Installation guide

Williams Sound
®
Helping People Hear
14
2 TO 16 Ω OR 70 V SPEAKER LINE
The T4 input can also be connected directly to 2 to 16 Ω or 70 Volt speaker lines. If you
choose to connect to 70 Volt Speaker line, be certain to set the Audio Select switch
correctly. Severe damage will occur if you do not. When making such connections, it’s
very important to avoid creating ground loops. Pin 1 of the
3-pin connector and the sleeve of the 1/4" jack are connected directly to the chassis.
Normally, one of these would be connected to the common output terminal of the power
amplifier connected to the speaker line. But in most installations, there cannot be an external
connection from the common terminal of a power amplifier to ground. To avoid this
unacceptable situation, use a connection scheme like those described in the section Avoiding
Ground Loops.
Set the Audio Select Switch for Line input when connecting to 2-16 Ω speakers. Set the
Audio Select Switch for 70 V when connecting to 25 V or 70 V speaker lines.
Speaker lines are most often equalized, making them an inferior signal source. Source signals
should not be equalized.
AVOIDING GROUND LOOPS
Sometimes the normal way of connecting a T4 transmitter to other audio equipment creates a
“ground loop”. If other ground conditions are favorable, ground loops can often be
eliminated by using the T4’s balanced input amplifier (even on unbalanced sources), and
connecting a capacitor in series with the audio line shield to the transmitter’s ground. This
method also maintains good RF shielding. Determining the effectiveness of this method for
your installation usually requires experimentation. (See Figure 7.)
CHOOSING A GOOD AUDIO SOURCE FOR THE HEARING IMPAIRED
In engineered sound systems, the designer can specifically provide an advantageous mix for
the hearing impaired. Here is a list of attributes that improve the installation’s usefulness:
The audio should be “dry,” with minimal reverberation, either natural or artificially
generated.
The signal should be “flat,” with no equalization as might be found in the feed for house
speaker amplifiers. Connect the transmitter’s input ahead of any equalizers. However,
equalization provided by the parametric equalizers on each input channel of a console
can be helpful.
The audio should not be subject to a compressor, limiter, or other signal processing
equipment. The transmitter has an effective audio processor. Additional compression is
.01 uF Ceramic
Disc Capacitor
In Phase
3–Pin Connector
.01 uF Ceramic
Disc Capacitor
12
3
Breaking A Ground Loop when
Connecting to an Unbalanced Line
Breaking A Ground Loop when
Connecting to an Balanced Line
FIGURE 7: AVOIDING GROUND LOOPS