Preamps make up an integral part to many audio signal chains. The basic purpose of a preamp involves getting a low signal up to a workable level. However, preamps can offer much more in the way of color and warmth to an audio signal. Tube preamps have a good reputation for warm sound, and a DIY tube preamp allows you to customize your preamp to fit your needs.
Deciding Application
Preamps work for numerous different applications---most commonly, microphones, instrument pickups and turntables. Each of these applications has different needs, such as different impedances and gain (volume) requirements. Even within the microphone classification, some mics need a high impedance preamp while others require low impedance for ideal operation. While a general tube preamp may cover all of these needs adequately, build a preamp built with components specific to your needs to allow it to function at a higher level.
Choosing Tubes
The quality of your tube preamp depends on the quality of the tubes. Because vacuum tubes are not made as commonly as in the past, finding new tubes can prove difficult and expensive. Purchase older, gently used tubes for better quality at a lower price. Take carte when buying used tubes, though, as mild damage can cause major sound degradation. Look for tubes with low noise and appropriate gain factor.
Function and Appearance
Before you start soldering pieces to the board, plan out the placement and function of each component, such as tubes, resistors and capacitors. Kits and pre-manufactured schematics generally come with a set placement. However, if you choose to start from scratch or modify an existing plan, take time to map out the location of each component before beginning to solder. Think about where you will place inputs, outputs and variable controls. Also think about the aesthetic of the preamp while planning the component locations. If you plan to place the tube preamp in a case, make sure the size and shape will work with the casing.
Putting it all Together
Soldering makes up a huge part of putting your tube preamp together. Make sure you have a good soldering iron, a sponge to clean the soldering iron tip and a solder sucker to clean up any messes. Practice soldering before putting together your preamp if you do not feel absolutely comfortable; poor soldering can quickly ruin your components. Keep a multimeter on hand, and if possible find an oscilloscope to test your preamp. Use great care when putting the components together, as they damage easily. Never work on the components with the preamp plugged in; electrical shock poses a real risk and can cause serious injury.
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