Saturday, June 30, 2018

"Retro-TubePhono" - MM tube phono preamp

Yet another Radi0Kit K318. The kit comes from Greece and has some good quality parts; Wima, Dale, ceramic sockets. polyprolene caps and some JJ 12AX7. I swapped the JJs out for some Psvane 12AU7 "T" series MKII. If there is any PS noise the A7s will bring it up. The Psvanes are matched and more expensive and provide a much smoother sweeter sound. The JJs can be a touch edgy. 

External PS from an earlier build in a Hammond extruded Al anodised enclosure. A 24V 1A wall wart provides the AC. Quality gold plated RCAs and fine silver plated wire-wrap wire is used for audio internal hookup plus silver solder make this another "nice" build. 

The larger enclosure is a Jaycar steel and Al project box. From the AC wall wart a step down tranni is used backward to regenerate 280V for the HT. Double Pi filtering is used with nearly 400uf of filter caps. The 12V filament voltage is rectified and filtered for DC on the filaments.

At the users house (on a very nice VPI TT and Denon Karat cartridge) an earlier Le Pacific MC step-up is used before the MM tube pre. The sound was "club". Smooth flowing, solid and with great image. Bass and treble sounded, once more; "club". Most enjoyable and highly believable. 

Tuesday, June 12, 2018

Lagoon - Two stage 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.