Advertisement
For a long time there has been faith in the technical community that eventually some objective
analysis would reconcile critical listener's subjective experience with laboratory measurement.
Perhaps this will occur, but in the meantime, audiophiles largely reject bench specifications as
an indicator of audio quality. This is appropriate. Appreciation of audio is a completely
subjective human experience. We should no more let numbers define audio quality than we
would let chemical analysis be the arbiter of fine wines. Measurements can provide a
measure of insight, but are no substitute for human judgment.
As in art, classic audio components are the results of individual efforts and reflect a coherent
underlying philosophy. They make a subjective and an objective statement of quality which is
meant to be appreciated. It is essential that the circuitry of an audio component reflects a
philosophy which address the subjective nature of its performance first and foremost.
Lacking an ability to completely characterize performance in an objective manner, we should
take a step back from the resulting waveform and take into account the process by which it
has been achieved. The history of what has been done to the music is important and must be
considered a part of the result. Everything that has been done to the signal is embedded in it,
however subtly.
Experience correlating what sounds good to knowledge of component design yields some
general guidelines as to what will sound good and what will not:
1) Simplicity and a minimum number of components is a key element, and is well reflected in
the quality of tube designs. The fewer pieces in series with the signal path, the better. This
often true even if adding just one more gain stage will improve the measured specs.
2) The characteristic of gain devices and their specific use is important. Individual variations
in performance between like devices is important, as are differences in topological usage. All
signal bearing devices contribute to the degradation, but there are some different
characteristics are worth attention. Low order nonlinearities are largely additive in quality,
bringing false warmth and coloration, while abrupt high order nonlinearities are additive and
subtractive, adding harshness while losing information.
3) Maximum intrinsic linearity is desired. This is the performance of the gain stages before
feedback is applied. Experience suggests that feedback is a subtractive process; it removes
information from the signal. In many older designs, poor intrinsic linearity has been corrected
out by large application of feedback, resulting in loss of warmth, space, and detail.
High idle current, or bias, is very desirable as a means of maximizing linearity, and gives an
effect which is not only easily measured, but easily demonstrated: Take a Class A or other
high bias amplifier and compare the sound with full bias and with bias reduced. (Bias
adjustment is easily accomplished, as virtually every amplifier has a bias adjustment pot, but it
should be done very carefully). As an experiment it has the virtue of only changing the bias
and the expectations of the experimenter.
As the bias is reduced the perception of stage depth and ambiance will generally decrease.
This perception of depth is influenced by the raw quantity of bias current.
If you continue to increase the bias current far beyond the operating point, it appears that
improvements are made with bias currents which are much greater than the signal level.
Advertisement
