The Circuit Design and Topology
There are no special features like meters, special switches, or filters. These are, after all, power amplifiers. They do have the usual triggers for remote off-on switching from a preamp, XLR and RCA inputs, and twin sets of speaker binding posts for bi-wiring. They also have exceptional fusing for power protection.
The only really different feature is a small removable grille panel in the rear through which you can see the two vacuum tubes in the driver stage glow (if for some weird reason you don’t like the distinctive form-follows-function design of the front panel and you feel you absolutely must place the amplifier back to front).
What BHK Signature 300s do have that really matters is unique circuit topology that reflects Bascom H. King’s years of design experience. PS Audio’s literature explains the amplifier’s superior performance by focusing on two key design features. First, it’s a hybrid design that uses a vacuum tube input stage. The text states, “A tube’s high voltage, empty space, rich numbers of electrons, and connection through unattached fields preserve details lost in the solidity of silicon. Tubes are the perfect interface between sources and power stages.”
Second, the BHK Signature 300 uses a different type of MOSFET output stage. PS Audio states: “MOSFETs handle power without the additional circuitry needed by tube power amplifiers, and they sound better than tubes or their solid-state alternatives, bipolar transistors. Field effect transistors were first invented by Julius Edgar Lilienfeld in 1925, and because they require very little current and operate with an invisible field, their sound is more closely related to vacuum tubes than transistors, without suffering any of the issues driving loudspeakers typical of vacuum tube power amplifiers.
“Not all MOSFETs are the same, their differences characterized by their relationship with the input signal and the power supply driving them: N-types for the positive going signals, P-types for negative. Years of design, measurement, research, and listening experience convinced King that N-type MOSFETs have lower distortion and perform better than P-types. Despite this anomaly, most power amplifier designs use both types of transistor in a configuration known as complementary symmetry. The BHK takes a different approach, one that avoids the problem of uneven performance between N and P devices altogether. Using only N-channel MOSFETs in its output stage, the BHK Signature produces a near-perfect balanced waveform without the degradation inherent in a complementary design.”
I have mixed feeling about such statements. Designers do make choices based on their hard-won experience and beliefs, but I’ve reviewed truly excellent amplifiers over the years that have had radically different technology mixes, as well as ones that sounded very different even though they had the same basic topologies and component elements and the same basic design technology. Just as I do not believe that there is one right amplifier or right set of sonic nuances, I don’t believe there is one right circuit.
In fact, Bascom King’s long career as a top designer proves this. He has been developing and refining different circuits for decades, and when I asked Paul McGowan to describe the Signature 300’s design process, he made it clear that amplifier design evolves rather than ends. He says, “In 2014, Bascom H. King set out to put his half-century of design experience to work in the creation of a statement amplifier that would bear his name. King had designed superb amplifiers for many major manufacturers, but had never seen his name affixed to his work.
“Reaching back to the Infinity HCA (Hybrid Class A) he had designed in 1979, King chose a hybrid architecture with vacuum tubes in the input stage, and transistors in the output stage. Of the HCA in 1979, Stereophile’s founder J. Gordon Holt had written, ‘It is the best-sounding amplifier I have ever heard.’ King felt that with the technology now available to him and a free hand in its design, his new amplifier could be far better than the HCA—and even his more recent designs, some of which sell for six figures.
“King realized what many do not: Power amplifiers play a critical role in preserving the minute details and subtle nuances of music. Overtones from plucked instruments, subtle cues defining placement, depth, soundstage width, transient decays-—all are often lost within the power amplifier. From his decades of research, he had identified three major elements within a power amplifier that, if poorly designed, severely compromise reproduction of music: the connection between the preamplifier and power amplifier; the input voltage amplification stage; and the output current amplifier/speaker interface.
“Of those three elements, King felt that the input stage was the area most critical for preserving musical nuances. If an input stage is badly designed, it is impossible for the rest of the amplifier to repair the losses incurred in the input.
“The input stage has the seemingly impossible task of passing both delicate and powerful musical information, sometimes simultaneously, without loss. To preserve both micro- and macro-details, King chose a vacuum tube as interface between preamp and power amp. Vacuum tubes are true voltage amplifiers with extremely high input impedance, and demand less of devices that send signals to them than any other type of interface.