DESIGN PHILOSOPHY
5
The circuity used in the FET CONTROL BUFFER 05
x
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 ).
T
he 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 sacrificing 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 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 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% 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 the 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.
5.1
Design Philosophy and Approach