Conrad-Johnson Premier Eleven power amplifier Eleven A Measurements
I performed a full set of measurements on the C-J Premier Eleven A, but I'll show just a couple here. In the measurements accompanying WP's review of the original Eleven amplifier, Tom Norton found a suspicious-looking distortion spuriae trace (fig.1) that implied the amplifier suffered from crossover distortion. It appeared that the culprit was the circuit that drove the biasing LEDs; the A revision of the amplifier was said to have been fixed in this regard. Fig.2, taken under much the same conditions as fig.1, shows that, indeed, it has been. The distortion is now primarily second-harmonic rather than the original's third-harmonic; both are generally regarded as innocuous unless present in much higher quantities than in the C-J.
Fig.1 Conrad-Johnson Premier Eleven A, 1kHz waveform at 10W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom).
Fig.2 Conrad-Johnson Premier Eleven A, 1kHz waveform at 2W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom).
Second, it has been postulated that all audible differences between well-designed amplifiers are due to the differences in frequency responses caused by the voltage-divider action between the loudspeaker impedance and the amplifier's source impedance. The latter measured between 0.48 and 0.56 ohms for the C-J, varying only slightly with frequency; and 0.28 ohms for the Krell KSA-50S, giving rise to response variations when loaded by the B&W Silver Signature (fig.3). The top trace is the Krell; the bottom, offset by 1dB for clarity, is the Connie-J. It varies by about twice as much as the Krell, reaching ±0.25dB. The tube amp's more depressed top two octaves were audible as a very slight lack of air, yet it was the C-J's lower mids that sounded warmer—the opposite of what these curves would suggest.
Fig.3 Krell KSA-50S (top) and Conrad-Johnson Premier Eleven A (bottom), frequency response at 0.5W into B&W Silver Signature loudspeaker (0.5dB/vertical div.).
Neither of these amps will be all things for all listeners. If you just have to have the most forceful presentation of rock music's low-frequency foundation, then the Krell will be the better choice. The Conrad-Johnson, on the other hand, will be the better amplifier for soundstage freaks and those in love with the sound of the human voice. You pays your money, you makes your choice. Be sure to listen to both.—John Atkinson