¨ 786 Precision Mic Preamp Owner's Manual WARNING CAUTION RISK OF ELECTRIC SHOCK DO NOT OPEN FOR YOUR PROTECTION, PLEASE READ THE FOLLOWING: WATER AND MOISTURE: Appliance should not be used near water (e.g. near a bathtub, washbowl, 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: with same type and rating only. Replace fuse A T T E N T I O N : RISQUE DE CHOC ELECTRIQUE - NE PAS OUVRIR W A R N I N G : TO REDUCE THE RISK OF FIRE OR ELECTRIC SHOCK DO NOT EXPOSE THIS EQUIPMENT TO RAIN OR MOISTURE 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 attempt 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. 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 service personnel or equivalent. 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 CIRCUMSTANCES 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. 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. 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 DECLARATION OF CONFORMITY ManufacturerÕs Name: ManufacturerÕs Address: declares that the product: dbx 786 conforms to the following Product Specifications: Safety: EN 60065 (1993) IEC65 (1985) with Amendments 1, 2, 3 EN 55013 (1990) EN 55020 (1991) dbx Professional Products 8760 S. Sandy Parkway Sandy, Utah 84070, USA 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. EMC: 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 CONDUCTOR L N LIVE NEUTRAL WIRE COLOR Normal BROWN BLUE Alt BLACK WHITE GREEN E EARTH GND GREEN/YEL 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. contents 786 Manual Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 About Mic Preamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Understanding Console Signal Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Connection To Your System Operating Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Front Panel Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Rear Panel Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Technical Support/Factory Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Warranty Registration Card Introduction 786 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 IVTM Conversion System with TSETM (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. 2 786 Introduction 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. 1/4"line input Mic / Line Switch XLR mic input Phase Rev. Switch Gain - Preamp Lo-Cut Filter Send 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" output Inserts 1/4" input Return Buss to Main Outputs and Group Routing Pan EQ in/out switch Equalization Mute Switch Fader Aux Sends 1/4" output Direct Out 3 Connection to your system 786 The 786 is the perfect partner for your microphone, providing you with two channels of audio purist quality preamplification. The 786 delivers up to 70dB of sonically transparent gain to reveal the detail and audio signature of even the most esoteric studio mics. It transforms their low level output signal to a clean high level, high current output for direct connection to the line level inputs of your mixer, DAT machine, sampler, or any other recording medium that accepts line level, analog input. 1. Bypassing a console's preamp with the 786. 1. Mount the 786 in a 2U rack space. The 786 requires a two rack-space height and a standard 19 inch rack-space width. It can ...