INTRODUCTION
After building a OPA2134AP CMoy / Grado RA1 headphone amplifier in a timbre box, I wanted to try this high performance OP amp in a line stage / preamp. Rod Elliott's High Quality Audio Preamp (Mk II) which is a two stage preamp with balance control seemed like a good choice.
DIY PREAMP AND POWER SUPPLY
Two small pc proto boards (Radio Shack 276-159 / Dick Smith H5601) were used to accommodate all components. Metal film polypropylene capacitors were used in the signal path. It would have made the whole project easier by using physically smaller caps but for me the sound was a priority. For the power supply I used a 1.2A 18V regulated SMPS wall wart. I definitely do not need 1.2A, but I wanted a gutsy supply. Two 1k 0.5W resistors make up the "virtual ground" split supply. 220 uF low ESR capacitors and 0.1 uF decoupling capacitors are used for channel. This simple supply configuration works very well and is dead quiet.
PREAMP ENCLOSURE
The case only measures 115 X 90 X 55 mm (same size enclosure that I used for my Nanoo LM3875 Gainclone) and placing all the components in this tiny space, including a dual gang volume control and balance control was going to be tough. Unless you enjoy working in ridiculously small spaces and/or you are masochistic, build this preamp in a bigger case. Even a slightly bigger enclosure would be better. For the finish I use an acid etch primer, allowed a few days drying and then the spray color over the top. This was the same technique used with the big red caps on the Synergy Roasting Pan LM3875 Chip Amp / Gainclone. 
RESULTS
I have used the preamp as a front end to my S-5 Electronics K-12 Tube Amplifier. The test setup comprised a NAD C542, modified bookshelf speakers and DIY Silver Interconnect Cables. The preamp added nothing and took away any CD player/ Valve power amp miss-match. For that reason I have named it Neutrino. The balance control has a very wide sweet spot and the volume control was non-twitchy, linear and easy to use. A very inexpensive project but it feels odd to build something that adds and removes NOTHING audibly in a cute box. Try it.
Mark Houston 
Other DIY Audio Projects by Mark:
31 March 2008
DIY OP Amp Based Preamplifier
01 May 2007
"Beauty" - A 12AX7 Tube Preamplifier
To accompany his recent LM3875 DIY Chip Amplifier (gainclone) project, Bruce has put together a DIY 12AX7 Tube Preamplifier Kit to drive the chip amp. He has named the tube preamp "Beauty", as it sweetened the sound of his LM3875 Chip Amplifier.
For the enclosure, Bruce used a readily available plastic and aluminum project box from Radio Shack. This is the same enclosure he used for the LM3875 Chip Amp. The photograph below shows the Tube Preamp and Chip amp together.
Similar to what Mark discovered with this economical (read dirt cheap) tube preamplifier kit is that you get a very quiet preamp that can compete with ones that cost much more. For more information about this project, see Bruce's project page, "Beauty" - A DIY 12AX7 Tube Preamplifier Kit.
Related Post:
What's Playing: The Cars - Bye Bye Love (Demo)
01 March 2007
DIY 5687 Tube Preamplifier
The latest addition to the DIY Audio Projects website is a fine looking tube preamplifier constructed by Vincent Wong. The tube preamp is based around a pair of 5687 twin triodes. Each channel uses both sections of a 5687 tube and LED biasing is used on each section.
To keep hum at a minimum, the filaments are heated using DC. The power supply is external to the preamp section and consists of a RCA-83 mercury rectifier and two 50H chokes.
For more project details including schematics and an enclosure construction guide, see: DIY 5687 LED Biased Tube Preamplifier.
Related Tube Preamplifier Projects:
- Mark's build of the DIY 12AX7 Tube Preamplifier Kit
- Bruce's build of the DIY 12AX7 Tube Preamp Kit
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What's Playing: Fleetwood Mac - Landslide
12 February 2007
12AX7 Tube Preamplifier Kit
This is Mark's build of a simple and inexpensive 12AX7 tube preamplifier kit from Silicon Chip magazine. What makes this kit inexpensive (and interesting) is that power comes a switch mode power supply, which is fed from a 17 volt DC wall wart! To silence the skeptics of a switch mode power supply in a tube circuit, Mark has completed a number of measurements on the power supply as well as the preamp. Mark's modifications to the preamp kit include the addition of snubbers and upgraded capacitors. The end result is a simple DIY tube preamplifier that sounds great and will keep some money in your pocket for your next DIY Audio project.
For more detailed information about Mark's latest project, see the project page, a DIY 12AX7 Tube Preamp kit.
Related Posts:
- Bruce's DIY 12AX7 Tube Preamplifier Kit
What's Playing: Pearl Jam - Glorified G
