6AS7 headphone amp with Sophia Mesh Plate 274B rec. tube

The power supply and amp section are on two die-cast Al bases. A Sophia Mesh Plate Princess 274B rec. tube does the rectification. The PS has its own power filter to reduce EMI\RFI and the mains cable is choked. The power tranni is Hammond and the 20H choke is Hammond also and these are both isolated from the chassis by rubber grommets to control vibration. The 6AS7 power tube is NOS Winged "C" Svetlana and a NOS Tesla E88CC gold pin for the driver and is directly coupled. XLRs connect the HT and AC filament current to the amp section. A single 6.5mm input upfront with dual headphone output. Attenuation is provided by an Alps Blue Velvet pot. Both enclosures are dampened with bitumanised Al foil and the paint is Epoxy Enamel. The attenuator knob is 4 ounces of solid brass.

The Sophia Mesh Plate 274B rec. tube and a 4uf PIO Russian  filter cap start the filtering network with seven stages of filtering and storage in all. I believe the great sound of this amp is due to the Sophia tube and the PIO cap. There is no PCBs or shielded wire used in this amp to dull the sound. Wiring is all point-2-point and component-2-componet. Fine silver plated wire warp wire carries all the audio. The only caps in the amp are the output caps. All PS caps are snubbed with polypropylene caps and the output caps with Russian PIO caps.WBT silver solder is used throughout.

The amp is is absolutely dead quiet. Through my Audio-Technica ATH700 Air headphones music is like I have never heard before. Harmonics are rich and deep, detail is fine grained and dynamics unlimited.This may well be the greatest piece of audio gear I have ever made.

Inexpensive MM\MC phono preamp MKII

For a 1000th the price of some SS phono preamps on the market these days I'm very happy with the second Bruce (of Oddwatt's fame) deisgn named "Mimic" This can be built in an afternoon and even with batteries cost about $40. Mine runs on 3 X 9V batteries wich should give 50 to 100 hours play with no change. The sound stage is detailed and expansive and instruments have there own place. Over-all tone is flat and natural.

Some better caps were used in this build over the earlier one. WBT silver solder, a dampened enclosure (bitumanised with Al foil)  and wire wrap wire hookup used throughout. A simple off-the-shelf enclosure keeps this preamp cheap. But sound is rich.

Mains DC blocker and RFI\EMI power filter

In Australia our mains supply has gone from clean power to not so clean power. For a number of years now I have built power filters into my power amps but not in media players or preamps.  DC on the mains can cause many problems with audio gear and particularly audio gear using a toroid transformer a DC mains blocker may improve sonic performance. This simple DC blocker also incorporates an RFI\EMI power filter. A choke on the input lead and 10A slow blow fuse round out a simple and neat solution to "dirty" mains power.

OPAMP phono preamp with high RIAA EQ accuracy

The creator of the oddawtt series of tube amps and kits has developed an OPAMP based, highly accurate, phono preamp. In my build you can switch between, MM and MC and in MC mode switch between 50 and 100ohms load. This MC loading suites my Ortofon Rondo Blue and I can switch between the two loadings on the fly to gauge the effect. I run my build on 4 X 9V batteries and on a 1.75" square single prototype cct. brd. I have a simple resistive network to provide +/- 18V. A couple of low ESR 220uf caps ensure smooth power to the chip an OPA2134. A battery test point is at the front.

The cap compliment is polies and MKT with a NP electro on the output. Better caps could have been used but a wanted a neat tight build. The preamp came out looking attractive and most of all sounds glorious. For an inexpensive build with great flexibility (two switchable TT inputs for my Rega (MC) and AT (MM) TTs)this would be hard to beat. Tested RIAA EQ tracking is better than 0.25db!

"Abstinence" - 6EM7 power amplifier

Yet another 6EM7 power amplifier. This time some expensive and different parts were use in construction and a different internal layout was employed. The power transformer and choke are isolated top and bottom by rubber grommets. This ensures no AC vibration reaches the tubes.The initial PS filtering is preceded with a on-board power filter, a 4uf\400V Russian PIO cap and 20H\100mA Hammond choke. The HT is supplied by a Hammond power transformer and a very expensive Sophia Electric mesh plate rectifier tube. The second stage of filtering is 47uf and 100uf electros. with a large limiting resistor ensure the amp is totally free from of hum and noise. All PS caps and cathode bypass caps are snubbed.

This amp sounds better than the last and I think it may be the Sylvania NOS 6EM7 tubes. Russian Teflon caps are used in the inter-stage coupling  and once more large NP caps for the bypass on the power tubes. For the first time I have used Chinese made H&K 15W OPTs, can't fault them. The look of the amp is almost under-stated "sexy" and the sound the same - sexy and natural.

"Number 9" - Tube and Chip 80W Integrated

From a $48 printed cct. brd. and a bag of parts I fashioned this very functional looking 80W amplifier. The parts include a 6N3 valve (preamp), two TDA7294 high performance power chips and all the parts required to complete the cct. brd. You have to source the rest and that's where the expense starts.

The amp has been constructed in attractive charcoal grey 2U 19" rack enclosure. Three sets of switchable RCAs on the back and 4oz volume knob and Alps pot to control loudness. Speaker protection is included on the PCB and there is a delay complete with flashing led before the amp is ready for use. Very clean sound, holographic presentation.

"Arabella" - 6EM7 amplifier on Acacia

Inspired to create by the sight of a beautiful Acacia chopping board I constructed this 6EM7 power amp. To hide the wiring and smaller components underneath a shallow Al skirt was fashioned. Larger power supply components are housed in a small enclosure at the rear which supports RCAs and binding posts. The amp sits on three polished brass cones.

  The OPTs are Edcor GXSE, power transformer and 20H choke, Hammond. The valves are NOS AWA 6EM7 from Holland and the rectifier tube, 5Y3. Arabella only produces a few watts of power but has a clear and open sound. The amp is dead quiet due to the large choke and two stage filtering. This is not only a beautiful looking amp but a beautiful sounding amp.

Electronic Crossovers for sub-woofers

I had never built an electronic crossover (EXover) before but do understand how they work. This EXover would be simple as it was to just work with stereo sub-woofers and reject frequencies above 60Hz. The single cct. brd. contains the differential PS required to power the LM833 OPAMPs left over from other projects. Due to the fact that this EXover was as simple as an OPAMP preamp a single cct. brd. solution was easy.

The cct. brd. is vero-board or strip board and fitted into a standard enclosure I use for headphone amps. The input\output RCAs were mounted on the top of the enclosure and power socket on the rear. A small ex-power drill charger suppliers 18V @ 400mA for the EXover which only requires a very small amount of current.  As stated above this is converted to +\- 9V by the power supply on the cct. brd.

The design was meant to produce a rollover at 60Hz but under test the rollover occurs at 52Hz. The slope is only -12db\octave. This gentle roll-off allows lower mid frequencies into the sub-woofers drivers but also allows for easy integration into my 2-ways speakers. The EXover performs well with inexpensive parts and is dead quiet in operation.

retro-34UL - Signature EL34

This amp is my new EL34 the "retro-34UL", Signature EL34 amp. On early listening the amp sounds fast, crisp and SS like. A very natural sounding amp. Valve compliment is NOS Svetlana EL34 and Svetlana 6N1P. Edcor GXSE 6omh ultra-linear OPTs and 10H choke and power tranni are Hammond. Inter-stage caps are Jensen copper, paper and oil with pure silver leads. Biploar caps are used as bypass caps on the EL34s with ceramic sockets for the audio tubes. Rectifier valve is a Golden Dragon 5U4. OPTs are rubber mounted.

Top plate is ABS plastic and base Australian Spotted Gum (oiled). Feet are filled with lead sinkers and rubber glue, silver solder and and silver plated wire used throughout. Topology is Single-end Ultra-linear.

retro-thermionic\Oatley K272C headphone amplifier

Oatley has continued to improve their headphone amp\preamp with the release of the K272C. I had something to do with this progression consulting directly with Oatley. In fact there was a K272B between the K272A and the “C”. Here are two K272C headphone amps, assembled from the new kits which sell for $40 from Oatley.

The improvements over the original K272 are:

• ·         18V working

• ·         Separate filament and B+ batteries

• ·         Filament and B+ battery health LED indicators

• ·         OPA2134 driver chip

The kits are very easy to assemble with the excellent PCBs being screen printed and solder masked with through hole plated. The sound has improved with better bass response.

In this build I Al lined the ABS compact enclosures and used switched 6.5 stereo phone jacks. Only with both source and headphone plugs inserted and the press button switched “on” at the rear will power flow. Wire-wrap wire has been used in the signal path and the battery health LEDs have been brought out to the front of the enclosure. The sound appears to have better bass than previous models but the tubes will ring if struck.

Stereo 12" sub-woofers

These 60ltr sub-woofers were built to complement my 2-way speakers "VoXDeuX". The drivers are 12" polycones with a free air resonance of 25hz. In the box they go down to 31hz where they are very active. The drivers are very efficient and are being powered by a compact 50W Class D amplifier. A stereo feed is taken from the preamp and passed through an electronic filter. The filter has a -3db point of 52hz and but employs only a -12db slope.

The internals of the box comprise of six vertical braces. Bitumanised heavy Al foil covers most surfaces. Pockets and chambers are formed at the rear by the bracing which carries additional dampening. These are then covered by a single layer of damping. This forms a number of air and dampening layers. With a gentle roll-off and the ability to control the volume of the sub-woofer's integration with VoXDeuX has been reasonably easy. The sub-woofers have been called "VoXItBassO" meaning voiced in the bass region.

Oatley K272C headphone amp in a tin can

Oatley has released a new version of their popular K272 headphone amp\preamp the K272C. In this version there is separate filament batteries (2 X AA) two LEDs to indicate battery health and 18V working throughout. Further more the driver chip is now the high performance OPA2134.

I was given an assembled prototype for evaluation but had no case to mount it in. My wife gave my a very nice tin can with a pop off lid and suggested I build an amp in it. I mounted the cct. brd. on a varnish ply plinth and slid the whole assembly into the can. To use the preamp, pop the lid, plug in and turn on.

This is the warmest sounding preamp to date. At this point I have only had a short listen but will add to these notes once I have spent some time with the preamp. The kit should be available from Oatley by the end of April 2013. Cost has not been finalised but I would assume somewhere around $30 to $40.

"Intermezzo" - 1920 UX-171-A power amp

This amp uses the UX-171-A 1920s tube (matched pairs) which are rare and produce only 1/2W. There are no resistors or capacitors in the audio cct. a Hammond 124B nickel core inter-stage trannie couples to the power tube. Edcor 10W OPTs in the metal case. Bias is provided by 3 X 9V batteries and 230VHT by two step-down trannies on in reverse. The rec tube is a very old "80". PS filtering under the top plate (ply timber). The base will house the power trannies and is a 1940s wooden pot stand. The amp sounds excellent but I have other tubes (Silver Dragon) which sound better with more power.

I have named the amp "Intermezzo". You build an amp like this because you want to use very old and excellent sounding tubes. This detracts in no way from the overall sound which is transparent, soft sounding and musical. Easy to listen to and when matched to a pair if speakers like the Fostex FE206 based dual chambered boxes you have more than enough volume and a unique setup.

"VoXDeuX" - Inexpensive two-way speakers

Though the components in this pair of speakers are quite inexpensive and bought from the local electronics store, their performance is anything bit "cheap". They compare or even outperform my $AU1,300 KEF iQ30s with their Al cones and teardrop shape. The woofer\mid is 5" shielded paper cone driver and the tweeter a 1" silk dampened driver. A 2nd order Linkwitz-Riley Xover is used to split the incoming signal at 3KHz. Bi-Wiring is supported from the external Xover right through to the speakers. Chokes are air core and caps high voltage metalized polypropylene. Speaker efficiency is about 87db.

Under test these small boxes went down to 50Hz (in room) with still good bass energy at 40Hz. The port on the back is only a few centimeters wide and deep but becomes very active from 40 to 50Hz. Voice sounds natural and highs mellow and extended. Ideal for classical music or laid-back jazz or string vocal tracks.

The finish is something I have developed myself. The stippling effect comes from using a roller to roll on a heavy paint layer then tease out the stippling with many semi-dry roller applications. The finish is light and acoustically non-reflective adding to the sound quality of these speakers.

Oatley K272B - "6418-B" headphone amplifier

Oatley Electronics of NSW Australia has been very supportive of the DIY audio community. Their release of the sub miniature valve, the JAN 6418, in a preamp\headphone amp caused quite a storm.  The K272 was released as a kit and worked from a single 9V battery. My post of the project on the Diyaudioprojets forum caused the order of these kits from Oatley to skyrocket.  Working with Oatley many other inexpensive audio kits have been developed and released. The latest is the K272B the third version of the original K272.

Here I have used an early prototype, pre-assembled by Oatley, to make a portable tube based headphone amp.  

I recently  made two CMoy style, OPA2134 based, HP amps but now prefer the better sounding, harmonically rich, K272B build.  Two 9V batteries are used in parallel to prolong battery life and switching phone jacks, power switch and “on” LED ensures it is difficult to accidentally leave the amp on. The enclosure is impact resistant ABS which is heavy Al foil lined. Volume control is via your player and the “6418-B” will drive low impedance phones.

Where the CMoy style, chip based, HP amps are clean, clear, bright and clinical, the “6418-B” has a much richer, more harmonically complex sound and a smoother all over delivery. Really it all gets down to taste.  I would recommend building the K272B when released around early February 2013. I suggest you upgrade the Oatley passive parts and chip socket to better quality parts.