A general reader of i-fi journalism and discourse—someone who isn’t an audio engineer, or the equivalent thereof in scientific expertise—has to push against a stiff headwind of technical terminology, specifications, and jargon. You have to resign yourself to encountering a certain degree of incomprehensible wonkishness, accept that it’s appropriate, and skim past it. Occasionally you’ll suspect that there’s a touch of professional grandstanding, obfuscation, or sheer mumbo-jumbo at work. But once in a while, the hi-fi layman is rewarded with a scientific exegesis that not only makes explanatory sense, but has a kind of poetry. The other day I was listening to an episode of Home Theater Geeks, a weekly TWIT Network podcast hosted by Scott Wilkinson. The guest was Bob Carver, a legendary audio designer, innovator, and all-around idiosyncratic genius who owns a long list of hi-fi patents. Carvers’s finest hour, I’d argue, came in 1985, when he made a bet with the editors of Stereophile that he could modify his $700 Carver Model 1.0 solid-state amplifier, on the spot, to match the sound of any reference (i.e., state of the art) amplifier Stereophile decided to throw at him. The magazine accepted the challenge, and chose the Conrad-Johnson Premier Four, a 100-watt tube amplifier that cost $3,000. When Carver emerged after a couple of days of tinkering in a hotel room, his modified amp was a perfect match for the Conrad-Johnson, and he won the bet. J. Gordon Holt’s rueful, impeccably fair-minded account of Stereophile’s unconditional defeat is an audio-journalism classic and a perennial parable of the danger of audiophile certitude. But back to the podcast. At the end of the hour, Carver was asked for his thoughts on the relative merits of solid-state amplification and tube amplification. The valves-vs.-transistors war has now been going on for decades (Stereophile’s “The Carver Challenge,”of course, was one of its great skirmishes) and it will probably go on forever. I prepared for an answer that would leave my usual mystified state intact. What I heard was something beautiful, lovely, and lucid. First, notwithstanding his 1985 triumph, Carver declined to pick sides in the endless feud:
I grew up with tubes. I love vacuum tubes. They glow in the dark. I have a wonderful warm feeling when I listen to my music with vacuum tubes. A vacuum-tube amplifier and a solid-state amplifier can be made to sound virtually indistinguishable. That’s the reality of it. However, when I listen to vacuum-tube amplifiers, there’s a magic there that I adore and love, and I’m not about to give it up. I have both amplifiers in my listening room, I have vacuum-tube amplifiers, I have our latest solid-state amplifiers. They both sound stunning to me.
Then Carver outlined the concept of “the feedback signal”:
If we listen to a vacuum-tube amplifier, when it produces a signal, it makes the loudspeaker move. The loudspeaker sends sound waves into the room, the sound waves bounce off the wall, they come back to the loudspeaker, and the sound waves make the loudspeaker move like a microphone. That loudspeaker motion makes a little voltage, just like a microphone would. The voltage is fed around the amplifier’s feedback loop back to its input. All amplifiers have two inputs. One is the audio signal that comes from the [playback source], the other is the audio signal that is the feedback signal. Both of these signals are the same. They’re the same amplitude, the same frequency response, everything.
But then Carver blew my mind:
What happens in a vacuum-tube amplifier, the amplifier makes another sound that is related to the sound it heard. In other words, the amplifier is able to listen to the room. Because it’s hearing reverberation, echoes, time delays, all of the components associated with the venue. The loudspeaker speaks, and the room speaks back to the loudspeaker. The amplifier hears it, via the signal going back around the feedback loop, and out it comes again. It’s not delayed by much; the real decay is the acoustic delay. That delay makes it sound spacious and big to our ear-brain system. We love sounds that have ambience, and echoes, and stuff like that.
He went on to explain why solid-state amplifiers behave differently:
The output impedance of a solid-state amplifier is so low that when it tells the speaker to move, the speaker sends the wave out, it bounces off the walls, comes back, and the solid-state speaker will not allow the speaker to move in response to the sound wave coming back and hitting it. The amplifier shorts the speaker out so that the speaker can’t move on the back wave. The amplifier is said to have a very low output impedance. It shorts the speaker out, basically. A higher output impedance is one of the things we hear when we listen to vacuum-tube amplifiers. It’s one of the things that makes a vacuum-tube amplifier sound so enjoyable and so nice and so spacious.
Needless to say, I have absolutely no way to judge the technical merits of Carver’s explanation. I’ve read that other factors, including things called “soft clipping” and “core saturation,” are said to contribute to the sound of tube amplifiers. The science may be in dispute, Carver’s disquisition an oversimplification. I don’t care. Bob Carver’s the man. He’s made me believe I finally have a fleeting grasp of the meaning of “output impdedance.” What he said, that’s how I’m gonna roll.
[Note: I’ve edited and slightly condensed Carver's recorded comments for the sake of readability.]
Photo by arbyreed