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AMP2-VSDA/AV and AMP2-VSDA/AM User Manual P/N 821576 Rev-B
AES Digital Gain Calibration Fine Adjustment (919050)
The digital gain calibration is set at the factory for +4 dB (Analog) = -20 dBFS (Digital). Three other values (+8 = -20, +6 = -9, and
0 = -18) may also be user selected via a DIP switch on the rear panel (Item H, page 14). However, if a fine adjustment of the
calibration is desired, perform the following procedure:
1) Remove the top cover. Save screws for later reassembly.
2) You must remove the 919083 PCB to access the trim pots (P1-P4) on the 919050 PCB (see Figure-3a below for trim pot
locations).
3) Connect a digital (AES) source of known digital amplitude and set the unit's controls to monitor that source. For convenience
sake, it would be best to use your facility's digital reference level, but any nearby level may be used with appropriate scaling of
output level required.
4) Using an accurate DMM (preferably one having direct reading in dBv/dBu), measure the level on the Converted Analog Output
phoenix connectors (Item I, page 14). Be sure to make measurement of the FULL balanced output, NOT just between one of
the outputs and ground!
5) Locate trim pots P1, P2, P3, and P4 (see Figure-3a below). Adjust P4 (channel 1) through P1 (channel 4) for the desired analog
level corresponding to the digital signal amplitude being supplied to the unit. For example, if your "house" Analog/Digital
correspondence is for a -18 dBfs digital level to correspond to an Analog level of +0dBu, it would be easiest to set the digital source
to -18 dBfs, set the gain calibration DIP switch for -18dBFS = 0dBu and then set P1 through P4 for 0dBu outputs on the phoenix
Analog Output connectors. But, if a digital source of -20dBfs is applied, even though the facility references are for -18dBfs, then
the analog output levels should be adjusted for a -2.0 dBu analog output level on the phoenix connectors.
Note: If the meter being used to measure the analog outputs is not so accurate at those relatively low signal levels, you may wish to
increase the digital signal amplitude by some convenient factor like 10 or even 20 dB. (But of course you may not increase above
digital full scale.)
© 2004 Wohler Technologies Inc. ALL rights reserved
Figure-3a:
AES/EBU to Analog Module PCB (919050)
Digital Gain Calibration Trim Pot Locations
P4 - Adjusts Channel 1
P3 - Adjusts Channel 2
P2 - Adjusts Channel 3
P1 - Adjusts Channel 4
Section 3: Technical Information
21
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