Sunfire Stereo Power Amplifier
Bob Carver is recognized as one of the most innovative amplifier designers of our time. Founder of both Phase Linear and Carver Corporation (he is no longer affiliated with either company), he has been responsible for several of the audio world's most unusual and controversial products.
Carver's goal in his so-called "magnetic-field" amplifiers of the early 1980's was to make a highly efficient amplifier whose power-supply voltage "tracked" the signal envelope so as to minimize the power dissipation in the output transistors. At the time, he was unsuccessful in achieving that goal, although a signal-tracking power supply was included in his 1979 patent. The closest he came to a true signal-tracking supply in his magnetic-field amplifiers was a three-step approximation. But that changed with the development of what he calls a "tracking downconverter." The tangible result is actually two amplifiers from two different companies - the Carver Research Lightstar amplifier and the Sunfire amplifier, the first product from his new Sunfire Corporation. Although the two amplifiers operate on fundamentally similar principles, they differ in numerous design details, as well as in price, weight, and so forth.
A simplified explanation of the operation of the Sunfire amplifier is that the DC output of a conventional power supply is pulse-width-modulated by the audio signal. The supply delivers narrow pulses when the signal level is low, increasing the pulse width linearly with signal level. The result is that most of the amplifier's output voltage comes from the power supply, leaving only about 6 volts across the output transistors. The action of the switching system (the "tracking downconverter") is analogous to that of a transformer, converting a high voltage at low current to a lower voltage at higher current, but without significant power loss or heat generation.
The Sunfire amplifier is fundamentally a voltage source, with a very low internal impedance. As a result, its output voltage for a given input level remains constant as the load impedance is reduced, whereas the current output (and thus the power output) is inversely proportional to the load impedance. That characteristic is reflected in the amplifier's rated output of 300 watts per channel (from 20 Hz to 20 kHz at less than 0.5 percent total harmonic distortion) into 8 ohms, 600 watts into 4 ohms, 1,200 watts into 2 ohms, and 2,400 watts into 1 ohm. (The 2-ohm and 1-ohm ratings are based on intermittent operation, since few home power outlets are rated to deliver the required current.)
Bob Carver says that the tracking downconverter is fully effective at frequencies up to about 7 kHz. Because of speed limitations in the switching system, the circuit's efficiency drops progressively at higher frequencies and at 20 kHz is little better than that of a conventional amplifier. That is not a problem in normal listening, however, since the higher audio frequencies are not usually present at high levels for an extended duration (if they were, few tweeters would survive). In addition, an internal ultrasonic filter sharply reduces the amplifier's response above 100 kHz.
The Sunfire amplifier is a fairly large and heavy unit, finished in black over its entire aluminum exterior. It has no switches or controls, only a single, softly lit meter in the center of the front panel. The meter, calibrated in joules, is marked power supply energy and is, for all practical purposes, a high-tech pilot light. It reads a constant 380 joules unless the amplifier is delivering its full output, in which case your speakers or your eardrums are probably in shreds!
Since the amplifier has no power switch, and would therefore normally be turned on and off from a switched AC line output on a preamplifier, we were concerned about the ability of a typical preamplifier power switch to handle its potential kilowatts of output. Bob Carver told us that the amplifier's drain on the AC line is minimal under no-signal conditions, however, and we measured it at a surprisingly low 40 watts or so. In fact, we left it on continuously for a week and it never became detectably warm to the touch. Even in normal listening, it was never more than faintly warm.
On the back of the Sunfire are separate inputs and outputs for the two channels. For each channel there are two phono jacks, marked lab direct and normal, along with a standard three-pin Cannon-type balanced input.
The normal jack rolls off the response at infrasonic frequencies, to -3 dB at 1 Hz, whereas the direct and balanced inputs will pass DC.
Each channel has two separate outputs, through gold-plated binding posts that accommodate banana plugs, lugs, or wires. The outputs are marked current source and voltage source and have respective source impedances of 1 ohm and nearly zero. The current source output simulates the output characteristics of a vacuum-tube amplifier, whereas the voltage source output is typical of good low-output-impedance (high-damping-factor) solid-state amplifiers. The two sets of outputs can be used simultaneously, if desired, with biwirable speakers. Mono operation of the amplifier is also possible, either by parallel operation of both channels (through the 1-ohm current source outputs) or by series bridging via the balanced inputs.
The Sunfire amplifier comes with a heavy glass plate that can support the entire chassis. This is suggested as a means of raising the amplifier above a carpet (if it's placed on one) to prevent blockage of the airflow under the cabinet, although that seems an unlikely problem in a normal home installation.
We tested the Sunfire into loads of 8, 4, and 2 ohms, driving only one channel at a time to prevent interruptions from circuit breakers tripping on our AC line (we did blow the amplifier's own 10-ampere fuses on several occasions and had to resort to temporary use of 20-amp fuses to push the Sunfire to its limit into 2 ohms).
The Sunfire amplifier easily surpassed its published ratings, which are based on a limit of 0.5 percent distortion. Its distortion below the clipping point was typically well under 0.05 percent, and its dynamic and clipping-level power outputs were practically identical, as one would expect from a true voltage source. Indeed, the Sunfire proved to be a superb performer in every respect - as close to an ideal amplifier as we have ever encountered. It ran cool, sounded great, and undoubtedly could drive any loudspeaker on the face of the earth with complete aplomb. Once again. Bob Carver is off to an exciting start.
