Tuesday, December 20, 2011

Two Stage Tube Phono Preamp

Tubecad, in a tutorial article, described a number of tube phono preamps. This is one of the simpler Tubecad designs and uses two 12AX7s in a two stage, passive RIAA equalisation, configuration. Golden Dragon tubes and quality tube rings are used here. The preamp employs a 24VAC wall wart which is step up to 260V and then well filtered. The HT filter is in two stages using a C-R-C-R-C network. The heater voltage is also rectified and filtered. There is a MM/MC switch to switch between 47K and 100 ohm loading. Gain is high.

All construction is on veroboard (strip board) and silver soldered. The four coupling capacitors are the Russian paper-in-oil (PIO) and the RIAA equalisation circuit uses Sprague “orange drops”. The enclosures are Hammond anodized Al cases. A separate power supply and substantial filtering helps keep noise out of the sensitive preamp cct. The sound is bright and very liquid. The preamp appears to work extremely well with pop, rock and jazz and most classical music.

Wednesday, November 2, 2011

"Paris" - 6EM7 Dual Mono 2W Integrated amp.

In this construction, "Paris", is a dual mono build. The preamp tube at the front of the amp is the only shared tube. Each channel has its own power transformer, 5Y3 rectifier tube, choke and 6EM7 tube. The transformers are rubber mounted, sockets are ceramic and an Alps Blue Velvet pot attenuates the output from the preamp section. A power filter and Edcor 15W OPTs in the base. A black chrome 4oz brass knob adds to the look and feel of the amp. The colour is called Sungold but some may say yellow. Paris contains two signal caps sets which can be switched between.

Two selectable inputs grace the back plate. This is a true integrated amp with a 4S Universal preamp section. Any 12A?? tube can be used for the preamp allowing the user to customise the "tone" of the amp. Sprague poly caps are used in the signal path. Power output is about 2.5W. Only sensitive speakers should be used. It has the best mid-range detail I have ever heard in any amp.

Monday, September 12, 2011

Universal - 4S preamp MKIV

This is the forth of these Super Simple Single Stage (4S) valve preamps. This was not designed by me but for me and has come after building three others and posting results on the diyaudioprojects forum. The others (see earlier posts) were specifically designed for the 12AU7 dual troide. This breaks away from that theme and will work with any of 12Axx available. This includes: 12AX7, 12AU7, 12AT7 and 12AY7.

Once more there is only one cct. brd. (this time strip board or veraboard) and holds both the PS and the amp itself. Expensive Solen caps were used in The Black but in this build MKP caps were used and less expensive parst and construction all round. With the right tube in place its sound is very close to The Black.

Housed in a Hammond Al case and quite sleek looking it is powered by the same 24VAC wall wart power pack the others are. It is quiet and has extensive gain. After some listening and comparing its performance across a number of different valves the Universal delivers a detailed sound with solid bass and airy mids and highs. The best valve tested with it so far is the 12AU7 Golden Dragon. Very easy to listen to all day!

Saturday, July 23, 2011

The Black - 4S preamp MKIII

This is the third of the 4S preamps I have built. It has been dubbed "The Black". This one carries quite a few schematic changes. No feed back from the plate to the grid with only an un-bypassed cathode resistor to act as feed back. Solen 680nf output caps, additional snubbing on the cct. brd. and different value blocking, load and cathode resistors.

A new Golden Dragon 12AU7 appears to work the best in this preamp which has good bass and a very balanced mid and high frequency repsonse. Detail and clarity abound. The gain with this build is very high, not something I have had with the previous builds. This is the best build so far.

Saturday, June 25, 2011

Single Stage Tube Preamp MKII

Shown here is the second build of the simple but good sounding preamp. In this version (MKII) two values were changed in the cuircuit which has caused a sharp drop in distortion (0.2%) and better performance from the whole preamp. Internally the build is neater and tighter. With even shorter signal path than in the previous build. The finish is beaten copper.

Again single brd. construction prevailed with no PCB. All component-2-component wiring using silver solder and wire wrap wire to connect to the pot and RCAs. An Alps Blue Velvet pot is used on the output with a solid brass black chrome knob. Any 12AU7 can be rolled to the provide different timbres and tone. The cast Al case is damped with tar/Al heavy foil and thick silicon feet fit-off this fine preamp.

Friday, June 17, 2011

Single Stage Tube preamp

This simple single stage preamp requires only a single 12AU7, a 24V wall wart and the balance of a good stereo system. The construction consists of a single board with PS at one end and a tube at the other, but how it sounds beguiles its simplicity. The tube is a 12AU7, blue velvet attenuator with 4oz solid brass knob on the output and all component-2-component wiring. No PCB to dull the sound and silver solder used throughout. Frequency repsonse is from 10hz to 130Khz (-3db points).

A small amount of negative feed back is used in a local sense. The way this is applied contributes to the excellent sonic signature. The detail, warmth and deep sound stage exceed most of the preamps I have ever built both in the tube and SS camps. This one was commissioned and will be employed in an elaborate theatre system to add warmth and detail to the centre channel. But in a 2 channel system this absurdly rudimentary preamp really shines. And now one for me.

Saturday, May 21, 2011

The Silver Dragon rises

The finished Shiraz-Silver Dragon I built for a member of the Melbourne Audio club sounds as good as it look. It is what you would expect from an amp made from very expensive parts. It has taken almost nine months to gather the parts and assemble the amp. The final finishing touch came in the form of a Golden Dragon 5U4G rectifier tube. A great touch with the tube supporting a large dragon on the front.

Feature components of this amp are: TJ Full Music 300BSE limited addition tubes, 50 Year Treasure 6SN7, Mundorf M-Tube poly filter caps, ceramic sockets, silver hookup wire in the audio path, Audio Note Silver and Mylar inter-stage capacitors, 2mm Al plate, 30W Hammond OPTs, 200mA 10H Hammond choke, regulated DC to the heaters and over-size speaker binding posts.

This amp has extended, deep and very controlled bass, rich fully fleshed mids and bright crisp highs. The sound stage is very well defined but is not etched. The amp appears tonally balance across the sound spectrum. It does everything right and nothing wrong. It would be the first amp I have made I would never want to see go. Ironically it must.

Monday, March 28, 2011

Shiraz-Silver Dragon 300B

Above is the most expensive amp I have ever made. This is only the amp section. The PS is separate. The parts alone came to AU$3.5K

The amp has been designed from the ground up by an American Electronics engineer. The components were selected by the future owner of the amp and the task of acquiring those components and assembling the amps fell to me. The power transformer was wound to our specifications locally in southern Victoria and the base made from Mirbeu timber.

The paint on the top and bottom plates is "Woodlands Grey" epoxy enamel and has been selected to compliment and contrast against the black OPTs. Gold coloured brass cones support the amp on thick brass pads and add to the look and performance of the amp as a whole. The chrome enhancements on the top plate are really just for show but may also add weight to the top plate and dampened resonances.