Odd or even?

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I arrived in Las Vegas last night to attend the annual Consumer Electronics Show (CES) and so the title of this post seems appropriate - except it's not about red or black, odd or even on the gambling tables - rather we're talking briefly about distortions. Several of you wrote and asked me why FETS and tubes sound "warmer" than bipolar transistors after yesterday's post and I wanted to briefly touch on that. Every device type we might choose to design an output stage or an amplifier of any kind has inherent distortion products. These distortions are different in a current amplifying device (like a bipolar transistor) than they are in a voltage amplifying device (like a FET or tube). The differences are in the type of harmonics they produce, odd or even. Harmonics are simply added signals at a higher frequency than what we started with. So, let's imagine we have a single tone of 1,000 cycles or Hertz. When this tone passes through our device we generate more than just this fundamental frequency: we also generate 2,000 Hz, and 3,000 Hz tones at the same time (and 4,000 and 6,000 and so on). These extra added "bonus" tones are called harmonics and you've probably heard the term THD? That stands for Total Harmonic Distortion - those distortions referred to in the THD acronym are the addition of these unwanted tones. Voltage amplifying devices produce more even tones and current amplifying devices produce more odd tones. Even refers to the tone being multiples of 2 (2K, 4K, 8K) and odd is multiples of 3. We humans find even tones more pleasing and warm than odd tones. Think of harmony in music: if you play two piano keys that are next to each other, the result isn't pleasing - but skip one and those two sound harmonious. That isn't an exact explanation so don't jump up and down please, but you get the point. The bottom line: if you have to accept distortion products, might as well make them pleasing. Now, should I choose red or black?
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Paul McGowan

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