Burwen laboratories 1000 Service manual

Dvn- mic
Model 1000
I".
FEATURES
Bandwidth Dynamically Controlled By theMusic
Noise Attenuation Up To
25 dB @ 30cpsand 22 dB @ 10kc
ResponseTo Musical Content Flat
* .2 dB
A TransientExtendsthe Bandwidth to 32 kc in
1 ms
AttenuatesNoise Above and Below the Audio Range
LessThan .l%TotalHarmonicDistortion
Dynamic Range 100dB
-. .l dB Insertion Gain
10dB Unweighted TapeNoise Reduction
Outputdc Coupled, kllVOpenCircuit
Delivers
18 dBm into 600 ohmsor 16dBm into 150ohms
1,2,3or4 ChannelsAvailableon 1-3/411Rack Panel
Stereo ChannelsGanged in Pairs or Independently
Plug-inEpoxy Encapsulated Modules for Ease of Servicing
Active TransformerInput, lOOk or600 ohms
HighestQuality Materials and Components Guaranteed for
Two Years
A
Signal ControlledAutomatically Variable
Bandpass FilterWhich ReducesNoiseWhen
Playing Any:
Master Tape
Multitrack Mix
Prerecorded Tape
Cartridge
Cassette
Record
FM Program
Video Tape Sound
with no audibleeffectoneither music or speech
LABORATORIES, INC.
209 MIDDLESEX TURNPIKE,
BURLINGTON,
MASS. 01803 TEL. [phone]
I

CONTROL
CONSOLE
I
DYNAMIC
NOlSE
, FILTER
FIGURE
1.TWO CHANNELDYNAMICNOISE FILTEF
PERFORMS 18 CHANNELNOISEREDUCTION. =
I
STEREO
RECORD
CUlTlNG
SYSTEM
I Dynamic Noise ~ilte;~odell000 I
h* *dpk
The Dynamic Noise Filter reauces noise ~y ?rtenuaring the high and low
frequencies when there isno music present.Asthe high frequency content
of the music increases the upper cutoff frequencygradually increases from
1100cps to 32kc. Similarly,lowfrequency musical content causes an
I
increase in the lowfrequency bandwidth from 350cps down to I3 cps.The
variable filter action (Figure2) is distortionlessand the wideband response
~
isextremely flat.Bandwidth increases are so rapid that there is no audible
loss
of high frequencytransients in the program material and the net effect
isa reduction of background noise without degradation of the program
material-such aswould occur with a fixedfilter.
No signal encoding
Any conventional audio source can be improved by passing the signal
through the Dynamic Noise Filter.Because no specialsignal preprocessing
isneeded it is unnecessary to use a Dynamic Noise Filterchannel for every
signalchannel.Instead a multitrack recording system (Figure1)can use a
two channel Dynamic Noise Filter to remove the cumulative noise from all
sources ahead of the filter.Tapes thus remain conventional and
interchangeable amongstandard tape machines.
Low kvel operation
It is'the extremely lowlevel operation of the Dynamic Noise Filter
(Figures3 and
4) that makespossible a 10dB noise reduction with no
audible lossof the high orlow frequencies when playing a normal two
track tape master.For a wide range of input levels the frequency response
isflat within
.2 dB from20 cps to 20 kc. Signal energy in the region of 6.6
kc dynamically controls the high frequencybandwidth.Signal energy in
the region of 85cps controls the low frequency bandwidth.Although the
bandwidth controller responds to other frequencies it hasconsiderably
lesssensitivity thereby assuring that the signal levelis sufficientto mask
the noise.
Fast attack
A sharp transient can extend the high cutoff frequency to 32kc in only
1 ms. Thisrapid attack was chosen tobe so fast that the ear cannot detect
anyloss of impact yet slowenough to prevent actuation by microsecond
noise impulsesfrom a phonograph record.Atlowfrequencies the
bandwidth can extend to
I3 cps in 10ms.
The decay times must be slow enough to prevent modulation of the signal
ataudiofrequencies yet fast enough to follow the normal decay of the
signalin order toprevent noise from being heard for a moment following
each note.A decay to within 10%of the final bandwidth occurs in50 ms
athigh frequencies and in500 ms atlowfrequencies. Smoothaction is
produced by a newly developed multistage nonlinear feedback fi!ter.
: .-
- *!?atent applied for
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MODULAR COMPONENTS
ATXU3 ACTlVETRANSFORMER
Thepassive audio input transformer is obsolete. Burwen Laboratories
uses the Active Transformer-a differentialinput dc amplifier that
overcomes the frequency response,distortion, andhum pickup
limitationsof the common audio transformer.TheAT200 consists of
aninput over-voltage protector that feedsa pair of unity gain
followers and then a bridge connected amplifier for common mode
rejection.An output buffer amplifier provides 0to30 dB voltage gain
selectedby anexternal resistor.
Brief specifications:Input ~mpedancelook, output impedance
.l ohms,
distortion .01%,output k11Vinto 10k,
t 18 dBm into600ohms, and
CMRR
85 dB min.
VF301 VOLTAGE VARIABLEBANDPASSFILTER
TheVF301uses precision multipliers andintegrators to forma
bandpass filter having dccontrolled high and low cutoff frequencies.
As the high frequency dc control voltage increasesfrom
+ .2V to
+ 5.0 V the multiplier X1 reduces the effective time constant of the
integratorA1and increases the upper cutoff frequency in direct
proportion to the dc control voltage. Similarly,an increasein low
frequency control voltage from +.2to +j.O V, inverted by the
divider,increases the effective time constant of A3toreduce the low
frequency feedback and thereby lower the low cutoff frequency in
inverseproportion to the control voltage. Attenuation beyond the
audioband eliminatesextraneousnoises and internal noise is
minimizedby the use of high frequency preemphasis before the filter
and deemphasis afterward.The frequency response is shown in
Figure 2for control voltages of .2V,
1 V, and 5 V each.
Brief specifications:Output 11Vpeak into 10k,
18 dBm into600ohms,
1 kc harmonic distortion .01%wideband, input impedance 200k,
voltage gain0dB or 10dB, anddynamic range 100dB.
BC302BANDWIDTHCONTROLLER
The BC302 produces the high and low frequency +.2 to +5 V dc
control voltages to actuate the VF301.The high frequency control
voltage isderived from the incoming signal by selectingfrequenciesin
theregion of 6.6kc and then full wave peak rectifying.Nonlinear
multistage filtering provides fast step response together with a medium
speed decay freefromaudio modulation. Similarly, the low frequency
dc control voltage isderived fromfrequenciesin the region of 85cps.
Separate high frequency andlowfrequency sensitivity controls and a
high frequency cutoff control optimize the performance for signals
having differentnoise levels.
A single BC302controls two VF30ls.
Brief specifications:Low frequency sensitivity 3.5 mV at85cps,250
mV at 1 kc; high frequency sensitivity .9 mV at 6.6 kc, 220 mV at 1 kc.

ATXX)
ACTIVE
TRANSMRMEI
PR401
ISVOLT WE-
PPLY REGULATOR
VOLTAGE VARlP
BANDPASSFlLl cn
BANDWIDTH
CONTROLLER
SY!+TEM DIAGRAM
A single channel consists of anAT200 Active Transformer feeding a
VF301VoltageVariableBandpass Filtermodule.A single BC302
Bandwidth Controllerreceives asinputs via the sensitivity controls
the sum of theAT200 output signalsfor channels
1 and 2.The
Bandwidth Controllerproduces high and low frequency dccontrol
voltages used to vary thebandwidth simultaneously for both
channels in stereo operation.Channels 3and 4 are similar and all
four use a common power supply.Independent operation can be
attained byutilizingchannels
1 and 3or2 and 4each having its own
Bandwidth Controller.
POVIR3LSLlPPLY
Thepower supply consists of a large toroidal power transformer
designedtoeliminate magnetic interference with adjacent
equipment, oversized rectifiers and computer gradefilter
capacitors, and a precision regulator module Model PR401.The
supply delivers
-t 15 V +5%, 2500 mA regulated to .01%.All
componentsare highly derated, andinterlocking and overvoltage
protection prevents damage to other modules in the event of a
power supplyfailure.
OPERATION
Innormal use the individualhigh and low frequencysensitivity
controlsare set so the hiss just begins to raise the upper cutoff
frequency and the rumble just begins to reduce the lower cutoff
frequency.Thebandwidth then increases to maximum for most of
the range of signal levels.For poorer material a limit can be placed
ontheupper cutoff frequency between
1 kc and 30 kc and for special
effects the bass can be attenuated at a fixed
6 dB/octave below 400
cpsor the treble canbe attenuated above 800cps.
REVIVING OLD RECORDS
Program material such asold 78rpm records whichmay have more
distortion and noise than useful signalin the
6.6 kc region can be
substantiallyimproved.First the signal is passed through a
12
dB octave, 4 to 6 kc fixedlow pass filter and then through the
Dynamic Noise Filter followed by a flexible equalizer to adjust the
tonal balance.
llDaOrmowmnm~~m
A 7-1/2 ipsDynamic Noise Filter demonstration tape recorded in
four tracks compatible with 1/4 and 1/2 track two channel stereo
machines is available for a $10.00deposit which will be refunded
upon return of the tape, postage prepaid, within
15 days.
LABORATORIES, INC.
209 MIDDLESEX TURNPIKE,
BURLINGTON, MASS. 01803 TEL. [phone]
I