BRUEL & KJAER 2706 User guide

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Input
I ,
"'•
Power Ampl ifi•r
T~ 2706
Gain Control • Power Amplifier
Type 2706
A low distortion power amplifier for
driving small vibration exciters and
for general purpose power amplifica-
tion.
It features protection against short
circuited
output and excessive
temperature. Clipping
of the output
signal is indicated.

POWER AMPLIFIER TYPE 2706
May 1972

CONTENTS
1. INTRODUCTION ........•.........................•.•. 3
2. CONTROLS .......................................... 4
2.1. Front Panel
2.2. Rear Panel
.................................. 4
5
3. OPERATION .•.......••......•.•......•..•.•.....•... 7
4. DESCRIPTION ...........•..•.....•................... 8
4.1. Attenuators and Preamplifier .................... 8
4.2. Differential amplifier and power amplifier
.......... 9
4.3. Current limiters and clipping
detector ............. 9
4.4. Temperature protection ........................ 9
4.5. Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Power output capacity . . . . . . . . . . . . . . . . . . . . . . . . 10
Distortion .................................. 12
Noise and Hum
.............................. 12
5. APPLICATIONS ............••........................ 14
5. 1. Generals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. Use with the B & K Vibration Exciters ............ 14
Accelerometer calibration . . . . . . . . . . . . . . . . . . . . . . 15
Vibration testing
............................. 16
5.3. Use with the Reverberation Processor Type 4422 . . . . 16
5.4. General use in Acoustics . . . . . . . . . . . . . . . . . . . . . . . 17
6. SPECIFICATIONS .................••......•.......... 19

1. INTRODUCTION
The Power Amplifier Type 2706 has been designed to drive small
vibration exciters, particularly the B & K Vibration Exciter Type 4809. By
switching to the 1.8 Amp. RMS output current limit, it is possible to drive
the
Mini Shaker Type 4810 safely at full rating.
Power capacity
of Type 2706 is 75 VA into a 3 n load. The overall
votage gain is 40 dB, and input attenuators allow to adjust the level at a
desired value when no level adjustment
is available on the signal source,
which makes
it ideal for use with the Reverberation Processor Type 4422.
Its very low harmonic distortion and low noise level make it suitable for
a wide range of applications in vibration and acoustics.
Type 2706
is fully protected against short circuited output and excessive
heat sink temperature. When clipping
of the output level occurs, a warning
lamp is lit.
3

2. CONTROLS
2.1. FRONT PANEL
Current Limit
Clipping
Input
POWER ON:
INPUT:
ATTENUATOR:
4
Power Amplifi er
Type 2706
t8A RMS Attenuotor Gain Control
30. ~ . 10
·8·
Bruel & KjCI!r
~
Attenuator
Gain Control
Fig.2. 1. Front Panel of 2706
On/off switch for mains supply. When the
instrument
is powered, the white lamp to
the left of the switch is I it .
Coaxial socket input accepting standard
B & K coaxial plug
JP 0101. Input
impedance is 15 kQ.
For attenuation of input signal in 10 dB
steps,
from 0 to 40 dB.

GAl N CONTROL:
CURRENT LIMIT:
CLIPPING:
OUTPUT:
2.2. REAR PANEL
VOLTAGE SELECTOR
POWER SOCKET:
A logarithmic potentiometer allowing
continuous attenuation of the signal down
to 0 V. Gives 0 dB attenuation when
turned
fully clockwise.
A two position switch for presetting
output current lim it to either "5.0 A
RMS" or "1.8 A RMS". These two values
are the current ratings of the 8 & K
Vibration Exciter Type 4809 and Mini
Shaker Type 4810 respectively.
An amber warning lamp which indicates
that output current exceeds the limit set
by the CURRENT LIMIT switch or that
output voltage exceeds 15 V RMS, and
that output signal is clipped. The amplifier
will continue to operate. However, input
level must be reduced or attenuated to
resume operation with a good waveform.
Output terminals for connection to the
load. The right-hand terminal is ground.
Output impedance is less than 0.04 n
between 10Hz and 5kHz and less than
0.08 n from 5 kHz to 20kHz.
For selection of correct mains supply
voltage, unscrew the central fuse, remove
the black selector bush, insert the
pin
associated to the window on the bush in
the socket corresponding
to the desired
mains voltage and plug
it fully in. The
correct mains voltage should now appear in
the
window. The fuse is 250 V, 1A rated.
Socket
for connection of AC mains supply.
For connections see Fig.2.3.
5

Voltage Selector Heat
Sink
Power Soc
Fig.2.2. Rear Panel of 2706
AC Supply
Chassis Ground
171170
Fig.2.3. AC Power Socket
6

3. OPERATION
Before the instrument is switched on, ensure the voltage selector is set to
the correct line voltage. If not, unscrew the central fuse, remove the black
selector bush, insert the pin in the socket corresponding
to the desired
mains voltage
and push it fully in. The correct mains voltage should now
appear in the
window.
Ensure that air can circulate normally around the heat sink. The Power
Amplifier should not be placed in a closed volume such as a box, etc. The
heat sink can
be warm (typically 80°C). Care should be taken not to leave
temperature sensitive objects (e.g. cables) in contact with it.
Connect the signal source
to the INPUT of the Power Amplifier, and
connect the load
to the OUTPUT terminals. Feed the signal to be ampIified
to the 2706, and adjust the ATTENUATOR and the GAIN CONTROL,
and/or the controls on the generator so that the desired output level is
obtained. If the CLIPPING lamp is lit, reduce the input level or attenuate
further.
If a frequency sweep is to be made, ensure that no clipping occurs
throughout the frequency range, especially when the Power Amplifier is to
be used in a regulated system.
Care should be taken to avoid ground loops. It is recommended to isolate
accelerometers
from ground, using an isolated stud and a mica washer, when
ground loops
are likely to occur.
7

4. DESCRIPTION
A block diagram of the Power Amplifier Type 2706 is shown in Fig.4.1.
Output
Fig.4. 1. Block Diagram of 2706
4.1. ATTENUATORS AND PREAMPLIFIER
The signal is fed to the Power Amplifier via the INPUT socket which
accepts
the standard B & K coaxial plug JP 0101. The input section itself,
including
the preamplifier, is DC coupled, but any DC voltage present at the
input will not appear in the output voltage as the gate following the input
section is AC coupled. The amplifier input impedance is 15 kil.
The input leads to the ATTENUATOR which steps down the signal in
10 dB steps to a maximum attenuation of 40 dB. It is followed by the
GAl N CONTROL potentiometer which allows continuous adjustment of
the voltage down to zero. This is a high resistance logarithmic potentiometer
and has no loading effect on the previous attenuator. This means that the
amplifier input impedance is independent of the ATTENUATOR and GAIN
CONTROL setting. The preamplifier is an operational amplifier with
resistive feedback. It has a fixed
20 dB voltage gain.
The signal passes
then through a gate which will ground the signal when
excessive
temperature is reached on the heat sink. See Temperature
Protection, section 4.4.
8

4.2. DIFFERENTIAL AMPLIFIER AND POWER AMPLIFIER
The differential amplifier is of the constant-current source type, which
provides high common mode rejection. On one
input is applied the signal
from the input section, while the other receives feedback from the power
section.
The main power
amplifier components are two pairs of complementary
transistors (MJE
2955 and MJE 3055) working in class AB. The standing
current
is small, ensuring very low cross-over distortion. The power
transistors
are placed on a heat sink situated at the back of the instrument
(see Fig.2.2). Care should be taken to allow air circulation around the sink.
A
fifth transistor is placed on the sink for stabilizing purpose. When
temperature
rises, this transistor will limit the standing current in the power
transistors which otherwise
would increase too much. On the sink is also
placed a
thermistor used to trigger the gating circuit (see section 4.4).
At low frequencies, temperature on the sink and in the transistors will
follow the current variations, possibly introducing distortion. Two
transistors are provided to compensate for this effect. The output of the
2706 is directly coupled in order to avoid the need for a bulky transformer.
The gain
of the power amplifier together with the differential amplifier is
20 dB, giving an overall voltage gain of 40 dB.
4.3. CURRENT LIMITERS AND CLIPPING DETECTOR
The current limiting circuitry will limit the output current to the selected
value
of either 1.8 A RMS or 5.0 A RMS. Current limiting is indicated by
the CLIPPING lamp. The clipping detector is triggered by excessive
instantaneous voltage across either
of the DC biased diodes connected to
each branch of the differential amplifier. The amber CLIPPING lamp will
then be lit. The amplifier will continue to operate, but the input signal must
be reduced or further attenuated to resume operation with a good
waveform,
as clipped stgnals introduce considerable distortion.
4.4. TEMPERATURE PROTECTION
Abnormal load conditions, high ambient temperatures or short-circuited
output could result in output transistor temperatures in excess of design
limits and subsequent transistor failure. To prevent such damage, an NTC
thermistor is placed on the heat sink together with the power transitors. Its
9

resistance is 47 kQ at 25°C, falling down to 1.2 kQ at 125°C. It is part of
the base biasing network of the gating transistor. The circuit is shown in
Fig.4.2.
-9V
/f12ooS
Fig.4.2. Temperature sensitive gating circuit of 2706
For normal working temperatures (around 80°C on the heat sink) the
DC collector-emitter voltage is kept to a value allowing AC signals to be
passed
to the gate output. If temperature rises, the thermistor resistance
reduces, causing
the base-emitter voltage to increase and therefore the
collector-emitter voltage is reduced. This introduces a limitation on the
voltage allowed to go pass the gate without clipping (the peak voltage being
limited to the collector-emitter voltage). Clipping will be indicated by the
CLIPPING lamp. If temperature on the heat sink is increased further up to
125°C, the collector-emitter voltage is reduced to zero and the signal is
grounded and therefore does not reach the power section.
When
the heat sink temperature reduces, the Power Amplifier will
automatically become operative again.
4.5 CHARACTERISTICS
4.5.1. Power output capacity
The overall voltage gain of the Power Amplifier is 40 dB ± 1 dB at 1 kHz.
Output voltage is limited to 5 Amps RMS or 1.8 Amps RMS. Power output
capacity is 75 VA into a 3 n load. For loads above 3 n power is limited by
the maximum voltage available (15 Volts RMS). Below 3 n, it is limited by
10
•

the 5 Amps RMS current limit. It is not recommended to use loads below
2 n as power dissipation on the heat sink will rise considerably.
When the 1.8 Amps RMS current limit is selected, it does not affect the
voltage limit which remains 15 Volts RMS. Maximum power will therefore
be available with a load of about 8 n. The 1.8 Amps RMS limit can be used
to drive the Mini-Shaker Type 4810 safely at full rating. Fig.4.3 shows the
maximum power available
as a function of load resistance for the two
current limits.
75
~
[
" 50
a.
"
0
x
~
25
2 3 4 5 6 7 8 9 10 11 12 13 14 15
r.nool
Load Resistance In I
Fig.4.3. Maximum Output Power for both current limits
A typical frequency response curve is shown in Fig.4.4. The specified
frequency
range of Type 2706 is 10 Hz to 20 kHz (± 0.5 dB).
1 ( \
\
-2
I \
I
i\
-3
1 10 20 50 100 200
500 1K 2K SK lOK 20K SDK lOOK
Frequency (Hz) ¥l.Z "" ~
Fig.4.4. Amplifier Frequency Response (10 ARMS into a2 n load)
11

Due to the generous feedback in the power section, output impedance is
very low. Fig.4.5 shows the variation of the output impedance with
frequency.
100
90
80
///
///
/
70 /
60 / I
/ I
V/ /// //
/ Sp~fic/io/ /
// //
V/ / /
/
50
40
30
20
10
0
10 20 50 100 200 500 1K 2K
5K 10K 20K
Fig.4.5. Output Impedance
4.5.2. Distortion
Frequence (Hz)
"r72ool
Heavy feedback in the power section and use of complementary power
transistors
working in class AB have resulted in very low harmonic
distortion.
At full capacity, it will be less than 0.5% over the frequency
range 20 Hz to 20 kHz and wi II be less than 0.2% between 20 Hz and
10kHz. Typical values at 500Hz are -66 dB referred to the fundamental
for both the second and third harmonics.
4.5.3. Noise and Hum
Careful design has resulted in low noise and hum levels, which 'should be
at least 70 dB below full output (i.e. <4.75 mV).
A
typical frequency analysis of noise and hum with short-circuited input
on the 2706 is given in Fig.4.6.
12

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Fig.4.6. Narrow band Frequency analysis of noise and hum. Short circuited
input
13

5. APPLICATIONS
5.1. GENERAL
The Power Amplifier Type 2706 has been designed to drive small vibra-
tion exciters, particularly the B & K Vibration Exciter 4809. When switched
to the 1.8 A RMS output current limit, it will safely drive theM ini Shaker
Type
4810 at full rating. However, the 2706 can be used for general purpose
amplification in vibration and acoustics applications.
Some
of the devices to be driven by the Power Amplifier could, for many
applications, be directly driven by a signal generator (for example the B & K
Beat Frequency Oscillator Type 1022), but the low impedance of a shaker
or a loudspeaker will cause considerable distortion of the generator output,
which may be a problem when measuring a frequency response curve. Also,
it may be difficult to get sufficient power. These problems can be solved
easily
by using the 2706. It will provide the desired power and reduce
distortion, as its input impedance (15 kQ) will have no loading effect on the
generator.
Distortion from the generator will therefore be reduced. Since
the Power
Amplifier Type 2706 itself introduces very little distortion, the
output signal will exhibit less distortion than when taken directly from the
oscillator.
Measurements on the Mini Shaker Type 4810 directly driven from a
generator have shown
that the third harmonic level could be only 15 dB
below the fundamental. For the same vibration level on the Mini Shaker, use
of the Power Amplifier reduces the level of harmonics by at least 20 dB.
5.2. USE WITH THE B & K VIBRATION EXCITERS
The Vibration Exciters Type 4809 and the Mini Shaker Type 4810 are
small vibration exciters having a wide range of applications, with force
ratings
of 10 lbf (45 N) and 1.5 lbf (7 N) respectively. The useable
frequency range of Type 4809 is 10Hz to 20kHz and the maximum input
current is 5 A RMS. The useable frequency range of Type 4810 is 20Hz to
18kHz and the maximum input current is 1.8 ARMS.
14

Both can be used for calibration of accelerometers, vibration of small
objects, mechanical impedance measurements. For more details on their
construction and performance, reference should be made to their
instruction manuals.
When the Power
Amplifier Type 2706 is used to drive one of the
vibration exciters, the CURRENT
LIMIT should be set to "5.0 ARMS" for
Type 4809 and to "1.8 ARMS" for Type 4810.
5.2. 1. Accelerometer calibration
A calibration set-up using the Mini Shaker Type 4810 is shown in
Fig.5.1.
r-
1
I
Be at Frequency O sci llator
1022
Comp ressio n I
L
______ _
_j
M ec hani ca l Drive Sh aft UB 0041
Fig.5.1. Example of Calibration set-up
Sensitivity of the unknown accelerometer is measured by comparison·
using the Sensitivity Comparator Type 2970 together with two Con-
ditioning Amplifiers Type 2626.
Calibration
is performed at a fixed frequency (e.g. 160Hz) and a relative
frequency
response curve can be recorded on a Level Recorder Type 2305
or 2307. Full details on the calibration procedure are given in the instruc-
tions manual for Type 2970.
15

5.2.2. Vibration testing
Fig.5.2 shows
an example of set-up for vibration testing using the
Vibration Exciter Type 4809. Filtering
of both compression and measured
signals allows response measurements at very low levels or in the presence of
high ambient noise.
r-
1
I
I .-. ·4 ·;
: Heterodyne Analyzer
2010
Heterodyne Slave Filter 2020 Level Recorder 2305 I
L----------------------~~~~~~--------------------m~
Fig.5.2. Example of Vibration Testing set-up
5.3. USE WITH THE REVERBERATION PROCESSOR TYPE 4422
16
Loudspeaker
Power
Amplifier
2706
Enclosure
Band Pass Filter Set
1615
Microphone
Audio Frequency Spectrometer
2113
Level Recorder 2305
---1
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I
I
I
I
I
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...a...JLJ-~·
. r-.
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L______L ________ ~~~~~~~~J
171420
Fig.5.3. Measurement of Reverberation Time by the Kuttruff Method

The design of the Reverberation Processor is based on the "Method of
Integrated Tone Burst" suggested by M.R. Schroeder and modified by H.
Kuttruff. The use of this method results in very smooth decay curves, giving
a very accurate determination
of the initial slope of the curve. The "Early
Decay Time" (the reverberation time extrapolated from the upper part if
the decay curve) can be directly read from the meter of Type 4422.
A set-up
for measuring reverberation times is shown in Fig.5.3.
Type 2796
is used to amplify the output signal from the Band Pass Filter
Set Type 1615. When calibrating the excitation pulse, the ATTENUATOR
and the GAIN CONTROL of the Power Amplifier are set so that the output
signal level is just below the clipping limit.
Reverberation curves measured using the set-up are shown in Fig.5.4.
For more details on the
"Method of Integrated Tone Burst" and on the
Reverberation Processor, reference should
be made to the instruction
manual
for Type 4422.
aoooao oo ooo o oooooo ooo oooooooo o ooooooooa oaoooaoo o ooao oa o
..... ...
~ Church in Nrerum
~ Tota l Range 25 dB
E=: soo Hz, .1/3 oct.
!== SIN -50 dB
~ Potent i omet er Rang e 50 dB
~ Writing Speed 500 mm/s
GP 1102
Schroeder/ Kut tru ff 's
Method
5 dB
Fig. 5.4. Reverberation Decay Curves, repetitive recordings
5.4. GENERAL USE IN ACOUSTICS
171417
The Power Amplifier Type 2706 will be found very useful in acoustics
measurements
to supply a loudspeaker at a suitable level for reverberation,
sound insulation, microphone
and loudspeaker measurements.
17

An example of set-up for sound insulation measurements is shown in
Fig.5.5.
Spectrometer 2113
18
Two-Channel
Microphone
Selector 4408
Level Recorder 2307
Random Noise
Fig.5.5. Sound Insulation measurements
Power Amplifier 2706