The unit uses a star earth system where all earths go to the 0V terminal using heavy cable.
Wiring and Fixing
Where connections are crucial we have soldered directly to the PCB. This forms a more reliable long-term connection than plugs. All cables are loomed and firmly cable tied to the board. The audio cable is OFC, again producing a small benefit over the long term. No external connectors are PCB mounting. This ensures long-term reliability.
The input DC decoupling capacitors are metallised polyester of 5% tolerance. The input is protected by four ultra fast diodes. The transistors are stacked to improve thermal coupling and reduce lead length. They are mounted in a turned pin socket. The gain switching uses a self-cleaning rotary switch which is screwed to the PCB, then soldered, and the switching shaft is fixed to the screwed down portion of the switch body so that there is little or no mechanical stress on the PCB. The pot (conductive plastic tracks) is also mounted on a bracket to minimise PCB stress. The filter stage uses 5% metallised polyester capacitors too, to give accurate and stable filtering. The amplifier stage is decoupled from the filter and trim stage by a 100uF bi-polar electrolytic capacitor. The trimmers are all low noise trimmers with a wide track. Throughout we have used Burr Brown OPA604s which are extremely high specification op-amps optimised for audio applications. In listening tests they have been preferred to NE5532s.
Noise and distortion
EIN is -133dB relative to a maximum output of +22dBu. Distortion is very low, with THD+N figures of around .00025% @ 1kHz (measured in the audio frequency range). Input impedance is ca 1kOhm.
-3dB point at 48kHz, roll off at 6dB/octave Flat to less than 20Hz on phantom with subsonic roll off at 6dB/octave, DC coupled on non phantom inputs.