DBX 786 Precision User Guide

A Discussion of the dbx 786 Precision
Microphone Preamplifier

White Paper

Jon Duffin
Engineer,dbx Professional Products

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dbx Professional Products
A Discussion of the dbx 786 Precision Microphone Preamplifier

Introduction
The dbx 786 Precision Microphone Preamplifier is the latest dbx Blue Series product to hit

the market. This paper discusses the design criteria, parts selection, and special circuitry
required in the 786 to make it a world class product.

Design Philosophy
To build a world class microphone preamplifier,we needed to start from the ground up and

design premium parts into our circuits. The signal path includes parts such as:gold plated
Neutrik® XLRs, Jensen® JT-16-A audio input transformers, gain switching relays with rare
earth magnets and gold contacts in a hermetically sealed nitrogen environment,
temperature stable metal-film capacitors,premium 0.1% and 1% resistors,and a proprietar y
Jensen® audio output transformer. But,it doesn’t end with the signal path, just look at a
786 and you will see: 1/4" machined aircraft aluminum front panel, machined solid
aluminum knobs,and heavy gauge stainless steel chassis.

We at dbx are always hard at work coming up with innovative circuitry to improve the
performance of our current and new products. The 786 is no exception. There are several
patented circuits and new technologies in the 786 which lead to its exceptional and
transparent performance.

And, of course,a product of this magnitude has required countless hours of listening and
beta testing in several studios. There has been well over one year’s worth of development
time put into the 786 tweaking,upgrading,listening,and improving the design.

Signal Path
The signal enters through gold-plated input XLRs, and then passes through the +48V

Phantom,Super Low Z,Pad,and Phase Invert circuitry. All of these functions are controlled
remotely by relays,allowing the signal path to be as short and clean as possible. The signal
is then bridged to the gain circuitry through a Jensen® JT-16-A microphone input
transformer,the best microphone transformer that Jensen makes. The JT-16-A affords a very
high input common-mode rejection ratio (CMRR) up to 300 volts,which means the 786 will
have no problem eliminating induced common-mode signals from sources such as RF
transmitters or SCR controlled light dimmers. Other “electronically balanced”input sta ges
which do not employ a transformer, can only reject common-mode signals of up to a few
volts. The JT-16-A also allows us to get rid of awful-sounding electrolytic capacitors which
are commonly used to isolate phantom power from the gain circuitry.

The heart of the 786 is its “M8” Microphone Preamp module. Electronics of the M8 are
potted in a zinc-aluminum alloy housing for maximum heat transfer and shielding. The M8

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A Discussion of the dbx 786 Precision Microphone Preamplifier

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contains proprietary electronics that perform the gain function of the 786 and also contains
DC servo circuitry which nullifies any DC offsets that are present,again allowing us get away
from using any DC blocking electrolytic capacitors. The 786 is entirely DC-coupled,input to
output. Gain of the M8 is controlled by an 11 step Gain switch which sequences a bank of
relays,keeping the signal path as short and as clean as possible. The bottom line is the M8
microphone preamp module amplifies signals to a clean, distortion and noise-free level.
Overall gain of the Gain control is +15dB to +65dB in 5 dB steps.

After the Gain control,the signal is fed through a Fine gain control,whose circuitry r uns of f
of high voltage +/-24 volt rails,enabling signal levels of up to +30 dBu. Translation – gobs of
headroom. The Fine gain control is sweepable to provide an additional -5 to +5 dB of gain.
Maximum overall gain of the 786 is 70 dB, which comes in handy for amplifying ambient
sounds and other low level signals without any added noise.

A unique feature of the 786 is the high frequency shelving equalizer. This equalizer is
frequency sweepable using the Spectrum control from 5 kHz to 40 kHz, while the Detail
Control is sweepable from -∞ to +16 dB. Patented EQ filters allow the shelving band to
“morph” between a typical shelving filter operation and that of a low-pass filter when the
Detail control is in the -∞ position. Slope is switchable between a gentle 6 dB/octave to a
more aggressive 12 dB/octave. A 12 dB/octave setting can let you create a sense of “air”
without stridency caused by too much boosting of upper midrange frequencies. If your
intent is to have the cleanest pre-amplified signal without the need for EQ,when the HF EQ
switch is not engaged,the EQ is hard-wire bypassed from the signal path.

And finally comes the output stage. Most equipment that incorporates a transformer in the
output stage will have a fair amount of distortion at high levels and low fr equencies.The 786
uses a patented circuit in conjunction with a proprietary oversized output transformer,
designed by Jensen, which overcomes these distortion problems. The output stage is a
discrete-based power amplifier capable of driving hundreds of feet of cable to +30 dBm
without any problems,allowing the 786 to be used on stage and drive long cables to FOH,
to a studio control room,or to a remote truck.

Power Supply
The over-designed power supply is very robust and is capable of supplying high current to

the 786 without hesitation. A toroidal power transformer is enclosed in a mu-metal can
which yields 30 dB of electrostatic and magnetic shielding. The power supply is completely
enclosed in a steel chassis and is easily removed for field service if needed.

dbx Type IV™ Conversion Output Option
When recording directly to D AT ,computer via a soundcard,digital multitrack,CDR,or various

other digital media,dbx offers an digital output option card for the 786. Featuring the dbx
Type IV™ Conversion System which captures the most dynamic range and true essence of
the signal, the card supports AES/EBU (XLR jack) or S/PDIF (RCA jack) formats. The A/D
converters have a 24-bit wordlength at a sample rate of 44.1 kHz or 48 kHz. Dithering to 16
or 20 bits is possible with TPDF and dbx SNR

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™ options. The card is capable of syncing

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A Discussion of the dbx 786 Precision Microphone Preamplifier