Friday, June 14, 2019

"retro-2308" - portable chip headphone amp

















I have been using the Oatley K272C kit in a portable HP amp enclosure for years now. It has spent a lot of time on trains, planes and overseas. Also good use when the wife is home on weekends. After building “LeManja” I realised how good the PT2308 dedicated HP driver chip was. Feeling the K272C a little on the large and heavy side for traveling I figured I could build a smaller lighter portable amp. I wanted to retain the high performance I have been use to in the past.

Above is that build which runs on four AAA batteries for over 100hours of play. That is with cheap AAAs. With lithium’s you could double that time. The enclosure is two tone ABS with matching knob. Al lining is used on the bottom and top of the enclosure and is earthed. The amp is built on Vera-board and is extremely compact. I didn't have much room in the enclosure for the cct. brd.

All caps are NP or LESR and all hookup wiring is silver plated fine copper wire. Safe on is provide with a switched input phone socket. If there is no plug in the input socket the amp will not run. There is also an off switch. The chip is the PT2308 (Class AB) with a THD of 0.001%. The sound is solid and crisp and drives both 250ohm and 32ohm HPs with ease. Light, compact, extended battery life and great sounding. Just what I wanted. 

Thursday, May 23, 2019

"LeManja" - Hybrid Tube and chip headphone amp




LeManja comprises a tube front-end and a chip backend. The tube section is a 4SU universal design so that any 12A?7 tube can be rolled with no bias adjustments required. The chip, which drives the headphones, is a PT2308. It is a dedicated very low distortion headphone driver. Generally you would require an input cap for the chip but by keeping the Alps blue velvet pot at the front of the tube I could use the tube section's ploy output caps to block DC. This means there is only two caps in the signal path. Two mutually exclusive inputs are on the back with a four pin socket for the three exernal DC volatges and common earth.  

The PS is external and has a choked (Hammond 5H) triple Pi HT filter. The last cap at the ceramic socket for the tube is a 4uf PIO and snubbed. The tube here is a new Tung Sol 12AX7 gold pin. DC is supplied for the filaments and a separate DC 30V source for the chip. The chip max voltage is 7V so a 6V reg. holds the voltage to the chip. All caps on the chip veri-board are LESR types. A 24VAC 1A wall wart provides the initial power from the mains. All audio path wiring is via silver plated fine single strand copper wire.

The amp is absolutely dead quiet. The back ground is completely black causing all music to standout 3D style with amazing dynamics. The chassis is a Hammond Wood Cheek and is sprayed black with Epoxy Enamel paint first, an etching primer for an outstanding finish. The wood cheeks are treated with a ceda oil. 




Saturday, April 6, 2019

"Altec-Faital" - Altec 511B horns with Italian FaitalPro drivers





I never really liked horns whether commercial or DIY but after listening to a friends Altecs I thought that if ever a cheap pair of horns only came up for sale I'd buy them. A few months later refurbished Altec 511Bs came up for $500. I always felt horns a bit brittle and to tame this I added Italian FaitalPro HF108R compression drivers. The Faital compression drivers use a polymer 1" diaphragm. I built a Xover to work with the horns and added a 12" woofer in a sealed 60 ltr box to handle 1100hz and down.

The Xover crossed the horns in at 1100hz on a third order and the woofers on 2nd order. Serious padding was need on the horns to balance them against the horns and the system now at about 90db+ efficient. The horns were boxed for better appearance and shock mounted to keep vibrations from them. Also the horns are wrapped in a sound deadening foam an foil to stop ringing. 

Driven by a 180W Class D amp there is plenty of deep bass (down to 30Hz) from the woofer and great clear, very dynamic mids and treble from the horns. The most natural sound I have heard rom any system. Pleased I tried them. I have since bought a second pair with top of the range FaitalPro drivers. 






Friday, April 5, 2019

"Njira" - 180W Tripath Class D in Hammond chassis







After a number very successful builds in these compact Hammond wood cheeked chassis I thought a matching power amp with external PS maybe the perfect compliment. The amp is a hifimediy 180W T4 Tripath Class D. All the PS is on board and all high quality components. The external PS is really just a big heavy 500VAC toroid.

The amp, as with "najah", produces excellent bass is dead quiet and runs cool. I've included some of the other builds just to show my "three of a kind" hand. 

Sunday, March 31, 2019

"Calibre 834" - tube phono MM preamp






Esoteric, some years back, sold a tube phono preamp called the EAR 834p. It got excellent reviews. The schematic is available and kits based on the original schematic are plentiful. This is one of the cheaper ones by not providing tubes. I have three pairs of match 12AX7s - Mullard, Sovtek and Tung-Sol. Time to put them to use. I used the Sovteks as they had the smallest plates which is good for microphonic control. The third tube is a cathode follower for driving the next stage. GNFB is used.

The kit comes with top shelf components;- Vishay resistors and capacitors, Dales and other quality parts. The brd. is silk screened and double sided plated through. A trick I have used in the past (and used here) is to mount the passives on the bottom of the brd. and not on top. This allows the tubes to poke through the top of the preamp. The brd. is easy to assemble. Also mounting it in yet another Hammond wood cheeked light Al chassis was easy. Once more an XLR plug and socket bring in the HT and heater voltage from a choked external PS. The PS itself runs from a 24VAC 1A wall wart.

WBT silver solder was used and silver plated wire-warp wire used in the signal path. A light multi-stand hooks up the HT and heaters. The brd. is designed to utilise two separate PSs but I joined them to one. Excellent sounding preamp and well worth the cost of parts.

Friday, March 22, 2019

"Overture" - LM4780 chip power amp





Built two Audio Sector LM4780 based amp modules. Each module is a stereo amp. They can be bridged and paralleled. On a request I finally put these compact modules into a finished amp. The enclosure is a 2U 19" rack mount which gave me plenty of space to work. The buyer was happy to spend money on better parts. These included Panasonic caps on the board, 2X 22000uf Mundorf MLytic audio grade filter caps. copper, palladium and Teflon RCAs and extra heavy duty speaker posts.

The signal wiring was with fine silver plated copper wire and heavier wiring is twisted pairs of solid core cat5. All caps are snubbed and the amps is absolutely dead quiet. Should produce near 60Wpc.

Excellent sounding amp and well worth employing these compact "Overture" modules. 

Saturday, March 9, 2019

"Icon" - CV-181Z (6SN7) preamp




After a very successful build of three similar preamps I wanted to make another using some excellent and less expensive Shuguang CV-181Z (6SN7) 50 Year Treasure tubes. I bought these some years back and it was wrong not to use them. Staying with external DC supplies and using similar construction methods I used in the past "Icon" was born. But this build had to stand out, be different and not just another Grace or Elite (see earlier posts). There are some similarities; same chassis, ceramic sockets and the use of an XLR socket to bring in the two DC voltages to power the preamp.

Where the big changes occurred were mainly in the passives (of course the tubes are very different). Firstly all wiring was with twisted pair or single strand solid core Cat5. I retrained the use of WBT 4% silver solder. The output caps are Mundorf EVO Al and oil types and the resisors are metal film instead of 1W carbon film. A single 4uf PIO cap (snubbed) was used on the HT at the plate instead of one cap for each tube.

I never use a cathode resistor bypass cap (Ck) because I want some local NFB and to supress 2nd harmonic. This time Ck was employed but not with the electrolytic suggested,\ - NP caps. This raised the gain a little but has produced a much fuller and even deeper bass. Also here three inputs instead of two and the selection switch at the front and not the rear. A cheaper stepper attenuator was used rather than the premium version used in Elite. Caps, tubes and stepper were cheaper in this build.

The preamp sounds glorious. It is rich and deep in tone and bass. A big full sound with real depth. Is it better than Elite? That gets down to personal taste, regardless it is an excellent and less expensive build.