Product Data

220 VA Power Amplifier – Type 2707

USES: ❍ Low distortion over wide frequency range
❍ Single and multiple exciter applications ❍ Adjustable exciter displacement limit
❍ Drives Exciter Body Type 4805 to full rating ❍ Adjustable RMS (including DC) output current limit
❍ Powerful variable frequency source in combination ❍ Internally protected against current overload
with frequency generator ❍ Extensive built-in protection with six indicator lights
❍ External protection possible

FEATURES: ❍ Automatic shut-down for malfunction in single and

multiple systems
❍ Front panel control for low or high output impedance
❍ Switchable phase reversal of output signal for mode
❍ Front panel control for DC centering of exciter table studies
❍ High AC power output and large DC capability in a ❍ Front panel oscilloscope monitor points
small cabinet
❍ Multi-range precise current and voltage monitoring
meters

Introduction

The Power Amplifier Type 2707 has

been designed to drive vibration ex-
citers, particularly the Brüel & Kjær
Exciter Body Types 4805 together
with associated interchangeable
heads.
The power amplifier has a useable
frequency range from DC to 100 kHz.
The full AC output is 220 VA over the
range 40 Hz to 10 kHz. Harmonic dis-
tortion content of the output is small
as heavy negative feedback is used. ing the oscilloscope monitor points cate the operation of these four pro-
The amplifier and the exciter head provided. tective functions as well as voltage
driving coil are directly coupled, to Fast-acting protective circuitry clipping due to excessive signal am-
provide static table centering and to gates-off the signal input to the am- plitude and also possible malfunction
exclude bulky output transformers. plifier if pre-set driving coil current of any output power transistor.
As well as power amplification, the and exciter head displacement limits The amplifier can be used as a
2707 provides system control, meter- are exceeded. In addition, excessive voltage generator with low output
ing and protective functions. During transistor temperature or opening an impedance and a flat voltage to fre-
operation the driver coil voltage and external interlock loop (e.g. failure of quency response, or as a current gen-
current RMS values are continuously fan cooling in a vibration exciter) also erator with high output impedance
shown on front panel meters and cause the removal of the amplifier and a flat current to frequency re-
their waveforms can be monitored us- input signal. Front panel lamps indi- sponse. In both modes it is a high-

Brüel & Kjær B K

power, low-distortion vibration-excit- FET gate to the preamplifier stage. Preamplifier Section
er driver, providing good protection The AC input is capacitively coupled The type of feedback from the output
from over-testing and system compo- and the DC input is direct-coupled. to the preamplifier stage is selected
nent failure. The feedback mode is The DC static centering signal is by the output impedance switch. Volt-
switchable and the amplifier can be applied to the preamplifier stage age feedback is used in the low im-
used in single- or multiple-vibration through a similar FET gate. Under pedance mode giving constant output
exciter applications. normal operating conditions the voltage and very low output imped-
The balanced preamplifier, opera- gates allow the signal to pass ance. Feedback proportional to the
tional amplifiers and sturdy solid- through. When the protection circuit- current flowing in the load is used in
state design provide the basis for ry is activated the gates are triggered the high impedance mode resulting
good temperature and supply line in constant output current and high
and disconnect the preamplifier stage
variation rejection. output impedance.
input from the static centering and
Impedance versus frequency
amplifier drive signal.
curves are shown in Figs. 2 and 3. In
The gate controlling the static de-
Description flection signal is purposely made
the high impedance mode even high-
er output impedance can be obtained
slow-acting. This ensures that large above 10 Hz by an internal shorting
The block diagram for the various cir- static offsets in table position are not link.
cuit functions is found in Fig.1. Cer- transmitted to the test object as fast Excessive signal levels at the input
tain features of the amplifier have transients when the equipment is will saturate the amplifier and cause
been omitted for simplicity. switched on or when the protection clipping of the output waveform. This
circuitry triggers. Resetting is simply will trigger the clipping detector
Input made by turning the amplifier gain which lights the yellow CLIPPING
Two inputs are provided, connected control fully counter-clockwise and lamp on the front panel. The power
via the amplifier gain control and a back again. amplifier will remain operative.

Clipping Transistor Peak

Current
Limiters

Amplifier
Gain
Preamplifier Amplifier
AC Input FET Output
+ Power Amplifier
Gates Current
– Monitor
20 mV/A
Current (10 mV/A)
DC Input
Feedback 3A
Quasi
High RMS
10 A
Direct + Rectifier
Low 30 A
Current
Output ± 10 A
High Current
Control – Meter
Voltage Voltage
Feedback Monitor
Power Section Low 1V/V
(0.1 V/V)
Low-High Z
Auxiliary Trigger Interlock 3V
Quasi
10 V RMS
Exiter 30 V Rectifier
Analogue Squarer Analogue
Interlock
Temperature and Displace-
Sensing Averager ment Voltage
Sensing Meter

Gating Signal
to FET Gates
Comparator and Trigger

Temperature Current Displacement

271062/1e

Fig.1 Simplified block diagram of power amplifier

2
 The preamplifier stage is followed 1000
by a high gain driver stage with 6 dB/ Ω
octave roll-off ensuring high stability 200
and high loop gain. 100
50
High Z
20
Power Output Section 10

Output Impedance
The power output stage is protected 2
by fast-acting fuses designed for sem- 1
iconductor protection. Transistor fail- 0.5
ure will blow one or more fuses and
0.2
light the TRANSISTOR warning
0.1
lamp. Transistor failure also acti- Low Z
vates a relay which removes the 0.05
three phase power line from the pow- 0.02
er output section and from the exciter 0.01
power output on the back panel. 0.005
Peak current limiting circuitry is
0.002
arranged to provide limitation of in-
0.001
stantaneous positive and negative 1 2 5 10 100 1 kHz 10 100
output current peaks. Frequency (HZ) 271050/1e
The small signal frequency re-
sponse across a resistive load of 0.5Ω Fig.2 Typical output impedance curves for Type 2707
is shown in Fig.3. Typical distortion
curves in the high and low impedance
modes are shown in Fig.4.

Protection
The Power Amplifier Type 2707 con-
tains extensive protective functions
for the power amplifier itself and con-
DISPLACEMENT. The front panel tirange front panel meters. An
nected vibration exciters. When trig-
control is used to pre-set the peak-to- additional zero-centre DC current
gered it turns off the two FET gates peak level of exciter table displace-
at the input, thus disconnecting the scale is provided for exciter head
ment at which the DISPLACEMENT centering purposes. The output volt-
drive and DC centering signals. Each circuitry trips. The control is contin-
triggered protective circuit also lights age and current waveforms can be
uously variable and can be set any-
a red lamp which gives an indication where between 2.5 and 50 mm (0.1 monitored at front panel BNC sock-
of the reason for equipment shut- and 2 inches). The circuit provides ets.
down. protection for both positive and neg-
Overload protection is provided ative displacement peaks. Use in Multiple Exciter-Amplifier
against excessive coil drive current SENSITIVITY. A 10-turn digital Systems
(CURRENT), exciter table displace- dial from 0 to 100 mm/Vs is used to In multiple systems the power ampli-
ment (DISPLACEMENT) and tran- calibrate the sensitivity of the exciter fiers are interconnected in such a way
sistor junction temperature increase used. that a trigger signal in any amplifier
(TEMPERATURE). A malfunction in TEMPERATURE. Abnormal load will remove the input signal from all
the external interlock loop will trig- conditions, failure of the heat sink
ger the EXCITER circuitry. the other amplifiers as well.
cooling fan, or high ambient temper-
CURRENT. A front panel control The high impedance mode is par-
ature could result in output transis-
is used to pre-set the true RMS out- tor temperatures in excess of design ticularly suitable when multiple ex-
put current including the DC compo- limits and subsequent transistor fail- citers are used for resonant mode
nent at which the CURRENT ure. To prevent damage to the output determination of structures. The forc-
circuitry trips. The limit can be set transistors under these conditions es generated by the exciters are in
anywhere between 2A and 24A. The the TEMPERATURE protective cir- phase or shifted 180° by a switch if
time constant of the averaging cir- cuitry will trigger. desired, adjustable in magnitude and
cuitry is 50 seconds, matching the EXCITER. This protective circuitry are insensitive to changes in imped-
minimum thermal time constant of is built-in to protect the connected
ance of the structure as frequency is
the exciter head used. This permits vibration exciter and is triggered by
swept. In this respect the operation
short-term current overload and an interlock loop.
means that the exciter system can be is similar to that of multiple exciters
used in a pulsed mode. The averaging Metering and Monitoring connected in series and driven by a
time constant can be simply reduced The RMS values of output current single amplifier, but it provides great-
to 2.5 seconds if a snap-action current and voltage for sinusoidal and Gaus- er flexibility in adjusting the relative
trip is required. sian waveforms are displayed on mul- forces generated by the exciters.

3
Use in Single Exciter Systems dB
The low impedance mode keeps the
voltage applied to the exciter inde- +2

Small Signal Frequency Response

pendent of test object changes. This DC Input
mode provides the best acceleration 0
waveform and is therefore preferable
for most single exciter applications. –2

However, the low impedance mode is AC Input

–4
also useful for multiple exciter sys-
tems in the lower frequency range –6
when the same motion is desired
from each exciter. An example is the –8
use of one exciter to push and anoth-
er to pull on a model of a bridge. –10
1 Hz 2 5 10 20 50 100 1 kHz 10 100
Frequency
Power Required 820846/1e
The power amplifier requires a three- Fig.3 Typical small signal frequency response curve
phase power supply of nominal volt-
age as indicated in the specifications.
For switching convenience the power
supply for the Exciters is also routed %
via the amplifiers power switch. The
0.6
power amplifier will be supplied from
the factory with mains cables at-
0.5
tached and fitted with a CEE plug
JP 0512 on the free end.
Distortion

0.4
High Z
System Interconnection 0.3
Interconnection between the amplifi-
er and exciters is made using three 0.2
cables; the power supply and inter-
0.1
lock cables to the exciter body, and
drive cable to the exciter head. For Low Z
0
the 4805, the interconnection is made 1 Hz 2 5 10 20 50 100 200 500 1 kHz 2 5 10
by routing a drive cable to the exciter Frequency
head and using a chassis-to-ground 820847/1e

wire to close the interlock loop. Fig.4 Typical distortion curve

4
Specifications 2707
CAPACITY: Response: • Safety Interlocks remove signal upon mis-op-
220 VA into 0.5 Ω exciter or resistor load Voltmeter: ±2% 20 Hz to 10 kHz eration of controls
Voltage: 10 V RMS DC to 10 kHz Ammeter: ±2% 20 Hz to 5 kHz; ±5% 5 kHz to • Multiple exciter-amplifier operation. Cross in-
Current: 10 A DC; 14 A RMS at and below 5 Hz; 10 kHz terlocking between amplifiers
24 A RMS 40 Hz to 10 kHz Accuracy at 1kHz: • Exciter power switching, system control
Voltmeter: ±2% of full scale
FREQUENCY RANGE: Ammeter: ±4% of full scale AC POWER REQUIREMENTS:
Full capacity: 40 Hz to 10 kHz 47 to 63 Hz
Reduced capacity: DC to 100 kHz PROTECTION: Three phase 380 V, 440 V, ±10%, RMS
Input signal is removed and an indicator 700 VA ±10% at full output
FREQUENCY RESPONSE: lamp is lit when the following parameters ex- Amplifiers comply with safety class 1 of IEC 348
Typical small signal response, low impedance ceed preset limits: Almost full output voltage capability down to
mode • Exciter displacement limit adjustable –15% of nominal mains supply.
DC input: DC to 10 kHz ±0.5 dB; DC to 100 kHz • Driver coil current true RMS (including DC) Exciter Body power is additional.
±3 dB adjustable limit
AC input: 15 Hz to 100 kHz ±3 dB • Power transistor junction temperature CABINET:
• Heat sink temperature Supplied as model A (light-weight metal cabinet)
INPUT IMPEDANCE: • Opening of an external interlock loop or C (as A but with flanges for standard 19”
at least 10 kΩ • Power transistor fuse blown racks)
Front panel indication is provided when the
GAIN AT 1 KHZ: following occurs (no shut-down): DIMENSIONS:
Low Impedance: 5 V/V ±1 dB • Output signal clippin A-model excluding connectors
High Impedance: 14 A/V ± 2 dB Height: 177 mm (7.0 in)
OTHER FEATURES: Width: 430 mm (16.9 in)
OUTPUT IMPEDANCE: • Electronic peak current limiting Depth: 500 mm (19.7 in)
Low Impedance: • Selectable output phase, 0° or 180° Weight (A-model): 27.9 kg (61.4 lb)
<0.02 Ω DC to 1 kHz • Voltage and current monitor points (front panel)
<0.05 Ω 1 kHz to 10 kHz
High Impedance:
>20 Ω 5 Hz to 1 kHz
>50 Ω 20 Hz to 300 Hz
>80 Ω 40 Hz to 100 Hz

HARMONIC DISTORTION:
0.5 Ω resistive load
Full capacity (Low Impedance): COMPLIANCE WITH STANDARDS:
<0.1% DC to 5 kHz
<0.5% 5kHz to 10kHz CE-mark indicates compliance with: EMC Directive and Low Voltage Directive.
Full capacity (High Impedance):
<0.2% DC to 2 kHz
<0.7% 2kHz to 10 kHz
Safety IEC 348: Safety Requirements for Electronic Apparatus
NOISE AND HUM:
Below full output EMC Emission EN 50081–1: Generic emission standard. Part 1: Residential, commercial and
Low Impedance: at least 85 dB light industry.
High Impedance: at least 75 dB EN 50081–2: Generic emission standard. Part 2: Industrial environment.
CISPR 22: Radio disturbance characteristics of information technology
DC STABILITY: equipment. Class B Limits.
For +5% to –15% variation of mains supply from FCC Rules, Part 15: Complies with the limits for a Class B digital device.
nominal, and for 10 to 40°C (50 to 104°F) vari-
ation in ambient temperature EMC Immunity EN 50082–1: Generic immunity standard. Part 1: Residential, commercial and
Less than 50 mV drift light industry.
EN 50082–2: Generic immunity standard. Part 2: Industrial environment.
DIRECT CURRENT OUTPUT:
Note 1: The above is guaranteed using accessories listed in this Product Data
Available at output socket, variable by front panel
sheet only.
control
At least ±2 V Note 2: RF disturbances above 3 MHz may result in demodulation and overload
conditions.
METERING:
Quasi-RMS indication rectifier circuits correct for Temperature IEC 68–2–1 & IEC 68–2–2: Environmental Testing. Cold and Dry Heat.
sinusoidal and Gaussian inputs Operating Temperature: +5 to +40°C (41 to 104°F)
Scales: Storage Temperature: –25 to +70°C (–13 to 158°F)
Voltmeter: 3, 10, 30 V RMS
Ammeter: 3, 10, 30 A RMS; ±10 A DC Humidity IEC 68–2–3: Damp Heat: 90% RH (non-condensing at 40°C (104°F))

Ordering Information
Type 2707 220 VA Power Amplifier includes the SC 0465 Self-adhesive Tag
following accessories: 10×VF 0022 5 Am 250 V Fuse, fast
JP 0035 screened BNC 75 plug VF 0039 400 mA, 250 V Fuse, time-lag
JP 0308 Cable 6×VF 0056 2 A, 500 V Fuse
JP 1005 6-pin plug VS 1274 Lamp, Festoon
QA 0112 Special Tool

Brüel&Kjær reserves the right to change specifications and accessories without notice

5
Brüel & Kjær B K

WORLD HEADQUARTERS:
DK-2850 Naerum · Denmark · Telephone: +45 45 80 05 00 · Fax: +45 45 80 14 05 · Internet: http://www.bk.dk · e-mail: info@bk.dk
Australia (02 ) 9450-2066 · Austria 00 43-1-865 74 00 · Belgium 016/44 92 25 · Brazil (011) 246-8166 · Canada: (514) 695-8225 · China 10 6841 9625 / 10 6843 7426
Czech Republic 02-67 021100 · Finland 90-229 3021 · France (01) 69 90 69 00 · Germany 0610 3/908-5 · Holland (0)30 6039994 · Hong Kong 254 8 7486
Hungary (1) 215 83 05 · Italy (02) 57 60 4141 · Japan 03-3779-8671 · Republic of Korea (02) 3473-0605 · Norway 66 90 4410 · Poland (0-22) 40 93 92 · Portugal (1) 47114 53
Singapore (65) 275-8816 · Slovak Republic 07-37 6181 · Spain (91) 36810 00 · Sweden (08) 71127 30 · Switzerland 01/94 0 09 09 · Taiwan (02) 713 9303
United Kingdom and Ireland (0181) 954-236 6 · USA 1 - 800 - 332 - 2040
Local representatives and service organisations worldwide
BP 0091 – 13 96/06