
depending which mode has been selected using the Phase button
• The Interlock LED Indicator lights red when the interlock circuit is activated
• The 0° or 180° LED Indicator lights green indicating phase shift from input to output
• The Distortion LED Indicator lights yellow when output voltage or current clipping takes
place
Current Limit A (RMS): Single-turn potentiometer for limiting RMS current output.
Amplifier Gain: Single-turn, logarithmic potentiometer for continuous adjustment of input sig-
nal level. To prevent a power surge, turn the knob fully anticlockwise for maximum attenuation
before switching the power on or off. Turning the knob fully anticlockwise past the click and
back again activates reset.
V/C Mode: Button for selection of feedback and output impedance modes. The positions are:

Power Amplifier Type 2720 – User Manual4
•Voltage Mode: Provides constant voltage characteristics independent of changes of test object
on the exciter. Gives the best acceleration waveform and is therefore preferable for most vibra-
tion tests
•Current Mode: Provides constant current characteristics, keeping the generated force inde-
pendent of changes of test object. This is the preferred setting for most modal test applications
Phase: Button to set the phase shift, output/input, 0 or 180 degrees.
Power: Mains On/Off button.
Back Panel
Fig.2.2 Back panel of Type 2720
Signal Input AC: BNC socket providing a capacitive coupled input to the amplifier.
Signal Input DC: BNC socket providing a direct-coupled input to the amplifier. It enables a
DC offset voltage to be applied for centring the exciter’s armature. The input impedance is
>10 kΩ..
Voltage Monitor: BNC socket providing output of the amplifier voltage waveform (including
DC component) for display on an oscilloscope. The monitor signal is the amplifier output
voltage attenuated 20 dB or 0.1 V/V.
Current Monitor: BNC socket providing output of the amplifier output waveform (including
DC component) for display on an oscilloscope. The monitor signal is the amplifier output
current in 0.1 V/A.
Output: Power output Neutrik®Speakon®socket accepting a 4-pin Neutrik®Speakon®plug or
connecting cable for connection of an exciter (see “Output Connections” on page 6). For a full
power output of 500 VA, the moving coil of the exciter or load should have a nominal load
impedance of 4 Ω.
Power Outlet for DC Static Centering Unit Type 1056: See “Optional Type 1056 Connec-
tion” on page 7 for connection information.
Mains Voltage Selector and Fuse: Voltage selector for operation of the amplifier from a 100,
120, or 230 V ±10% (50 to 60 Hz) single phase, AC mains supply. To select the correct voltage
setting or to change the fuse, see “Mains Supply Connections” on page 7.
Mains Input: Input socket accepting the power cable provided. For connection of a mains
supply see “Mains Supply Connections” on page 7.

5
Chapter 3
Operation
Preliminary
Environment
Power Amplifier Type 2720 is designed for operation at ambient temperatures between 5 and
40°C (41 to 104° F). Operation at full power will cause the Power MOS Output Transistors’
heat sink to heat up. This is normal. However, it is important to ensure a free flow of cooling
air through the input vent in the front panel to the output vent in the back panel, otherwise the
input drive signal will be automatically blocked and Type 2720 will go into Reset State to
prevent overheating of the transistors.
Rack Mounting
Power Amplifier Type 2720 can be used free standing on its four rubber feet. Its design also
allows it to fit nicely in a 19-inch standard rack system with the rubber feet removed.
System Checks and Connections
Before connecting a mains supply, go through the following connections and system setup, to
ensure the correct function and safe operation of the apparatus.
WARNING: Disconnect all power from the amplifier before removing or replacing its
protective panels. Internal adjustments with mains power connected should only be carried out
by skilled persons who are aware of the hazards of dealing with live circuitry.

Power Amplifier Type 2720 – User Manual6
Fig.3.1 Typical system setup using Type 2720
Output Connections
The amplifier’s power socket accepts a 4-pin Neutrik®Speakon®plug. The pin connections are
shown in Fig.3.2.
Fig.3.2
Output sockets for Type
4825 (external view)
Note: The over-current safety input limits the maximum current to the exciter to one half. The
current limit is activated in the event of either a short circuit or a voltage above 8 V.
For connection to Modal Exciter Type 4825, use Drive Cable AQ 0649 provided with the exciter.
The pin connections are shown in Fig.3.3.
Optional
Signal from
Generator
Coaxial Cable
BNC to BNC
AO 0133
AN 0010
Drive Cable AQ 0649
9-pin Sub-D
Female to HR10
Cable
AQ 0654
010250/1
UH-1035
AF 1101
DC Static Centering
Unit 1056
Power Amplifier 2720
Serial Cable
AQ 0651
Mains
Power
4825
Drive Signal
(Coil Start)
Over-current
Neutral
(Coil Finish)
-1 -2
+2
+1
010167

CHAPTER 3
Operation 7
Fig.3.3
Drive Cable AQ 0649 for
connection of Type 2720
and Modal Exciter Type
4825 (view from soldering
side)
Over-current Safety Feature
The built-in over-current safety feature will limit the maximum current output to one half,
thereby protecting the armature coil when used without forced cooling. The safety feature is
activated (reduced current) in the following events:
1) If cooling is not sufficient for Modal Exciter Type 4825 (operation).
2) Short circuit or voltage difference above 8 V across control input and ground.
Optional Type 1056 Connection
When using DC Static Centering Unit Type 1056 for applying a pre-tensioning force to the
tension wire between the modal exciter and the input transducer, the signal generator is connect-
ed directly to the BNC input connector of Type 1056. Using the standard BNC cable AO 0133
provided with Type 1056, connect its BNC output to the DC input of Power Amplifier Type
2720.
Type 2720 provides power to Type 1056 via the standard serial cable AQ 0651, which is also
included with Type 1056.
Mains Supply Connections
Mains Voltage Setting
Type 2720 can be powered from a 100 V, 120 V or 230 V ±10% (50 to 60 Hz), single-phase AC
mains supply. To select the correct mains voltage setting, use a small screwdriver to remove the
voltage selector with integrated fuse holder from the combined voltage selector and mains inlet,
and then put it back again so that the white arrow is aligned with the appropriate voltage
printed on the selector.
Fuse Check and Replacement
The mains fuse is part of the voltage selector. It can be released using a screwdriver. For 100 V
or 120 V operation, the fuse should be a T10A, while for 230 V operation it should be a T4A.
Make sure that only fuses of the required rated current and of the specified type are used for
replacement. Do not use mended fuses or short-circuit the fuse holder.
Brown
Yellow/Green
Blue
Black
-1
-2
+2
+1
-1
-2
+2
+1
010113/3
Drive Cable Connections

Power Amplifier Type 2720 – User Manual8
Mains Socket Connection
Once the mains voltage setting and fuse are correct, use the power cable provided to connect
the mains to the mains input socket of the amplifier. Note, that for maximum operating safety
the protective (green/yellow) conductor of the cable should be connected to a suitable earth,
such as the earth contact of a mains outlet socket. The use of an extension cable without
protective conductor should be avoided for safety reasons. In countries where use of mains
socket outlets without protective conductor is standard, the mains supply should incorporate an
authorised earth leakage current circuit breaker (a Residual Current Circuit Breaker – RCCB).
Operating Procedure
After following the instructions described in “System Checks and Connections” on page 5, use
the following procedure to start operation:
1) Set the amplifier controls as shown below:
2) Connect the output of a signal generator to the AC or DC input on the back panel of the
amplifier.
3) On the power amplifier set:
4) Set the generator to the required frequency. Then slowly increase the output voltage until
the exciter reaches the required level of vibration. With setups where the vibration level is
controlled by a feedback circuit, the voltage level will be increased automatically.
If the amber distortion lamp lights or the maximum displacement limit of the exciter is
exceeded, causing the armature to knock against its end-stops, reduce the generator output
voltage to a lower level in order to resume operation.
For frequency sweep vibration testing, adjust the generator to the lowest frequency of interest
to check that the exciter's low frequency limit is not exceeded.
5) To set the amplifier to stand-by during a test, turn the Amplifier Gain to Reset. At the end
of a test, always put the amplifier on stand-by before switching the Power Off.
Power: Off
Amplifier Gain: Reset click-stop position
Current Limit A (RMS) Maximum current limit of the exciter’s moving coil. Consult
the manufacturer’s data for the exciter in use.
V/C Mode: Select Voltage Mode for the best acceleration waveform or
Current Mode for force-related tests. See “Power Output”
on page 11.
Power: Power button to On. The Status, Mode and Phase LEDs in
the display unit will light up green.
Amplifier Gain: Fully clockwise to position Cal.

CHAPTER 3
Operation 9
Warning Lamps and Fault Detection
If one of the red warning lamps lights up, a fault has occurred in the system. Under such
circumstances, the test is automatically stopped to protect the amplifier and exciter. To help
establish the cause of the shutdown, some probable faults are given in the table below.
If incorrect amplifier control settings or exciter connections cause shut-down, turn the Amplifier
Gain to Reset and make the necessary adjustments. Normal operation can then be resumed by
returning Amplifier Gain to the position used for test. If a shut-down is caused by an internal
fault in the amplifier or exciter, stop the test by setting Amplifier Gain to Reset and switch the
amplifier off. Then consult your Brüel & Kjær service representative.
Warning Lamp Probable Fault
Current Input drive level too high for Current Limit A (RMS) setting
Wrong output connections to exciter
Temperature Overdrive at low frequencies
Wrong output connections to exciter
Vibration laboratory temperature too high
Forced air cooling system of amplifier blocked

Power Amplifier Type 2720 – User Manual10

11
Chapter 4
Characteristics
Signal Inputs
The signal inputs of Type 2720 have a minimum input impedance of 10 kΩ. The AC signal
input is for connecting an exciter controller or generator and is capacitive-coupled giving a
–0.5 dB lower limiting frequency of approximately 15 Hz. The DC signal input is direct-coupled
enabling a DC offset voltage to be applied for centring the exciter armature.
Power Output
In Voltage Mode (low impedance), a fraction of the voltage developed across the moving coil of
the exciter is used as feedback. This gives the amplifier constant voltage characteristics –very
low output impedance and constant output voltage with frequency –producing the best acceler-
ation waveform. It is therefore suited to most single-exciter applications as well as to multiple-
exciter applications at low frequencies, where it is important that the exciters have the same
motion.
In Current Mode (high impedance), feedback is proportional to the current flowing in the excit-
er’s moving coil. This gives the amplifier constant current characteristics –high output imped-
ance and constant output current with frequency –necessary to obtain a constant force with the
exciter regardless of changes in the test object. This is useful for single-exciter fatigue tests and
multiple-exciter resonant mode studies of structural test objects.

Power Amplifier Type 2720 – User Manual12
Fig.4.1 Output impedance of Type 2720 as a function of frequency and impedance mode
The power output of Type 2720 is direct-coupled. Its output impedance, shown in Fig.4.1,
depends on the feedback setting selected using the V/C Mode switch.
Fig.4.2 Maximum output current rating of Type 2720 as a function of frequency
010203
20
10
5
2
1
0.5
0.2
Output Impedance (ohms)
10 20 100 200 500 1k 2k 5k
10k
20k 50k 100k
Frequency (Hz)
0.1
0.05
0.02
0.01
5
2
150
50
100
200
500
1k
0.005
0.002
0.001
High Impedance Specification Limit
Low Impedance Specification Limit
20
18
16
14
12
10
8
Maximum Output Voltage (V) and Current (A)
10 20 50 100 200 500 1k 2k 5k
10k
20k 50k 100k
Frequency (Hz)
6
010202
4
2
0
5
2DC1
11.25A
11.25A

CHAPTER 4
Characteristics 13
As shown in Fig.4.2, the maximum output current is frequency dependent. The maximum pow-
er output is 500 VA, which can be obtained using exciters with a nominal load impedance of
4Ω.
Frequency Response
The full power output of 500 VA is available in the frequency range from 40 Hz to 15 kHz. At
lower power levels the amplifier has a useful frequency range from DC to 100 kHz. This
depends on the signal input socket used and the setting of the V/C Mode switch, as shown by
the response curves for small signals given in Fig.4.3.
Fig.4.3 Frequency response of Type 2720 in Voltage Mode for small power output levels
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
Relative Amplitude (dB)
10 20 50 100 200 500 1k 2k 5k
10k
20k 50k 100k 200k 500k 1M
Frequency (Hz)
-7
010201
-8
-9
-10 521
Low Impedance DC Input Specification Limit
Low Impedance AC Input Specification Limit
DC Input
AC Input Low Impedance
voltage

Power Amplifier Type 2720 – User Manual14
Distortion
The percentage of harmonic distortion produced by Type 2720 is shown in Fig.4.4. Considering
the 500 VA power output rating of the amplifier, the amount of distortion is very low. This can
be attributed to the generous amount of feedback applied and the use of a direct-coupled input.
Fig.4.4 Typical harmonic distortion percentage curves for Voltage Mode and Current Mode
output impedance –500 VA in a 4 Ωload
010200/1
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Output Distortion (%)
12510 20 50 100 200 500 1k 2k 5k 10k 20k 50k 100k
Frequency (Hz)
H
L
High Impedance Specification Limit
Low Impedance Specification Limit Voltage Mode
Current Mode

15
Chapter 5
Specifications
COMPLIANCE WITH STANDARDS
compliance with EMC Directive
compliance with EMC Requirements of
Australia and New Zealand
Safety, EMC Emission and Immunity: According to
relevant standards:
EN/IEC 61010 –1, UL 61010–1,
EN/IEC 61000–6–2, EN/IEC61000–6–4, CISPR22
Class A limit, FCC Rules Part 15, EN/IEC 61326
Temperature: According to IEC 60068–2–1 and
IEC 60068–2–2
Operating temperature:
+5 to +40°C (41 to 104°F)
Storage temperature:
–25 to +70°C (–13 to 158°F)
Humidity: According to IEC 60068–2–78, Damp
Heat: 90% RH (non-condensing at 40°C (104°F))
Mechanical: Non-operating according to IEC 60068–
2–6, IEC60068–2–27, IEC60068–2–29
POWER OUTPUT CAPACITY
500 VA into a 4 Ωexciter or resistive load, at 25°C and
nominal mains voltage
(4-pin Neutrik SPEAKON socket on rear panel)
OUTPUT VOLTAGE CAPACITY
45 V RMS, DC to 15 kHz
OUTPUT CURRENT CAPACITY
5 A RMS at or below 5 Hz
11 A RMS, 40 Hz to 15 kHz
MAX. VOLTAGE GAIN
25 dB (±2dB) @ 1kHz
FREQUENCY RANGE
Full Capacity: 40 Hz to 15 kHz
Reduced Capacity: DC to 150 kHz (–20 dB)
FREQUENCY RESPONSE
Typical small signal response (–20 dB) in low-
impedance mode:
DC Input: DC to 15 kHz ±0.5 dB; DC to 150 kHz ±3dB
AC Input: 15 Hz to 15 kHz ±0.5 dB
(2 separate BNC sockets on rear panel)
THD AT FULL OUTPUT CAPACITY
Low-impedance mode:
<0.1% (40 Hz to 5 kHz)
<0.2% (5 kHz to 15 kHz)
High-impedance Mode:
<0.2% (40 Hz to 2 kHz)
<0.8% (2 kHz to 15 kHz)
INPUT IMPEDANCE
>10 kΩ
DC STABILITY
Less than ±100 mV drift from 0 V for ±10% variation
of mains supply from nominal, and for 10° to 40°C
(50° to 104°F) variation in ambient temperature
CONTROLS
Power on/off
Continuously variable gain control, 0 to Cal. (25 dB)
with integral reset
Continuously variable current limit control, 1 to 11 A
(RMS)
Switch for voltage mode or current mode operation
Switch for phase inversion (0° or 180°) between input
and output
INDICATOR LAMPS (LED)
Clipping
Temperature overload
Current overload
State
Voltage mode
Current mode
Interlock