Coda 02b operation manual Design Philosophy and Approach

DESIGN PHILOSOPHY 7

I. Design Philosophy and Approach

The circuitry utilized in the FET PREAMPLIFIER 02b 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 every facet of each design choice regardless of how small. Analytical, as well as subjective 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 explanation. With this in mind, we present here a few design highlights and concepts.

All voltage gain is derived from FETs. While careful design can yield good results from any device type, FETs consistently seem to 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 output current. In other words, it “senses” the audio signal without drawing 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 caps yields the best possible signal integrity. The DC stability of the circuit eliminates the need for any questionable DC servo circuits or 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 performance throughout the audio region and well beyond. These choices include the use of top quality dual FETS in differential configurations for all voltage gain. Because the signal in these stages is handled in a balanced manner, rejection of unwanted noise and modulation from external sources is extremely high. This rejection extends even to noise which may originate within the circuit from support circuity such as current sources. Stray RF signals are also rejected well.

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.

To accomplish balanced inputs and outputs, the FET PREAMPLIFIER 02b uses simple very high performance inverters. The most commonly acknowledged advantage of this is rejection of stray noise pickup, but improvements in distortion and bandwidth may occur also.

The FET PREAMPLIFIER 02b controls provide an off position for the recording output. This completely disconnects the recording system from the main system to prevent possible interactions which may affect sound quality.