BA660.doc
01/01/97
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The tip in adjustment is necessary because the point that the tube
conducts initially is both noisy and abrupt. This point varies
greatly from one tube to another, which is why the offset voltage
between the output of the gain control system and the point of
conduction is adjustable. This point is set so that with no limiting,
the tube conducts approximately 2 db. The reason for this is that
without it, the transition to limiting is sonically abrupt and
thumpy. More than 3 db of limiting doesn't really seem to help
and it comes right off the top of the maximum available limiting
which is tube bound to approximately 20 db.
Gain Cell Control
The audio signal is converted into a DC voltage that is an average of
the LOG of the input signal. This is then presented to a
logarithmic charge pump, whose charge time is adjustable linearly
in time/db. This results in an attack time that tracks the average
level of the input signal and is not affected by its crest factor. The
parameters of ratio, attack and threshold are set with linear dc
voltages. The release time is also determined via a log converted
charge pump, which resets the past memory of the average signal
level at a fixed recovery time. The objective was to provide
transparent gain control without removing the life from the
performance.
PRESETS
A linear DC voltage controls all of the parameters of the signal
dynamics, making remote and preset operation easy to implement.
The control elements are voltage dependent, not current, so that
they can have some variability in their switch elements. This
enables simple FET gates to provide the matrix selections. The
manner in which the preset voltages were obtained is a trade secret
but for some good PR we will tailor them for YOU; consult your
dealer or the factory for details.
Output Stage
The same circuitry from the previous gain stages is employed for the
output driver. The output stage itself is something else however.
The power supply winding that feeds the 60 VDC to the output
stage is balanced and floating in relation to ground. The circuit
can operate in balanced or unbalanced configuration like an
output transformer and allows either polarity of the output to be
grounded. An unbalancing switch is provided on the rear panel to
select the desired signal output to be grounded via an internal
relay.
