© 2005 Wohler Technologies Inc. ALL rights reserved
14
VM SDI Digital Series
User Manual P/N
821625
Rev-
A
Section 3:
Technical Information
General Mechanical Observations
Elimination of cabinet and component sympathetic vibrations (resonances) requires considerable attention to mechanical details.
Because of this, and the physical constraints of the speakers' acoustic enclosures, even minor changes to any of the mechanical
details of the unit can seriously impair its acoustic performance. This especially applies to the speaker mountings. If mechanical
work on the unit is necessary, be sure to make adequate notes to permit accurate reassembly.
The proprietary method of magnetic shielding in the 2U models is usually degraded slightly by any disassembly of the unit, except
removal of the rear panel. Almost any maintenance or repair will require removal of the cover. If an immediately adjacent video
monitor shows magnetic interference after reassembly of the unit, it may have to be returned to the factory to restore the shielding
completely.
General Audio Circuitry Observations
Since a single-sided power supply is used, all amplifier sections are “biased” with a 1/2 supply reference, so all opamp signal
terminals on the main board should have a DC level of +12V, +/-0.7V. Signal inputs to the main audio board from any of the input
select circuits are via the balanced input stage, in lieu of the analog inputs on a basic unit. The signal pick-off for the headphones is
after the volume and balance controls. Speaker muting is controlled by circuitry that senses connection of headphones to the jack.
The power amps are attached to an aluminum heatsink plate (which is also connected to the circuit common for these devices). The
heatsink plate forms an operational module separate from the chassis, which allows access to the solder side of the circuit board
while power is applied to the circuitry. To avoid thermal shutdown of the power amp(s), they should NOT be operated without their
tabs being fastened securely to the heatsink plate.
Variations in the frequency response of different production runs of drivers sometimes requires minor adjustments in the equalization/
crossover components in individual runs of units. Some of these components may have values slightly different than those indicated
in the schematic, which are the nominal ones. If any of the drivers (speakers) are replaced, it may be helpful 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 program 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.
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 necessary for them.
Level Adjustment
The
VM SDI Digital Series
units are set at the factory for nominal line levels
(digital = +20 dBFS
;
analog = +4 dBU)
. Other line
levels require either a custom order or require that the customer adjust the unit. To adjust the unit for the first time, apply a digital signal
(test-tone) of the nominal line level. Adjust the trim pots
P5
and
P8
so that the balanced analog output is at the nominal analog line
level. Apply a balanced analog signal of the nominal line level, select that signal for monitoring, then adjust the trim pot
P2
and
P3
until the balanced analog output is at the same level as the balanced analog input. Then adjust the trim pots
P6
and
P7
so that the front
panel bargraph meters read zero level.
See
Figure-3a
on the facing page for the trim pot locations on the
VM
main PCB (P/N 919141-1).
General Technical Observations
