Coda FET Phono Preamplifier 06X - Manual (Page 8)

dESIgN PHILOSOPHy 8
The circuitry utilized in your phono preamplifier is the result of an advanced and complete
design process combining innovation and proven fundamentals. This process avoids both
the limitations of total adherence to convention and the flaws resulting from inappropriate
application of clever circuit gimmicks. our approach demands painstaking consideration
of ever facet of each design choice regardless of how small. analytical, as well as subjec-
tive techniques are all applied in an open-minded fashion with “no compromise” musical
perfection as the goal. The resulting refinement of the product escapes simple explana-
tion. with this in mind we present here a few design highlights and concepts.
Primary voltage gain is accomplished with FeTs. while careful design can yield good re-
sults from any device type, FeTs consistently have the edge in voltage gain and interface
applications. This is borne out by superior sonic qualities observed in subjective testing. FeTs
are inherently transconductance devices, meaning that an input voltage controls an out-
put current. In other words, it “senses” the audio signal without drawing substantial current
from the source to provide an output. This eliminates complex interactions with the source
allowing maximum performance from each system element and greatly reducing the
chance of cable characteristics altering the sound. The absence of input current in FeTs
also allows high bias currents for linearity and speed without sacrificing dC parameters.
while excellent capacitors for coupling use exist, there can be no doubt that a signal
path free of coupling capacitors yields the best possible signal integrity. The dC stability
of the circuit eliminates the need for any questionable dC servo circuits or input or ouput
coupling capacitors. we make no aC compromises for dC performance, however. our
key design choices provide not only inherent dC stability but also optimized aC pefor-
mance throughout the audio region and well beyond. These choices include the use of
top quality dual FeTs in a differential configurations. Because the signal in these stages is
handled in a balanced manner, rejection of unwanted noise and modulation from ex-
ternal sources is extremely high. This rejection extends even to noise which may originate
within the circuit from support circuitry such as current sources. stray rF signals are also
rejected well.
since the necessary equalization requires the use of capacitors, we use only high grade
film-type capacitors. These and other equalization components were carefully chosen to
maintain response to within .2dB of rIaa standards. an additional subsonic rolloff at 14hz
serves to reduce IM distortion. Noise is kept low by multiple parallelling of input devices,
careful selection of circuit impedences, and pre-screening of devices.
The class-a complimentary followers used to drive the preamp output are of such speed,
linearity and low output impedance that no feedback correction is required or used. The
advantage of this is that the circuit’s perfect stability and transient response are ppre-
served into a wide range of difficult and unpredictable loads. variation in sound which
could occur through interactions with interconnect cables and other system elements are
thus avoided. a simple, very high performancei nverter provides a fully balanced output
on the phono preamplifier which allows the user to take advantage of the balanced
inputs on many preamplifiers. The most commonly acknowledged advantage of this is
rejection of stray noise pickup, but improvements in distortion and bandwidth may occur
also.
The requirements of a power supply for flawless audio reproduction are straightforward
but important. The supplies in the 06x take a very direct approach to high performance.
First, a top quality shielded toroid transformer with plenty of reserve current capability is
used. The shielding eliminates strong fields which could induce hum into sensitive circuitry.
about 25,000 uF of capacitance iwth very low esr and inductance provides good pas-
sive filtering. a reference voltage is developed by delivering a constant current to zener