SpotFire PR5 phono preamp

Australian designed RIAA phono preamplifier equaliser.

The Red Roo PR5 has two phono inputs, both configured for Moving Magnet (MM) cartridges. One of these inputs may be converted to provide Moving Coil (MC) cartridge support through the installation of two Lundahl LL9226 moving coil step up transformers.

Circuit description

The phono input is amplified by two DC coupled twin-triodes configured as Constant-Current-Draw-Amplifiers (CCDA) with cathode-follower output stages. The cathode-follower of the first CCDA drives a passive RIAA equalisation network (so the RIAA characteristics are not significantly affected by tube ageing). The second CCDA stage amplifies the output of the RIAA network to line-level, and its cathode follower output stage is capable of driving low impedance loads or very long/high capacitance cables. There are only two coupling capacitors in the entire signal path (audio grade supplied). The PCB accommodates larger components, if required.

Key features

• Incredibly quiet whilst offering high gain (50dB)
• No solid state devices in the signal path
• Channel selection via front panel
• No mains power enters the equipment
• High frequency DC-DC inverter (50kHz) reduces the size of filtering components resulting in a smaller, lighter circuit
• Differential and common-mode filtering plus electromagnetic shielding ensures pure output
  
• Inputs: one MM, one MC (2 x Lundahl LL9226)
  

Record Restore vinyl cleaner sample included

• 8ml sample sufficient to clean 2 sides of 1 x 12" LP

Moving Coil cartridges exhibit very low output voltages, usually around 10% of a MM cartridge. Without a step up transformer the resulting signal does not present a satisfactory input for the phono preamp stage.

LL9226 is an MC transformer based on (and pin compatible with) Lundhal's classic LL9206, but with reduced copper resistance and level capability. The new design has resulted in an even better frequency response but still with enough no load impedance to maintain the LF bandwidth. The transformer is built up from two coils, each coil with one secondary winding surrounded by two primary windings. Advantages with this structure are excellent frequency response and high immunity to surrounding magnetic fields. All winding ends are available on the pins. Thus, the transformer can be used with a set of different turns ratios.

The Red Roo phono preamplifier is designed with interfaces to easily accept the LL9226 for moving coil cartridge support. Installation of two LL9226 transformers converts one of the two MM interfaces to MC.