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The ILD252 Induction Loop Driver has been designed as a high quality
power driver for small and medium size audio frequency induction loops.
Ease of installation and use have been major factors in the design, combined
with optimised performance, and freedom from R.F.I. generation to meet all
technical requirements.
To ensure compliance with all technical standards, it is essential that the
equipment is installed by a person who is technically competent in
professional audio, and who has the necessary installation skills.
Warranty Information
This product carries a 5 year warranty which could be invalidated if the
following installation instructions are not followed correctly, or if the unit is
tampered with in any way.
The 5 year warranty is dated from the time the equipment leaves Ampetronic
and NOT when it is installed.
Inspect the equipment upon unpacking, and check for damage.
Install the unit in the place where it will be used. Care must be taken that this
location provides satisfactory ventilation for the equipment. In order to
ensure this, the unit should not be installed in a tightly enclosed space.
Enough room must be available to permit free airflow across the equipment.
The unit uses forced ventilation by using an internal fan, and the air intake
grille at the left side of the unit, as well as the exhaust on the rear must be
unobstructed. If the unit is installed in an enclosed environment, sufficient air
flow must be provided through this enclosure. The amount of heat generated
depends on the loop size, and wire gauge, but can be such that the reliability
of the equipment will be reduced if the ventilation is poor..
The unit can be installed freestanding, rackmounted or wall hung with special
adaptor brackets.
Prepare the input signal connection as described in section 3.
Connect the loop cable to the rear panel Loop Connector. Ensure that no
stray wire ends protrude from the terminals. The polarity of the loop is not
important, unless a specialised low–spill system is being installed. For sizing
of the loop, see section 4.3 on Loop Design. It is important that the loop cable
ends form a twisted pair between the amplifier and the loop, to reduce the
magnetic field generated by the loop cable near the amplifier position
The equipment can be driven from various sources, such as a microphone, a
P.A. system, any other audio system, or separate microphone preamplifier.
A single microphone should be connected straight to the ‘Mic’ Input. All other
sources should be connected to the ‘Line Input’ socket.
Microphone Input.
This input provides an electronically balanced input on an XLR connector for
direct connection of a microphone. Phantom supply can be selected with the
switch on the rear panel
Modern electret microphones normally have a high output level. The rear
panel gain boost setting should be off, unless a low output microphone is
High quality balanced screened microphone cable is necessary for this
connection to ensure correct operation. Low quality, or poorly screened
cable may result in serious instability of a full system, or non–compliance
with the EMC requirements.
Line Input.
This input to the equipment is a balanced line high impedance input
connection, which also permits single–ended operation. The connection is
made via a 2-pole (unbalanced) or 3-pole (balanced) 6.3mm jack plug.
Unbalanced inputs must use less than 3 metres of cable.
To prevent earth current loops causing hum effects, it is usually best to use
the balanced input mode, coming from a single–ended signal using a good
quality two–core plus screen cable as shown in the figure below. The
connector sleeve is connected to the screening braid, the ring connected via
one of the two cores to the source ground, and the tip to the source signal. If
hum is encountered, then the earth lift switch can be used to disconnect the
amplifier signal circuits from the AC power ground.
Preferred connection of 3-pole Plug
with unbalanced signals.
100 Volt Line
The ILD252 can be connected to a 100V line system via the ATT100 adaptor.
This allows any line configuration, from single–ended to balanced mode.
The leads from the 100V speaker line are wired to the connector which plugs
into the adaptor, which is plugged directly (no extension cable!) into the Line
Input socket. Similarly, the ATT30 can be used to connect the ILD252 to a
low-impedance speaker system.
Microphone Modules
The equipment can be used with separate microphone preamplifiers, which
are available in different configurations (see data sheet for details). The
power for these amplifiers can come from the Preamp Power socket, and the
preamp output is connected into the Line Input socket. To ensure EMC
immunity, cable length between preamp and Loop Driver should be less
than 1 metre.
Slave (I/O)
The insertion of a 6.3mm 3–pole jack breaks the link between the input stage
/ compressor and the power driver. The tip of the plug will be the preamp
output (after compression) and the ring connection is the input to the power
amplifier (see below). This connector is mainly used for the connection of the
special signal processor used in low–spillover loop installations where the
master unit controls the signal gain, and the slave unit operates purely as a
power driver. This is essential to ensure full tracking between amplifiers.
Under no circumstances should this input be used as a normal input, as
this bypasses the automatic compression circuit. This compression is
essential to the correct operation of other circuits which prevent RFI
generation. Cable length must be less than 3 metres.
Slave (I/O)
The output on this connector is the signal needed for driving equipment used
with low–spill systems (see 3.6). It is also valuable for driving other audio
equipment such as tape/cassette recording equipment, as the signal has
been processed by the compressor, and therefore the dynamic range of the
signal is reduced by the amount of compression. This can also be
understood as an automatic gain control network. To obtain this recording
facility the tip and ring of the 3–pole jack plug must be electrically joined. Do
not use 2–pole plugs.
Loop Monitor
This jack socket on the front panel allows connection of standard, good
quality stereo headphones for monitoring the current in the loop (do not use
extension cables for this signal). The headphones are at this point connected
in parallel with the loop current sensing resistor, which is of very low value
(110 mW) and therefore the signal monitored at this point is exactly the
current which is fed into the loop. A buffer resistance prevents the current
sense resistor from being shorted.
Loop Design
The design of the actual loop, type of cable to use, etc. is covered by a
separate publication.
Loop Output
The loop itself is connected to the ILD252 via a special 2way connector, capable of handling the current. The cable
from the loop to the amplifier should normally be a twisted
- Remove screw from top of plug, remove cover.
- Insert one wire into terminal 1, and the other wire into
terminal 2, and tighten screws to secure wires in place.
- Replace cover and refit screw.
Modern building construction often includes a large
amount of metal in the structure, including mesh in
reinforced concrete floors and ceilings. Under some
circumstances this can be a serious problem, as hysteresis losses in this
metal cause a loss which is frequency-dependent. The actual value of the
loss can only be found by measuring the site performance. Ampetronic have
significant experience in this area and should be consulted prior to
installation if significant amounts of metal are likely to be present. The loss
varies from 0 to 3 dB per octave, with a lower corner frequency between 100
Hz and 0.01 Hz. As such, the loss in the middle frequency band can be very
significant. A corrector is build into the ILD252 to correct the frequency
response, but the power loss can only be overcome with additional power
into the loop. This may require a more powerful driver.
Initial setting up
During the initial commissioning of the equipment it is essential that the
following procedure is used to ensure a satisfactory end result.
Ensure that all connections are made correctly, including power and loop.
Turn the “MIC”, “LINE” and “LOOP CURRENT” controls fully anti–clockwise,
i.e. minimum signal. Set the “Metal Loss correction” on the rear panel to“0”.
Provide a continuous input signal, preferably from a small tape or CD player
with wideband music, connected directly (i.e. not via an external audio
system) to the line input.
Switch unit on. After a short time the green Power LED should stop flashing.
If the LOOP ERROR LED is illuminated, then check for loop continuity. The
resistance of the loop must be within the specification for the unit to work.
After correcting the loop error, the unit must be switched off and on again in
order to reset the loop monitoring and system enabling circuits.
Increase the “LINE” gain until 2 of the compression LEDs are illuminated
under peak signal conditions. This establishes a reference level for the
output power driver. Increase the LOOP CURRENT setting until the desired
output current is achieved (section 5.2). Where the current needed is a value
in between two LED readings, position the control by interpolating, bearing in
mind that consecutive LEDs illuminate at 2 dB intervals. Having achieved
this setting of the LOOP CURRENT, check with headphones plugged into
the loop monitor output socket that a satisfactory sound quality is obtained
from the loop current. If a standard field–strength measuring unit is available,
then check that the field has the correct strength. From this point onwards,
the LOOP CURRENT control will not need re–adjusting, as this only affects
the peak field strength.
Before connecting to the sound system, check the entire sound system for
crosstalk from the loop into the audio inputs. While still playing a music tape
or CD, check every input circuit of the sound system, and if a significant
amount of signal is picked up, identify the reason and correct. This ensures
adequate stability for the complete system.
Connect the cable from the audio system to the line input, and readjust the
input gain control for optimum compression. If only the microphone input is
used directly, without another sound system, then set up the entire system
from a normal sound source driving the microphone. The level is set with the
“MIC” control. When using separate pre-amps, then adjust the gain controls
on these pre-amps, and the “LINE” gain control to achieve a satisfactory
Adjusting Metal Loss Correction.
Correct adjustment of the frequency response requires the use of
specialised test equipment to measure the frequency response of the
system, as specified by international standards (such as EN60118-4)
A simple method is to listen to the sound with a good quality receiver such as
the ILR2. Using the same headphones, listen first to the loop current signal
obtainable from the Loop Monitor outlet on the ILD252 with metal loss
correction set to minimum.. Then listen to the loop signal using the ILR2 and
adjust the “Loss Correction” to obtain a similar sound quality.
A full plot of the actual response can be made with suitable equipment, using
the CMR2 calibrated receiver. This can be done with Pink Noise, or a
frequency sweep (which must be done at 12 dB below normal maximum loop
current). Please contact Ampetronic Ltd for further advice.
Optimising setting of MIC / LINE controls.
In order to obtain the greatest possible dynamic control range from the
compressor, it is now necessary to establish the highest level of input signal
which the equipment may receive in the operational installation. This will
often be loud, close talking into a microphone. If the MIC / LINE gain is
adjusted so that the amber 36dB LED does not illuminate (just), then the
compressor will maintain the highest possible level into the loop for faint
speech, etc. Monitoring with the output headphones will indicate the clarity of
the signal under all levels of compression. It should be pointed out here that
back ground hum and noise from equipment earlier in the chain, such as a
P.A. system may sound very troublesome when subjected to some 20–36db
extra amplification. When this occurs, the gain must be kept at a lower level.
This may also have to be done in the case of marginal magnetic feedback via
dynamic microphones, etc. Where the dominant signal is music, it may be
important to keep the compression level low to prevent serious degradation
of the music dynamics. Experience will indicate which level to use.
REMEMBER: once the LOOP CURRENT control has been set, only adjust
the MIC/LINE controls, otherwise the correct operation of the system is
Loop Condition Testing.
The ILD252 incorporates two different modes of testing for correct loop
condition. At startup, a full test is made to check that the loop impedance is
between 0.3 W and 3 W. This test is done with an internal test signal . Any
loop measuring outside these impedance values will cause the system to
detect a loop error.
Additionally, during continuous operation, a monitor uses the actual audio
signal to check for open circuit / high-resistance loop conditions, and when
detected the unit will again indicate loop error condition. This will ensure
early indication of system failure due to defective loops.
If you have any difficulties in calculating the loop design, or experience
difficulties with the operation of the equipment, then contact your supplier or
Ampetronic Ltd. It is useful to have all the relevant data available when
contacting our technical staff who will be pleased to help you.
Please have the following information available:
Loop dimensions, loop position, conditions under which problem occurs,
building usage, equipment type.
The following are known trouble areas:
– Strong hum field, mainly from fluorescent light fittings, or electrical wiring
where current flow and return are not in the same cable or duct.
– Electric guitars used in single–coil mode. Twin coil/humbucker mode is
generally necessary to prevent pickup of the loop signal into the guitar.
– Loop cable installed where it is in close proximity to microphone (or other
audio) cables for an appreciable length. Telephone cabling can also be very
sensitive to this coupling.
A 20mm fuse is incorporated in the rear panel power input socket. It is
necessary to remove the power cord before extracting the fuse holder. The
fuse rating and type are printed on the rear panel.
Specialised Mounting
A rackmount kit is supplied for installing the ILD252 in a standard 19" rack,
taking up 1 rack space (1U).
Wall mounting brackets (WM252) are also available for fixing the amplifier to
a wall.
Microphone Input:
Suitable for driving from 200–600 W microphones.
Electronically balanced, XLR connector.
Phantom voltage: +15V DC available
Input Sensitivity: -70dBu (gain boost position). Gain boost is 15dB
Overload level -20dBu.
Line Input:
Impedance 1M W each side, 2M W differential.
Sensitivity: -30dBu. Overload at +20dBu
Balanced signal input on 6.3mm 3–pole jack socket. Can be used
unbalanced with mono plug.
Slave Input / Output
Input Impedance 100k W Sensitivity 1V rms, +2.2dBu.
Source Impedance 100 W Output level 1V rms +2.2dBu.
Signals are unbalanced, with a 3–pole jack socket used as an insert
point. See chapter 3 for connections
Metal Loss Correction
Loss correction adjustable from 0 to 3dB per octave.
Gain remains constant at 1 kHz. Lower frequencies are attenuated.
Higher frequencies are boosted.
Output Current:
7A Peak signal current into SINGLE TURN loop
Absolute maximum peak current >8 Amp.
Metering via front panel LEDs. These LEDs indicate the
peak current, with intervals of 2dB.
Output Voltage:
Greater than 19V peak to ensure good frequency response.
Loop Resistance:
Must be less than 2.75 W, greater than 0.3 W for normal operation.
Resistance range for Loop Condition Monitoring 0.3 W to 3 W
Error will also be detected during normal operation, when loop goes
high resistance, using normal audio signals for detection.
Compression range 36dB before overload. Front panel
indication of compression level. Efficiency: less than 0.25dB
output change for 25dB input change. Attack and Decay
time constants optimised for speech.
Frequency response:
80Hz to 5.5kHz ±1.5dB at low level, measured as loop current.
High frequency high signal level response is a dynamic variable
and is a function of loop size, loop current and signal content to
ensure that no RFI generation takes place.
Internal time constants are very short.
8.10 Pre–amp power:
±15V DC regulated at up to 0.2A.
8.11 AC Power input:
230V nominal, 45-65Hz. 120VA Peak power.
Fuse fitted to input power connector type T 0.63A L
Factory setting for 115V operation available upon request.
8.12 Dimensions : Width: 430mm Height: 44mm Depth: 220mm
8.13 Weight: 2.9 kg.
The ILD252 is designed and manufactured in England by Ampetronic Ltd.
Ampetronic Ltd.
Northern Road, Newark,
Nottinghamshire, NG24 2ET.
United Kingdom.
Declares that the product:
Induction Loop Driver
Type Name:
Conforms to the following Directive(s) and Norm(s):
Directive 89/336/EEC
EN55103 (1 & 2) 1997
Directive 73/23/EEC
EN60065 (1995)
September 2000
L.A. Pieters
Managing Director
Ampetronic Ltd.