BA660.doc
01/01/97
19 of 25
Level Meter
The 10 segment level meter employs a unique combination of 2
separate audio level meter circuits. The output of a VU driver and
a Peak driver are hard wire Or'ed to obtain the dual metering
function. In order to distinguish between the ballistics of the two
metering circuits the visual intensity of each scale is varied
according to signal level. The peak scale is dim for the first 7
LED's and then is bright, whereas the Vu scale is Bright for the
first 7 LED's and dim for the last 3 segments. This is set so that a
wide dynamic range signals can be simultaneously displayed along
with subtle 1 dB changes in signal level. The signal is selected at
the insert return point and just prior to the mute relay at the
output. signal is attenuated by 18 db to obtain the dynamic range
necessary for operation. The signal is converted to single ended at
the metering detection circuitry. The signal conditioning is
performed on the gain cell control PCB, and the DC meter drive
is presented to the front panel board independently.
Gain Cell
The gain cell employs the same medium Mu twin triode as the gain
stages. The tube is used as a transconductance shunt element.
What this means in layman's terms is that the conductance of the
vacuum tube, (i.e. resistance) is a function of the ratio of the
Cathode current to the grid bias. If you will look at sheet 7, and
note R12 and R13, these are build out resistors for the tube acting
as a shunt resistor. The cathodes are shorted together, so the more
the tube conducts, the closer the two plates are to each other the
more attenuation through the circuit. The transistors above and
below are there to control the bias and no audio flows through
them! The plate circuit and the cathode circuit are forced by the
gain control line which appears on the right side of R6 to increase
the current through the tube, in equal and opposite directions.
The two tube sections are part of the same tube, therefore will
track one another. The transistors are part of a super-matched
array, so they will track one another, and U1A sets the quiescent
bias point with no limiting. The zener Z13 clamps the cathodes to
+6 v above the grid so that the tube doesn't become a diode.
Whenever the control line goes above .75V the gain cell clip
indicator comes on and shortly thereafter distortion ensues.
The gain reduction bypass is a relay that simply cuts the tube off by
putting a large negative bias on its grids. We also insert a 2.5db
loss via shunt resistor elements to accommodate the tip in
characteristic of the tube.
