The Calla 60w monoblock is a nominal sixty watts. It is possible, and recommended, to be used without a transformer between the output tubes and the speaker. There is included, however, a matching transformer for very low impedance speakers. The calla 60w uses eight 6AS7 dual triodes, for a total of 16 triodes eight vs. eight in a bridged push pull configuration known as a Circlotron. The circuit is arranged so that the output tubes cannot be driven into grid current or operate push push therefore giving them the class ‘A1' designation. Because of this configuration we are able to get away without global feedback. In every amp we have built before we had to compromise between high distortion and the compressed sound of feedback. The lack of feedback lends to the amp’s open quality. Note: The noise in the square waves is due to the test equipment.

Included are snapshots from an oscilloscope the first two are a square wave at 40w. and 1w respectively. The extremely fast rise time of the Calla 60w is reflected in how close to vertical the rise lines are. The third is a plot of voltage vs. frequency from 4Hz. to 1MHz. Flat to 100Khz, and only down about six dB at 1 Megahertz, not bad. The last snapshot is a linear spectrum analysis of a 50Hz. sine wave note, how small the harmonics are.