Solid State Preamp Reviews

The experience left me doubting my ears. After I'd performed all the measurements of Ayre Acoustics' KX-R preamplifier ($18,500) to accompany Wes Phillips's review in our November 2008 issue, I spent a weekend listening to it. To my astonishment, the sound of my system with a Transporter D/A processor feeding the preamplifier was better than when the DAC fed the power amplifier directly. Through the KX-R, images sounded more tangible, and the sound was better focused, despite the signal's having been passed through not just another set of interconnects but also through the preamp's input and output socketry, switches, a volume control, printed circuit-board traces, and active and passive parts. Logically, you'd think that having nothing in the signal path would have less of a degrading effect than so many somethings. But no, that was not what I heard, much as I would have preferred otherwise.

After that, the KX-R went back to Ayre Acoustics, and over the next five years I tried several other preamplifiers in my system. But it wasn't until Pass Labs' three-chassis XP-30 ($16,500) came along, and I reviewed it in April 2013, that I felt I'd equaled my brief experience of the KX-R.

Then, at the 2014 Consumer Electronics Show, Ayre celebrated its 20th year of operation with the debut of its KX-R Twenty preamplifier. The new preamp looks identical to the original KX-R, but, at $27,500, costs $9000 more. I signed up for a review sample, which arrived in the summer, in time for me to use it while preparing my continuing series of loudspeaker reviews.

A brand new outfit
The Twenty looks the same as and is functionally identical to the original KX-R; I refer you to our 2008 review for a full description. Briefly, it has four sets each of balanced and unbalanced inputs, and two sets of main balanced outputs. Recessed, sculpted knobs flanking a central, blue display panel select inputs and control volume via Ayre's Variable-Gain Transconductance (VGT) topology.

So what's different? Via e-mail, I asked Ayre's Charles Hansen how the Twenty differed from the nominally identical KX-R.

"The KX-R Twenty is not an upgrade, it is a complete redesign. If you have an original unit, the only parts that don't get touched are the chassis, the connectors, and the transformers. We start by rebuilding the volume control for greater reliability. We replace the logic board to reduce the noise of the volume control by over half. Then we replace all of the audio circuitry, including the power-supply regulators. The first thing that we changed was the preamplifier's output stage. The Diamond output on the X-5 series (footnote 1) worked so insanely well that we put it into the KX-R Twenty. Never content to rest on our laurels, we further optimized it for even better performance. There was a massive performance improvement over the previous solution of paralleled complementary JFETs.

"The next thing that we did is something that I have never seen before, although I can't imagine that it hasn't been done, even if only by a DIYer in a home-built unit. Specifically, all power-supply regulators, whether three-pin ICs using massive amounts of feedback or the discrete zero-feedback designs we have always used in Ayre product ever since the V-3, 21 years ago, have at their heart a single series-pass active element (tube, FET, or BJT). When the load (the audio circuit) draws more current, the pass element decreases its resistance (turns on harder), to keep the voltage to the load constant.

"For the KX-R Twenty, we added a second circuit element (in our case, a BJT) that also pulls down. So when the load (audio circuit) draws less current, the second transistor draws additional current to ground. An analogy would be like going from a single-ended output stage in a power amp to a push-pull complementary output stage. This 'locks' the output voltage of the regulator, regardless of any fluctuations in the load. We call this the AyreLock regulator.

"The last thing that we changed was the voltage reference for the EquiLock circuit. This was introduced in the MX-R power amplifier, and it is (one of many variations) on a cascode topology. The cascode is an active device (transistor, FET, or tube) that holds the voltage across the amplifying device constant. To hold the voltage constant, we need a voltage reference. Compared to the original voltage reference in the original EquiLock circuit, the voltage reference in the new EquiLock has over one million times (120dB) greater isolation from the power-supply rail in the front end.

"That's it. Three massive changes to the audio circuitry and one massive change to the power-supply regulation. The changes were far too extensive to allow us to perform our normal 'trick' of upgrading a product by simply changing or adding parts—or even adding a daughterboard. Instead, the entire printed circuit board containing the circuitry is simply replaced wholesale."

Hence the $9000 price increase, I suggested.

"The large price increase was inevitable, unfortunately," explained Hansen, who instanced the high-precision resistors used in the volume-control arrays. The control uses more than 400 of these resistors, in different values, and the price to Ayre has risen from 90õ to $2.80 each.

"The other factor that led to the price increase," Hansen elaborated, "was six years of inflation in parts costs. This wasn't just the normal 'inflation.' This was the period when increased demand from China for raw materials led to a tripling in the cost of copper (transformer wires and higher PCB costs) and a doubling in the costs of aluminum and transformer-grade steel. Transformer prices have doubled, and as each KX-R chassis starts off as a 75-lb slab of aircraft-grade aluminum, the cost of the chassis has increased significantly.

"The Twenty is literally a new design in the same chassis as the KX-R," Hansen summed up. "And since everything is new, other than the box and transformers, KX-R owners who have their units rebuilt—the rebuild costs the same as the price difference between the old and new models, $9000—receive a complete five-year warranty from the date of the rebuild. Even if the original five-year warranty has expired."

Times change but the fascination stays
Setup comprised sitting the KX-R Twenty atop three of Ayre's inexpensive Myrtle Blocks, so that the preamp's plastic feet were in the air. I bought 20 of these domino-sized blocks a few years ago; irrespective of their effect on the sound, they are most useful in providing clearance between warm-running components.

The first word that popped into my mind after I set up the KX-R Twenty was robust. There was a solidity to the system's sonic signature that reminded me of when I reviewed the MP upgrade to Ayre's K-5xe preamplifier, in 2011. But the palpability of images and the sense of musical flow that had impressed me with Ayre's $3500 preamplifier was magnified with the Twenty. When I listened to a recording of pianist Katy Mahan playing Debussy's The Engulfed Cathedral, recorded by German engineer Rainer Maillard with Ayre's QA-9 A/D converter (24-bit/192kHz WAV file, available from the Ayre Store), her Steinway was present in my room. I would have preferred this recording to have been made in a more characterful acoustic than the Emil Berliner Studio, but with the KX-R Twenty driving Pass Labs XA60.5 monoblocks, such quibbles became irrelevant, and I could appreciate Mahan's hauntingly evocative touch on the keys.

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Footnote 1: For a discussion of the Diamond circuit, see Art Dudley's review of the AX-5 integrated amplifier in August 2013.