Balanced Audio Technology VK-D5 CD player I Sing the Body Electric

Sidebar 2: I Sing the Body Electric (or Let's Get Differential)

Designer Victor Khomenko goes through a lot when designing BAT products. For one thing, he "listens" to the circuit boards and components.

Victor: "We have to listen to the noise they make to know how they're behaving. Listening through a normal home audio system, you'd hear no noise and think everything's fine. But we have to go deeper than that. Like a doctor with a stethoscope, we amplify the completed boards to check their heartbeat and breathing. The nature of the noise tells us a lot. It should sound uniform, clean, and, well, beautiful—nothing forward or abnormal. We also listen to the sound of individual components, their reactions to vibration and shock. And we analyze the boards to find their resonant nodes so we can tie them down with judiciously placed screws rather than at random. And this, of course, affects the boards' layout."

When I mentioned the no-jitter advantage of single-box designs, Victor surprised me. "Well, we keep jitter down by using a proprietary servo board that synchronizes the Philips transport mechanism to a single master clock. But I think what's more important is to look at the entire circuit. For instance, after the DACs there are several elements that must be in place. First there's current/voltage conversion, then an analog reconstruction filter to remove the sampling frequency, and then a gain stage typically followed by a buffer. The buffer is often an op-amp, which is also used quite a lot for the current-to-voltage conversion. Did you know there are a number of designs that claim to be vacuum tube, but in reality are nothing more than typical [solid-state] implementations with a tube cathode-follower buffer? If you take out the cathode followers on these players, it changes nothing! It's just 'tubes-in-the-box' marketing.

"Our own circuit is quite unique," he finally offered. (What took him so long?) "The I/V converter, filter, gain stage, and buffer are all packaged in one circuit that has a particular frequency-response characteristic. This accomplishes another goal, that of reducing cost by using fewer components. Since most balanced designs have four separate circuit elements multiplied four times for differential operation, well, that's a lot of circuits. But by combining individual building blocks, BAT's gain stages operate with only four elements.

"The output of the DACs is delivered to a unique current/voltage converter using high-quality Vishay bulk foil resistors in a passive, differential configuration. The signal is then amplified by a tubed circuit using high-current plate-loaded triodes. It's an extension of our Unistage construction [as] found, for example, in the VK5 preamp. Plate loading uses a much lower degree of local feedback than typical cathode-follower designs, which use 100% local feedback."

Victor ruminated further on true differential operation: "It's common in many designs to see all four phases of the balanced signal treated separately. But you understand that the signal is not really in all four phases, it's in the difference between each pair—the differential signal. The rest is simply garbage and noise. So we totally decouple the differential signal from ground, and in doing so eliminate common-mode 'intrusion.'

"Here's how it works. You've got two signals, common-mode and differential. Common-mode can be of a very large value, but the heart of the signal is the small difference between the two phases of the differential signal—it's quite minute. Often these small signals arrive on top of 100V of common-mode signal! Looking for small variations in large values can throw you off. It's far better to look directly at the much cleaner small signal.

"The problem arises with these other designs that treat the four phases as independent signals. That reintroduces large-value common-mode signals that contaminate and blur the picture. At all times the signal within the VK-D5 is treated differentially as a combined music signal derived from the difference between the two phases."

And don't you forget it.—Jonathan Scull

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