A M S - 1 0 2 7 A M P L I F I E D S P E A K E R S T A T I O N
3 - 2
any of the drivers (speakers) are replaced, it may be necessary to change some of
these components to achieve maximum flatness of response.
The operating threshold of the woofer limiter is critical to both satisfactory
reproduction of musical transients and preventing damage to, or destruction of,
the speaker itself. The side speaker output limiter circuits are similarly important,
though not as critically adjusted.
Note: The woofer power amps are arranged in a bridge configuration; care must
be taken to avoid letting either speaker terminal contact the chassis (common) or
the grounded lead of any test equipment so as not to short out the power amps.
The side speaker outputs are single-ended, so these precautions are not as
important for them.
GENERAL LEVEL METER AND LEVEL GAIN CALIBRA-
TION OBSERVATIONS
The display driver circuit provides a total of 23 dB of visual audio level
indication via a 10-segment LED bargraph. Meter calibration adjustments are via
two recessed trim pots (right and left) on the rear panel labeled “LEVEL ADJ.”
Please note that these trim pots also simultaneously adjust the gain at the left and
right input connectors. The range of adjustment is -10dBu to +10dBu
(nominal). The driver PCB connects the LEDs in the bargraph in a
series
string.
The anode of the LED corresponding to the
lowest
input level (-20) is connected
to V+, with the other 9 LEDs following, and the outputs of the bargraph
connected to the node between each LED.
Note: Only one output of the bargraph is active (low) at any one time, so if the
LED string is “broken” (open) at any point, the bargraph will go OFF whenever
the input level is at or above the level corresponding to the break.
AUDIO CIRCUIT DESCRIPTION
INPUT SECTION
The balanced input is buffered by a differential input of the audio signal to dual
op amp ICs 1a and 2a. The output of ICs 1b and 2b goes through P1 and P2,
which simultaneously sets the input volume gain and VU meter driver levels and
then proceeds to the input of the quad VCA (U4) after receiving a slight treble
boost. The output from the quad VCA (U4) then goes to a dual rectifier (IC5),
used to drive the VU meters. This setup also allows the inclusion of unbalanced
signals via the RCA jacks on the rear panel.
VOLUME CONTROLS
Volume controls are implemented through a quad VCA chip (U4). The control
potentiometer for each VCA is buffered by IC3 and IC4. Sections A and B of the
dual IC each buffer one volume control. Counter-clockwise rotation of the
volume pot decreases the wiper voltage causing the IC3A and IC4A output to
The display driver circuit
provides a total of 23 dB of
visual audio level indication
via a 10-segment LED
bargraph.