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.

Menatzchim- C3m metal valve preamp

These petite C3m metal valves are quite inexpensive. After making a 6N7 (also metal valves) preamp which sounded positively excellent,  I decided I would make a preamp with the C3m(s) in a very attractive 1UR 19" enclosure. I could find no preamp schematics incorporating them but a fellow DIYer in the States designed one for me.

This preamp uses the same power supply I have used in the last dozen preamp builds. An external 24VAC 1A wall wart is stepped up to 250VAC by using a step down transformer backwards. Double filtration and a DC filament supply ensure a dead quiet preamp. Wire wrap wire is used for signal hookup wire and heavy duty gold plated RCAs are employed. No shielded wire or PCB is used. An Alps blue velvet pot is on the output for attenuation. Jensen Copper, paper and oil caps with silver leads are used on the output. Three sets of input RCAs are selected by two small power switches.

The sound stage is extremely deep and voice and instruments have a uncanny natural sound and feel about them. With symphonic pieces you get the unnerving feeling you are in row twelve and right in the center. I would highly recommend the construction of this preamp.

"Heavy Metal"- 6N7 metal valve preamp

In a box of very old valves, I was given recently, was a new looking pair of black metal can RCA 6N7s. These are dual triode with a common cathode. I could find no schematic for a preamp but a DIY buddy came up with a design. Construction is in a timber box. There are three switchable inputs and power is supplied by an external 18V 2.2A AC power pack I was also given. Internally 255V is developed with a linear PS and regulated 12V for the heaters in series.

An Alps blue velvet pot is fitted to the output for attenuation with a fine solid brass knob to compliment the gold right-angle RCAs, name plate and ventilation holes. Wire Warp wire is used in the signal path with PS and amp on separate brds. No PCBs or shielded wire is used. Bitumanised heavy Al foil lines section of the preamp for resonance control.

The preamp is dead silent at idle with stunning drama and dynamics. It's fast, crisp, clean and makes every track sparkle. Best preamp I have ever built, SS or valve based. Nothing comes close to this one. Nothing!