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.