6
DESIGN PHILOSOPHY
The circuity used in the FET PREAMPLIFIER 07x is the result of an advanced and com-
plete 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 con-
trols 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 sacri-
ficing 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
usedto drive the preamp output are of such speed, linearity, and low output imped-
ance 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 difficult 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% typical-
ly),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 chan-
nel 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 attenua-
tors 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.