Impedance and other matters

OTL Asylum

OTL, Output Transformerless Amplifier User Group.

0

Re my previous post about determining
output impedance in an amp:
I neglected to make it clear that many of the
calculations we use in tube-audio designs are
APPROXIMATIONS. The commonly-used formulae were
usually derived many years ago, and left out some
of the lesser terms to simplify the calculations
in the era of slide rules.
In general, if calculated figures are within 20%
of the actual measured performance, you are doing
well. An additional influence is the simple fact that
individual sample tubes can vary 20% in bias point,
transconductance, and mu plus in other quantities.
Analog design remains an art as well as a science.
I've noticed in the past that many beginning
DIYers expect calculations to be spot-on. Often
they are disappointed to hear that it usually
is NOT possible to get exactitude from common
formulas. The ONLY way to get exact figures is
through modeling, of the PSPICE or other type,
using actual measured electrical behavior of the
components. And real-world component variations
can still "screw up" your simulation.
As I said, design is an art and a science.
And tubes are relatively well-

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Topic - Impedance and other matters - Eric Barbour 11:30:49 06/01/98 (0)