Tube DIY Asylum: Re: Partial slew rate distortion by Lynn Olson Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by Lynn Olson (M ) on June 6, 2002 at 13:16:23

In Reply to: Re: Partial slew rate distortion posted by Lynn Olson on June 6, 2002 at 12:44:55:

Quick Note: So far we've been discussing straight (resistive) and elliptical (reactive) load-lines. This assumes distortionless loads! In reality, the lines themselves are kinked or bent a little ... with a transformer, for example, things get really weird as we approach 20 Hz. Our pretty little ellipse actually gets dented, something no tube can correct for. The presence of DC, and the choice of core material, have a big, and clearly audible, effect on the linearity of transformers and chokes.

Don't think we're home free with active loads either; slew rate considerations *do* apply for current sources, so even fancy active loads can add unexpected colorations. Current sources sound quite different from each other, and who knows which one is "right." Those tiny little semiconductor pF's are a lot more audible in the midrange than you'd think. (As capacitors go, bipolar and MOSFET transistors are very low quality, with base/gate capacitance modulated by current, voltage, and temperature.)

So even a dumb load-line has slope, elliptical reactance, and nonlinear distortion (which in turn is frequency-dependent). All this stuff is cross-multiplied by the plate curves of the tube itself. Just think, all this in one tube stage!

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Kimber Kable

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