What fun! Today we will build a passive preamp. Passive means that there are no tubes or semiconductors in the signal path. There is no AC or power supply source.
Passive preamps are the best thing to use when you have a good signal source like a tuner or CD source. Any source that can drive your amplifier by itself will work. Passive preamps (actually there is no amplification) offer you the lowest distortion and coloring of the sound. You will hear things you never heard before. You will more than likely never again use your other preamp, there will be no going back!
The preamp you are building is a "shunt" method passive preamp, where the signal does not go through a variable pot, but a fixed value, high quality resistor. You might want to experiment with the value of the pot to get a smooth control over the volume range. The selector switch can be one from Radio Shack. It is up to you on how many inputs you would like to have. Simply select the appropriate selector switch. Understand that the schematic represents one channel, two will have to be built for a stereo setup.
Gather the items listed below, then using the schematic below, get to building. Try to use silver solder, which you will find at Radio Shack. The resistor should be the highest quality you can get since this is the only part the signal will travel through. Morrow Audio uses a 1/2 watt Tantalum film resistor sourced from www.partsconnexion.com . If they are out, ask them what they have that would be similar. All parts can be obtained from them for the build.
Parts List (One Channel):
- Some good quality hookup wire.
- Silver solder from Radio Shack
- Phono plugs for the input and outputs (Radio shack)
- Some kind of box or chassis to house the parts.
- 1/2 Watt, 10K Ohm Tantalum or other high quality resistor: http://www.partsconnexion.com/
- 5K Ohm pot
- Selector switch
Tweak Of The Month:
By Dr Harvey "Gizmo" Rosenberg...
HOT GLUE, LIKE YOU
As a cunning young hunter I recognized that my mother, who was an artist, had a valuable skill ...she was into the hot glue gun in very creative ways. I don't think there was anything that she didn't try to stick together in her search for a new objet d'art. For a professional gizmologist, work with audio gizmos would be impossible without the hot glue gun. The tube savant must master two different weapons...the hot soldering iron, the hot glue gun...and it is the responsibility of every father to teach their son how to use these tools.
TUBE CREATION OF CEREMONIAL TRIBAL GARB: This is the secret substance for creating Tube Crowns, and attaching tubes to your clothes, hats, condoms, furniture. To impress your fellow workers hot glue a tube to a tie. If you are single and hanging out at bars, hot glue a tube to each of your collar points...and check out what happens.
VINYLMANIACS MANNA: Nothing is better for attaching your phono cartridge to your headshell in terms of 100 percent mechanical connection. Make sure the glue is super hot so you only have to use a very ULTRA thin layer. Press the cartridge body to the tonearm with great force. Hot glue is also one of the best tuning materials for the Decca cartridge. Any moving magnet cartridge that has a removal stylus...use a tiny glob of hot glue to the side of cartridge body where the removable style makes contact with the magnet structure. What is so cool about hot glue is you can easily pop it free. Use hot glue on your tone arm as dampening...try bands and spots. A few tiny dabs will keep your record mat in place. Why is hot glue such a great dampening device? Drop a stick and see how dead it is, and it perfectly bonds to the surface it is applied to.
SPEAKERMANICS. Use for attaching felt to your speaker faces. Use for building your external passive crossovers. Hot glue on film capacitors can make them sound better. Use to attach better dampening material to the inside of your speaker cabinets.
LINE STAGES/PHONO SECTIONS: If you are going to retro-fit grid chokes, or plate chokes, or capacitors, hot glue can be sued to them in place either on the chassis or PC board.
DIY PROJECTS: Keep your soldering iron and your hot glue gun hot, because you can use that hot and sticky stuff to keep wires, parts and connections in their places. I use to keep heavy parts, like plate chokes, the interstage transformers in place while I am building my prototypes.