Audio Developments AD144 - Manual (Page 26)

25
Output limiters - calibration involves four presets per output; however, only two are
involved with a particular parameter. Using 'A' output as the example…
Biasing and threshold are adjusted as follows: VR104 biases the limiter circuit to the
point of correct operation and VR103 sets the threshold. (Labelled SET 0 and TH
respectively on the left-hand PCB.)
With no signal present, VR104 should be adjusted to give a reading in the
range -1.5V to -2.5V at PIN 12 of IC7. VR103 should be adjusted to give a reading
in the range -2.0V to -3.5V at PIN 10 of IC7.
Apply a signal at 1kHz to the mixer to give a level of 0dBu at 'A' output. Select the
limiter function and adjust VR104 until the output signal starts to fall (typically 0.1dB).
The DC voltage at PIN 12 of IC7 should be approximately -2.0V.
After setting VR104, adjust the output signal to just greater than +8dBu with the limiter
deselected. Introduce the limiter and adjust VR103 until the output signal falls to
+8dBu. This is the THRESHOLD setting. (If a different threshold setting is required,
alter the signal levels accordingly.) The DC voltage at PIN 10 of IC7 should be
approximately 0.6V different from that at PIN 12.
The other outputs can be set using this procedure, but the preset and IC numbers
change to correspond with the output being calibrated.
The LINK function has no individual setting of its own. To ensure this works within
specification it is important that A & B and C & D outputs are set up in pairs. After
following the above procedure, the DC voltages at PINS 10 and PINS 12 of the ICs
are identical for each pair of outputs. (Tolerance ±0.05V with typical figures at PIN
10 of -2.6V and PIN 12 of -2.0V.)
The ATTACK time is varied by VR102. The lower the resistance across
VR102 (counter-clockwise), the faster the attack.
The RELEASE time is varied by VR101. The lower the resistance across
VR101 (counter-clockwise), the faster the release
(If appropriate timing or measuring equipment is unavailable, VR101s and VR102s
should be set in the same position, as accurately as possible, by eye.)
For correct operation, it is essential that the attack times for each pair of outputs and
the release times for each pair of outputs be the same, ie A & B and C & D.
(The two PAIRS may have different settings.)