Design Philosophy - Coda 07x Operation Manual

DESIGN PHILOSOPHY

6
The circuitry used in the FET PREAMPLIFIER 07x is the result of an advanced and complete
design process combining innovation and prove fundamentals. This process avoids both the
limitations of total adherence to convention and the flaws resulting from inappropriate
applications of clever circuit gimmicks.
Impedance isolation and matching is derived from FETs. While careful design can yield good
results from any device type, FETs consistently have the edge in voltage gain, low noise, low
interaction, and interface applications.
FETs are inherently transconductance devices, meaning that an input voltage controls an
output current. Unlike conventional transistors, FETs have extremely high input impedance
(about 10 meg ohm - similar to vacuum tubes). The FET "senses" the audio signal without
drawing current from the source. This eliminates complex interactions with the source, allows
maximum performance from each system element, and greatly reduces the chance of cable
characteristics altering the sound. The absence of input current in FETs allows high bias
currents for linearity and speed without sacricing DC parameters.
Noise is kept low by multiple paralleling of input devices, careful selection of circuit
impedances, and pre-screening of devices. The Class A complimentary followers used to
drive the preamplifier 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 preserved into a wide range of dicult and unpredictable
loads. Variation in sound, which could occur through interactions with interconnect cables and
other system elements are thus avoided.
Until recently, perfect volume controls (attenuators) did not exist. This is because
conventional stereo potentiometers have serious channel mistracking (20% typically),
become noisy with age, wear out, and have poor resolution of level - particularly when
operated by a motor as required for remote operation. In an attempt to solve these problems,
many manufacturers have been using "switched attenuators", which are discrete, resistor
ladders built on rotary switches. But while these eliminate channel mistracking, they introduce
new problems, including limited resolution, stepping transients, and cannot be operated
remotely. And they still wear out.
They limit resolution because the most contacts available on rotary switches are thirty-one.
This requires two or three dB steps in order to get enough range out of the attenuator, which
isn't a fine enough resolution for most listeners. Switched attenuators also introduce switching
transients (a "click" or "pop") each time they change positions.
This noise comes from two sources. The first is mechanical noise from the switch itself as its
ball-detent mechanism moves from detent to detent. But a more troublesome source is the
voltage difference caused by the change in the musical waveform during the time it takes the
switch to move from one position to the next. The greater the voltage difference, the louder
the transient.