Tube Preamp Reviews

A capacitor shoot-out is a method for selecting capacitors that have the potential to produce the best sound. It does not involve measurements—Convergent Audio Technology's Ken Stevens has found that measurements of capacitors taken with even the most advanced laboratory instruments reveal very low correlation with sound quality. On the other hand, such factors as termination technique, film tension, core construction, and wrap style—all of which have virtually no effect on traditional capacitor measurements—may profoundly affect the sound. The only way to tell how good a capacitor will sound in a circuit is to listen to it.

Stevens is adamant in not regarding these tests as "subjective"; he considers them to be objective, in that they're "repeatable with trained listeners with access to a credible system and familiar with the sound of instruments and human voices." Typically, CAT's capacitor shoot-outs involve Stevens and two other listeners, who reportedly find themselves in almost consistent agreement about the sonic characteristics of the capacitors tested—if not necessarily their preferences, which Stevens considers to be more subjective.

The procedure involves inserting various makes of capacitors, all of the same nominal value in microfarads/picofarads, in a part of the circuit that is highly sensitive to capacitor performance, such as the interstage position in a preamplifier's line stage. They then listen to highly familiar recordings, selected on the basis of their ability to reveal differences. In the last capacitor shoot-out, the first recording was Diana Krall's Live in Paris (CD, Verve). According to Stevens, they listened for the overall "feel" of the performance as well as for specific details, such as the naturalness of Krall's voice and her working of the piano's pedals: "With really bad capacitors, including some of the expensive audiophile brands, the pedals just become a low-frequency disturbance of unknown origin." That first test eliminated many capacitors from contention. The three listeners then played additional recordings, to focus on specific sonic characteristics such as timbre and macrodynamics. Sometimes, using IC pins on the board, they plugged in and removed caps without turning off the system.

The same sort of listening tests are used to select other component parts of CAT products, including various support feet. The squishy feet used for the SL1 Renaissance Black Path Edition are the best CAT has found so far, but they're always on the lookout for something better.—Robert Deutsch