September Remember the essays you had to write in high school? Topic sentence, introductory paragraph, supporting paragraphs, conclusion. The conclusion being, say, that Ahab in Moby Dick was a Christ-like figure. So I'm going to try to give the other side of the story:
The Genesis I've been preparing to write this article for many years! In order to make this investigation, I needed to create a laboratory-grade distortion processor, which I've just completed designing and building.
Let me introduce you to Bob's Blender Fig 1. In a moment we'll discover its features and controls. The underlying topologies in analog amplifiers, preamps, and equalizers have been evolving for decades.
At first there were only tubes: Then, of course came solid state, the germanium bipolar transistor which was not particularly good-soundingthen the silicon bipolar transistor, different types of FETs, discrete opamps which can be tube or solid stateand integrated circuit opamps which often contain hybrids of bipolar and FET elements.
Some people have claimed that the choice of individual components governs the sound of the circuit e. So the circuit and layout are just as important as the component topology.
That said, the first amplifier component ever designed, the triode tube invented in the year !
For my Blender, I chose a low voltage, low gain, ultra-miniature, metal and ceramic Nuvistor tube, the one for each channel. The Nuvistor's footprint is smaller than a dime and it's not even as tall as the first joint of my thumb!
My tube blender is flat, not even a tenth of a dB drop at 89 kHz, the limit of my measurement system, so I don't know where it drops off! Of course bandwidth also depends on the circuit, so one designer can make a 12AX7 extend higher in frequency than another through use of optimized impedances.
Bob's Blender is a processor that adds calibrated, controllable and repeatable amounts of second harmonic distortion, as little as 0. The ultra-miniature Nuvistor, from Wikipedia: Photo By Jim Rees en: The sonic nature of triode distortion and saturation.
Triodes have been variously characterized as sounding "smooth", "sweet", "round", "creamy", "warm", "three-dimensional" and "present". This is largely thanks to the presence of second harmonic, the triode's dominant harmonic component. Distortion is signal compression, so as the input level goes up, the triode softens transients by adding more second harmonic twice the fundamentaland eventually other harmonics when levels get very high.
In contrast with many solid state circuits, it starts to round off the corners in a gentle way.
Other devices and topologies including tetrode and pentode tubes are more prone to adding higher harmonics, which are known to sound harsh compared with the second harmonic that predominates in triodes.
Witness the warm sound quality of the Mjolnir Pure Bipolar headphone amplifier and many of the Pass products as well as the Bricasti power amp and power amplifiers from Krell, many others.
From on, many designers have been seeking ever lower harmonic distortion figures. The earliest solid state devices exhibited similar distortion numbers, but these values quickly lowered, and by solid state audio circuits produced lower distortion than the analyzers of the day were capable of measuring.
I wonder if the race for the lowest distortion figure was sponsored by the measurement equipment makers! I remember in my youth lusting over an analyzer which could measure as low as 0. That analyzer was soon superseded by another and yet another, each successor capable of measuring an order of magnitude lower.
Some of today's amplifiers measure as low as 0. Lower is good, right? Not so fast, number crunchers! Let's cut to the chase, it was the increasing use of negative feedback that engendered ever-lowering distortion figures. Negative feedback is a perfectly good mathematical technique that works very well.
But the designer has to realize that the circuit may not sound good even if its measured THD total harmonic distortion is very low. If the designer pays attention solely to the THD value, he may not realize that sometimes the "good-sounding, lower harmonics" get suppressed at the expense of the "bad-sounding, higher harmonics".
Research by Norman Crowhurst points out that feedback mostly reduces the level of the 2nd and 3rd harmonics, leaving the upper harmonics more or less alone, or sometimes at higher level than before feedback.
It doesn't take very much level of a 5th, 7th, or 9th harmonic to make the sound "bitter", "dry", "clinical", "cold" "small" "dynamically limited" or "harsh". This is all part of the psychoacoustics of distortion and masking.
In today's investigation I aim to get a handle on how to use distortion to improve the sound quality of our playback gear. Yes, I said "improve", because we're going to redistribute the harmonic emphasis of our signal chain.Ben!
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September Remember the essays you had to write in high school? Topic sentence, introductory paragraph, supporting paragraphs, conclusion.
The conclusion being, say, that Ahab in Moby Dick was a Christ-like figure. Oy.