dbx 128 User manual

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HOW TO USE THIS MANUAL .
To achieve maximum benefit from your 128, we
recommend that you read the entire dbx 128 Operator’s
Manual and keep it handy for future reference. However, if
you are familiar with expansion and compression, and with
the use of dbx II tape noise reduction, the FUNCTIONAL
CALLOUTS and BRIEF OPERATING INSTRUCTIONS
should provide all the information you need to connect
and operate the 128. These sections also serve as quick
reference guides for occasional review.
The INTRODUCTION explains why and when to use
dbx {I tape noise reduction, expansion, compression, peak
limiting and peak unlimiting. Combination applications are
also covered. The remainder of the 128 Operator’s Manual
is a detailed set of connection diagrams, operating
instructions and explanations of the 128’s functions.
WARNING: TO PREVENT FIRE OR SHOCK
HAZARD, DO NOT EXPOSE THIS APPLIANCE
TO RAIN OR MOISTURE.
INDEX
FUNCTIONAL CALLOUTS 2
BRIEF OPERATING INSTRUCTIONS 4
INTRODUCTION TO THE dbx 128 6
WHEN TO USE COMBINATION APPLICATIONS 9
“DETAILED OPERATING INSTRUCTIONS —————~<C~s~“‘C=S:*«SC
“CONNECTIONS —————...—.
OPERATION
14
“HINTS TOGETTHEMOSTOUTOF ——S20~
dbx TAPE NOISE REDUCTION
“USING THE DYNAMIC RANGE ENHANCER SYSTEM 21
HOW TO SET UP COMBINATION APPLICATIONS 24
SYSTEM PERFORMANCE 29
TECHNICAL DETAILS OF dbx PROCESSING 30
“COMPRESSOR/LIMITER AND EXPANDER, ~—~—=«33
A SUMMARY
SIMPLE PRECAUTIONS WHEN USING ANY 35
PROGRAM EXPANDER
SPECIFICATIONS 7 36
SCHEMATIC 38
dbx PRODUCT WARRANTY 39
FACTORY SERVICE 39
dbx GLOSSARY I

FUNCTIONAL CALLOUTS
Fig. 1 — Front Panel Callouts
BELOW, ABOVE THRESHOLD (L.E.D.’s)
When the amber indicator is illuminated, the
input level to the 128 is below the threshold
level adjusted by the LEVEL control. When
the red indicator is illuminated, the input
level to the 128 is above the threshold level.
The L.E.D.’s do not measure output tevel.
DYNAMIC RANGE ENHANCER: PRE, POST
(switches} When the 128 is in TAPE mode and the
PRE switch is depressed, compression or expansion
takes place ahead of the tape recorder’s input (and
before tape noise reduction encoding if the 128
is in REC mode); when the POST switch is
depressed, compression or expansion takes place
after the tape machine's output (and after tape
noise reduction decoding if the 128 is in PLAY
mode). When the 128 is in DISC mode press the
POST switch.
PLAY LEVEL MATCH (control) Adjust
this control if the playback level from a
dbx I] encoded tape is considerably differ-
ent from the playback level of a non-
encoded tape played back in the bypass
PILOT (L.E.D.
indicator light)
Shows that the AC
power is ON
(illuminated), or
OFF (dark).
POWER (switch)
Controls the AC
power to the dbx
128. Push once
for ON, again
for OFF.
US
NY
Gi
¥
rd
DYNAMIC RANGE
ENHANCER
Lonoise reouction I
ABOVE THRESHOLD/LINEAR
(switch) In the LINEAR position
(switch in), the compression/
expansion section of the 128
operates in a normal fashion over
the entire program dynamic range
(see Figure 14 A & C). In the
ABOVE THRESHOLD position
(switch out), the compression/
expansion section acts only on
program levels that exceed the
threshold set by the LEVEL con-
trol (see Figure 14 B & D). This
changes the 128 into a peak limiter
(corresponding to COMPRESS
settings of the COMPRESS/EXPAND
control) or a peak unlimiter (cor-
responding to EXPAND settings of
the COMPRESS/EXPAND control).
Passages.
LEVEL (control) Sets the
“threshold’’ level above
which compression or ex-
pansion begins when the
ABOVE THRESHOLD/
LINEAR switch is in the
ABOVE THRESHOLD
position. When the ABOVE
THRESHOLD/LINEAR
switch is in the LINEAR
position, set the LEVEL
control so that ABOVE
THRESHOLD illuminates
during loud passages, and
BELOW THRESHOLD
illuminates during quiet
VANCE
COMPRESS/E XPAND (control)
Counterclockwise rotation of this
control past the 1.0 setting turns the
128 into a compressor with a ‘‘com-
Pression ratio” given by the numbers
on the surrounding scale.* Clockwise
rotation of the control past the 1.0
setting turns the 128 into an ex-
pander with an ‘expansion ratio”
given by the scale numbers.* When
the COMPRESS/EXPAND control is
set at the “1.0” setting (12 o'clock),
the 128 does not compress or
expand, and signals pass unchanged.
mode. The setting is not critical.
TAPE NOISE REDUCTION: REC, BYP, PLAY
(switches) When the REC switch is depressed, the
128's tape noise reduction circuitry is ready for
recording and encoding a tape with dbx {I tape
noise reduction. When the PLAY switch is de-
pressed, the 128's tape noise reduction circuitry
is ready for playback and decoding of an
encoded tape. When the BYP switch is depressed,
the 128's noise reduction circuitry is bypassed for
recording or playback of non-encoded tapes.
TAPE/dbx DISC (switch) Set this switch to
the DISC position, and press the PLAY
switch for playback of dbx II encoded discs.
The TAPE/dbx DISC switch should be in
the TAPE position for recording or play-
back of dbx II encoded tapes.
*The term “ratio,” whether associated with
“expansion” or “compression,” refers to the
amount of change in output /evel versus the input
level. A compression ratio of 2:1 (a COM-
PRESSION setting of 2.0) means that for a 2dB
change in input level there is a 1dB change in the
output level. An expansion ratio of 1:2 (an
EXPANSION setting of 2.0) means that for a
1dB change in input level there is a 2dB change
in the output level. A ratio of 1:1 (a setting of
7.0) suggests neither compression nor expansion,
so that for any change in input level there is a
corresponding change in the output level.

. RECORD LEVEL This screw-
Fig. 2 — Rear Panel Callouts driver control is factory
adjusted to match the record
levels encountered in most
home stereo equipment. Ad-
justment is not critical for
encode/decode performance; PLAY INPUTS
if you have too much or too Connect the cables
little level at the input to from your tape
your tape recorder, see page 14 : recorder “playback”
for adjustment instructions. (or ‘‘“monitor’’) out-
puts to these inputs.
q
4
WH
QUAD COUPLER For
compressing or expanding
four channel programs,
connect the QUAD
COUPLER jacks of two
dbx 128’s together. This
allows the compression/
expansion circuits in both
128’s to track” each
other so that the ‘‘quad
image” remains stable.
if
}
RECORD
HOSS
LEANING
i
ROM PREAMP TO TAPEBRECORDER FROM TAPE
TAPE OUTPUT AUXEBARY OR RECORDER
LINGINPUT OUTPUT
SERIAL NO.
A
f
py
WY
PLAY OUTPUTS Connect the cables
RECORD
INPUTS
For
connec-
RECORD
OUTPUTS
from
these
outputs
to
the
tape
(‘’play-
tion
of
the
cables
from
your
Connect
the
cables
back"’)
input,
preamp
input,
or
tape
preamp’s
record
output
or
tape
from
these
outputs
into
monitor
inputs
of
your
preamplifier
output.
If
there
is
no
tape
out-
the
“record”
inputs
of
amplifier
or
receiver.
Before
installing
put,
use
the
main
preamp
out-
your
tape
machine
the
dbx
128,
these
cables
came
directly
put.
Before
installing
the
dbx
(line
level
or
auxiliary
from
the
tape
recorder
“playback”
(or
128,
these
cables
were
plugged
inputs,
not
the
mic
“monitor’’)
outputs.
(Figure
3,
Page
7
directly
into
the
‘‘record’’
inputs
inputs).
illustrates
these
connections.)
“
of
your
tape
machine.
Micro-
7
phones
cannot
be
connected
NOTE:
Never
connect
any
other
directly
to
the
128;
a
preampli-
electronic
equipment
between
the
128
fier
or
mixer
must
be
used
to
and
the
tape
recorder.
See
page
9
provide
line-level
signals
to
for
connection
of
a
graphic
equalizer,
the
128.
reverb
unit,
etc.

BRIEF OPERATING INSTRUCTIONS
To Play a dbx-Encoded Phonograph Disc
1. Set your preamplifier’s INPUT SELECTOR Switch to
PHONO, and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, PLAY and POST Switches,
and set the COMPRESS/EXPAND Control to the ‘’1.0’
Position.
3. Play the disc and adjust the preamplifier’s VOLUME
Control for the desired listening volume. If the volume is
considerably louder or quieter than normal, adjust the
128’s PLAY LEVEL MATCH Control.
To Make a dbx-Encoded Tape
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s TAPE, REC and POST Switches, and
set the COMPRESS/E XPAND Control to the ‘1.0’
Position, Do not switch the 128 while recording.
3. With your tape machine in the Record Ready (Pause)
mode, set recording levels for peaks typically 3 to 6 VU
lower than the levels normally used. (With dbx processing,
significantly lower recording levels are often required to
avoid tape saturation, particularly in cassette recording.
Experiment during the initial setup to find the appropriate
levels.)
To Play a dbx-Encoded Tape
1. Set your preamplifier’s TAPE MONITOR Switch to
TAPE.
2. Press the 128’s TAPE, PLAY and PRE Switches,
and set the COMPRESS/EXPAND Control to the ‘1.0’
position.
3. Play the tape. If the volume is considerably louder
or quieter than normal, adjust the 128’s PLAY LEVEL
MATCHControl.
Fig. 3 — Basic Hook-Up Diagram

To Expand a Program
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
LINEAR position (IN).
3. Set the COMPRESS/EXPAND Control to the desired
degree of expansion (usually between 1.2 and 1.4). Do not
over-expand, especially on classical music.
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights on loud passages and the
BELOW THRESHOLD LED lights on quiet passages.
To Peak Unlimit a Program
1, Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
ABOVE THRESHOLD position (OUT).
3. Set the COMPRESS/EXPAND Control to the desired
degree of peak expansion (usually between 1.2 and 1.6).
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights-only on musical peaks, or for
the desired effect.
To Compress a Program
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
LINEAR position (IN).
3. Set the COMPRESS/EXPAND Control to the desired
degree of compression (usually between 1.2 and 3.0). Too
much compression can cause unwanted side effects.
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights on loud passages and the
BELOW THRESHOLD LED lights on quiet passages.
To Peak Limit a Program
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
ABOVE THRESHOLD position (OUT).
3. Set the COMPRESS/EXPAND Control to the desied
degree of peak limiting (usually 10.0 or higher).
4. Set the LEVEL Control so that the ABOVE.
THRESHOLD LED lights only on musica! peaks, or for
the desired effect.
To Record with Peak Limiting and
dbx I! Tape Noise Reduction
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e, PHONO, TUNER or BUN).
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC (IN) BYP and POST Switches,
and set the ABOVE THRESHOLD/LINEAR Switch to the
ABOVE THRESHOLD position (OUT).
3.
Set
the
COMPRESS/EXPAND
Control
for
the
desired
degree
of
compression.

4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights only on musical peaks, or for
the desired effect.
5. Now press the 128’s TAPE (OUT) REC and PRE
Switches.
6. With your tape machine in Record Ready (Pause)
mode, set its input level controls.
7. Record the tape.
8. To play back the tape just recorded, press the 128’s
TAPE (OUT), PLAY and PRE Switches, and set the
COMPRESS/EXPAND Contro! to the 1.0 position. Play
the tape.
To Play a dbx-Encoded Tape or Disc and Add Peak Limiting
1. Press the 128’s PLAY and POST Switches, and either
the DISC or TAPE Switch depending on the source.
2. Set the ABOVE THRESHOLD/LINEAR Switch to
the ABOVE THRESHOLD position (OUT).
3. Set the COMPRESS/EXPAND Control for the desired
degree of peak limiting (usually 10.0 or higher).
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights only on musical peaks, or for
the desired effect.
5. Play the tape or disc.
INTRODUCTION TO THE dbx 128
A live musical performance is an exciting experience,
the kind of experience that record manufacturers and radio
stations attempt to capture for reproduction in home
listening.
Unfortunately much of the excitement of the original
live performance is lost, even when using the best speakers,
amplifiers, tuners, turntables, and tape machines. The
primary reason for this lack of excitement and realism is
the dynamic range limitations in the recording and
broadcast mediums.
Dynamic Range and Why It Is Important
Dynamic range can be defined as the difference in
sound level, measured in dB, between the loudest and the
quietest passages in music. Acoustical loudness is measured
in decibels of sound pressure level, or dB SPL. The dB
expresses the smallest change in sound level that the average
ear can detect. 0 dB SPL is the threshold of human hearing,
the faintest sound most people can hear at a midrange fre-
quency (1,000 Hz). The loudest sound the average person
can tolerate without physica! pain is approximately 120 dB
SPL (some people can tolerate 130 dB, others leave the
room when the sound level reaches 110 dB). Thus, human
hearing has a maximum dynamic range of about 120 dB SPL.
Noise is the jower restriction of the dynamic range of
most recorded material. Typical sources of noise are phona-
graph disc surfaces, turntable rumble, tape hiss, and FM
broadcast hiss. When the original program’s quietest sounds
are too low in level, they are buried in the noise. ““Rumble”
and “scratch” filters can remove some of this noise, but
unfortunately, they also remove parts of the program. A
simple but effective way to remove or reduce noise
significantly without subtracting anything from the music
has long been sought. One method to avoid noise-imposed
restrictions in the recording and broadcast mediums would

be to raise the overall program level, allowing the louder
sounds to become even louder. However, when the pro-
gram’s loudest sounds are too high in level, they cause
distortion. Thus, the difference between the noise level
and the maximum undistorted level of a tape machine, disc,
or radio broadcast is not wide enough to accommodate the
full dynamic range of a live performance.
To get around the inherent dynamic range limitations in
a given medium, the original program is often ‘‘compressed.”’
This means that its dynamic range is restricted by making
the loud passages quieter and/or the quiet passages louder.
Compression thus avoids the dynamic range limitations of
the medium by reducing the dynamics of the program.
Compression is effective, but unfortunately it also removes
the program’s excitement and realism. Given the necessity
for compression, engineers long ago realized that if there
were some way to reverse the process, restoring the
dynamic range of the compressed program,the quality of
recorded music could be significantly improved. Until
recently, this ‘reverse compression” process, known as
“expansion,” had not been successfully applied in any
consumer product.
dbx has developed techniques that make a dramatic
improvement in the dynamic range of any recorded musical
material: (a) the patented dbx II Tape Noise Reduction
System for tape machines and phonograph discs, and
(b) the linear decibel compression/expansion system. The
dbx 128 incorporates both of these systems which may be
used independently or in combination with each other.
You can use the 128’s dynamic range enhancer to reduce
the noise of records or FM broadcasts, or you can capture
all of the widest range program material on your conven-
tional cassette or reel-to-reel tape recorder using dbx II
tape noise reduction.
dbx Il Tape Noise Reduction
dbx 11 tape noise reduction circuits ‘‘encode”’ a program
before it is recorded on a tape recorder and “‘decode” it
upon playback. This process reduces tape noise to inaudi-
bility and retains the original program’s dynamic range.
This means that a recorded program will not have any more
audible noise than the original source. Also, the dbx-
encoded program will retain the dynamics and excitement
of the original source without causing tape saturation.
When to Use the dbx I! Tape Noise Reduction System
Use the 128’s dbx tape noise reduction circuitry to
avoid
audible
tape
noise
and
increase
headroom
when
you
record a tape of a live performance, dub (copy) a con-
ventional disc onto a tape, or record any other source,
such as an FM broadcast.
The 128’s tape noise reduction circuits can also be used
to
decode
a
dbx-encoded
phonograph
disc.
dbx
encoded
discs have been recorded with dbx I! noise reduction
processing, and already have all the advantages of dbx
noise reduction.
dbx tape noise reduction processing cannot reduce noise
on an existing tape if it ha’ been recorded without dbx
encoding. Similarly, the dbx tape noise reduction circuits
cannot reduce noise on conventional phonograph records.
However, the dbx 128’s expander circuits can reduce
these types of noise, as explained in the next paragraph.
dbx Expansion and Compression
For the most part, noise in an FM broadcast or ona
phonograph disc is only audible when the program levels
are quiet. dbx linear decibel expansion increases the
program dynamics so the loud levels become fouder and
the quiet levels, inciuding the noise, become quieter.
Simultaneously, expansion restores much of the dynamic

range and, therefore, the excitement of the original per-
formance. The same circuits that do the expanding can be
used to compress a program for background music or for
making car stereo tapes, making the loud levels quieter,
and the quiet levels louder. Depending on front-panel
control settings, the linear decibel compression/expansion
circuitry can also “‘peak limit’ or “peak unlimit.” Peak
limiting is simply compression with the 128 set for “above
threshold” operation. This prevents program levels from
rising above an adjustable level (threshold), but has no
effect on levels below that point. Conversely, peak un-
limiting is simply expansion withthe 128 set for ‘‘above
threshold’”’ operation. This increases the amplitude
during the loud passages of the music, without affecting
the program below the set threshold level.
When to Use Expansion
Use the 128’s expansion capabilities to reduce the noise
present in the original source. For example, you can expand
a highly compressed and older recording and increase its
dynamic range to the point where it is comparable to your
best conventional phonograph discs and, at FM broadcast
time, significantly reduce the hiss. Or you can expand a
conventional disc and dramatically reduce surface noise
and turntable rumble at the same time you increase its
dynamics to recreate sound like a live performance.
When to Use Compression
Compression is used for recording a program which
must have a relatively constant level, as for example when
playing or recording background music. At the low listen-
ing levels of background music, quieter passages must be
kept loud enough to be heard, yet loud passages must not
be too loud. Similarly, if you are recording music for play-
back in your automobile or any other high noise area,
compression can keep the quiet sounds above the noise,
while it holds the loud sounds below distortion-causing
levels. Compression is also useful for a live recording of
several people with different voice levels. In this case, the
compressor acts like an automatic volume control and
saves you the task of “gain riding.”
When to Use Peak Limiting
If a tape must be recorded without the benefit of dbx Il
tape noise reduction (for later playback without dbx
decoding), then peak limiting is very useful because it can
prevent musical peaks from causing tape saturation. Peak
limiting can also be used to protect speaker systems from
extremely loud musical peaks, or from loud ticks or pops
on a damaged phonograph disc.
When to Use Peak Unlimiting
Many record manufacturers, and all radio stations use
some form of peak limiting in order to record or broadcast
high average program levels without having sudden musical
peaks saturate the tape or overmodulate the radio
transmitter.
“Unlimiting’’ (peak expanding) with the 128 can correct
the dynamic alteration created by peak limiting operations.
For example, suppose you have a particularly good record-
ing of a concert, one with excellent overall dynamics, how-
ever, some passages may lack the expected dynamic impact,
giving an unnaturally controlled feeling where instruments
should stand out. Such a recording probably had very little
original compression, but had peak limiting applied to high
level passages. The 128’s peak unlimiting action could make
this recording sound better.

WHEN TO USE COMBINATION APPLICATIONS
You can simultaneously use the 128’s tape noise reduc-
tion circuitry plus any one of the following: expansion,
peak unlimiting, compression or peak limiting.
When to Use dbx Ii Tape Noise Reduction Plus Expansion
By recording and playing back a program with dbx I|
tape noise reduction and then expanding it, you can end up
with a program that actually has lower noise and better
dynamic range than the original. This is especially effective
on noisy phonograph discs.
When
expanding
or
peak
unlimiting
a
program
that
has
been
recorded
with
dbx
II,
the
128’s
expansion
circuitry
should
come
after
the
dbx
I!
tape
noise
reduction
circuitry
(push
the
POST
button).
This
allows
the
dbx
tape
noise
reduction
circuits
to
properly
decode
the
program,
before
the
expansion.
It is not necessary to use the 128’s expansion circuitry
when playing a dbx-encoded disc, since the encoded disc
already has an extremely wide dynamic range. To avoid
over expansion, always press the PRE button and set the
COMPRESS/EXPAND control to the “1.0” (12 o’clock)
position when playing a dbx-encoded disc.
When to Use dbx II Tape Noise Reduction
Plus Compression
Normally, when making a recording with dbx Tape
Noise Reduction, additional compression is not necessary.
Compression might normally be used to restrict dynamic
range, and to “get the program on the tape,’’ but dbx
encoding alone overcomes the dynamic range limitations
of recording tapes. If you use compression plus dbx tape
noise reduction, the compression would primarily be for
special effects purposes.
When playing a tape which has been recorded with
dbx II tape noise reduction, or when playing a dbx-
encoded disc, you can use the 128 for peak limiting
(above threshold compression).
When adding compression or limiting while playing back
a dbx-encoded tape or a dbx-encoded disc, the 128’s com-
pression circuitry should come after the 128’s tape noise
reduction circuitry (press the POST switch).

10
DETAILED OPERATING INSTRUCTIONS
GENERAL NOTES
Mounting
The 128 is supplied in a handsome cabinet that can be
placed almost anywhere. Avoid mounting the 128 above any
source of heat or strong electro-magnetic fields, such as those
generated by power amplifiers or vacuum tube equipment.
Level and Impedance
The input levels, output levels and impedances of the 128
are designed for most hi-fi and semi-pro equipment. The128’s
input impedance (for both the noise reduction and compres-
sion/expansion sections) is 50k-ofims, which means it can be
driven from high or low impedance sources. The 128 input
will accept a wide range of voltages. The output will drive
up to 7 volts into 5000-ohms or greater. This means that
you can confidently connect the 128 to almost any type of
equipment found in a normal hi-fi or semi-pro system. See
the specifications section on Page 39 for greater detail.
Grounding
The 128 is not grounded to the power mains through its
power cord. !t obtains a ground via the shields of the inter-
connecting audio cables from the preamplifier (or receiver)
and/or from the tape machine. No special ground wires need
be connected between the 128 chassis and any other equip-
ment. If you hear hum, try reversing the AC plug, and if that
does not help, check to make sure all audio cables are prop-
erly installed, and that there are no broken conductors in
any cable. Also try cleaning the connectors on your cables
since hum and radio station pickup are sometimes caused
by microscopic dirt particles between the mating surfaces of
the connectors.
Cable Length
Some installations will tolerate longer cables, but when-
ever possible it is a good idea to keep signal cable runs under
10 feet. This will minimize hum and noise pickup, and also
help prevent high frequency losses naturally caused by cable
Capacitance.
dbx and Dolby®* Together
The dbx II tape noise reduction system provides at least
20 dB more reduction of noise than Dolby “B” tape noise
reduction systems, and 15 dB more than Dolby “A”, so there
is no need to use Dolby processing. If your tape machine has
internal Dolby processing, leave it off, and use only the dbx II
encoding and decoding. Dolby processed tapes are not com-
patible with dbx I] decoding (and vice-versa); if a tape was
recorded with Dolby encoding, it must be Dolby decoded.
A Dolby decoded tape may then be expanded.
NOTE: It is possible to combine both dbx 11 and Dolby
processing, though we do not recommend it. Any imperfec-
tions in the Dolby encode/decode processing will be magni-
fied by dbx I! treatment. Since dbx II processing alone re-
duces noise to inaudibility, there is really no need to use
dbx !1 with another system.
*“Dolby” is a trademark of Dolby® Laboratories Inc.

CONNECTIONS
Where to Connect Your dbx 128
If your preamp, receiver, or integrated amp/preamp has
tape monitoring provisions, the 128 should be connected
within the tape monitor loop. {n other words, connect the
TAPE OUT of your preamp to the 128’s RECORD INPUTS,
and connect the 128’s PLAY OUTPUTS to your preamp’s
PLAY or TAPE INPUTS (Figure 4).
For preamps, receivers or integrated amps that do not
have tape inputs and outputs, connect the PREAMP OUT-
PUTS to the 128’s RECORD INPUTS, and connect the
128’s PLAY OUTPUTS to your main amp INPUTS (Figure
5). (NOTE: When making recordings with this setup, keep
all filter and tone controls set at ‘‘flat,’’ or whatever setting
yields the flattest frequency response. Tonal adjustments
can be made when playing back the tape.)
Where to Connect Your Tape Recorder
Connect the 128’s RECORD OUTPUTS to the RECORD
INPUTS, or LINE INPUTS of your tape recorder. Do not
connect the 128’s RECORD OUTPUTS to the MIC INPUTS
of your tape recorder; the output level from the 128 is too
high for most mic inputs, and could cause distortion. Con-
nect the PLAY OUTPUTS, MONITOR OUTPUTS, or LINE
OUTPUTS of your tape recorder to the PLAY INPUTS of
the 128 (Figure 4 ).
Caution: Make sure that the power is OFF on all equipment
when installing the 128. As a precaution, turn down the
volume on your amplifier prior to switching on the 128 for
the first time. Inadvertent decoding of non-encoded programs
(128 in PLAY mode) can create surging to high volume levels.
y SEES
GI ZO N
(kOe) ’
Fig. 4 — Connecting the 128 in a Tape Monitor Loop
11

12
AN
y SENS
DDIM RON RCH
CO?
Fig. 5 — Connecting the 128 when there is No Tape Monitor Loop
NOTE: Using different preamp VOLUME CONTROL settings
changes the THRESHOLD and LEVEL MATCH settings.
Graphic Equalizers
If you use a graphic equalizer with your hi-fi, the equal-
izer must not be connected between the 128’s RECORD
OUTPUTS, and the tape recorder’s inputs, or between the
recorder’s outputs and the 128’s PLAY INPUTS. Ideally the
128 can be installed in the tape monitor loop, and the
graphic equalizer installed between the preamp output and
the power amplifier input (Figure 6 ).
Equalized Speaker Systems
If you have an equalized speaker system, such as a Bose
or Electro-Voice ‘‘Interface: A,” then the 128 should be con-
nected in the tape monitor loop of the speaker equalizer and
the equalizer should be installed in the tape monitor loop of
the preamp (Figure 7 ).
Note: Never connect any other electronic equipment between
the 128 and your tape recorder inputs. This could cause in-
accurate encode/decode tracking and loss of quality in the
recorded signal.

13
Re: Fig. 6 & 7 — If you are
using an equalizer with your
amplifier, you can hook it up
either as shown in Fig. 6 or
Fig. 7. Either hookup works
equally well.
MODEL 128
GRAPHIC EQ OR
SIMILAR UNIT
Fig.
6
—
Where
to
Connect
a
Graphic
Equalizer,
Electronic
Crossover,
Reverb,
Matrix
4-Channel
Decoder,
Etc.
(Special
signal
processing
follows
the
dbx
encode/decode
process.)
PREAMP
:
TAPE
a
t
R
L
Rp!
SPEAKER
R
L
R
@
@
@
L
a) © G EQUALIZER ©
°
oc
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<
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=
uerTTO
TQ
FO
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FO
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O
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9
ge
TO TARE RECORGER — EROM TAME Toe awe tape OF e
auunay oF RECORDER WONSTOE INPUT 56 60m
pasa eure es
AMDT LON
|
Fig.
7
—
Connecting
the
128
with
an
Equalized
Speaker
System.
(The
dbx
processing
is
done
inside
the
tape
monitor
loop
of
the
speaker
equalizer,
which
itself
is
in
the
tape
monitor
loop
of
amplifier.

14
OPERATION ~ (/f a rotary control setting is not
indicated, the setting does not affect that operation.)
Kev
viod
them?
mot
Jb»
-Codverant
Making a dbx-Encoded Tape
1. Set the 128’s front panel switches and controls as
shown.
=,
=
THRESHOLD mone. at tae =
ts >
ve y
sow C) Caso 128 MI a
GYNAMIC RANGE ENHANCER e =
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‘SYSTEM
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3
:
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ones S 2° zo
A LG
2. Using an input signal that is similar in level to the
program to be recorded (or using the program itself), place
the tape machine in RECORD READY (PAUSE) mode and
set the machine’s recording level for maximum VU meter
readings of about OVU (—3 VU for cassette recorders).*
*Record Level Match Control
The REC LEVEL MATCH control on the rear of the 128 is
factory adjusted to perform well with home hi-fi or semi-pro
equipment, Exact level matching is not necessary for accurate
encode/decode operation; the control is provided for use only
when there is an extreme level mismatch between the output of
the 128 and the input of your tape recorder.
if you have to turn the input level controls on your tape
recorder to extreme high or low settings to make the VU meters
read in their normal range, then adjust the REC LEVEL MATCH
control as follows:
Place your machine in RECORD READY mode (PAUSE}
with the input level controls at a normal position (about 120’ clock
on most machines).
These, or even lower recorded levels may be used with suc-
cess because the dbx processing prevents audible tape noise.
3. Feed a normal level program (from a phonograph
disc, FM broadcast, etc.) into the tape recorder through
the 128, and adjust the REC LEVEL MATCH controljuntil
the tape recorder’s VU meters display a proper maximum
level. (about OVU for reel to reel machines, about —3 VU
tor cassette machines). Alternatively, for greater precision,
use a test record with a 1kHz tone. (Frequencies between
400Hz and 1kHz are acceptable.)
w edev abe Vd dhe “ todixelen A uf gains oh
Playing Back a dbx-Encoded Tape
1. Set the 128’s front pane! switches and controls as
shown.
move puny
. LEVEL mATEN
.
DYNAMIC RANGE EANANCER. | %
NOISE REDUCTION
. SYSTER
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BS
=
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=
=
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BY
2. Place the tape machine in PLAY mode and play
the tape.
3. If the playback volume is considerably louder or
quieter than the original recorded volume, adjust the
PLAY LEVEL MATCH on the 128.

Playing a dbx-Encoded Disc
1. Set the 128’s front panel switches and controls as
shown.
THRESHOLD: ope.
secomC> ©) aove
fay
128 =
DYNAMIC RANGE ENHANCER. , =
aie MOISE REDUCTION.
ale. € SYSTEM
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PB PLEA ECR VAG
Y
2. Set your receiver or preamplifier as follows: Selector
Switch in the PHONO position, TAPE MONITOR Switch
to TAPE.
3. Play the disc.
4.
|f
the
average
playback
level
is
much
quieter
or
louder
than
that
of
a
normal
disc
without
the
128
decoding,
adjust
the
128’s
PLAY
LEVEL
MATCH
control.
Dubbing (Copying) an Encoded Disc onto a Tape in
Encoded Form
To dub a dbx-encoded disc onto a tape and retain the
dbx-encoding:
1. Set the 128’s front panel switches and controls as
shown.
THRESHOLD M0064 wt
LeveL Mari
eecow > © sfove 1 Eater
DYNAMIC RANGE ENHANCER, .
10 NGISE REDUCTION
LE ie eve SYSTEM
€
ba ota 2 rd
* c
© .5EB.0 Ba
2
mi"
se
viet
Oo
Ra,
PRE
ost
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BYP
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ay remy &
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POWER LPNEAR LEVER
@-»a Ozortt
2. Set your receiver or preamplifier as follows: Selector
Switch in the PHONO position, TAPE MONITOR Switch
to TAPE.
3. Play the disc, and record normally. You are not
decoding the disc, but are merely copying it in encoded
form. The resulting tape can be played back in the same
manner as any dbx || encoded tape.
15

Dubbing an Encoded Disc Onto a Tape in Non-Encoded Form
To record a dbx-encoded disc in non-encoded form onto
a tape for future playback without dbx processing:
Normally the cables from the RECORD OUTPUTS on
your preamplifier, amplifier or receiver are connected
directly to the RECORD INPUTS on the 128.
1. Connect the PLAY OUTPUTS of the 128 directly
to your tape recorder’s RECORD INPUTS.
2. Set the 128's front panel switches and controls
as shown.
3. Place your tape recorder in RECORD READY
(PAUSE) mode and start the disc. Adjust the recorder’s
input controls to approximately the same setting used for
normal recording. Then adjust the 128’s PLAY LEVEL
MATCH for maximum tape recorder OVU readings (about
—3 for cassette recorders).
4. Play the disc and record normally. Note that this
recording, because it is not dbx encoded, will have about
as much noise and distortion as any conventional,
non-dbx {| encoded recording.
THRESHOLD. ibis. may
BELOW C} © asove 128 EEVEGMATCH
DYNAMIC RANGE ENHANCER | -
ve 38 NOISE REDUCTION
+ SYSTEM
: E
Oo oa eta \
x
@ ;
ABOVE 4
THRESHOLD |
z
30, a PRE POST REC 8 8¥P PLAY «TAPE
5
Et
@0 ow ZO BOC O®
DYNAMIC RANGE aon
wuancen Linosenroucront ose dk
POWER NEAR LEVEL
MODEL 128
BS dbx
x
PREAMP 5
LEFT TO
Out ”
® b bee © ©® @
DY LR 27)
SE RIEL te
Ne se sen
Fig. 8 — Special Connections for Dubbing an Encoded Disc onto a Tape in Non-Encoded Form

Tape Copying (Dubbing)
Making
a
dbx-encoded
copy
from
a
dbx-encoded
master
™
provides
a
copy
with
no
more
audible
noise
than
the
master
tape.
1.
Since
the
encoded
tape
does
not
require
additional
dbx
II
processing
in
order
to
be
copied,
the
master
machine's
PLAY
QUT
can
be
connected
directly
to
the
slave
machine's
RECORD
INPUT.
If
the
signal
passes
through
the
128,
it
is
not
necessary
to
disconnect
the
unit,
just
press
the
BYP
button.
(Figure
Q)
2.
Adjust
the
record
levels
on
the
slave
machine
as
if
recording
with
dbx
II
processing.
3.
Tape
copies
encoded
on
the
128
can
be
played
through
any
dbx
I!
decoder
(120
series),
and
will
have
no
more
audible
noise
than
the
original
master
tape.
See
page
30
for
a
discussion
of
the
encoded/decoded
processing
dbx-encoded tape on
Master machine
(machine in PLAY
mode)
Blank tape on Slave
machine (machine
in RECORD mode}
Fig.
9
—
Copying
dbx-Encoded
and
Unencoded
Tapes
Directly
from
One
Tape
Machine
to
the
Other.
*in
the
discussion
of
copying,
we
use
the
term
“master
machine”
to
refer
to
the
tape
machine
with
the
program
to
be
copied.
We
use
the
term
“slave
machine”
to
refer
to
the
tape
machine
which
is
to
record
the
program.
17
differences
in
dbx
professional
systems
and
dbx
II
systems.
(Making
a
non-dbx-encoded
copy
from
a
non-encoded
master
is
identical
to
the
procedure
outlined
above,
although
average
recording
levels
should
be
raised
to
conventional
levels.
i
A
wpa
bcs
ort
f
db
K
Won
Nowme
[
bau
il
Making
a
dbx-Encoded
Copy
from
a
Non-Encoded
Tape
This
is
occasionally
desirable.
The
copy
will
not
have
any
more
audible
noise
than
the
original.
1.
If
your
preamp
(or
receiver)
has
two
tape
monitor
loops
with
switching
for
“Dub
A
to
B”
or
similar
copying
provisions,
connect
your
system
as
shown
in
Figure
10.
2.
Set
the
128’s
front
panel
switches
and
controls
as
shown.
TARESHOLD
sao)
PUAY
‘he
LEVEE
MATCH
:
Becow () © ssove 128
DYNAMIC RANGE ENHANCER %
me, TR NOISE REDUCTION
TS sgh SYSTEM
. & > = & a
O
_asove
6
ts
oat
7
THRESHOLD | M20,
ma
R30
cre
ene
|
POST
REC
BYP
PLAY
TAPE
yams
Riaee
@0O
wun
"@O0@00O0O
wt § ae
Pam 9 DYNAMIC RANG
re ex
power
REan
_KEVEE
Tevaxcer
Linoise
renuction—}
aise
&Th
@-.2 OzourE
3.
Set the
preamp
to
dub
from
the
Master
(machine
A)
to
the
Slave
(machine
B)
and
copy
the
tape.

18
Ar
abe
alone
db,
vere
Abevemdbtetom
PY
coicaadh
Making a Non-Encoded Copy from a dbx-Encoded Master
This is sometimes necessary so that the copy can be
played without the benefit of dbx I| tape noise reduction.
The copy will be similar to an original recording made with-
out the benefit of dbx processing: in other words, with
restricted dynamic range and audible tape hiss.
1. The encoded master tape must be dbx-decoded prior
to feeding the slave machine. This may be accomplished in
a number of ways depending on the nature of your
installation.
2. If your preamplifier, amplifier or receiver has two
tape monitor loops with switching for ‘‘Dub A to B” or
similar copying provisions, connegt your system as shown
in Figure 10.
3. If your preamp (or receiver) has only one tape monitor
loop, there are two ways to make a non-encoded copy.
Machine A
Either disconnect the dbx PLAY OUTPUTS from the pre-
amp’s monitor inputs, and connect the cables instead to the
slave machine’s RECORD INPUTS, or simultaneously con-
nect the dbx PLAY OUTPUTS to the preamp and to the
slave’s RECORD INPUTS by using a “Y” adapter as shown
in Figures 11-& 12.
4. In either case, set the 128’s front panel switches and
controls as shown.
THRESHOLD MmOooEL
Puay
BELOW () © ssove 128 CevELMatCN
DYNAMIC RANGE ENHANCER, “s
2 10 NOISE REDUCTION
1 8 gle SYSTEM
. =
O _ avove Sta we NN
i ees pie oa
E .
@0O Fi. “s@OC0®
rag s ie)
tomy ae S55
POWER LINEAR CeveL
q
MAME OSSD
IN
Fig.
10
—
Copying
dbx-Encoded
and
Non-Encoded
Tapes
with
a
Preamp
having
Two
Tape
Monitor
Loops.

q
MODEL 128
dbx
5. Place the slave machine in record mode and the
master machine in play mode and copy the tape.
py ADEE
©® © ©#© @
SAMA LONI
MRD.
Fig. 11 — Making a Non-Encoded Copy from dbx-Encoded Master
withoina
Preamplstiers
dbx
encoded
‘‘master’’
blank
tape
(machine in PLAY mode) (machine in REC mode)
dbx encoded ‘‘master’’
(machine in PLAY mode)
blank tape
(machine in REC mode)
“Y"" cable adapter
MODEL 128
°o
@acsrPo 0 0 0 O
AMP/PREAMP (OR RECEIVER}
AW? COSA
WAAAY
Fig.
12
—
Making
a
Non-Encoded
Copy
from
a
dbx-Encoded
Master
without
the
benefit
of
Two
Tape
Monitor
Loops.