DBX 786 User Manual
¨
Precision
Mic Preamp
786
Owner’s Manual
WARNING
FOR YOUR PROTECTION, PLEASE READ THE FOLLOWING:
WATER AND MOISTURE:Appliance should not be used near water (e.g. near a ba thtub, wash-
bowl, kitchen sink, laundry tub,in a wet basement, or near a swimming pool,etc). Care should
be taken so that objects do not fall and liquids are not spilled into the enclosure through openings.
POWER SOURCES: The appliance should be connected to a power supply only of the type
described in the operating instructions or as marked on the appliance.
GROUNDING OR POLARIZATION: Precautions should be taken so that the grounding or polarization means of an appliance is not defeated.
POWER CORD PROTECTION: Power supply cords should be routed so that they are not likely to
be walked on or pinched by items placed upon or against them, paying particular attention to
cords at plugs, convenience receptacles, and the point where they exit from the appliance.
SERVICING: To reduce the risk of fire or electric shock, the user should not attempt to service
the appliance beyond that described in the operating instructions. All other servicing should be
referred to qualified service personnel.
FOR UNITS EQUIPPED WITH EXTERNALLY ACCESSIBLE FUSE RECEPTACLE: Replace fuse
with same type and rating only.
MULTIPLE-INPUT VOLTAGE: This equipment may require the use of a different line cord,attachment plug,or both, depending on the available power source at installation.Connect this equipment only to the power source indicated on the equipment rear panel.To reduce the risk of fire
or electric shock, refer servicing to qualified ser vice personnel or equivalent.
SAFETY INSTRUCTIONS
NOTICE FOR CUSTOMERS IF YOUR UNIT IS EQUIPPED WITH A POWER CORD.
WARNING: THIS APPLIANCE MUST BE EARTHED.
The cores in the mains lead are coloured in accordance with the following code:
GREEN and YELLOW - Earth BLUE - Neutral BROWN - Live
As colours of the cores in the mains lead of this appliance may not correspond with
the coloured markings identifying the terminals in your plug, proceed as follows:
• The core which is coloured green and yellow must be connected to the terminal in the plug marked with the letter E, or with the earth symbol, or
coloured green, or green and yellow.
• The core which is coloured blue must be connected to the terminal marked
N or coloured black.
• The core which is coloured brown must be connected to the terminal
marked L or coloured red.
This equipment may require the use of a different line cord, attachment plug, or
both, depending on the available power source at installation. If the attachment
plug needs to be changed, refer servicing to qualified service personnel who
should refer to the table below. The green/yellow wire shall be connected directly to the unit's chassis.
WARNING: If the ground is defeated, certain fault conditions in the unit or in the
system to which it is connected can result in full line voltage between chassis and
earth ground. Severe injury or death can then result if the chassis and earth
ground are touched simultaneously.
U.K. MAINS PLUG WARNING
A
moulded mains plug that has been cut off from the cord is unsafe. Discard
the mains plug at a suitable disposal facility. NEVER UNDER ANY CIRCUM-
STANCES SHOULD YOU INSERT A DAMAGED OR CUT MAINS PLUG INTO A
13 AMP POWER SOCKET. Do not use the mains plug without the fuse cover
in place. Replacement fuse covers can be obtained from your local retailer.
Replacement fuses are 13 amps and MUST be ASTA approved to BS1362.
The symbols shown above are internationally accepted symbols that warn of
potential hazards with electrical products. The lightning flash with arrowpoint in
an equilateral triangle means that there are dangerous voltages present within the
unit. The exclamation point in an equilateral triangle indicates that it is necessary
for the user to refer to the owner’s manual.
These symbols warn that there are no user serviceable parts inside the unit. Do
not open the unit. Do not a ttempt to service the unit yourself. Refer all servicing
to qualified personnel. Opening the chassis for any reason will void the manufacturer’s warranty. Do not get the unit wet. If liquid is spilled on the unit, shut it off
immediately and take it to a dealer for service. Disconnect the unit during storms
to prevent damage.
CAUTION
ELECTROMAGNETIC COMPATIBILITY
This unit conforms to the Product Specifications noted on the Declaration of Conformity.
Operation is subject to the following two conditions:
• this device may not cause harmful interference, and
• this device must accept any interference received, including interference that
may cause undesired operation.
Operation of this unit within significant electromagnetic fields should be avoided.
• use only shielded interconnecting cables.
DECLARATION OF CONFORMITY
ManufacturerÕs Name: dbx Professional Products
ManufacturerÕs Address: 8760 S. Sandy Parkway
Sandy, Utah 84070, USA
declares that the product:
dbx 786
conforms to the following Product Specifications:
Safety: EN 60065 (1993)
IEC65 (1985) with Amendments 1, 2, 3
EMC: EN 55013 (1990)
EN 55020 (1991)
Supplementary Information:
The product herewith complies with the requirements of
the Low Voltage Directive 73/23/EEC and the EMC
Directive 89/336/EEC as amended by Directive
93/68/EEC.
dbx Professional Products
Vice-President of Engineering
8760 S. Sandy Parkway
Sandy, Utah 84070, USA
January 19, 1998
European Contact: Your Local dbx Sales and Service Office or
International Sales Office
68 Sheila Lane
Valparaiso, Indiana
46383, USA
Tel: (219) 462-0938
Fax: (219) 462-4596
RISK OF ELECTRIC SHOCK
DO NOT OPEN
ATTENTION: RISQUE DE CHOC ELECTRIQUE - NE PAS OUVRIR
WARNING: TO REDUCE THE RISK OF FIRE OR ELECTRIC
SHOCK DO NOT EXPOSE THIS EQUIPMENT TO RAIN OR MOISTURE
CONDUCTOR
L
N
E
LIVE
NEUTRAL
EARTH GND
WIRE COLOR
Normal Alt
BROWN
BLUE
GREEN/YEL
BLACK
WHITE
GREEN
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
About Mic Preamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
Understanding Console Signal Path
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Connection To Your System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Operating Controls
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Front Panel Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Rear Panel Description
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Technical Support/Factory Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Warranty
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Specifications
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Block Diagram
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Warranty Registration Card
contents
Manual
786
2
Introduction
About mic preamps
When you use a microphone to record vocals and acoustic instruments, make recordings to DAT, or
sample acoustic sounds, you want the best definition and character possible from your microphone.
Sometimes the electronics of recording or PA consoles “color” the sound of a microphone, or add noise
to the signal. In the 1960s, it became popular to use an outboard mic preamp rather than using the one
usually installed on a console. Engineers and technicians took the preamps out of famous vintage consoles and installed them in “lunch boxes”, or cases with a power supply and audio connectors. This was
done to bring the benefits of the characteristic sound, or transparency of sound, to another recording or
PA console. In years since, the business of designing stand-alone preamps has grown significantly, as
the quality and space dedicated to “stock” preamps installed in most middle-cost consoles has declined,
usually consisting of only one op-amp and some supporting transistors without a transformer. During
this period of years’ worth of development, it has been proven that better performance is inherent in
designs that incorporate larger circuit boards dedicated to the mic pre, and larger transformers on both
input and output stages, resulting in better frequency response and dynamic range, as well as better
noise specs.
There are two differing philosophies when it comes to preamp design: whether to color the sound with
the preamp, or not to color the sound with the preamp. The dbx 786 is designed with the latter philosophy in mind. We wanted to create a pristine mic pre that would carefully reveal the very subtle nuances
of any source device. To that end the 786 uses premium parts and as short a signal path as possible to
colorlessly amplify a signal to useable levels. With the increased popularity of digital recording techniques and equipment, it has become absolutely necessary to provide a preamp that operates within
these new “transparent” parameters. With the optional digital output featuring dbx TYPE IV™ Conversion
System with TSE™ (Tape Saturation Emulation), a signal may be amplified and converted to the digital
domain, ready for use in another digital medium. This allows the user to choose a mic for any application with the knowledge that the qualities of that mic as well as the essence of the recorded signal will
be preserved through the amplification and conversion process. Mic inputs on the 786 are transformer
isolated, eliminating the traditional electrolytic DC blocking caps in the signal path which, as audiophiles
know, can color the sound of a mic, especially if they are not premium quality. Another benefit of the
dbx 786 is the high Common Mode Rejection Ratio (CMMR) with its ability to reject hum and Radio
Frequency (RF) interference. With the published frequency range of 2Hz to 200kHz at -3dB, as well as
a specially designed high frequency, hard-wire bypassable EQ circuit, the 786 will effectively and cleanly amplify any signal presented at the input, while preserving the desired charactersitics of both the
source and the microphone. All coarse gain adjustments are done through relay switching, as opposed
to audio pots, keeping every possible component out of the audio signal path, in keeping with the pristine design philosophy.
Introduction
786
3
Understanding Console Signal Path
Recording and PA consoles operate at a nominal level of +4dBu, while microphones operate at a level
of typically about -50dBu. Microphone signals must be amplified to the appropriate level for a console
to process it properly. If a signal is not raised to the appropriate level, the signal will be “masked” by
the console’s internal noise, especially when one tries to increase the level of a low signal using the console fader or gain knob. All consoles have a preamp (usually labeled “Gain”) as the first stage of an input
strip, allowing the user to boost a signal to an appropriate level before the other processes of the input
strip act upon the signal (EQ, dynamics, panning, etc.) Unfortunately, console preamp circuits are often
not given enough room on the circuit board, or high-quality parts. It is for this reason that outboard preamps have become so popular. See Figure 1 below for a typical input strip’s signal flow.
Introduction
786
Figure 1: Signal flow of
Typical Console Input
Strip.
The gain, or preamp stage
occurs before all audiocritical processes act upon
the incoming signal. The
preamp’s “color” is then
processed as part of the
signal.
The only processes ahead
of the preamp stage are the
input selection stage, and
the phase reverse switch.
1/4"line
input
XLR mic
input
1/4" output
1/4" input
Send
Return
Equalization
Inserts
Mic / Line
Switch
Phase Rev.
Switch
Gain
Lo-Cut
Filter
EQ in/out
switch
- Preamp
Mute
Switch
Pan
Buss to
Main Outputs
and Group
Routing
Aux Sends
Direct Out
1/4" output
Fader
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