Coda 01 - Manual (Page 10)

balanced inputs on many power amplifiers. The most commonly acknowledged
advantage of this is rejection of stray noise pickup, but improvements in distortion and
bandwidth may occur also.
While many manufacturers intentionally omit recording buffers on the grounds of signal
path simplification, the potential advantages of their use were worth an investigation.
The resulting experimentation ended with the development of a circuit of such simplicity
and performance that its transparency was guaranteed. The circuit is simply a direct
coupled, class A, FET follower. Because of the FETs inherent characteristics, the buffer
provides total isolation of the recording system from the main signal. In addition, it
provides a relatively high, purely resistive load to the source, and a relatively low, purely
resistive drive to the recording system. This assures consistent stability and freedom from
interactions, including cable effects.
DESIGN PHILOSOPHY 9
The requirements of a power supply for flawless audio reproduction are straightforward
but important. The supplies in the FET PREAMPLIFIER 01 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 20,000 uF of capacitance with very low ESR and inductance
provides good passive filtering. A reference voltage is developed by delivering a
constant current to zener diodes. The resulting voltage is heavily filtered and delivered
to each stage through independent class A followers which completely decouples the
stages. The resulting non-reactive low impedance over an extremely wide bandwidth
yields a perfect power source for the individual circuits. The simplicity and absolute
stability of the supplies removes the chance of unpredictable interactions which may
occur with the more elaborate, high feedback circuity often used.
Most companies in the upper end of the audio industry use either sheet metal or formed
aluminum. By contrast, the FET PREAMPLIFIER 01 has all structural parts made of machined
extruded aluminum. The advantage of this over an all stamped chassis is that the
machined metal can be worked more precisely allowing us to work on tighter tolerances
and use PC mounted parts more easily. Moreover such a
design allows easier servicing
either for repair or for future upgrading. From the standpoint of appearance, a
machined surface can be contoured in a far more precise manner, giving the final
product a more seamless appearance.