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V M Q - 4 U s e r G u i d e
I n s t a ll a t io n R e c o m m e n d a t i o n s
Electrical Interference
Magnetic Shielding
4
© 2 01 0 Wo h le r Te ch n ol og ie s , I n c . A l l r i g h t s r e s e rve d .
As with any audio equipment, maximum immunity from electrical
interference requires the use of shielded cable; however, satisfactory
results can sometimes be obtained without it. The internal circuitry
common is connected to the chassis. In the VMQ-4 (standard analog
inputs), unbalanced sources may also be applied to the balanced
inputs. For analog input signals, the gain is 6 dB greater with a single-
ended source connected across both "hot" terminals than when
connected between either input and common. Note that the unused
input terminal of a balanced input should be connected to common,
rather than being left floating, to insure best immunity to noise.
We have defined five levels of magnetic shielding. Level One
represents the most minimal shielding of the steel chassis compared to
the stray field of the speakers without any ferro-magnetic enclosure.
Level One typically results in a field strength of two Gauss at a distance
of two inches from the enclosure surface. (Because of the inverse square
law, such a field is eight Gauss at one inch from the surface!) Because of
the purity of color of a CRT display, it is extraordinarily sensitive to
magnetic fields making Level One shielding inadequate, even if the
CRT is several rackspaces away. (CRT color purity may be disturbed by
fields as low as 1/2 Gauss.)
Level Two shielding, specified as two Gauss or less anywhere along the
top and bottom surfaces and front panel, was an interim level, and is
sometimes the best available for highly customized units, but is
satisfactory in many applications.
The VMQ series employs proprietary methods to achieve Level Three
shielding, which is specified as one to 1.5 Gauss or less at the surface,
anywhere along the top or bottom of the chassis and front panel. (The
VMQ has stray field of about 1 Gauss.) For all situations we are aware
of, this level has proved to be entirely satisfactory. With Level Three
shielding, the unit may be located in the rackspace immediately above,
below or to either side of a color monitor, usually without disturbing
the purity at all.
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