Ayre Acoustics V-3 power amplifier
I knew this last one to be untrue because I'd had a V-3 hand-delivered to me last year, but before I could even get it connected to my system, I had to send it back. Ayre needed it to demo at HI-FI '95—they couldn't keep up with demand.
As a rule, I applaud any company that takes the time to build its dealer network and shake the bugs out of its products before submitting them to the audio press. Companies that build products that excite dealers and satisfy consumers usually have produced mature designs that—not so coincidentally—excite and satisfy reviewers as well. But I was losing patience with Ayre—surely, by anyone's definition, the V-3 was mature. "Enough with satisfying customers already," I whined. "Send me the damned amplifier!"
"We'd just love to, Wes," purred Marketing Director Bruce Van Allen, "but we're sold out. How'd you like to hear our new K-1 preamp instead?"
"I'd just love to, Bruce. But I consider it unfair to audition a balanced preamp through a single-ended amplifier—which is all I have around the house. I couldn't really work it in until someone who makes a truly balanced amplifier managed to get me one to use with it."
"[sigh] We'll find you a V-3."
God, I love my job.
Resistance is futile
As Sam Tellig details in this issue's "Sam's Space," the V-3 utilizes some innovative technology—as well as some really ancient knowledge. To understand how this affects the sound of the amp, we need to examine how an amplifier works in the first place.
Quick! How does an amplifier work? Most people would answer that amps boost or magnify the small signals coming into them—but that's not quite right. In reality, they create a new signal that's a copy of the original. The amplifier reacts to changes in the input signal so that the voltage at the output changes in proportion. As Charles Hansen says, "The little input signal modulates the greater output voltage of the amplification stage, which keeps it proportional to the input. We've made a copy of it. The output is in scale with the original signal and resembles the original, but it's a new version of it."
There are two other factors at work. First, the speakers are connected in series with the internal impedance of the amplifier—this is where the issue of feedback comes in (footnote 1). At the same time, current flows from the power supply directly into the speakers. We can thus draw a few conclusions about amplifier sound quality: 1) the quality of the sound depends upon the accuracy of the copy of the input signal; 2) the speaker becomes part of the amplifier's circuit; and 3) the power supply has a direct influence on the sound coming from the speaker.
So what does Ayre do differently? To begin with, they've pared down the chain of stages making copies of the original signal. It's not unusual for amplifiers to have five stages: input, voltage-gain, pre-driver, driver, and output.
"Each time the signal goes through a transistor, you get a copy—so you can imagine that you eventually are copying a copy of a copy of a copy," explains Hansen. "You lose resolution every time. So we've tried to reduce the chain to the minimum number of stages. We have an input stage, which is a complementary differential quad, which drives a folded cascode—which, I'll grant, is a stage, although it's a debatable point—that, in turn, drives the output stage.
"Reducing the number of stages means we have to be clever about the design, though. MOSFETs are easier to drive than bipolar transistors—if we used bipolars, we'd need to have an emitter-follower triple (an emitter-follower driving an emitter-follower driving an emitter-follower) in order to have a low enough input impedance. We just have the MOSFETs, so we don't need that extra driver stage. But because we drive the MOSFET directly with the folded cascode, which has no current gain, we have to have enough current in the cascode to drive the input capacitance of the MOSFET. This means the input stage has to have the same amount of idle current as the cascode. That's why, when you look inside the V3, the input transistors are TO220-packaged MOSFETs—they look like a lot of amps' output devices!"
The use of MOSFETs, which offer low distortion and high current, also minimized the need for overall negative feedback. But mention the power supply and Hansen really waxes rhapsodic. "When you understand how an amplifier works—that you're making a copy of the signal by modulating the power supply—then you see that the power supply directly drives the speaker. Anything that's not right about the power supply is going straight into the speakers. That's why we use such a huge power supply—take off the lid and you'll see that about 90% of the innards are the power supply. We have a giant transformer, stacks of chokes, and all those filter capacitors to regulate the front-end. Compared to the audio circuitry, whether you're looking at board-space or cost, that stuff represents 80-90% of the design.
Footnote 1: Typically, loop negative feedback is used to lower distortion and reduce output impedance. Because the feedback voltage is proportional to the ratio between the internal impedance of the amplifier and that of the speaker, changes in the speaker's impedance can cause changes in the feedback level—which affects the sound.—Wes Phillips