Ayre Acoustics K-1 preamplifier Page 2
Looking inside the K-1, one is tempted to comment upon how empty the box seems. There's a circuit board attached to the front panel to handle logic functions, and affixed to both the right and left side-walls are the phono modules. But what dominates the interior is the massive, component-packed board that runs across the height and breadth of the rear panel. Attached to it is a second panel that houses the Rube Goldberg-like gears and ladder-belts which connect the motor to the complementary volume controls and allow for stereo source and recording switching. The device looks more mechanical than electronic, the only wires evident being the ones connecting the phono modules to the primary boards.
Everything for the line stage is on that huge double-sided circuit-board, densely packed with high-quality parts, just behind the rear panel—all the connectors, circuitry, regulators, switches, volume controls. The signal never leaves that board. There are no traces longer than an inch. And unless you add the optional phono stage ($1600), it's wireless. The most dominant features on the board are the silver contacts for the volume control—an amazingly precise stepped attenuator that Hansen credits with being the preamp's sonic heart. Since the preamp is balanced and dual-mono, there are actually four sets of silver contacts—a pair on each face of the double-sided board.
The phono circuit uses the same three-stage topology that Charlie Hansen refers to as "the Ayre building-block" (input differential, cascode, source follower). "Actually," he confessed," we had to cascade two of those to get the gain we needed. We leave out the first buffer because we don't need it—so the phono has an input differential, then a cascode, then another input differential, and another cascode, and then the output buffer."
As phono cartridges are, by nature, balanced generators (unless hard-wired with one side of the coil connected to ground), the phono boards connect to XLR inputs on the rear panel (occupying the B2 position). This means that most users will have to re-terminate their tonearms with new cables. While the appropriate harness is not yet a stock item, most cable companies can supply it. Both gain and cartridge-loading are user-adjustable via small metal-film resistors that plug into terminal strips on the boards. The gain is factory-set at 50dB, but resistors are available to offer gain adjustment in 10dB increments from 40 to 60dB. "We can give you more," Hansen avows. "I just can't imagine that you'd need it."
The default cartridge loading is 50k ohms, but additional resistors give you the option of loading to 100, 249, 499, and 1000 ohms—hey, if you need something less standard, just get the appropriate resistors and plug 'em in. Ayre likens the complexity of changing gain or cartridge loading to that of changing a phono cartridge. It's actually a lot simpler than that, but what I think they mean is that, if you're comfortable changing your cartridge, plugging in the resistors should pose no problem. Although the resistors are really tiny, the phono boards are reasonably accessible, so it's not that difficult. Except, that is, for holding the resistor firmly enough to plug it into the terminal strips; you could use tweezers or needle-nosed pliers as Ayre recommends, but, in my opinion, the real tool for the job is a pair of hemostats.
I got distracted when I went to change the loading—the phono boards are remarkable pieces of work. The EQ is purely passive and split into two parts, which, Hansen explained, "allows us to separate the HF and LF sections. Typically, all of the RIAA stuff is done in the feedback loop, and the turnover points interact with one another. Our way works better and is dead-nuts quiet."
The logic board mounts to the front panel and has two basic functions: mute and remote. Muting turns out to be more complex than I would have thought: There's a timer, which mutes the circuit upon turn-on. But there's also a voltage monitor, so the mute will engage if the wall voltage drops to 90V; then, 90 seconds after the voltage comes back up, it will disengage. In addition, there are two ways to control the mute circuit: from the switch on the front panel and from the optional remote (footnote 2).
The rest of the logic board involves the remote function. "In the case of the volume control, it has to do some extra thinking," Hansen laughed. "It took a while to figure out how to do it. We use a stepper motor to move the rotary volume knob, which has 46 positions. The problem is that stepper motors themselves have four positions—A, B, C, and D—then they recycle. So the motor can only keep track of A and B and C and D, but not which A, B, C, or D it's set to. If you use only the remote, you'd be fine. However, if you also use the manual knob, the motor might think it was at A when it was on C. So I used the ladder-belts to gear the stepper motor so that every click on the volume knob took the motor through a full four-step cycle. That way the motor is always set to A, so the two never get out of phase."
The volume knob is a stiff one; you have to put some real muscle into turning it. And the remote does cycle through with a strangely satisfying whir, whir, whir, whir, thunk for each step. I am frequently frustrated by stepped attenuators—the precise level I want always seems to rest between clicks. Not this time: I never felt that I lacked the ability to fine-tune playback to the precise level that a recording required.
"The problem with most remote controls," maintains Hansen, "is that you have a microprocessor in there. That means there's a high-frequency quartz oscillator generating a 4MHz signal, and a 4MHz RF generator is not a good thing to have inside a preamp. Rowland is smart—he has them shut down, except when they're functioning. We do it differently—we designed a whole discrete logic circuit that operates at 100Hz, and only long enough to perform the function."
Footnote 2: The remote adds $250 to the price of the unit and controls volume and muting only. All input switching must be done manually. The hand-held remote is carved out of a solid block of aluminum and feels satisfyingly solid in the hand.