Ayre K-5xeMP line preamplifier Page 2

I wasn't happy about a decrease in the measured performance of an 'upgraded' product, no matter how much better it sounded. So we went through a long period of developing a circuit using bipolar transistors that would sound as good as the JFETs. We were finally able to do this, but at the cost of an extremely complex circuit. The final result was a great-sounding design [it's used in the QB-9—Ed.], but it was simply too complex to retrofit to the K-5xe in a manageable fashion.

So we are plundering our precious stash of Toshiba JFETs to make the MP upgrade to the K-5xe. It measures great, sounds great, and is easy to retrofit to existing units."—Charles Hansen

What
The K-5xeMP is an elegant-looking preamp conforming to Ayre's house style, with brushed aluminum panels set off by a central blue display. (A black-anodized finish adds $250 to the price.) There are four inputs—two balanced, two single-ended—and both balanced and single-ended outputs. Unlike the K-5xe, the MP abandons the astronomical symbols used to identify the inputs and returns to the plain old B1, B2, S1, and S2 used on the original K-1. The input selector buttons select not just the desired source but also the ground connection on that input, minimizing noise pickup from the grounds of other sources. Also to reduce noise, the control microprocessor is awake only when executing a command from the front-panel buttons or the remote control.

Inside the chassis, the power supply and audio circuitry are carried on a large, double-sided printed-circuit board running the full width behind the rear panel, and mounted directly on it are all the input and output sockets. Unusually, this board is coated with clear solder mask, which I believe is because Ayre thinks this enhances ultimate sound quality. Other than the power supply, the layout is dual-mono, with wide physical separation between the left and right circuits. Through-hole components are used exclusively, the only surface-mount parts being on the control board behind the front panel.

The volume knob on the front panel operates a shaft encoder, and control signals from the front-panel board are passed to the audio board via a ribbon cable. The volume control operates with 66 steps of 1dB each, but there is also a fixed unity-gain Theater mode for use with surround-sound processors, which can be assigned to any of the inputs.

Both the volume control and the input switching appear to be implemented using 74HC4052 CMOS multiplexer chips, this a high-performance part with a low On resistance. Many, many years ago, I designed and built a small mixing console that used an early version of the 4052 chip for switching. One of the things I remembered was that the On resistance of those then-primitive FET switches was modulated by the signal—you needed to drive the 4502 with a buffer having a low source impedance and follow it with a stage having a high input impedance so that the modulation would not affect the signal. Looking at the Ayre's circuit board, the output buffer comprises two pairs of those valuable Toshiba JFETs for each channel, but there also seems to be a discrete buffer stage ahead of the array of 74HC4502 chips.

The interior of the chassis is dominated by a large power transformer painted blue. This is fed juice via Ayre's proprietary RFI filter, which is mounted behind the IEC AC socket. (This filter was also featured in the K-5xe.) Noticeable by their absence in the K-5xeMP's power supply are the usually ubiquitous three-terminal voltage-regulator chips; unless these are hidden under the board, it looks as if Ayre either doesn't use regulation for the voltage rails or uses discrete devices.

Why
It's the music that matters, of course, and with two of the three preamplifiers that had recently graced my system—Ayre's own KX-R ($18,500) and Simaudio's two-chassis Moon Evolution P-8 ($15,000)—having been cost-no-object designs, I wasn't sure quite what to expect. As I have written before, the preamplifier is the heart of an audio system, determining the nature of the overall sound. And I had never got to grips with the forward balance of the K-5xeMP's predecessor.

COMPANY INFO
Ayre Acoustics, Inc.
2300-B Central Avenue
Boulder, CO 80301
(303) 442-7300
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