Tuesday, June 12, 2018

Lagoon - Two stge tube preamp with cathode follower






On request from an owner of one of my Parley preamps I build this cathode follower. Parley had been used as a tube preamp to drive some large SS amps. Parley had a tube frontend with SS backend to drive into lower impedances. This new build has a 4SU (Single stage universal tube preamplifier) frontend with a cathode follower backend. Now an all tube preamp capable of driving low impedance SS amps. You still have the ability to swap any 12A?7 tube at the frontend. A 12AU7 has been selected as the logical driver tube.

The 2U enclosure is steel with Al front and back plates. The finish is a fine black stipple. Six selectable RCAs are on the back with two small power switches to select your input. Power switch is also on the rear. The enclosure is fused and earthed. The power cord has a ferrite choke and a RFI\EMI internal dedicated power filter helps provide clean mains AC. A 3A over-size EI power tranni supplies 24V for rectification and breakdown for filaments and to drive a stepdown tranni backward for HT. 

The PS uses 1A diode bridges for HT and DC filament voltage. The bridges are snubbed. HT filtration is a double Pi the first with two 100uf caps and a 5H Hammond choke the second Pi with a small limiting resistor and a further 200uf of smoothing. DC for filament is single Pi and limiting with 10W resistors. The preamp is dead quiet. Mains earth, filament and HT negative are all connected. 

An Alps Blue Velvet 250K audio pot and 4oz solid brass gold plated knob provide attenuation. The tube compliment is Genalex Gold Lion gold pin 12AX7 preamp tube and Psvane's top of the range 12AU7 "T" series MKII gold pin. Sockets are on a turret board and are ceramic with gold pin sockets. Russian PIO military metal-over-glass caps are used in the inter-stage and the output. Metal film resistors use on the amp board. All signal path wiring is silver plated unshielded fine copper wire wrap wire. All soldering is WBT 4% silver solder. A low glow 3mm retro-yellow led indicates on. 

On only a brief listen the sound was bright and clear and sounded very balanced with good drive. This preamp will require about 50hours burn in so how it sounds in the end I will never know. I would imagine the preamp to mellow a little. With easy tube changes with the preamp tube the final sound is quite tailorable. Looks smart and sounds very smart.

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