Audio Developments AD149 - Manual (Page 64)

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Main-output limiters - calibration involves two presets per output.
Using the 'L' output as the example and referring to FIG 12 …
Biasing and threshold are adjusted as follows: VR1 biases the limiter circuit to the
point of correct operation and VR2 sets the threshold. (Labelled SET 0 and THR
respectively on the left-hand PCB.)
Determine whether the left-hand PCB is issue 3 and below or issue 4 and above. If
issue 4+, read ‘PIN 5 of IC2’ instead of ‘PIN 12 of IC1’ in the following paragraphs.
(Issue 4+ does not indicate an early improvement in circuitry; simply a change in
layout to make testing simpler.)
With no signal present, VR1 should be adjusted to give a reading in the
range
-1.5v to -2.5v at PIN 12 of IC 1. VR2 should be adjusted to give a reading in the
range -2.5v to -3.5v at PIN 10 of IC 1.
Apply a signal at 1kHz to the mixer to give a level of 0dBu at 'L' output. Then select
the limiter function and adjust VR1 until the output signal starts to fall (typically
0.2dB). The DC voltage at PIN 12 of IC 1 should be approximately -2.0v.
After setting VR1, adjust the output signal to just greater than +8dBu with the limiter
deselected. Introduce the limiter and adjust VR2 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 IC 1 should be
approximately 0.6v different from that at PIN 12.
The right output can be set using the above 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 L and R outputs are set up as a pair. After following
the above procedure, the DC voltages at PINS 10 and PINS 12 of the ICs
should be identical. (Tolerance ±0.05v with typical figures at PIN 10 of -2.6v and
PIN 12 of -2.0v)
There are two DIL switches associated with the attack and release times: one for the
left output, the other for the right output.