Thank you for choosing an Adastra LA series induction loop amplifier for your assisted hearing installation.

This unit is designed to offer high quality hearing assistance for hearing aid users.

Note: All induction loops in the UK should be installed in accordance with BS7594.

Front panel

Rear panel

Installation

The LA series loop amplifiers can be operated free-standing or fitted into a standard 19” rack cabinet using the supplied rack ears. Ensure that the unit is positioned on a stable surface with adequate cooling ventilation.

There are 3 input channels which can accept inputs via balanced XLR or 6.3mm jack connection (14) or unbalanced RCA connection(15).

The XLR/jack connectors can be set to accept microphone or line level via the Mic/Line DIP switches (16), which are also accompanied by switches for activating phantom power (+20V) for use with condenser microphones.

Multiple loop amplifiers can be used in master/slave mode using the combined slave input/output jack (17). Connecting a stereo 6.3mm plug to this socket gives 2 wiring options as shown below.
Connecting a mono 6.3mm plug will function as a slave output. A TRS plug with signal connected to the ring will be needed for a slave input from another loop amplifier.

Alarm contacts (13) are provided, which will emit a siren on the induction loop when the connection is made.

These can be connected to an emergency switch or alarm panel contacts to activate the siren for emergency evacuation, indicated by an LED on the front panel (7)

Alongside the alarm contacts are the screw terminals for the induction loop (12). Connect the 2 ends of the loop wire to these terminals, ensuring no stray strands or shorted connections. See below for details about installing the loop wire before connecting mains power to the loop amplifier.

Mains is connected via a rear panel IEC connector (11) with a built-in mains fuse.
Use the supplied lead and ensure that the correct mains voltage is set on the voltage selector (10).

Induction loop cable installation

Prior to installation, it is important to check if there will be any equipment in the vicinity that may interfere with the loop’s magnetic field, such as large transformers, high power cables or substations. It is also important to ensure that no equipment will be adversely affected by the loop field, such as sensitive data or signal cables. In these cases, it may not be possible to cover some or any of the required area with an induction loop.

The cable gauge will depend upon the total length of the induction loop. Usually, the loop will be installed around the perimeter of the listening area. However, there are various techniques which can be employed to build arrays to vary the shape and strength of the field. In general, calculations outlined here are based upon a perimeter installation.

It is recommended to use good quality insulated pure copper cable for the induction loop. The cable gauge used will need to be determined by the total length of the cable run. The LA-series loop amplifiers are designed to operate with a load of between 0.2Ω and 2Ω. The following equation can be used to calculate the total cable resistance, which will show if the gauge is correct.

R = Cable length (m) x 0.01786Ω*mm²/m (specific resistance of Copper)

Cable CSA (mm²)

So, for a cable 80m long with a Cross-Sectional Area (CSA) of 1.5mm²… R = 80/1.5 x 0.01786 = 0.9525Ω

So, this gauge of wire (1.5mm²) would be OK for the 80m run because it is between 0.2Ω and 2Ω

As a quick reference, refer to the table below.

The wire may be run in plastic conduit but not in any metal containment so as not to impair the magnetic flux. The ideal height to install the cable depends upon the width of the loop (i.e. the narrowest dimension relative to the listener). The optimum output will be achieved if the cable is installed 14% of the loop width higher or lower than the plane of listening (ear height).

In practical terms, the loop is usually installed onto skirting boards, under flooring or at ceiling height so that it is unobtrusive. It is recommended to avoid varying height levels as much as possible to avoid anomalies in field strength.

For irregular shaped areas or larger than the loop amplifier’s capacity, multiple induction loops may be required for coverage. Connecting further loop amplifiers using the Slave connection will enable multiple loops to be used together.

Cancellation Loops

If there are areas adjacent to the loop where the magnetic field would cause problems, it is possible to avoid this by use of a “cancellation loop”, which is a narrow loop parallel to the main loop at the problem area.

This loop is the opposite polarity to the main loop and causes the magnetic field to narrow along the adjacent edge of the main loop to control the spill of the loop field.

Connection

Connect any line sources (e.g. CD player, mp3 player) or microphones to one or more of the input channels.

If a microphone is connected, select “MIC” on the DIP switches and connect via 6.3mm jack or XLR input.

If the microphone is a condenser microphone requiring phantom power, select “PHANTOM”.

For standard dynamic and battery powered microphones, set the Phantom switch to “OFF”

For line inputs, select “LINE” on the DIP switches.

For channel 1, there is an option for “PRIORITY” over the other two channels, which will mute the output of channels 2 and 3 when there is an input to channel 1.

If the loop amplifier is to be used in conjunction with a PA system with loudspeakers, connect the line or aux output from the PA system to a Line input on the loop amplifier via RCA, jack or XLR connection.

Ensure that this input is set to “LINE” and Phantom is set to “OFF”

Before connecting the induction loop to the amplifier, use a test meter to check the D.C. resistance in the wire. Ensure that there are no shorted or grounded points in the wire and that the total resistance is not less than 0.2Ω

Twist the loop wires from where the loop ends to the amplifier terminals to avoid creating an extended loop to the area where the amplifier is situated.

With the power switched off (9), connect the loop cable to the loop terminals (12) on the rear panel of the loop amplifier.

Operation

The front panel controls on the loop amplifier are recessed and can be adjusted using a flat blade screwdriver. This is to avoid tampering or mal-adjustment after the correct settings have been applied.

Where new or unsupervised operators are making adjustments to equipment in the venue, they may not be aware if they have adjusted the induction loop settings unless they themselves are a hearing aid user.

Set the Ch.1, Ch.2, Ch.3 and Loop Current controls (1, 2, 3, 6) to minimum and keep the Bass and Treble controls (4, 5) in the mid position (12 o’clock)

Power up the loop amplifier and increase the Loop Current control (6) part way.

Whilst playing a signal into one of the input channels, gradually increase the level (1, 2 or 3) until the peak current indicator (8) shows movement.

The optimum operating level is achieved if the channel level controls are set so that if all connected signals are at maximum level, the Peak Current indicator does not show overloading.

It is recommended to use an induction loop receiver (952.855UK) to check the audio from the induction loop.

Adjust the Loop Current control to set the main audio level from the induction loop.

The Metal Loss Correction control (18) enhances higher audio frequencies which may be absorbed by metal architectural features and appliances within the induction loop area. If the building has a lot of metal structure or reinforced concrete in the vicinity of the loop, use this control to adjust the audio quality of the loop signal.

Adjust Bass and Treble controls (4, 5) to set the tonal characteristics of the audio from the induction loop.

The mid-point (12 o’clock) is the zero position. Turning to the left of the mid-point will cut Bass or Treble and to the right of the mid-point will boost Bass or Treble.

When the induction loop is not required, ensure that no hearing aid users are remaining within the loop area and switch off the loop amplifier (leaving the settings in position will ensure that the induction loop is set up correctly for the next usage).