dbx 4BX User manual

Model 4BX
Three-Band
Dynamic-Range Expander
with
Impact Restoration
and
Remote Logicontrol
Owner's Manual

INSPECTION and INSTALLATION
Your unit was carefully packed at the factory in acarton designed to protect it.
Nevertheless, we recommend examining both carton and contents for any signs
of damage that may have occurred during shipping. If there is evidence of dam-
age, don't destroy the carton or any of the packing material, and notify your dbx
dealer immediately.
In any case it is agood idea to save the carton and packing materials should
you ever need to ship your unit in the future.
In addition to amodel 4BX and this instruction manual, the carton should
contain:
1) a set (two pairs) of hookup cables with RCA phono, or pin, plugs;
2) awarranty/registration card;
3) apair of brackets for mounting the unit into astandard 19-inch(483-mm)-
wide equipment rack (see below).
No special ventilation is required in any installation; other components may
be stacked above or below the 4BX provided that they themselves do not get hot.
ACAUTION
risk of iirctmc shock
00 NOt OPtN
CAUTION: TO REDUCE THE RISK OF ELEC-
TRIC SHOCK. DO NOT REMOVE
COVER (OR BACK).
NO USER-SERVICEABLE PARTS INSIDE.
REFER SERVICING TO QUALIFIED SER-
VICE PERSONNEL.
This symbol is intended to
alert you of the presence of
uninsulated dangerous voltage
within the unit's enclosure that
may be of sufficient magnitude to
constitute arisk of electric shock
to persons.
This symbol is intended to
alert you of the presence of
important operating and maintenance
instructions in the literature
accompanying the unit.
WARNING: To Prevent Fire or Shock Hazard, Do Not Expose This Appliance to
Rain or Moisture.
"dbx" is aregistered trademark of dbx Inc

CONTENTS
Operating instructions:
Front panel and remote control
Connections:
Rear jacks and controls
About impact restoration, expansion, and dynamic range
Typical hookups
Usage notes and cautions
Specifications and notes
Warranty and factory-service information
Important information for users in the UK

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OPERATING INSTRUCTIONS:
FRONT PANEL and REMOTE CONTROL
4BX
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TRANSITION IMPACT SOURCE TAPE PRE POST BYPASS DISPLAY
LEVEL RESTORATION
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APOWER ON/MUTE. Push this button to turn the
4BX on. It always comes on muted; push the button
asecond time to unmute the sound. (The very first
time you do this the sound comes on at -15 dB;
subsequent turn-ons and unmutings make the
sound come on at the level it was at before the unit
was last turned off, so be careful. If you're in doubt
about how loud it was the last time, press VOLUME
down acouple of seconds before unmuting or, bet-
ter yet, press VOLUME up and don't unmute at all.)
To mute the sound again, as for answering the
phone, press the button athird time. The muting is
not total: the sound is lowered by some 40 dB from
the 0-dB indication, however loud that is. (See the
"Volume" discussion, below, for advice on setting
4BX and preamplifier* levels.)
The LED (light-emitting diode) above this button
goes on whenever the sound is at this -40-dB level,
whether muted or just turned down; it doesn't indi-
cate "power on."
BOFF. Push this button to turn the 4BX off. The LED
above it, which lights up whenever the power is on,
will go out.
Note: When you turn the 4BX off, it is only re-
moved from the signal path; it does not shut down
the sound. If you've been listening at a-20- to
-30-dB level (again, see the "Volume" section) and
press OFF, the sound suddenly will increase by a
lot. This is most likely to happen when using the
remote control. So the point is that the 4BX's remote
is significantly different from aconventional TV
"armchair" control, whose Off button shuts things
off entirely. The 4BX's OFF bypasses the unit, rein-
stating the sound at the preamp's level (that is, at
unity gain). MUTE is what you press to lower the
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VOLUME
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EXPANSION
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TRANSITION LEVEL
IWACT RESTORATION
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4BX
REMOTE
LOGICONTROL
volume. Push OFF after the rest of your system is
shut down, perhaps, or at least press it cautiously,
not casually.
CVOLUME. These buttons raise and lower the vol-
ume, or gain, of the sound, according to their ar-
rows and in the amount shown by the LED column
above them. The 0-dB setting in this column is the
unity-gain position, where the level of loudness is
the same as from your preamp, the 4BX making no
increase or decrease. The volume range is 50 dB,
the last 10 coming between the -30-dB LED fully lit
Tt>r the sake of simplicity, "preamp" will mean apreamp, receiver,
or integrated amplifier.

and fully dark. The VOLUME down button does not
turn the sound off; it just lowers it to the muted
(-40 dB) level.
In establishing levels with your 4BX, it is best to
begin by setting the volume to its maximum, +10
dB. Then turn up the preamp's volume control to as
high alevel as you would normally listen to comfort-
ably. In this relationship the 4BX always will be
lowering the volume from this high level, and the
probability of alarming level jumps when the 4BX is
put into or taken out of the signal path is reduced.
(See the discussion of the OPF button.) Still, it's
possible, if the levels are set as suggested here and
you're listening at below -30 dB and press OFF, for
the sound to jump 35-40 dB, astartling but probably
not damaging occurrence. Again, caution is war-
ranted with the OFF button.
You'll be interested to know that each 10-dB in-
crease in level calls for 10 times as much power,
even though the difference to the ear is only what
most people call twice as loud. In fact, each 3-dB
increase (which you'll have to approximate through
the partial illumination of the next VOLUME LED),
even though it sounds small, requires twice as much
power.
EXPANSION. These buttons control, according to
their arrows, the amount the 4BX expands the dy-
namic range of the sound. Asetting of 1.2 results in
a20-percent increase in dynamics, 1.4 a40-percent
increase, and so on. In the latter case, an input
signal with 40 dB of dynamic range will be ex-
panded to 56 dB and asignal with 50 dB will go
to 70 dB. The maximum expansion, as marked, is
50 percent.
GAIN CHANGE and TRANSITION LEVEL. The
GAIN CHANGE LED rows show the amount of gain
(volume) change in each of the three frequency
bands the 4BX divides the audio program into. HF
stands for high frequencies, above approximately 6
kHz; MF is the mid-frequencies, between about 150
Hz and 6kHz; and LF is the low frequencies, below
approximately 150 Hz. The red LEDs in each, to the
right, show upward expansion (volume increase) to
+12 dB, and the yellow LEDs, to the left, show
downward expansion (volume decrease) to -20
dB. (The amount of HF expansion can be separately
adjusted to suit various kinds of music and different
speakers, and to vary the overall expansion "mix";
see Connections: Rear Jacks and Contmls.)
The point between upward and downward, the
center or unity-gain point, is set by the TRANSITION
LEVEL buttons, according to their arrows, ami is
shown in the LED column above them. When the
incoming program signal is above (louder than) this
set level, the4BX expands it upward, making it
louder still. When the signal is below (softer than)
this level, the 4BX expands it downward, making it
still softer. And when the signal is exactly at (he level
set by these buttons, it goes through unexpandod.
Set the TRANSITION LEVEL so that the red LKDs
in the three GAIN CHANGE rows glow during the
louder parts of the music and the yellow LEDs glow
during the quieter parts. Please note that this setting
needn't be exact; the buttons are there for your
convenience in setting and matching levels among
sources.
In controlling the ratio of the sound that gets
expanded upward to that which get's expanded
downward, the TRANSITION LEVEL also functions,
whenever the EXPANSION level is above 1.0: Las n
sort of sub-volume control (after the VOIUMI- but-
tons). We mention this because if you have the
TRANSmON LEVEL set on the low side and press
BYPASS (which, as will be discussed presently, re-
moves only the expanding action of the 4BX from
the signal path, not the VOLUME action), the sound
can jump up as much as 20 dB. Likewise vice versa:
if the TRANSITION LEVEL is set high and the 4BX is
taken out of BYPASS and put back into operation,
the sound will increase suddenly. This may well be
startling but, as was the case with hitting the OFF
button, it is unlikely to do damage.
FIMPACT RESTORATION. These buttons, of course,
increase and decrease the amount of impact restora-
tion the 4BX provides, as shown by the LED column
and the LED row. "Impact restoration" describes a
dbx expansion circuit that works on (increases the
volume of) the initial attack of amusical transient,
e.g., adrummer's rim shot. All the LEDs in the row
are red (there is no downward expansion of tran-
sients) and are read from left to right, up to +12
dB. This upward expansion of attacks adds consid-
erable crispness and, well, impact to the sound,
and does so musically. Live music has this exciting

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VOLU»* EXPANSION
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MULTI-BAND EXPANDER
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TRANSITION IMPACT
LEVEL RESTORATION
CD nil CD
SOURCE TAPE PRE POST BYPASS DISPLAY
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quality, and we think that the 4BX is quite successful
at making compressed, limited, and clipped re-
corded music approximate live (see About Impact
Restoration), You will quickly find the best-sounding
settings for various kinds of music, classical, jazz,
and/or popular.
GSOURCE and TAPE. These buttons choose what
program you listen to. Push SOURCE for your record
player, radio (tuner), or Auxiliary inputs (e.g., your
TV/VCR). Now you can expand these sources, i/
you wish. Push TAPE to listen to your deck play a
tape and/or to "access" (bring into the signalpath)
any signal processors connected in the 4BX's tape
loop (see Rear Connections). To expand a tape being
played, the POST button, discussed next, must be
pressed; to expand one of the other programs
(SOURCE), either PRE or POST may be pressed.
If your tape deck has monitoring capability (three
heads), you can alternately push SOURCE and TAPE
to check on atape being recorded.
HPRE and POST. These buttons control where the
4BX's actions take place with respect to taping.
The more common application will be expanding
atape on playback, with both TAPE and POST
pushed in. This places the 4BX after the deck's out-
put (of course the deck is in play, not record). If you
have not atape deck but asignal processor in the
4BX's tape loop, pressing POST and TAPE places the
4BX expansion etc. after the signal processor, which
most often is where you'd want it to be.
If, however, you push PRE and TAPE, the 4BX
comes before your deck, making it possible to expand
and "punch up" the signal before it gets recorded
onto the tape. But be advised that this calls for con-
siderable care, for a number of reasons.
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IWACT RESTORATION
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4BX
REMOTE
LOGICOrJTROL
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First, in almost all cases, extreme expansion, and
impact restoration especially, will make the program
dynamics wider and "hotter" than any cassette
deck has the capability of handling. (All cassette
decks are readily overloaded by powerful signals;
open-reel recorders have an easier time of it.) Con-
servative recording levels and moderate expansion
and impact-restoration settings will help, although
conventional mechanical recording-level meters
(needles) cannot respond fast enough to something
as quick as impact restoration. It is best if dbx noise
reduction is employed, since it gives you more room
than the other noise-reduction systems, and if the
4

tape deck has electronic (lighted-bar) meters set to
their peak-reading position, in order to give you an
accurate idea of how potent the 4BX's effects are.
Second, if PRE is used, the 4BX's other controls
also come before the recording —that is, VOLUME,
POWER ON and OFF, MUTE, and BYPASS. Therefore
they must not be pushed thoughtlessly during the
recording. This means, among other things, that
the loudness level in the room either has to be as
you like it for the duration of the selection or has to
be controlled by your preamp only. Of course it is
possible to adjust the 4BX's LOUDNESS and the
tape deck's recording controls beforehand so their
levels are appropriate for both listening and taping.
And in many instances it might be agreat conven-
ience to have an instant remote capability for lower-
ing or muting applause, commercials, etc. after the
selection being recorded is over. It's just that you
will want not to press any 4BX buttons casually
when you're recording in PRE.
For foolproof results, then, we recommend that
you do your expansion and impact restoration on
playback. But if you do choose levels and settings
wisely and monitor carefully, you'll be able to reduce
noise in the original program, add impact, expand
dynamics, and preserve all these changes on the
tape.
Note that when you've pressed SOURCE (say, to
compare the tape recording with the original), PRE
and POST still affect the taping even though they
don't affect SOURCE. Thus if you're making atape
and using SOURCE and TAPE to check it (to monitor
it), be sure not to press PRE and POST back and
forth.•Probably it is best to monitor atape being
made of aPRE-expanded program using the deck's
own Tape/Source switch.
IBYPASS. Pressing this button removes the 4BX's
expansion and impact restoration from the signal
path —but that's all. Everything else still works.
Which means (among other things) that you can
still control your stereo system's loudness remotely,
even with no expansion or impact restoration at
work.
JDISPLAY. This controls the brightness of the 4BX's
LEDs (except for the one which indicates power,
above OFF), from full intensity to very dim.
KREMOTE LOGICONTROL. As we have been say-
ing all along, the handheld remote lets you control
all of the 4BX's functions except SOURCE/TAPE and
PRE/POST. And by extension, naturally, it is nremote
volume control for your entire stereo. At Inst, you
can lower or mute the sound in the room without
having to get up and go over to your system.
The control is not particularly directional, and
can operate over along range (25 Tcct or so). Since it
transmits infrared light, furthermore, its signal can
be bounced off walls and windows, as necessary.
However, its action is certain only when it's aimed
at the 4BX's receiving window, above the DISPLAY
slider. (The adjacent LED indicates reception.) There
are times when too great an off-axis direction of aim
or too much distance from the 4BX can result in the
loss of remote capability. Similarly, too much light
shining on the front of the 4BX may create an optical
interference or "background noise" that the Logi-
control must be stronger than in order to get
through; if the room lights are too bright, it will fail.
Generally, the Logicontrol will enable you to do
what you want with the 4BX at any reasonable
distance.
The supplied 9-V alkaline battery should last a
year or more in typical use; replace it with the same
type, i.e., alkaline.

CONNECTIONS: REAR JACKS and CONTROLS
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AFROM PREAMPTAPE OUTPUT. Connect one of
the sets of "Tape Out"* jacks on your preamp to
these 4BX inputs, left to left and right to right.
(Right commonly is designated by red cable ends;
for some people it helps simply to remember that
"red" and "right" start with the same letter.) You
can use either Tape 1or Tape 2.
BTO PREAMPTAPE INPUT. Connect these outputs
to your preamp's "Tape In'.'** Be sure that this
signal return goes to the same-number tape-monitor
loop on your preamp as it came from, i.e., no. 1or 2
or whatever. Also note that in order for you to use
your 4BX in this hookup, your preamp must always
be switched on its front panel to the corresponding
Tape position (Tape 1or Tape 2, etc.).
CFROM TAPE RECORDER OUTPUT. Connect your
deck's PLAY, OUTPUT, TAPE OUT, LINE, or MONITOR
outputs to these 4BX inputs.
DTO TAPE RECORDER INPUT. Connect these out-
puts to the INPUT, LINE IN, REQORD) or whatever
on your tape deck.
Note: regarding these two tape-deck hookups, if
you have adbx noise-reduction unit or other signal-
processing equipment, see the connection diagram
on page 12.
EFUSE HOLDER and POWER CORD. Replace a
blown fuse with the same type (as marked), but if
the new fuse blows, have the unit checked. Connect
the power- cable to the appropriate power source;
the correct information is printed on the 4BX's rear
panel next to the cable's entry hole. The 4BX may be
damaged if it's not connected to the proper power
source (US/Canada, 117 Vac, 50-60 1\z; elsewhere,
also as marked). If your unit has aswitch for differ-
ent power sources, be sure it's set correctly.
Aword about switched and unswilched ac outlets
on other components to plug the 4BX into. If the
4BX is in an unswilched outlet, its memory still
operates when it's off, and when you turn the unit
on, all functions are in their last mode, as noted
earlier. However, on "first power" (which Is every
time if the 4BX is in aswitched outlet, or after a
power failure or unplugging of the cord if not), the
unit reverts to SOURCE and PRE, the VOLUME will
come on muted and then will unmule to about the
-15-dB level, and EXPANSION, TRANSITION
LEVEL, and IMPACT RESTORATION all will be cen-
tered at about their mid-, or 50-percent, settings.
One reason for putting the 4BX in one of your other
components' switched jacks, if you don't care about
not maintaining any of your 4BX function settings,
is that then the 4BX will be turned on and off with
your system, and you won't get in the habit of using
its own OFF and potentially causing sudden level
changes.
FIMPACT RELEASE RATE. This control affects the
rate (speed) at which impact restoration decreases.
That is, the end of the attack may be made to die
more and less quickly. There's no one correct setting
for all music, just as there's no perfect all-purpose
setting of the front-panel IMPACT RESTORATION,
but you'll probably be able to find one position that
suits most of your listening. Much popular music
benefits from afaster reduction of its transients'
decay (turn the contn>! clockwise); the rhythmic
drive is enhanced appropriately. Such asetting,
Also called TAPE REC, REC(OUT), TO TAPE REC, TO TAPE IN(PUTS),
(OUT) TO TAPE, TAPE OUTPUT, etc.; Ihere probably are olhers.
"Also called PLAY, TAPE PLAY, PLAYBACK, (IN) FROM TAPE, FROM
.TAPE OUT(PUrS), etc.; olhers here too, probably.

however, may make gentler, longer-phrased music
sound alittle out of breath or sometimes staccato
and jabbing (e.g., piano), so aslower release rate is
more suitable, to preserve the proper spaciousness.
And these are just the extremes of music. Experi-
ment to find what sounds best to you most of the
time; the factory setting is just astarting point. The
control is easily turned with asmall flat-bladed
screwdriver, such as often come with phono car-
tridges.
HF TRANSITION LEVEL. This control raises and
lowers the transition level (see the Front Panel discus-
sion) of the high band only, the band most likely to
require changing for different kinds of music and
speakers. Altering this band's transition level means
increasing and decreasing the high treble, the up-
per-range brightness, of the sound (above approxi-
mately 6kHz). Alower transition level (turn the
control clockwise) produces more upward expan-
sion of the very high frequencies relative to the rest
of the sound; ahigher transition level (turn the
control counterclockwise) does the opposite, taming
the sound abit (if it's hissy, for example) and provid-
ing more noise reduction.
The control is set at the factory for what we be-
lieve is typical program material, but feel free to
adjust it to your musical tastes —violins don't need
to sound as intense as cymbals, and dull or spitty
recordings and broadcasts don't have to stay that
way. When you do your experimenting, make sure
that the 4BX is set at its EXPANSION midpoint (1.3:1
or above, say) and that the front-panel TRANSITION
LEVEL, which governs the proportion of upward to
downward expansion for all of the frequencies, is
set such that the red and yellow LEDs in the MF
and LF bands are lighting up about equally.

ABOUT IMPACT RESTORATION, EXPANSION, AND DYNAMIC RANGE
Music works largely through our anticipation of it: the pleasure of knowing,
but not knowing precisely, what's going to happen next. Whether the form is
fugue or blues, arock refrain or Haydn's Smjmse Symphony, the lasting fun
comes from our expectations being satisfied -but not too predictably so. In a
good piece of music, over time, the startling becomes familiar, even as the famil-
iar stays startling.
This effect, this freshness, frequently is enhanced by the attack of anote or
chord. Depending on the percussiveness of the instrument and/or on the loud-
ness of the attack compared with what came just before, these initial transients
have aconsiderable impact. It's not just that Haydn put surprises in his famous
symphony's slow movement, it's the nature of their sound, an orchestral sfor-
zando that comes delicately out of nowhere.
This is what the 4BX's unique impact-restoration circuit aims to do: to increase
the snap, the immediacy of these musical attacks that enhance our anticipating.
Many of these transients get clipped off or otherwise dulled and muted anyway
during the recording or broadcasting process, because of the compressors and
limiters that are customarily employed. But even if the peaks aren't blunted in
the processing, virtually all reproduced music sounds as though at least some of
its excitement is missing. Conventional dynamic-range expansion, as we shall
see, recovers much of this, yet cannot restore the excitement altogether. So with
impact restoration we have tried to bring back some of the heart-stopping,
breathtaking quality of live performance. After you have experimented with dif-
ferent amounts of impact restoration for various kinds of music, we think you
will agree.
As for dynamic range itself, it may be simply defined as the difference in level
between the loudest and the softest sounds in any given musical performance or
recording.
In aperformance, the loudest sounds are limited only by the musician's ability
to strike, bow, or blow into the instrument, by its own limitations (e.g., apipe
organ), or, with amplified music, by the amp/speaker system's ability to fill a
given space loudly without objectionable distortion. True (instantaneous) peaks
may be more than 15 dB* above the average level, so any measurements of the
"maximum" levels really should reflect the instantaneous peaks achieved.
•The decibel is aunit thnl expresses relative difference in power or sound level. The figure of 1dlJ is
usually given as (he smallesl sonic change delectable by most people, although over awide spec-
trum of music or sound and under controlled conditions, athird of this amount is delectable to
many. The threshold of hearing (the point at which one no longer can hear anything even at the
most sensitive frequency range of 1-3 kl \z, in the upper midrange/lower treble) is approximately
"0-dB SPL" (sound-pressure level, astandardized measurement), and the level at which most people
put their hands over their ears is about 115-120 average SPL. Some can tolerate 130-dH-SPL averages
which are harmful over time; others leave the room at tlO; the dynamic range of human hearing thus
is customarily said to be around 120 dB. Note that the relationships among SPL terms are widely
misunderstood and misidenlified. If the true (instantaneous) peaks are around 115-dB SPL, then the
standard fast readings of maxima are about 103-106 -and these are often labeled "peaks" even
within the audio world, especially in discussions of power-amplifier requirements for reproducing
conventionally measured fast maximum SPLs -and the average (slow readings) level is around
100-102 or so.

In ahigh fidelity stereo system, the highest levels that can be reached likewise
are determined by the entire system's ability to play the loudest parts ofdhe mu-
sic without distortion becoming objectionable. Often the source of Ins distor-
tion is tire strained amplifier(s) and loudspeakers, but it may come from the
record player and/or stylus/cartridge or the tape deck, and frequently it salready
present in the record, tape, or broadcast itself.
The softest sounds in aperformance occur just after the musicians stop play-
ing the echo and reverberation of the music dying out in the hall Surprisingly,
this reverberance may still be heard even when its level is below the hall sown
ambient noise -the ear has aremarkable ability to pick out wanted signals in
the presence of alarge amount of (unwanted) noise. Nonetheless aconvenient
and conservative figure for specifying the softest sounds practical malive per-
formance is the level of background noise in the hall, weighted to take into ac-
count the ear's decreased sensitivity to low-level (soft) low-frequency sounds.
The true peaks generated in alive performance can be up to 115 dB SI'L for a
full orchestra and somewhat higher for amplified music. A. the other end the
background level (correctly weighted) of aquiet ball often is below 30 dB SPL.
Tbe difference, then, is around 90 dB, auseful figure for specifying the dynamic
range of live unamplified music.
Unfortunately, the two most popular forms of storing recorded music that are
currently available -vinyl analog records and audio cassettes -have dynamic
ranges much, much less than 90 dB. Conventional records do well to reach be-
yond 50-55 dD of dynamic range; the most carefully prepared "audiophile
pressings may have slightly over 65 dB (provided they're no. decoded discs,
which upon decoding offer more than 90 dB of dynamic range). And even tbe
best metal-tape cassettes cannot reach 60 dB on their own without .,noise-re _
duction system. Clearly, the audio signal must be manipulated in order to fit the
dynamic range of live music into the constricted limits of tbe popular storage
media.
This manipulation almost invariably is accomplished by compressing the sig-
nal and/or riding the gain in the recording or broadcast: loud passages are re-
duced in level and soft passages are increased. The overall dynamic range is
thereby restricted. More than 30 dB in dynamic range has to be gotten r,d of in
'The wol.-kn.wn Hctcher-Munson curve, for single .ones, have ^^^^
ber of researcher. Among Ihe most ^^^^^^^^^^
demon*.* Ih» leaned .sensitivity. R* example, if a2-kt ,sound ,
mUsic) aHU \7tone has to be at about 115-dB SPI. to sound equally loud, adeference of some 20
7a 'lower levels, the difference is greater. A2-U sound at 60-dB SPLtquiet chamber music)
el. dinLdnessby.SO-lfe^^
Zsound as loud). A, Lis below 60-dn SPL, which are common in the home, the relative differ-
ences are greater,
Ml we remember Iha, .he ear can bear musical ,eve,be,a,lons Iba, lie be.,,.,. .*«| ^
Honed, Iba! many recording and performance slluallnns are considerably qulele. Iha 3MJ SX
Ibeir besl, and Iba, background-noise figures lend lo be aggregale, nnl .eadmgs
-
speclra. it become* clear Iha. even 90 dB of dynamic range is no, enough. Ilence .he need for d,g,l»l
recording systems of 110-dO dynamic range (and up), such as Ihe dbx 700 ser.es.

order for the soft sounds to be recorded or broadcast above the noise floor and
for the loud sounds to stay below an unacceptable distortion level.
If we want to restore this lost dynamic range in listening to arecording or
broadcast, an expander is required. The dbx 4BX does just that —it enables alis-
tener to recover substantial portions of the dynamic range missing on existing
conventional recordings and during conventional broadcasts. It works by in-
creasing the volume of high-level passages and decreasing the volume of low-
level ones, the opposite of the compression and gain riding used to constrict the
program in the first place. Since it splits the audio signal into three bands and
processes each separately, the 4BX can expand compressed material without the
usual artifacts —audible clues —that tell the listener that processing is taking
place. By flexibly expanding the dynamic range of a compressed program —not
to mention re-creating and/or augmenting the impact of its transients —the con-
trasts between loud and soft are restored and those of the original performance
more closely approximated. AH of which brings back much of the excitement
and involvement of listening to music live.
In lowering the characteristic noise levels of tapes, records, and broadcasts
and restoring the "punch" of loud moments and the whisper of quiet ones, a
4BX can give new life to old record and tape collections and can make FM broad-
casts worth listening to closely. And used carefully with adbx tape-noise-reduc-
tion system (such as the 222 or 224), a4BX lets you make copies of records, FM
broadcasts, and other tapes that actually sound better than the originals. With all
of these capabilities, it probably will become one of the most valued components
in your home music system.

TYPICAL HOOKUPS
Caution: be sure that the power is OFF on all equipment when you hook up
your dbx unit. Further, turn your volume control all the way down before
switching on the 4BX for the first time.
S«Mt7fi
rotjuf o* MctivtR
HEWTOH. U* .
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O O
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FREAMP
LEFT
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9
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5
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TAPE
A°ut Tape Deck
(1
Basic Signal Connections
The 4DX goes between the preamp and the tape deck, in the former's tape-
monitor loop, as shown. The tape deck then goes in the 4BX tape bop.

PREAMP
x
<
LEFT O
RIGHT O
TAPE
Z
OOQ~Q
O<)
~Optional
TOI'RfcAMI'
TAI'E INPUT 10 IAPE
RF.COKOr.R IN
fKOMI HI AMI'
tAif ounni fKOM TAI'I
HKOKDfROUr An automatic equalizer (the dbx
10/20) or aspeaker-only equal-
izer (e.g., dbx, Bose) goes here
Agraphic equalizer used on
program material goes here
224X
PiF.AMP OR RECEI
G--O
rwu ppcaxp
TAPC OUTPUT
Noise-Reduction Unit
TAPE RECORDER
RLR L
0-rO OyO
FROM TAPS TO APC
BttXW* OUTPUT PCCOflP fl KFVT
Using the 4BX with adbx Noise-Reduction System
The 4BX comes first in the hookup (and last in the signal path
except for an automatic EQ). Connect it direclty in one of the
tape-monitor loops of the preamp, as shown. Then connect the
no«se-reduction unit directly in the 4BX's tape loop. Finally
the deck itself goes directly in the noise-reduction unit's tape-
monitor loop. Red plug =right channel.
.Those whose stereos surpass this in complexity should investi-
gate the dbx 200X and 400X Program-Route Selectors.
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t
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Tape Deck
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USAGE NOTES
In general, {he amount of expansion to use is amatter of taste. However, some
guidelines might prove useful as you experiment with different program mate-
ria] and different signal sources.
Classical music, for example, probably will benefit most from expansion at the
lower half (left side) of the scale. You might want to start at 1.1:1 or so and range
from 1.05:1 to 1.25:1 for most classical material. Using much more than this may
make alot of classical music surge unnaturally, for usually it has not been se-
verely compressed.
Popular music, especially broadcast over FM stations, can use much more
expansion much of the time —say, from 1.1:1 for material with good dynamic
range to 1.4:1 and above for material whose loudness levels hardly differ. Some-
times this music gets so compressed that full expansion by your dbx unit won't
be enough, really, but improvements still will be marked.
With jazz you might want to start somewhere between these classical and pop
settings, although aquiet and well-recorded small acoustic combo won't need
much expansion at all.
Non-stereo-LP sources (regardless of the program) often are badly constricted
in dynamic range. We mentioned pop FM above, and the same frequently is true
of video signals, from VCRs, video discs and cassettes, and TV tuners and sta-
tion transmissions in the first place. Even classical music in such media usually
sounds badly squashed, and a1.4:1 setting might be the place to begin.
CAUTIONS
Any expander can make greater demands of your power amplifier and loud-
speakers; after all, that's its job. On the other hand, because noise —rumble,
hiss, and such —are reduced (expanded downward) by an expander, the ampli-
fier is being used more efficiently. Whether agiven amp is powerful enough
isn't always easy to determine after an expander is added, for it depends on,
among other things, loudspeaker sensitivity, the distortion characteristics of the
amp, room size, and any changes in your listening-level habits.
Set for full expansion, the 4BX can expand agood recording to nearly 90 dB of
dynamic range, and to realize fully such arange requires a hefty power amp and
speakers that can take high power. Fortunately, such capacities in your compo-
nents aren't essential for full enjoyment, and as we said in Usage Noles, more
often than not you will be using your expander at less than the maximum, 1.5
setting and it will be on music that won't stress your equipment.
The point simply is that if your speakers and amplifiers cannot handle the
widest dynamic range and the expander tries to drive them to that range, over-
driving —the excessive distortion of amp clipping and/or speaker breakup —
may be the result at the higher sound levels. If this occurs with good speakers
and areasonably large power amp, it probably will be noticed only with pro-
gram material that has agood dynamic range to begin with, music that actually
doesn't need much expansion. In which case what's called for is areduction in
the expansion setting, not more-powerful equipment.

Beyond this, when you also add impact restoration to the musical program
you will want to be especially careful that your equipment doesn't generate any
unwanted transients. Pops and clicks from noisy controls and knobs (or
indeed, from ascratchy record) and the tick of the stylus setting down will all be
boosted, just like musical attacks. In other words, don't drop (he toncarm To
be safe, back ofT on impact restoration and/or turn (he volume down whenever
you change functions or begin to play arecord.

SPECIFICATIONS
Expanslon From none (I: I) to 50% (1 :1.5),
each band
Impact-restoration gain To potential +15 dB, each band
Dynamic range Greater than 105 dB
Frequency response ±0.5 dB 20 Hz -20 kHz al no
expansion
Total harmonic distortion (THD) Less than O.I5%al no expansion
Inlermodulalion distortion (IMD) Less than 0. 1% at any setting
Equivalent input noise 90 dB below IV
Attack rales Program-dependent, optimized for
each band
Release rales Lincar cxpan(jcr program-dependent,
optimized; impact restorer adjustable
(see manual)
Transition-level range (mid-band) 30 111V -300 mV
Volume-control range -40 to +10 dB
Maximum input and output levels 6V
Power requirements Sec rear of unit
Dimensions 3]/2 "hx 17 I5/I6"wx 12 l/4"d
(8.89x45.56 x31.12 cm)
Weight (shipping) "14 |bs.(6 4kg)
NOTES
1) Specifications are subject to change without notice.
2) All voltages arc rms (root-mcan-squarc).
3) Dynamic range is defined as the difference between the maximum rms
signal and A-weigh(cd noise. All noise figures arc A-Wcightcd.
4) Frequency-response figures for noise-reduction units/decoders and expand-
ers are Tor pink noise or typical music-program material.
5) The dbx 4BX is designed to be used with components whose output impe-
dance is less than or equal to 10k-ohms. It is designed to drive loads or at
least 5k-ohms in parallel with 1000 pF or less.
6) All measurements of intermodulation distortion (IMD) are made by the
SMPTE (Society of Motion Picture and Television Engineers) method, that
is, 60Hzand7kHz mixed 4: 1.
7) Units for use with line voltages other than nominal 117 VAC are available
outside the USA; contact dbx Inc.

WARRANTY and FACTORY SERVICE
All dbx products are covered by alimited warranty (warranties for products
purchased outside the USA are valid only in the country of purchase and the
USA). For details, consult you warranty/registration card or your
dealer/distributor.
The dbx Customer Service Dept. will help you use your new product. For
answers to questions and information beyond what's in this manual, write to:
dbx Inc.
71Chapel St.
Newton, Mass. 02195 USA
Attn: Customer Service
You also may call (6 17) 964-32 10 during business hours (USA Eastern lime).
The Telex is 92-2522.
Should problems arise, consult your dealer or distributor. If it becomes
necessary to have your equipment serviced at the factory, repack the unit,
including anote with adescription of the problem and the date of purchase, and
send the unit freight prepaid to the above address, marking it Attcn: Repairs.
FOR USERS IN THE UNITED KINGDOM
Important
The wires in the unit's mains lead arc coloured in accordance with the
following code:
Blue: Neutral
Bfown: Live.
As the colours of the wires in the mains lead of this apparatus may not
correspond with the coloured markings indentifying the terminals in your plug,
proceed as follows:
The blue wire must be connected to the terminal that is marked with the letter N
or coloured black;
The brown wire must be connected to the terminal that is marked with the letter
Lor coloured red.
Ensure that all terminals are securely tightened and that there are no loose
strands of wire.
Warning
This unit must be protected by a3-amp fuse, preferably using afused plug.
Also, do not remove the cover without first disconnecting the unit from the
mains supply.