Altec 1566A:Vacuum Tube Preamp & Direct Box
First appeared in the January’97 EQ
Original ©1996 by Eddie Ciletti Most recent update June 2004
There are all sorts of tube
circuits, some of which actually deliver The Sound people
associate with Vacuum Tubes. Did you know, for example, that asymmetrical
distortion generates even-order harmonics while symmetrical distortion
produces odd-order harmonics? Neither vacuum tubes nor transistors
are exclusively responsible for either type of distortion, it’s really
about circuit design. Figure 1 shows distortion variations
in red and violet. Notice how the violet wave follows the green wave
perfectly during its positive excursion but falls short of the mark during
the negative swing. The red wave is more symmetrical, but does not "clip"
like an overdriven IC op-amp. This is what I like to call "The Sonic Airbag,"
or Electric Crayon effect. Op-amps are linear-clean right up until
the point of clipping, some discrete transistor and vacuum tube designs
have a non-linear window between clean and clipped. More...