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PS Audio BHK Signature 300 Mono Power Amplifier

Over time, reviewing the high end’s best power amplifiers has become increasingly more difficult, particularly those with solid-state output stages, high power and high current, and damping factors sufficient to help control a speaker—in other words, the kind that have become the norm for most audiophiles. Amplifiers seldom sound alike, but more and more, they differ only in more subtle nuances rather than in dramatic sonic differences.

In many cases, the nuances an audiophile actually hears also depend heavily on how revealing all the other elements—active and passive—in his or her system are. The sonic nuances of a whole high-end system are inevitably shaped largely by that system’s most colored components, as well as the interactions between a particular speaker and a particular listening room. As a result, these colorations and interactions will often mask many of the more subtle nuances in power amps.

What’s more, I find A/B listening tests to be an uncertain way (at best) to audition components. The colorations caused by other components, variable room interactions, and often the noise from other people, create too many masking effects and problems—as do short listening periods and any form of blind testing.

Understanding the nuances in given components takes time and patient listening to a wide variety of recordings and music. It is all too easy to become confused by switching too rapidly, hearing differences caused by unfamiliar recordings, and by minor shifts in loudness. (This is a reason why I strongly recommend working with the kind of dealer who will really let you listen, and possibly offer the loan of a really expensive amplifier so you can hear it in your system.) While some people talk about A/B testing as if it were a simple exercise, it is subject to all the problems that affect every aspect of any operations research that involves human testing—factors such as primacy and regency, focusing on the wrong variable, etc.

There also is no one right answer to choosing the best amplifiers; rather, it involves the sonic preferences that suit you and your particular system and room best. Today’s top designers and design teams—from legends like Nelson Pass, John Curl, and Bascom H. King to young Turks and collective design efforts—are producing a wide range of amplifiers that can present as musically realistic a sound as the recording and the rest of the components will permit.

Additionally, one must consider the specific variable qualities of music reproduced through a high-end system, including timbre, soundstaging and imaging, dynamics, and musical contrasts. Your listening position also impacts what you hear. Similarly, these same sonic aspects differ across various positions within a concert hall. No two venues or collections of instruments sound exactly alike, and every known brand of microphone and every other stage in the recording process has colored the music to some degree even before you begin the playback process. Ultimately, the key question is never, “What is truth?” It is rather, “What is the most satisfying and seeming realistic illusion of music?”

Enter the PS Audio BHK Signature 300
And yet, some amplifiers really are distinctive successes, and the PS Audio BHK Signature 300 monoblock is definitely one of them. Its BHK namesake references the amp’s designer—one of the audio legends I mentioned above—Bascom H. King’s initials. The BHK Signature 300 also reflects the deep involvement of PS Audio founder Paul McGowan and Arnie Nudell, who headed Infinity when it was one of top speaker firms in the world, and here served as a key listener.

This amplifier isn’t cheap, although it represents a bargain in sound quality compared to all too many of its higher-priced competitors. A pair of BHK Signature 300 monoblocks costs $14,998, and a BHK Signature 250 stereo amplifier costs $7499. Top-of-the-line prices should mean exceptional sound, and any amps that are not truly exceptional at any comparable or higher prices are simply a rip-off.

You do, however, get your money’s worth with the BHK Signature 300s. They have truly exceptional imaging, depth, and natural soundstage width, and they really can get the best out of the naturally miked and produced recordings that have a real-world soundstage. Once they are broken in, they have very little sound character of their own, but to the extent they do, they have a rich, natural musical timbre without any loss of highs or air.

The 300s’ dynamics are extraordinary, particularly in the lower octaves that seem to be in even more demand for audio reproduction today than the upper octaves (higher levels) that had previously been the key design challenge—until great performance at high powers became the rule, rather than the exception. Bass goes as deep as your speakers will permit. The amp’s control with difficult speaker loads is excellent, while the upper bass and lower midrange—which dominate most actual musical sound—have no touch of leanness or lack of natural warmth.

This performance is not simply a matter of power or “watts,” although the BHK Signature 300s are scarcely shy in this regard. They provide 300 watts minimum into 8 ohms, 600 watts minimum into 4 ohms, and 1000 watts into 2 ohms. They can drive a friend’s early Apogee speakers—candidates for most demanding speaker ever made. They handled both my Wilson Alexias and Spendor BC-1s with ease—both loads that aren’t that easy by any standard. In fact, the Signature 300s did better with demanding dynamic passages and sudden sharp musical contrasts than an amp rated at 600 watts.


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.


“The BHK Signature features a classic dual-triode vacuum tube in a modern configuration. Hand-matched pairs of Russian Gold Lion 6922s are self-biased with constant current and high voltage, through a MOSFET current source and MOSFET-regulated plate supplies. The input stage zero negative feedback design achieves low timing and phase distortion errors, allied with fast step response. The balanced-input vacuum tube stage runs in Class A configuration, and is coupled to the following balanced-configuration current amplifier stage by way of REL film capacitors.

“Hybrid amplifiers are not new (as evidenced by the 36-year-old Infinity HCA), and most incorporate a low-loss input interface, voltage amplification through vacuum tubes, and high power/low-distortion current amplification and delivery through MOSFET outputs. While these basic elements are present in the BHK Signature Amplifiers, it is King’s utilization of a new topology of MOSFET amplification that makes the BHK Signature Amplifiers not just possible, but far superior to ordinary designs.

“To maximize the benefits of the two different amplification devices (vacuum tube and MOSFET) inside the amplifier, King felt the amplifier should be built as if it contained two separate, independent systems, each with a power supply tailored to fit. Thus, the BHK Signature has two power transformers, one for the vacuum tube input stage, the other for the MOSFET power output stage.

“Regulation of the power supply for the input stage is important. Separate and discrete MOSFET regulators are used throughout to preserve details found in the music and keep noise levels to a minimum. The path music takes through the amplifier must be pure and without a polluting sonic signature. Each passive component is hand-selected to provide the cleanest path possible. PRP resistors, film and foil Rel caps, and the finest-sounding parts possible are hand-soldered onto BHKs circuit boards. King insisted that no surface-mount parts be used in the signal path; assembly is by classical through-hole means.”

You don’t produce a superb-sounding amplifier by choosing the wrong circuit. At the same time, I believe Paul’s comments do more to reflect how much a search for the best possible sound, the experience of the design team, and a deep commitment to creating the best product contribute to an amplifier’s success. The way products evolve are a critical part of its design, and often a part that explains a great deal more about their character and performance than any technical data.

In practice, high-end audio is not shaped by finding a singular solution. It is the result of the work of dedicated audiophiles who make what they listen to, and who become the equivalent of auteurs in sound.

The Music and Sound Quality
In any case, the end result is outstanding, so I think you will quickly find that this is a superb-sounding amplifier, and one with some unique sonic qualities. I’ve already provided a summary description of its merits, but when it comes down to the details, as I’ve stated, the BHK Signature 300 provides exceptional imaging and depth. You will need equally good components, wires, and speaker placement to hear this. You will also need source recordings that are recorded naturally and simply enough so you can get a real-world soundstage.

These are scarcely demanding conditions for any real audiophile system, however, and once they are met, the BHK Signature 300 makes the soundstage come truly alive. The sound presentation is somewhat more forward in character than many of the BHK Signature 300’s best rivals. For example, the Pass Labs 160.8 monoblocks I use as one of my references provide an excellent soundstage with the same degree of realism, but one whose sonic perspective is more mid-hall.

But both types of soundstage are musically realistic, and I have not heard another power amp that does as well in providing the same kind of detail in a musically natural, concert-hall way, or one that provides the same kind of slightly forward imaging and soundstage width and depth without slightly exaggerating the upper octaves.

The Signature 300’s soundstage performance is particularly striking with some older recordings such as the best of those from RCA Red Seal label and Mercury, chamber music made on the Accent label, and the top Smithsonian Jazz pressings. These were produced at a time when the focus was on providing a natural soundstage with minimal analog or digital editing, extra mikes, or other production tweaks.

Some of the current focus on the sound quality of LPs and analog tape almost certainly comes from the fact that they were originally recorded at a time when the processes were more natural; there was less emphasis on close miking and on detail over natural timbre, and it was far harder to “assemble” the music by joining different parts of multiple takes into a single performance. Better small human flaws than robotic, post-performance production perfection.


The Signature 300s were equally excellent with the best modern digital recordings that place similar emphasis on musical life and a natural soundstage, such as those by experts like Peter McGrath or ones that you find on the 4L and Channel Classics Labels. These were exceptional in providing depth with chamber music and choral performances that I’d heard live while the digital recording was being actually being made. As for natural image size and placement, centerfill, and soundstage width, the BHK Signature 300s do as well as any amplifiers I’ve heard at any price. As stated earlier, the limits are not set by the power amplifier; they are set by the way the recording was miked and produced, by the need to use only two speakers, and by the other components.

Moreover, this soundstage excellence is consistent on both solo instruments and voice, and small chamber and jazz groups. Indeed, these amps test the soundstage limits of the best large jazz band, symphonic, and operatic recordings. No recording in a high-end system in a real-world home is going to sound exactly like a live performance of a full jazz band, grand opera, or Mahler symphonic spectacular in a real hall. The BHK Signature 300s do, however, come as close to putting you in Row D to Row K as the recording will allow. Once again, I’ve only heard a handful of power amps do as well—and all put you more in Row L to Row P.

Once the BHK Signature 300s are broken in, they also have an exceptionally natural timbre at any dynamic level throughout the entire frequency range. Some otherwise excellent amps lack the ultimate in “grunt” and power in the deep bass. Others seem to be voiced to slightly emphasize musical detail with either a bit too much energy in the upper midrange, or a bit too little energy in the upper bass and lower midrange. The BHK Signature 300 gets it as right as the rest of your components will permit, and has by far the most natural timbre of any PS Audio amplifier I have ever heard.

I can’t speak to the degree to which having Paul McGowan and Arnie Nudell working with Bascom H. King in the listening phase led to this excellence, but Paul’s sudden conversion to vacuum-tube drive stages does seem to have come in part from this collective effort. It has really made a difference. Just listen to a really great recording that emphasizes the violin, the full range of the grand piano, percussion detail, or mixes of male and female voice (especially soprano). Timbre may not be king in voicing high-end gear, but it is absolutely critical.

The same excellence applies to musical dynamics and contrasts, air, and fine musical detail. One aspect of power amplifiers that has always puzzled me is the number of high-power amplifiers that can produce very high dynamic peaks, but lack the dynamic life and natural energy of better amps with far less rated power. Part of this difference almost certainly comes from the fact that most acoustic music depends on low-level detail, dynamics, and air, but this clearly is only part of the story. Being able to handle high SPLs doesn’t prevent some amplifiers from lacking life and natural musical realism—even in the parts of the music that are really loud.

You won’t have any such problems with the BHK Signature 300s. Like other truly excellent power amplifiers, they seem to perform almost effortlessly, and often transparently. You hear the compromises in the recording rather than the limits of the amplifier—whether you are listening to a song cycle, solo instrument, or the most demanding passages in music such as Saint-Saëns’ Third Symphony. One of my friends who plays bass guitar loves the BHK Signature 300s, and so does another friend who is an opera buff, and both stress the natural life they bring to the sound.

Interface and System Compatibility
The BHK Signature 300 should be an outstanding performer in virtually any system I can think of, with the possible exception of extremely efficient horn systems with limited tolerance for power. These amps are truly powerful.

They certainly are not particularly sensitive to speaker load, or to a particular brand or model of speaker cable or interconnect. You will, however, hear the colorations and nuances of your speaker cable and interconnects more clearly simply because this amplifier—like all of its best competitors—is more revealing. One of the key tests of a great component is that it reveals the rest of your system’s colorations. This was not a problem with my best AudioQuest, Kimber, and Transparent Audio cables, but I could hear the differences between them in ways that only emerge with really good amps.

The only caution I would give you about setup is that the BHK Signature 300s deserve good AC power input and power cables, and that you may have to change some of your other components to fully hear how superb this amplifier really is. You are going to want your other components to be worthy of such illustrious company.

Summing Up
A truly excellent amplifier. Scarcely cheap, but less expensive than most of the amps that challenge it in quality—and the stereo version is half the price of the monoblocks. I’m not giving up the Pass Labs 160.8s, but I am putting a second reference system together, and I’m keeping the BHK Signature 300s. Two of the best power amplifiers around can be better than one.


Output power: 300W into 8 ohms, 600W into 4 ohms, 1000W into 2 ohms
Inputs: RCA (unbalanced), XLR (balanced)
Outputs: Two pairs of gold-plated copper binding posts
Gain: 30.5dB +/-0.5dB
Sensitivity: 1.2V
Noise: <-85dBV, 100Hz-20kHz
Input impedance: Unbalanced 100k ohms; balanced 200k ohms
Output impedance: <0.1 ohms
Frequency response: 10Hz–20kHz +/- 0.1dB; 10Hz–200kHz +0.1/-3.0dB
Dimensions: 17.1″ x 8.7″ x 14″ chassis only; 15″ deep including connectors
Weight: 83 lbs. each
Price: $14,998/pr.

4826 Sterling Drive
Boulder, CO 80301
(720) 406-8946


By Anthony Cordesman

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