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Hearing Loop Signal Conditioner

Want to drive a hearing loop using a conventional voltage (audio) amplifier? This Hearing Loop Signal Conditioner includes signal compression and has a treble boost control to compensate for high-frequency roll-off due to loop inductance. It uses low-cost parts and is easy to build.

by John Clarke

Main Features

• Balanced or unbalanced input

• Stereo mixing

• XLR, 6.35mm jack or phono (RCA)

• Phono (RCA) or 6.35mm jack  
   socket unbalanced output

• Low-pass and high-pass filters

• Level and tone boost adjustments

• Signal compressor

• Optional compressor bypass

• Power switch and indicator LED

• Several power supply options


Signal-to-noise ratio with respect to 1V in and 1V out:

(1) Compressor out: 90dB (20Hz to 20kHz filter); 99dB “A” weighted.

(2) Compressor in: 75dB (20Hz to 20kHz filter); 78.5dB “A” weighted.

Frequency response: -3dB at 43Hz and 6.6kHz, -10dB at 10kHz (no treble boost).

Treble boost: up to +16dB at 5kHz with C1 = 5.6nF. Response complements loop treble attenuation.

Signal compression: typically 2:1 to -20dB input (with respect to 1V) – see graph.

If you are installing a hearing loop, you are going to need an amplifier to drive it. Commercial amplifiers specifically designed for the task are available but if you want to use a standard audio (voltage) amplifier, some form of signal conditioning is required.

For loops that are smaller than 5 x 5m, signal compression is usually all that’s required. This ensures that the loop signal is adequately maintained for a wide range of input signal levels.

In addition, the frequency response should roll off above about 5kHz but this is normally taken care of by the inductance of the loop. Larger loops, however, will have greater inductance and so will roll off the response earlier. This means that the input signal must be treble-boosted before it is fed into the amplifier, to compensate for the subsequent inductive losses in the loop.

Signal conditioner

The Hearing Loop Signal Conditioner described here is designed to provide both compression and treble boost. The latter can be set by the user, so that you can tailor the signal to suit your particular loop installation. In addition, the user can vary the signal level that’s fed to the amplifier.

Fig.1 shows the block diagram of the unit. As can be seen, the input stage can accept either mono or stereo line inputs and these are fed in either via RCA sockets or via a 6.35mm jack socket. Alternatively, it can accept a mono balanced input or an unbalanced input via an XLR connector.

From there, the signal is fed via level control VR1 to a low-pass filter stage. This filter rolls-off the response above 6kHz and has a Q of 0.9. The response of this stage is flat to about 5kHz and is designed to provide optimum results when the signal is subsequently fed to the treble boost stage that follows the compressor.

The compressor stage provides a nominal 2:1 compression, so that high-level signals are reduced by a factor of 2. By contrast, low-level signals are boosted by a factor of two. As a result, the compressor ensures a more or less constant signal level at its output, regardless of input signal variations, thereby preventing overload in the power amplifier.

This signal compression in turn ensures a relatively constant field strength level in the hearing loop and this can greatly improve the audibility of speech signals. Link LK4 enables the compressor stage to be bypassed if compression is not required.

The treble boost stage is the next in line. As previously stated, this provides boost at the higher frequencies to compensate for treble losses due to loop inductance. However, this boost stage is not like a normal treble tone control where the amount of signal boost is constant for all frequencies above the turnover frequency. Instead, it acts more like a single band boost stage in a multi-band equaliser (ie, the signal rolls off sharply at frequencies higher than the boost frequency).

The idea here is to ensure that the power amplifier is not fed boosted high frequencies above about 10kHz, as this could cause instability. If instability did occur, the loop would radiate RF signals that could interfere with other equipment.

The output from the treble boost circuit is unbalanced and is fed to an RCA socket and a 6.35mm jack socket which are wired in parallel. If necessary, a 6.35mm jack-to-XLR lead can be made up to connect to an XLR input on an amplifier.

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