Thursday, September 5, 2019

Lagoon Take 3

Finally finished this JJ12AU7 preamp - SRPP. Finish is blue hammer tone on a Hammond wood cheek chassis. The cct. is constructed on a turret brd. with metal film resistors and Russian PIO caps. Pot is Alps blue velvet with 4oz solid brass gold plated knob. External PS supplies 260V HT and DC filament current. Two selectable inputs. Sound is very fast and snappy with good bass.

Sunday, August 25, 2019

"Gothic" - all FET headphone amp

Oatley Electronics Australia had been selling headphone (HP) kits for some years. These were based around the JAN6418 subminiature pentode and PT2308 HP driver chip. I've built and sold quite a few. Branching out from the normal this all FET HP amp has a two jFET input stage then two small power FETs arranged in push-pull to drive the HPs. A unique design but after  a few prototype brds. were produced the amp was dropped.

I got two of the brds, an "A" and "B". The "B" had better layout and LEDs were added. From the brd. I made "Gothic" so named because it is all black and quiet weighty. The compact cast Al enclosure was prepped, etch primed then sprayed Epoxy Enamel black gloss. RCAs on the rear with 6.5mm phone input. Front has volume, on LED phone out and ON/off switch. Quite simple.

There are two external power supplies. A compact SMPS of 24V @1A and a larger linear PS of 20V @ 2A. I woulkd have thought the HP amp to sound better with the linear PS but the SMPS may be the better sounding. Either PS is dead quiet and the amp works extremely well. Though in Class A the amp runs very cool. The small power FETs which do the driving are mounted on small heatsink sections of the cct. brd.

The overall sound is rock solid and weighty. You get the feel the music goes way back into the black background. With my Sennheiser 700 there may be too much detail but with the Technica AIRs there is a better balance. Overall an excellent HP amp.

Tuesday, July 30, 2019

"Overture" - LM4780 chip power amp

Originally this 1U rack mount enclosure was intended for another chip amp which really didn't work well.  Now with a LM4780 cct. brd. and repurposed this low profile amp really sings. Currently being used around the TV system and controlled by a single stage tube preamp. Lots of power (60W) into high efficiency single driver Mark Audio speakers and backed with a 175W sub-woofer. Pleased wih the outcome and the repurposing of the chassis and robust PS. Under test in the second image. 

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.