"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.