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Build A Hearing Loop Receiver

This receiver is designed to pick up the signal from a hearing loop and will drive a pair of headphones. You can use it with a hearing loop you install in your own home or with commercial loops already installed in halls and churches.

By John Clarke

Specifications

Current Consumption....... 10mA

Frequency response....... -3dB at 100Hz with stereo 32Ω headphones connected. Upper response to beyond 5kHz

Signal to noise ratio (with a stereo 32Ω headphone connected)....... -67dB A-weighted with respect to a 400mA/m field strength and VR1 at mid setting. Noise is dependent upon background environmental noise from mains wiring and equipment

Battery voltage indication....... Down to 7V

Elsewhere in this issue we introduce the concept of hearing loops for those with hearing loss. They’re specifically intended for use with hearing aids fitted with T-coils (the other article explains T-coils).

But there are many people in the community who have hearing loss and, for various reasons (cost, denial and vanity are the main ones!) don’t own or want a hearing aid, particularly one of the more advanced types.

This project, in fact this whole series of related projects, is intended for them – and anyone else who “suffers in silence” (or perhaps suffers in muffles!).

You might have experienced it in your own household: someone who wants the TV or stereo turned up beyond everyone else’s comfort level so they can hear it.

Connect a hearing loop to your TV or stereo system, use this Hearing Loop Receiver and an earbud or two – and they will be able to hear everything in the program, with no need to have the volume cranked up!

Our Hearing Loop Receiver

It’s housed in a small case which can attach to a belt or slip into a pocket, so it’s fully self-contained. The user can walk around without the sudden jolt of reaching the end of a headphone lead! It’s equipped with a power switch, power on LED, volume control and of course a standard 3.5mm jack outlet for headphones or earphones.

Current consumption is about 10mA, which should give up to 40 hours of use before the 9V battery needs to be changed (a rechargeable battery could be used). The power LED also functions as a battery indicator where its initial brightness when power is applied is dependent upon battery voltage.

By now, we hope you’ve read the article in this issue on the design and installation of a hearing loop. That will give you a much better understanding of how the Hearing Loop Receiver works, so we won’t go into a lot of detail here.

But if you haven’t seen that article, a hearing loop at its most basic simply consists of a loop of wire around a room, driven by a standard audio amplifier. The magnetic field it produces induces the audio signal into a coil in a hearing aid equipped with a T-Coil or in this case, our Hearing Loop Receiver.

Circuit description

The circuit for the Hearing Loop Receiver is shown in Fig.1. It comprises two low-cost ICs plus a handful of other low-cost parts.

Click for larger image
It's all housed inside a "remote control" case which is small enough to fit into a pocket, or clip to a belt via an optional clip. So if Grandpa forgets he's wearing it and gets up to walk around, he won't leave his head back in his easy chair!

The magnetic field from the hearing loop is detected using inductor L1. This is actually the secondary winding of a standard Xenon flash tube trigger transformer (eg, Jaycar MM2520). Because of the very large number of turns, it has a high inductance – around 8.2mH. Best of all, it is quite cheap and is suitable for the task of hearing loop monitoring.

One side of L1 is biased at about +4.05V using two 10kΩ resistors connected in series across the 8.1V supply. A 100μF capacitor bypasses this half-supply. The 4.05V rail biases the output of IC1b so that its output can swing symmetrically within the available power supply rail.

Tying one side of the transformer secondary winding to the +4.05V supply means that it is effectively grounded while the other end of the winding provides the signal to op amp IC1b. The DC resistance of inductor L1 is 27Ω, presenting a low source impedance at low frequencies to the non-inverting input of IC1b and thereby minimising low-frequency noise.

A 2.2kΩ resistor is connected in parallel with L1 to lower the inductor’s Q and prevent the possibility of oscillation. The 220pF capacitor that shunts high frequency signals to ground also assists in this. Furthermore, the input of each amp stage has a 10Ω “stopper” resistor to help prevent oscillation.

Any signal induced in L1 will rise in level with frequency, at about 6dB per octave, because the induced voltage is proportional to the rate of change of the magnetic field.

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