Solid State Preamp Reviews

For the past decade or so, I haven't been using a preamplifier. The D/A processors I have been using all have volume controls, so I have been feeding their outputs directly to the power amplifiers. It would seem logical that having nothing in the signal path would have less of a degrading effect than a preamp's input and output sockets, switches, volume control, printed circuit-board traces, and active and passive parts, not to mention an additional pair of interconnects. However, with some of the preamplifiers I have auditioned in my system, there was no doubt that the sound quality improved compared with the direct connection from the digital processor.

The most recent of these preamps was the MBL N11 that Jason Victor Serinus reviewed in July 2021, which was preceded by the Pass Labs XP-32 I reviewed in March 2021, the Benchmark LA4 Kalman Rubinson reviewed in January 2020, and going back even further, the Ayre Acoustics KX-R Twenty I reviewed in December 2014, which was one of the products Ayre released to celebrate its 20th year of operation.

The KX-R Twenty, which cost $27,500 at the time of my review, is currently priced at $40,000. Ayre's more affordable 8-series products have been getting positive coverage in Stereophile. Ken Micallef reviewed the EX-8 2.0 Integrated Hub D/A integrated amplifier ($8450) in November 2021 and the VX-8 power amplifier ($6800) in October 2023. I am now reviewing the KX-8 line preamplifier, which costs $6500 in basic form. Three optional digital input modules are available: the X-8 DAC module ($1500) offers one AES3 input, one coaxial S/PDIF input, and two optical inputs; the X-8 USB2 module adds USB and costs $1000; the X-8 Net2 Ethernet module, which turns the X-8 into a streaming DAC, costs $1500. The review sample had all three optional modules installed, bringing the as-equipped price to $10,500.

What it is
The KX-8 is housed in a slim, aluminum enclosure that resembles the enclosures of the other 8-series products. The front panel has a rotary volume control on its right, with a single-ended headphone output on a &188;" jack and a balanced stereo headphone output on two 3.5mm jacks to the left of the control. On the left of the front panel are two buttons with a multicolored LED between them, and there is a small rectangular, turquoise-on-black OLED display to their right.

The back panel features two pairs of balanced analog inputs, three pairs of single-ended inputs, and one pair each of balanced and unbalanced outputs. Above those are the input ports for the digital modules. In addition to the AES3 and optical/coaxial S/PDIF inputs mentioned earlier, there are three USB-A ports, two labeled "Host" and one for updating the firmware, a USB-B port for connecting to a host computer, and an Ethernet port. There are also two AyreLink ports for communicating with other Ayre products and a Word Clock output.

The KX-8's zero-feedback, fully balanced analog circuitry uses discrete devices rather than op-amp chips and what Ayre describes as a "High-Speed" circuit-board material. It features Ayre's Equilock gain stage, which uses transistors operating primarily in current mode, and the company's Diamond circuit, which is used for both the main outputs and the headphone outputs.

For more than a decade, Ayre has been using variations on the Diamond topology, which the company's founder, the late Charley Hansen, uncovered while reading a 1967 patent by Richard Baker of MIT. The Diamond topology uses a bridge network of four bipolar transistors. On the signal-input side are a PNP transistor and an NPN transistor tied together at their bases; on the signal-output side are another NPN transistor and a PNP transistor in opposite polarity. The emitters of the left-hand transistors are tied to the bases of the adjacent right-hand transistors. The PNP's collectors are biased negatively, the NPN's positively.

Baker described the Diamond topology as having several strengths, including the ability to operate effectively in a floating- or above-ground condition, with considerable power gain. More importantly, Hansen felt that the Diamond circuit simply sounded better. Of all common solid state push-pull topologies—in which two phases of a signal are recombined to form a full wave—the Diamond is the only one in which the two half-signals are joined at a single point in the circuit, with no intervening circuitry. Thus, the Diamond circuit creates an output that should be more faithful to the shape of the input.

A significant feature of the KX-8 is its use of Ayre's Continuously Variable Gain Transimpedance (CVGT) volume control, a variation on the VGT volume control introduced in the KX-R preamplifier in 2008. Unlike a traditional volume control, which uses a variable resistor connected between the maximum output voltage and ground to attenuate the signal, the CVGT adjusts the input circuit's gain, using different resistors for each setting. The benefit of this approach is that the noisefloor is lower at lower volume control settings, maximizing resolution.

The optional digital module uses an ESS ES9038Q2M DAC chip coupled with a Xilinx Spartan FPGA to operate in 16× oversampling mode. The S/PDIF digital inputs are asynchronously coupled to the KX-8's local oscillators to minimize jitter. The reconstruction filter is Ayre's short minimum-phase type.

Control Options
With the preamp powered up via the back-panel switch, a single press on the leftmost button on the front panel wakes up the KX-8 from Sleep mode. Pressing it again mutes the output. The rightmost button switches between the inputs. The two buttons can also be used to access a setup menu for each input. Pressing and holding the left button for three seconds puts the preamplifier into Sleep mode; subsequently pressing and holding the right button for three seconds activates the setup menu's primary page, which allows access to the Input, Display, Network (including Wi-Fi setup), and System settings.

Each input can be activated, renamed, and set to unity-gain Pass-Through mode. There is also an option to use the AES3, S/PDIF, and optical inputs in Video Mode, which forces the KX-8 DAC to use incoming word clock data rather than the preamp's asynchronous clock for the selected input. Each input can be used with an AyreLink connection to an Ayre source component. The user can choose display timeouts of 5s, 15s, 30s, 60s, or "Never"; turning off the display reduces RF noise and maximizes the life of the module. (Ayre warns that selecting "Never" can cause faster-than-normal display fading.) The System option lets you access auto-sleep (on or off), view the firmware revisions and which options had been installed, and perform a factory reset.

How it sounds
I started my critical listening to the KX-8 with its balanced outputs connected to a pair of Parasound JCA100 Tribute monoblocks, which in turn drove the MoFi Electronics SourcePoint 888 loudspeakers I reviewed in the March issue. The source was the balanced outputs of the MBL N31 D/A processor, which converted audio data from my Roon Nucleus+ server. (The MBL's volume control was set to its maximum rather than the –13dB I usually set it to for my preamp-less auditioning, and as usual I selected its minimum-phase filter.) In addition to the KX-8, Ayre also sent a VX-8 power amplifier, which I substituted for the Parasounds. After the SourcePoint 888s had been returned to MoFi, I used the VX-8 first to drive GoldenEar BRX standmounts, then the Stenheim Alumine Two.Fives that are in for review.

With the KX-8 in between the MBL and the power amplifiers, while the depth of the soundstage on the recordings I had made in the 1990s of the Santa Fe Chamber Music Festival was well-developed, it was not quite as deep as it was without the preamp in the system. When I recorded works by Mendelssohn and Brahms in 1997 (Encore, Stereophile STPH011-2), I had to place the microphones closer to the musicians than in previous years due to the effect of an acoustic shell intended to throw the sound toward the audience. I therefore used a judicious amount of artificial reverberation when I mastered the recordings, matching the character of that reverb to the sound of the empty hall. With the KX-8, the images of the individual instruments were palpably positioned in the soundstage, but I wished I had used a little more artificial reverb.