Juice will flow down the shields and broadcast hum into the signal wires they
were supposed to protect or wind up imposed on the reference ground of the receiving
equipment. This situation manifests itself as the all too familiar buzz of a ‘ground loop’.
The intensity depends on many variables but can go from unnoticeable to raging. Some
people in desperation resort to using AC plug “ground lifts” to defeat the mains safety
grounds in a random fashion until the system quiets down a bit. This in our view (and the
view of the safety standards organizations) is an unacceptable method of taming ground
buzzes. The simpler way is to make sure that all the gear has a good mains ground and to
lift the shields on the receiving ends of the audio cables. The principals at Dangerous
Music have wired up large, world class facilities using this scheme and have brought
room after room online with
no buzz problems
from the moment of power up. This is
why many gear manufacturers have shield lift jumpers inside their equipment. If a noise
problem crops up, changing the jumper position will almost always cure the problem.
Planning out the wiring system to minimize the formation of ground loops solves
problems before they happen.
Many powered speakers seem to get their audio reference ground from the input
cable. This means that these shields should not be lifted at the XLR. One favored
technique for trouble shooting buzz issues is to make a short XLR cable with the shield
lifted on the male connector and use this to test whether or not an XLR interface warrants
a shield lift.
Alignment
The Dangerous Music Monitor is carefully aligned at the factory. The alignment
procedure is provided here in case the user wants different levels on the D/A, Meter, or
Dim circuitry. Please don’t attempt to align this unit without proper laboratory equipment
and knowledge of its use. Refer to the main board diagram on page 8 for pot locations.
1.
Set an analog oscillator to d4dBu output and patch it to Analog Input 1.
2.
Select
Analog Input 1
and turn the speaker level control all the way up.
3.
Meter the Main Speaker Output and note the level and press the
Mono
button.
4.
Adjust P1 for the same level noted above. Release the
Mono
button.
5.
Turn the oscillator up to +18dBu and press the
Dim
button.
6.
Adjust P2 and P3 for 0dBu output. Release the
Dim
button.
7.
Unplug the oscillator. Obtain a digital oscillator and set it to -14dBfs.
8.
Feed the oscillator to Digital Input 1 and select
Digital Input 1
and
D/A
.
9.
Meter the Main output and adjust P8 and P9 to the levels measured in step 3.
10.
Meter the A-MTR feed and adjust P4 and P5 for +4dBu output.
11.
Push the -6VU button and adjust P6 and P7 -2dBu output.
To set other reference levels for the D/A converter, run the preferred digital level and
adjust P8 and P9 (along with the gain ranging jumpers H8 and H9) to read +4dBu at the
meter feed. Remember to have the
-6VU
button disengaged while aligning the converter.
(How many times have I forgotten
that
one, Doh!)
-10-
