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.

Tuesday, April 10, 2018

retro-Oatley 6J6 Headphone amp (take 2)

















Yet another Oatley 6J6 tube headphone amp. See an earlier posting for more details. A different chopping board for this mount and different beige boxes for connection points. The big improvement is the toroid power transformer. Where the earlier prototype hand an EI tranni which induced hum into the OPTs this toroid version is dead quiet.

An excellent HP amp to listen to. A pretty cct. brd. to show off. Good sounding and good looks make this a winner.





Friday, March 23, 2018

"Mimic - Carbon II"

















On commission this "Delicious Red" Mimic Carbon II was born. Staying with an earlier formula of 1/4W carbon resistors and poly caps. Fashioning my own cct. brds. from Vero-brd a more compact Mimic can be made. Large polies on the output where the standard design suggests NP caps. Also polies used in the EQ network. Once more the differential PS is built on the same cct. brd. A really excellent MM phono preamp.

The enclosure is cast Al primed and sprayed Delicious Red. Epoxy paint may take some time to dry but the finished is illustrious, lush and tough. Heavy gold plated RCAs provide connection to the outside world. Silver plated wire-wrap wire is use for hookup WBT silver (4%) solder glues the few parts together. Bituminised heavy Al lines part of the base to control resonance and two 9V power the unit. Battery test point up front. As usually an OPA2134 OPAMP does the grunt work on a mil spec socket.

Probably the best sounding I have made to date. Sounded so good with my Garrott K3 I really didn't want to orphan it to its new owner. All I can do is make another.




Saturday, February 24, 2018

retro-Hiraga Le Monster

















I bought the cct. brds. for the Hiraga Le Monster over twenty years ago and populated them shortly after arrival. But that's where it stopped. An enclosure I had designed for the amp was incomplete and thus the whole project stopped. Using an off the shelf 19" 2U enclosure the amp is now complete. The PS uses a 160VAC 12-0-12 volt high current toroid with 80,000uf total of filtering\storage. Bias is 700mA with a 10A bridge and 4ohm\20W filter\limiting resistors.

All wiring is Cat5 solid core twisted pair this includes most of the power wiring. No shielded cable is used and RCAs and speaker binding post are heavy duty and high grade gold plated. A internal 10A RFI\EMI power filter ensures clean power in the amp and the enclosure is earthed and fused.

Not only does the amp look smart with an ash black stipple finish but sounds excellent. With only 8W of pure class A power efficient speakers only will deliver good volume levels. The amp has an extremely natural relaxed sound about it I've not heard from other amps. Being a DC amp bass is extended and treble clean with controlled sibilance. It is an amp which has to be heard to understand.




Tuesday, December 12, 2017

"Sanctorum" - simple headphone amp on a Cypress base




















Another simple two stage, direct linked headphone amp, Again using a RCA 6AS7 (also pictured with a 6080) and this time a E88CC Genelex Gold Lion gold pin. Ceramic sockets, choked PS and a stunning Cypress timber base. Alps Blue Velvet handles the volume levels and all Cat5 solid core copper wire hooks it all together.

Looks great sounds great and is quiet in operation. This proves once more that simple uncomplicated schematics can produce excellent sounding DIY gear and are easy to construct.



Sunday, November 26, 2017

Xover with ferrite choke and PIO cap



After experimenting with different Xover points, with my high efficiency Beyma speakers (VoXGrandious), I finally settled on 1100Hz. Also I went back to a first order Xover. I wound the choke on high frequency ferrite as a short rod. This allows the use of heavier wire and reduced turns compared with air core. The cap is a PIO Russian Military cap with another PIO across it. Total inductance is 1.16mH and total capacitance is 4.47uf into 32ohms (tweeter and limiting resistor).

VoXGrandious is a very dynamic speaker typical of most high efficiency drivers. I found that by using Ist order Xover I could hear the greatest dynamics. Also the PIO cap presented the clearest and most dynamic treble. All Cat5 wire used with heavy brass and gold binding posts.



Saturday, November 25, 2017

retro-Oatley 6J6 Headphone amp
















Its been some time since I have bread boarded an amp but this prototype headphone amp from Oatley was just to pretty to hide. I happened to have a Pete Evans chopping board unused. So as not to mark the board all cct. brds. etc. are stuck down with either double sided tape or sticky standoff. Once dried these are hard to remove but not impossible.

To zippy boxes house the 240V power cord,switch and fuse while a front box holds the headphone connection, pot and RCAs. I did not assemble the brd. it came fully assembled. My job was just to bring it all together. The valves are 6J6 in two stages. Input goes direct into the phase splitter then into the push-pull power stage. Custom wound 35K/32ohm OPTs couple the headphones. 

The PS is separate in an effort to isolate any hum. The power transformer is also custom wound and though gets very hot is within spec. The passive parts are quite ordinary which when this is released as kit, from Oatley Electronics, will keep the price down. Better parts can be substituted.

A ground plane runs through the two brds. which I have connected to mains earth along with the power tranni and OPTs. DC is provided for the filaments and two stage power filtering helps to make a quiet amp. High and low output taps help cater for different phones. The amp sounds good on two of my pairs of 32ohm headphones.