Coda 06x Phono Stage

Design Description

The circuitry utilized in the 06x phono preamplifier 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 ever 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.

Primary voltage gain is accomplished with FETs. While careful design can yield good results from any device type, FETs consistently 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 substantial 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 capacitors yields the best possible signal integrity. The DC stability of the circuit eliminates the need for any questionable DC servo circuits or input or output 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 a differential configurations. 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 circuitry such as current sources. Stray RF signals are also rejected well.

Since the necessary equalization requires the use of capacitors, we use only high grade film- type capacitors. These and other equalization components were carefully chosen to maintain response to within .2dB of RIAA standards. An additional subsonic rolloff at 14Hz serves to reduce IM distortion. 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 difficult and unpredictable loads. Variation in sound which could occur through interactions with interconnect cables and other system elements are thus avoided. A simple, very high performance inverter provides a fully balanced output on the phono preamplifier which allows the user to take advantage of the balanced inputs on many preamplifiers. The most commonly acknowledged advantage of this is rejection of stray noise pickup, but improvements in distortion and bandwidth may occur also.

The requirements of a power supply for flawless audio reproduction are straightforward but important. The supplies in the 06x 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 25,000 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 circuitry often used in similar equipment.

Most companies in the upper end of the audio industry use either sheet metal or formed aluminum chassis. By contrast, the 06x 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 upgrades. From the standpoint of appearance a machined surface can be contoured in a far more pleasant manner giving the final product a more seamless appearance.

The amplifier’s chassis is made from heavy-gauge steel with a half-inch machined aluminum faceplate. All exterior metal parts are anodized or powder coated for durability.

Printed circuit boards are fiberglass epoxy with gold plating over a tin/nickel barrier. The gold layer will not corrode, while the barrier layer prevents the gold from migrating to the lower copper layer.

All resistors are precision metal film; 1% tolerance for 1/4-watt and 5% tolerance for 1-watt.

Capacitors have been eliminated wherever possible. No electrolytic capacitors are used except in the power supply, where several high-capacitance electrolytics provide outstanding filtering of the supply output.

All semiconductor devices are of very high grade. Voltage gain is accomplished with an extremely high-quality, matched dual FET, chosen for it’s exceptionally low noise characteristics.

All audio input and output connector contacts are gold-plated, and XLR receptacles are manufactured by Neutrik of Switzerland. Wire is used as little as possible in the signal path – only to connect the RCA jacks and speaker terminals to the circuit board – and what is used is 141-strand silver-plated copper with silicone insulation.