"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!

4S Universal valve preamp in a Tea box (4SUTea)

All housed in a Twinnings timber Tea Box: Bitumanised Al foil controls enclosure resonance and Russian paper in oil caps link the modulated HT to the real world. Silver solder is used throughout. Heavy duty high grade gold plated RCAs grace the back and all is powered by a 24V 1A plug pack. There are two switchable inputs, Alps blue velvet pot and black chromed solid brass knob.


Power supply and amp reside on a single piece of vero (strip) board. No PCB or shielded wire is used. Any 12A?? tube can be inserted with no components changes. Sound is big, bold and fast with a Mullard 12AX7 in place.

retro-34UL power amp

In an effort to use-up so many spare parts I have I put together another EL34/KT88 amp. Part of the inspiration to build the amp came about when I was given four 1996 NOS Svetlana EL34s. These bare the old Svatlana logo. Also I had dismantled a bread board amp which had a large power tranni and choke which were perfect for this build.

The base is Australian Spotted Gum and the power tranni and choke are Hammond. The OPTs are Edcor 15W open frame types, reside in the base and are shock mounted. The feet are filled with lead sinkers for resonance control. A CLCRC filter network ensures the amp performs dead quietly and has an extremely high head room. Frequency response is to 40Khz and power is 5W RMS into 8ohms. Russian paper in oil caps are used in the inter-stage and bipolar caps and low ESR caps are used in the cathode bypass. Silver plated wire-wrap wire hooks up the gold plated RCAs to the nearby 6N1P and connects  the secondary of the OPTs to the binding post.

The paint work is epoxy enamel on 2mm high grade Al plate. Silver solder is use in the driver stage. Component-2-componet wiring is used to ensure signal path length is kept to a minimum. There is no shielded wire or PCBs to degrade the sound. This is a fast amp with a huge sound stage and a rich lush EL34 sound.

This amp has been built to sell.