BRUEL & KJAER 2604 User guide

Microphone Amplifier Type 2604
The instrument is designed for the amplification
and measurement of small a.c. voltages . The total
amp Iification is 100 dB. By means of a switch
the built-in rectifier and meter circuit can be
switched to measure either the arithmetic average,
the true r.m.s. or the peak value of the input
signal. The instrument also contains the inter -
nationally standardized weighting networks for
sound level measurements. Connections for Cond ·
enser Microphone and Level Recorder.
Nrerum, Denmark . ~ 80 05 00 . ~ BRUKJA, Copenhagen . Telex: 5316
88 2604

Microphone Amplifier
Type 2604
October 1965

Contents
0. Introduction
/
Page
5
Vibration and Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The Decibel Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The Detection of Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
The Detection of Vibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.
Description ....................................................... 10
General .......................................................... 10
Input Amplifier .................................................. 11
Output Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Rectifier and Meter Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Overall Performance of the Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Description of the Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2. Identification of Control Knobs, Terminals etc ..................... 19
3.
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Voltage Measurements . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Measuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Sound Level Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Setting up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Measuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2·5
Recording of Sound Level ...................................... 27
Calibration by Means of a Calibrated Sound Source . . . . . . . . . . . . . . 28
Influence of Background Noise ................................ 30
4. Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
External Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Condenser Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
The Two-Channel Microphone Selector Type 4408 . . . . . . . . . . . . . . . . . . 37
Response Test Unit Type 4409 .................................... 38
Artificial Ear Type 4152 .......................................... 40
Hearing Aid Test Box Type 4212 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Vibration Pick-Up Preamplifier Type 1606 .......................... 41
Input Transformer TI 0001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5. Applications
Page
44
A. Electrical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Distortion Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Automatic Recording of Electrical Impedance .................... 45
B. Acoustical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Checking of Hearing Aids ...................................... 46
Measurement of Acoustic Absorption Coefficients . . . . . . . . . . . . . . . . 48
Recording of the Frequency Response of Sound Projectors ........ 49
Directional Characteristics of Projectors ........................ 50
Recording of Frequency Response of Hydrophones . . . . . . . . . . . . . . . . 53
C. Mechanical Measurements ...................................... 54
Vibration Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Recording the Frequency Response of Accelerometers . . . . . . . . . . . . 56
Measurement of Microphony in Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

0. Introduction
The Microphone Amplifier Type 2604 is a very versatile measuring instru-
ment which, when used in conjunction with suitable transducers, can be
employed in a great variety of fields. It may be used as a vacuum tube volt-
meter, as a sound level meter, as a vibration meter, and, when supplied with
external band pass filters such as Band Pass Filter Set Type 1612, also as
a
frequency analyzer. As the name implies, however, it may predominantly
be used in conjunction with a condenser microphone and used for sound
measurements. A brief summary of the most common but important terms
concerning sound and vibration thus follows.
Vibration and Sound.
It is generally known that sound is a transmission of energy through solid,
liquid or gaseous media in the form of vibrations. These vibrations con-
stitute variations in pressure or position of the particles in the medium.
Sound may also be defined as the auditory sensation evoked when such
vibrations, normally in air, impinge upon the ear. As an auditory sensation
sound is limited to frequencies in the range from about 20 Hz to 20ooo Hz.
Pressure fluctuations outside this range will not generally produce the
sensation of sound.
The Decibel Scale.
Acoustical instruments for measuring pressure variations are usually cali-
brated in dB (decibel). A dB value is a measure of relative power, i.e. so
many dB above a reference power level:-
p
dB= 10 log Po
where Po is the reference and P is the actual power measured. However,
the power transmitted by a sound wave is proportional to the square of the
pressure variations so that we have
p2 p
dB =
10log2 = 20log -
po po
where po is the reference pressure and p is the root mean square value of
the pressure variations. When sound pressure is measured in dB re o.0002 ,ubar
with equal weight given to all frequencies it is termed sound pressure level.
The logarithmic scale has been found very convenient because of the large
range of sound intensities that the human ear can handle. It can · detect
5

pressure variations as low as o.0002 j-tbar and can also withstand levels
higher than 200 j-tbar. This is a ratio of more than 106: 1 which on the
logarithmic scale is represented by 120 dB.
In Table 0.1 are given some commonly encountered sound pressure levels in
order to give a better appreciation of the dB scale.
Sound pressure
Sound pressure
I
Environmental conditions
in bar
level in dB
1 140 I
134 dB T
Pain Threshold
mbar
130 T
Pneumatic C~ipper
120
100 -114 dB T
Loud automobile horn (dist. 1 m)
,ubar
110
Trnside
airliner (DC 6)
100
Trnside
10
94dB subway train (New York)
~-tbar
90
Trnside
motor bus
80 T Average
traffic on street corner 74dB
~bar
70
Tconversational speech
60
o.1 54 dB T
Typical business office
.,ubar 50 T
Living room, suburban area
40
o.01 34 dB
TLibrary
~-tbar
T 30 T
Bedroom at night
I 20
0.001
14 dB
T Broadcasting studio
,ubar
I 10 T
Threshold of hearing
0
0.0002 I I
,ubar
Table 0.1. Some commonly encountered sound pressure levels.
The Detection of Sound
The human ear is a remarkably sensitive instrument for the detection of
sound waves. Its response to a certain sound pressure level depends however
6

upon the frequency of the sound. The sensitivity is greatest at 1ooo-6ooo Hz
and falls off both for higher and lower frequencies.
140
)
130
'0
.
120
N
...
""
t-
...........
'"
1'-
~""-...
"'
130...,
""
I
r--
,._
"""'
!"' I
1'\.
120-..
....... j.../
E
!
110
N 100
0
0
0
0 90
i 80
....
1
70
~
60
c
"'
r--
1'--
110-..1'-....
1--.
'\ t---
r-.....
.._
'\.i'\ f'.
100 t--....
1-
I'\
"'
r--....
9..-
["-..
I/ ~
l\. !'-.
-~--
!-.......
1\.
........_
80
t-.. / 1\.
l\. r--.
-..
.....
-
'-"
'"
............
f'..
\.
\ -
.....
70
--~-.....
v
\ '\
f'. -......~-.....
I/ \.
t'-....
-~--
60
-t--.... v
,
..
50
~
'1l
!
r--. !'--....
50
1'-- I/ '\
............
1--
-t--....
'-"
f'. t'-....
........
40
v
~
......
-r-
1/
] 30
.............
I'-
30
17
"-.:._/
""
I'---
-r-
1/
'\. I
i 20
~
10
,
c
::>
0
Vl
1/
. ' '
1'--.t-
17
'V'
' r-
20
-r- '\...7.'
_ Normal Binaural '
t-
V'
,_,
I
f-
Minimum Audible 10
--~-.....
I
1----
-Field (MAF) -I
Phon
--
II II I ' --
I II II II
20Hz 30 40 60 80 100 200 300 400 600 lm1000
2kHz 3 4 6 8 10 15
(c/s) (kc/s) Frequency
162381
Fig. 0.2. Equal Loudness Contours.
A set of so-called equal loudness contours is given in Fig. 0.2. The curves
show the intensity levels in dB re o.0002 J-tbar, which at various frequencies
are judged by the average human being to sound equally loud. Other sets of
equal loudness contours which deviate from these curves in certain respects
have been published by various investigators but the curves shown in Fig. 0.2
have been recommended as standard by the International Organization for
Standardization (ISO/R 226-1961 (E)).
The reference level is usually set up at 1ooo Hz and the curves give the
sound pressure level in dB necessary for a tone of a different frequency
to sound equally loud. The loudness level is measured in phon and at
1ooo Hz the phon value is equal to the dB value. It will be seen that the
curves of constant phon become more and more straight as the loudness
level is increased. At a level of 120 phon the ear is approximately equally
sensitive to all frequencies in the audible range, while at 0 phon the variation
in sound pressure level with frequency is great.
7

Although the response of the human ear depends on many other things
beside frequency, modern sound level meters usually contain weighting
networks in order to try and incorporate in the meter a ~requency response
similar to that of the human ear. Three different curves have been inter-
nationally agreed upon and standardized. These are referred to as the A, B
and C curves and are shown in Fig. 1.5(a), p. 17. When sound pressure is
measured using one of the weighting networks and quoted in dB re o.0002
. ~bar it is termed sound level. The weighting network used should always be
stated clearly e.g. if the sound level measured with the A weighting network
is 70 dB, it should be quoted as 70 dB (A).
The Detection of Vibration.
A phenomenon which is closely related to what we usually think of as sound
is vibrations in solid materials. Such vibrations are in most cases the source
of sound and may have to be located in order to reduce their effect. (Noise
control). Man's sensitivity to vibration is extremely limited, especially for
the higher frequencies, so that measuring instruments are needed in order
to determine accurately the vibrations in question . By substituting an accelero-
meter for the microphone, a sound level meter can usually be adapted to
measure vibration instead of sound level and thus serve a dual purpose in
the fight against excessive noise.
Vibration in a mechanical object is an oscillation related to certain reference
conditions of the object. In vibration often three quantities are dealt with, dis-
placement s, velocity v and acceleration a. The coherence between the three
quantities are given in the following table . (For the sake of convenience the
relationships ar~ also given with respect to pure sinusoidal vibrations).
Transferred to-
Given
Quantity:-,
s I v
ds
s
v= --
s = S0 sin (cot)
dt
v = coSo cos (rot)
v s
=~ vdt
v = V0 sin (cot) s = _
_!_ V0 cos (rot)
co
a s = ff
adt
2 v =
~ adt
a = A0 sin (cot)
s = _ _ l_ A0 sin (cot)
v = - _!_ A0 cos (cot)
w2
co
In the m.k.s. measuring system the units will be
s in m
v in m/sec.
a in m/sec.2
I a
d2s
a= --
dt2
a = - co' S0 sin (rot)
dv
a= --
dt
a= co V0 cos (cot)

Frequently a is expressed in the unit G which is the acceleration derived from
the gravitational force of attraction of the earth. As the gravity alters with
latitude and altitude a standard gravity is chosen to G = 9.80665 m/sec.2 =
32.1739 ft/sec.2.
9

1. Description
General.
The Microphone Amplifier Type 2604 is primarily designed for acoustical,
elecroacoustical and vibration measurements.
Basically it consists of two independent amplifier sections which may either
be connected in cascade or have a filter system interposed between them (see
Fig. 1.1).
The power supply contains the necessary rectifiers and filters for
the operation of the amplifiers and can supply the external B & K Condenser
Microphone or Preamplifier in use.
Micr:
lnp.
Ampl.
lnp.
·Input Potentiometer•
Weighting
Networks
·Range Multiplier" -f6M + z
Fig. 1.1. Block diagram of the Microphone Amplifier.
The input and output amplifier sections can , as mentioned, be cascaded, this
being accomplished by means of a switch on the front panel. By this means
a high gain amplifier with a linear frequency characteristic in the range 10 Hz
-200 kHz is obtained.
If a frequency analysis is required, external filters, such as the Band Pass
Filter Set Type 1612 can be inserted between the amplifier sections. The
set-up will then be similar to the Audio Frequency Spectrometer Type 2112.
In the case of vibration measurements the Preamplifier Type 1606 or Cathode
Follower 2612, 2613, 2614 or 2615 with Accelerometers Type 4312 to 4315 can
be connected to the CONDENSER MICROPHONE input of the Amplifier.
The switch on the front panel also enables a series of built-in weighting
networks, which conform to IEC Standards, to be inserted between the
amplifier sections. When using these built-in weighting networks in con-
junction with the B & K Condenser Microphones Type 4131 and 4133 the
specification matches that for a standard precision sound level meter.
Some of the main requirements in the IEC specification a re quo ted here:
A precision sound level meter shall include at least one of the three weighting net-
10

works called A, B or C, and should cover the frequency range 20 Hz to 20ooo Hz
within certain tolerances.
The microphone shall be of the omnidirectional pressure type. Permissible tolerances
on the variation of sensitivity with angle are given, and it is suggested that the diffuse
sound field sensitivity (i,e. the root-mean-squ are of the sensitivities for all orientations)
should by some means be brought within the tolerances for the specified incidence.
A
square law indication instrument is specified, i.e. it must be capable of correctly
summing two pure tones according to the root-mean-square law.
Input Amplifier.
The two first stages of the amplifier consist of resistance-capacitance coupled
triodes, while the third stage is built as a cathode-follower. A large amount
of negative feed-back is introduced to stabilize the amplifier and to ensure
a low source impedance of approximately 12 Q, at the FILTER INPUT
terminal. The heaters of the amplifier tubes are supplied from a stabilized DC
supply. In this way a hum level lower than two microvolts, with reference
to the grid of the input tube has been achieved. The use of specially selected
tube types and coupling components also avoids disturbance from microphony
in the amplifier, even when a signal of only a few microvolts is being
measured.
The input amplifier is provided with two different input sockets, one of
which is seven-poled and corresponds to the plug for the B & K Condenser
Microphones and Preamplifiers. The other input socket accommodates a
14
mm coaxial plug and can be connected to the input circuit of the amplifier,
either directly, or via a variable potentiometer for relative measurements. A
three-position INPUT SWITCH marked "Condenser Microphone", "Direct",
and "Input Potentiometer" gives individual selection of these three input
facilities. When the INPUT SWITCH is selected to position "Condenser Micro-
phone" the input socket marked CONDENSER MICROPHONE has to be used.
In the other two instances the voltage to be measured should be fed into the
AMPLIFIER INPUT socket.
From the INPUT SWITCH the signal voltage is fed to a voltage divider, which
is operated by the knob marked METER RANGE on the front panel and
contains five steps of 20 dB with a total impedance of 1.1 megohms. The
different positions of the METER RANGE switch are marked for several
values of input voltages between 10 mV and 1ooo volts. A further position
marked "Ref." serves to check the sensitivity of the instrument. With the
METER RANGE switch in this position a square-wave type signal with the
frequency of the mains is supplied to the grid of the input tube. The voltage
is
stabilized by a zener diode and will vary less than o.3% for a 10% mains
variation.
The calibration adjustment to the red mark on the instrument scale should
be carried out with the METER SWITCH in position "RMS", INPUT SWITCH
in the "Direct" or "Input Potentiometer" position and the FREQUENCY
RESPONSE SWITCH set to "Linear 10-200ooo". With the FREQUENCY
RESPONSE SWITCH in this position, the r.m.s. value of the reference signal
11

coincides with the red mark on the scale, while a deviation of up to 4 dB can
be expected when the METER SWITCH is switched to "Peak" or "Average".
The deviation from the correct value of the peak and average values is mainly
caused by phase distortion in the filter, which will be predominantly notice-
able on peak measurements of complex signals that are just above the lower
limiting frequency, see also Appendix.
The output of the input amplifier is paralleled by an OVERLOAD INDICATOR.
The indicator is so designed that it gives a visual indication when signal peak
voltages exceed 4.5 volts on the output of the input amplifier. The signal to be
monitored is amplified in a triode connected pentode and fed to a gas tube,
indicating overload by a red light. Re fer also to item " Overdriving".
Output Amplifier.
Input Circuit. The output amplifier consists of three stages and shows an
input impedance of 146 kohms.
In the input circuit a voltage divider is inserted, containing four steps of
10 dB and operated by the knob marked RANGE MULTIPLIER. With the
RANGE MULTIPLIER switch in position "X 1 (0 dB)" the amplification of
the output amplifier is 10 times. Maximum amplification is obtained with the
RANGE MULTIPLIER switch in position " X o.01 (-40 dB) " and is then
1ooo times {60 dB).
Sensitivity Adjustment. The output amplifier is provided with negative feed-
back, the amount of which may be varied by two potentiometers. The
sensitivity of the apparatus can then be adjusted and thereby calibrated, the
reference voltage being used as calibration voltage. One of the potentio-
meters is in circuit when the INPUT SWITCH is in position "Condenser
Microphone", the other when the switch is in either of the two positions
" Direct" or "Input Potentiometer". Separate sensitivity adjustment is there-
fore obtainable for sound and voltage measurements, this being an additional
convenience during practical use of the amplifier. The potentiometers can
be adjusted with the aid of a screwdriver through holes in the front panel,
which are marked SENSITIVITY -- CONDENSER MICROPHONE and SENSI-
TIVITY- AMPLIFIER INPUT .
Output. The output circuit of the amplifier is connected both to the output
terminal marked RECORDER and to the rectifier and meter circuit. The
RECORDER terminal is provided for connection to level recorder, head-
phones, oscilloscope, etc. Additionally it can be connected to the compressor
input of a Beat Frequency Oscillator Type 1022 or 1013. The output im-
pedance at the terminal is 50 .Q.
The amplifiers are constructed in such a way that they do not clip the signal
if its peak value, after amplification in the output stage, does not exceed a
value
of 13 dB higher than is indicated for full deflection on the meter, i.e.
10 volts + 13 dB = 45 volts peak. When the instrument meter gives full
deflection, a voltage of 8- 10 volts r.m.s. is available across the RECORDER
terminals.
12

Rectifier and Meter Circuit.
A switch in the rectifier circuit marked METER SWITCH enables either the
true r.m.s., the arithmetical average or half the peak-to-peak value of the
input signal to be measured by the moving coil instrument.
R.M.S. Measurements. The characteristic current vs. voltage curve for an
instrument which should measure the r.m.s. value, is a parabola. By means
of a specially constructed circuit of resistors and semiconductor diodes this
parabola is simulated. The rectifier circuit employed is a full-wave rectifier
and the combined circuit is developed to measure the r.m.s . value of signals
with crest factors fc as high as 5,
fc = Sp ea k = 5.
Srm s
However, due to the peak limiting (45 volts) of the output amplifier, the
highest deflection for r.m.s . value of the meter pointer when measuring
signals with crest factors 5, should not exceed:-
Speak 45
Srms = -- = - = 9 volts
fc 5
For full deflection the crest factor fc should not exceed: -
fc =
45
= 4.5
10
At low frequencies the meter pointer will start to fluctuate when the METER
SWITCH is in the position "Fast". When the fluctuation due to the low
frequency is too large, the r.m.s . value read is no longer the correct value and
the METER SWITCH should therefore be set in position " Slow ". The lower
limiting frequency in position "Fast" is approximately 20 Hz (sine wave).
Arithmetical Average Measurements. The circuit employed is a full-wave
rectifier, enabling the average value of the complete sign al to be measured.
Care should be taken not to overdrive the amplifiers when measuring the
average value of a complex signal which has high peaks. It is therefore
necessary to know the highest peaks, positive or negative, which occur in
the measured signal. By means of the two factors, crest factor fc and form
factor ft, below,
f _ Speak
c- Srms
fr=~
Save rage
the peak value of the signal may be calculated, provided the average value
(read on the meter) and the two factors are known.
Speak = fc X f f X Save rage
As the 45 volts peak should not be exceeded, when the meter pointer gives
full deflection for average value, the highest permissible value for
13

45
fc X ft = lO = 4.5
If the meter pointer has not full deflection, the product fc X ft could be
proportionally higher.
Peak Measurements. The circuit for measuring peak values is a normal
full-wave rectifying circuit. The signal from the output amplifier is applied
via a capacitor C1 , vide Fig. 1.2. Due to this capacitor the deflection on the
meter corresponds to the half peak-to-peak value of the signal. This should
be taken into account when the positive and negative values of a signal,
applied from the output stage, are not equal. In such circumstances the
peak value read on the meter is thus not the maximum that occurs. A
¥60228
Fig. 1.2. Electrical circuit of peak rectifier.
further limitation of the peak indication is due to phase distortion in the
amplifiers and is discussed in the Appendix.
As indicated in Fig. 1.2 the output resistance of the output stage is approx-
imately 50 ,Q, which is the charge resistance for C1 and C2. The discharge
resistance for C2 is approximately 45 kohms. Due to these finite quantities,
signals with very sharp peaks and high crest factors will be somewhat
clipped. The ideal case would be to use a zero charging resistance and an
14
v
~~~--- - ----
Zero line ·
r----------- .....:....:::=---
time-
¥60229
Fig. 1.3. Example of unsymmetricnl signal with a crest factor of
approximately 5.

infinite discharging resistance which is, of course, impossible to realize in
practice. However, even when measuring positive or negative pulses with
crest factor 5 (refer passage "R.M.S. Measurements"), the clipping of the
signal will be negligible. Fig. 1.3 gives an impression of a signal with a crest
factor of 5.
At
low frequencies, i.e. below 20Hz (sine waves), the METER SWITCH should
be set in position "Slow". The reason is as under "R.M.S. Measurements ".
A description of the working principle of the complete rectifier circuit is
given in the B & K Technical Review No. 3-1958.
Damping and Graduation of Meter.
The moving coil indicating instrument is furnished with two different damp-
ing characteristics " Fast" and " Slow" which are in accordance with the
standards for sound level meters.
The meter is graduated 0- 10 and 0- 31.5 as well as 0 to 20 dB. In addition,
scales permitting the direct reading of acoustic absorption coefficients are
provided. These scales should be used when measurements are taken with
the Standing Wave Apparatus Type 4002. When utilizing the first three
graduations electrical measurements can be read either in mV, volts or in
dB re 1 volt, and acoustical measurements in Sound Pressure Level (SPL)
or Sound Level (SL) re 2 X 10-4 uUbar. When measuring SPL or SL one
of the B & K Microphones should be used in conjunction with the Amplifier.
For use of the scales concerning the Standing Wave Apparatus, reference
should be made to the Instruction Book for this apparatus.
Overall Performance of the Instrument.
Signal-to-Noise Ratio. When the RANGE MULTIPLIER switch is in position
".X 1 (0 dB)", the signal-to-noise ratio for full scale deflection on the meter
is better than 55 dB for any setting of the METER RANGE switch, see also
Fig. 1.4. By using any of the other four positions of the RANGE MULTIPLIER
switch, the signal-to-noise ratio for full meter deflection decreases by the
same number of dB, corresponding to the increase in amplification. To
obtain the best ratio between signal and noise voltage, the RANGE MULTI-
PLIER switch should thus, during measurements, be set to give as low ampli-
fication of the output amplifier as possible, preferably "X 1 (0 dB )" .
Overdriving. When measurements are carried out where filters are inserted
between the two amplifier sections, care must be taken not to overdrive the
input amplifier. To allow possible overdriving of the input amplifier to be
observed a visual indicator is connected in parallel to the output of the input
amplifier, as previously described. The indicator lights up when signal peak
voltages on this point exceed 4.5 volts (3.16 volts r.m .s. sinewave) i.e. 10 dB
higher than the 1 volt r.m.s. sine-wave, as required for o.1 % nonlinear distor-
tion , The 4.5 volts peak voltage indication limits the measurements to signals
with crest factor fc lower than 4.5, which also is the highest crest factor
allowable at full r.m.s. drive.
15

Signal Sine Wave
Level RMS
~6~938
Fig. 1.4. Sketch showing the signal levels at different points in the Microphone
Amplifier. The dotted lines indicate the noise levels.
The higher peaks in the signal have to contain a certain energy to be indicated.
When for example measurements are made on noise signals with Gaussian
distribution the indicator will only react for noise peaks, which are lower than
approximately 2.5 times the r.m.s. value, i.e. fc lower than 2.5.
To be able to check on how far the input amplifier is driven in making
selective measurements, the FREQUENCY RESPONSE SWITCH of the Micro-
16

phone Amplifier should first be set to position "Linear 10-200ooo" and
RANGE MULTIPLIER to ".X 1 (0 dB)". The r.m.s., average or peak drive of
the amplifier can then be read on the indicating meter. Full deflection cor-
responds to 1 volt on the output of the input amplifier. From the above can
be seen, that in selective measurements, where the input amplifier is driven
to maximum, the sensitivity of the Microphone Amplifier can only be in-
creased
by RANGE MULTIPLIER.
Frequency and Phase Response. For the Microphone Amplifier with the
FREQUENCY RESPONSE SWITCH in position "Linear 10-200ooo", typical
frequency and phase response curves are shown in Fig. 1.5.
The two curves shown in the high frequency range of the phase response
characteristic indicate the lowest and highest typical phase distortion,
Amplification
i _:~
-20
-30
-40
-50
-60
uri. 10Hz -200000Hz
lo-'
..... ~.--
,.,
n:
,..I-"
/
'C
~lo-'
/
~
·'
~~
lj
/
,~~
: I
I v
I
li
1 2 5
10 2
....
0
..
200
000
800
600
,00
200
0"
200
'00
60"
ao•
00"
2
r-- Meter switch:
r-- Off I
Peak-.::r-b- f
r-- RM~~ "/
r-- Averag~
I I tJll
-
Li~ear
""'
1/
10-200 000 Hz
a.
1Hz 3 5
10
30 50 100
300 500 1kHz
b.
~~
C+B
~·
......
'!!t..A
'~
5 10
5 2 Hz 5
10
6
Frequency~~~ ·
lbltJ 7
Most deviating / /
attenuator
I I I
positions --
100mV x 0,3
I
10Vx0,01 /
~ ·
v /
...--- e--
Example of phase responses
of amplifier Type 260' I
--
3 5 10
30 50 100
300 1000kHz
1'(97/
Fig. 1.5. Typical frequency (a) and phase responses (b) of the Microphone
Amplifier Type 2604.
17

obtained for a single apparatus, for all combinations of the METER RANGE
and RANGE MULTIPLIER switches.
When analysing or measuring complex signals containing many significant
harmonics the upper limit of the frequency range should be noted as the
harmonics may be attenuated, thus obtaining an incorrect measurement.
Common amplifiers show a phase distortion which is perceptible a decade
or more higher than the lower frequency limit and a decade or more lower
than the higher frequency limit. This phase distortion influences especially
the peak value and to some extent the average value of complex signals.
Therefore, when measuring the peak or average value of complex signals
with the Microphone Amplifier its phase response should be taken into
account, see also Appendix.
When the Type 2604 is used with the Band-Pass Filter Set Type 1612 the
combination becomes similar to an Audio Frequency Spectrometer of the
Type 2112, and reference should then be made to the instruction manual
for this instrument.
Description of the Power Supply.
The power supply unit of the apparatus contains two rectifiers, one of which
supplies the heater current for the instrument and the other the heater cur-
rent to the B & K Condenser Microphones or Preamplifiers via the CON-
DENSER MICROPHONE input socket. The third rectifier supplies the
necessary anode voltages for the amplifiers and the polarization voltage for
the microphone cartridge of a Condenser Microphone when used. These two
voltages are stabilized with the aid of two neon tubes. A socket marked
POLARIZATION VOLTAGE on the front panel of the apparatus makes it
possible to measure the polarization voltage for the microphone cartridges
by means of an ordinary sensitive voltmeter (current consumption not
more than 50 ,fkA). This voltage can be adjusted around 200· volts by means
of a screwdriver-operated potentiometer on the back of the instrument.
Before the instrument leaves the factory, the voltage is set to 200 volts.
Fuse and Mains Voltage Selection.
The equipment should be operated from a 50 to 400 Hz power line and can
be set for mains voltages of 100-115 -127- 150-220 or 240 volts. The power
consumption is approximately 50 watts.
The primary side of the power transformer is protected by a fuse placed
on the rear of the apparatus. The fuse is incorporated in a compined voltage
selector and fuseholder, and to select the appropriate line voltage it is
necessary first to remove the fuse and to turn the selector by a coin or similar
means until the white mark is lined up with the desired value .
To operate the equipment from an accumulator or other type of DC supply
the use of a DC. to AC converter is necessary.
18