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VTX-WM1 Installation Guide v1.0 13

Impedance Tests

The VTX-WM1 card includes a variable

frequency test oscillator which may be

programmed to apply a test tone (at 40 dB

below the amplier’s maximum output level) to

the amplier output at a future date and time,

and, if desired, at regular intervals thereafter.

(It is recommended that tests are scheduled

for the hours of building non-occupation).

The amplier output voltage and current are

monitored during the test and hence the load

impedance checked. This gives the engineer

a rst-line conrmation that all the speakers

connected to each amplier channel are

present, connected and functional.

Clicking Tests (click the amplier button

rst) opens the Test List page for the

amplier:

In order for the impedance tests to be

meaningful, an initial Load Conguration

test must be run, to establish a reference

impedance at the frequency to be used. The

upper pane of the Test List page is concerned

with this initialisation.

Conguring the Load

The amplier must be powered on and each

channel connected correctly to its normal

speaker(s). Select each channel in use from

the drop-down list under Channel. (If any

channels are in Bridge Mode, select the lower-

numbered channel of the pair.) Select the

frequency to be used for the test from the

drop-down list under Frequency. If using a

bi-amped (or tri-amped, etc.) speaker system,

the frequency for each channel will need to

chosen to match the relevant driver type. The

frequency selected should be approximately in

the middle of the frequency response range of

the speakers/drivers connected to the channel.

Consideration should be also be taken of any

loudspeaker enclosure resonant frequencies

– these may be particularly evident at low

frequencies, and should be avoided, as their use

will give misleading results.

Click the Cong button. This will generate

a dialogue box warning that a test tone is

about to be fed through the audio system.

Click OK if you are happy for this to occur.

A progress bar is displayed, and when the

test is complete an impedance value will be

displayed in the Expected eld. To re-check

the expected value, set a Tolerance value

(30% is recommended initially) and click the

Test button. A second test will be run, and

the results displayed.

The Remove button may be used to cancel

the current reference impedance value for the

selected channel. This should be done if, for

example, a loudspeaker has been removed or

replaced.

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