PS Audio BHK Signature Preamplifier
The single most important element of design
Over Bascom’s half-century of designing high-end audio products for companies like Infinity, Marantz, Conrad Johnson, Counterpoint, Constellation Audio, and now PS Audio, one overriding principle has dominated all others when sonic compromise isn’t an option: use of a vacuum tube on the input. The isolation and near-perfect linearity of a well-designed vacuum tube input stage cannot be duplicated by solid-state means, though many have tried.
"Vacuum tubes are simply more convincing and musically accurate than any other input configuration I have ever tried" said Bascom, and the BHK Signature preamplifier is demonstrative of this observation.
The BHK Signature Preamplifier features two 12AU7 dual triode vacuum tubes in the input stage that act as a zero feedback balanced input amplification pair. A unique self biasing constant current source and user selectable switch combination allows those who like to roll their own tube selections to place any acceptable 6, 7 or 12 series vacuum tubes inside the preamp. The twin hand-selected vacuum tubes that ship with the BHK are the preferred models we recommend and use in PS Audio’s Music Room One reference system.
The BHK Signature Preamplifier features 5 identical balanced and single ended inputs. Each of the inputs, whether single ended (RCA) or balanced (XLR) are immediately converted to a pure balanced signal by the dual triode vacuum tube for lowest distortion, highest linearity performance. Input switching is accomplished through hand-selected double-pole double-throw, gold plated relays that we’ve chosen for their sonic neutrality. The signals, once selected, are passed directly the revolutionary new multi-stage attenuator controlling both volume and balance.
If the preamp is going to be part of a home theater system controlling the main left and right channels, each input can be user-assigned as a home theater bypass.
Controlling the volume is a critical task
Perhaps the most difficult challenge facing preamp designers is the volume control. Simple in concept, controlling the level fed to your power amplifier, without damaging the sound, can be one of the most daunting design tasks. During high-end audio’s formative years it had become standard practice to incorporate a high quality rotary potentiometer to control the volume. Over the years designers discovered even the finest of those controls imparted an unacceptable sonic thumbprint, robbing the music of life and openness. Various schemes at removing the potentiometer’s degradation were tried with varying degrees of success. They included stepped attenuators, conductive plastic impedance elements, electronic gain circuits, relay, CMOS switching, and a host of other innovative ideas.
Bascom King decided to try a fresh and novel approach in the Signature preamplifier. A combination of an input stepped attenuator, built around the highest quality passive components for course volume adjustments, coupled with controlling the actual gain of the vacuum tube for finer steps, turned out to be the most sonically transparent level control we have yet heard. This unique topology, which can be controlled from either the front panel rotary controller or the remote control, provides the lowest possible impact on music passing through–and is one of the primary reasons the BHK Signature presents music in such a revealing, open manner.
Once amplified the signal must interface with the outside world
A designer’s best efforts at preamplification can be wasted if the interface between the preamp’s output, the interconnecting cables, and the receiving power amplifier are not optimized for lowest impact, best performance. Cable interactions and power amp input stages can test the mettle of any design, so it is critical a preamp’s output stage provide a high current, class A, low impedance design. The BHK Signature preamplifier output stage is a high-bias class A MOSFET design that is unaffected by cable and power amplifier interface challenges. Simultaneous connections of both single ended (RCA) and balanced (XLR) outputs have minimal impact on the delicate vacuum tube gain stage and provide remarkable isolation between source components and the power amplifier of your system.
This high current low impedance Class A output stage is part of the reason the BHK Signature delivers music in such an effortless fashion. One of the first qualities owners of the preamplifier will appreciate is its remarkable ability to isolate source and control from the power amplifier – and one of the principle factors in the BHK Signatures extraordinary life-like presentation of music, preserving every nuance in your media.
The headphone amplifier
Not every system is based exclusively around the loudspeaker and power amplifier. Many of today’s finest systems incorporate high-end headphones as either an adjunct to the main system or are the main system. Built into the BHK Signature is a state of the art headphone amplifier capable of delivering near perfect performance for any high-end headphone. Take the time to click on the video tab above these words to hear what designer King has to say about his design philosophies when it comes to headphones. The BHK Signature headphone amplifier was borne from years of work on such products as the Audeze King (designed by Bascom) among others. Extremely low–near zero–output impedance, zero feedback, transformer coupled, MOSFET and discrete, the internal headphone amplifier inside the BHK Signature is a tour de force that honors the music the designer worked so hard to preserve.