Feeding a Hearing Aid Loop from
an External Speaker Socket.

Before we start. Being of perfect hearing, I cannot justify getting a hearing aid. If there is a manufacturer of hearing aids that would like to contribute one for my experimental use, then I would be most grateful if you would contact me.

Received by email......

I am interested in a hearing aid loop system for my father in law for use in his lounge. Would the jack socket on the front of his TV have enough drive or do I need some form of amp.

We cannot knock these commendable efforts. But is everything is as easy as it may seem. Let's investigate.

Before tackling the issue of connecting to the TV, let's investigate the amount of drive required for a typical lounge area. Ok. So there are large lounges, but where is a person who is hearing impaired likely to sit? Favourite chair? More than likely. So the area required is likely to be no more than about 4x4 metres. This makes a loop circumference of 16m.

My favourite cable for this purpose is typical indoor telephone or alarm cable. Each strand exhibits about 10Ω/100m. We will want between 40 and 80m total strand length to get between 4 and 8Ω being a typical speaker impedance. As this cable usually comes with 4 strands, 16m would give us a total of 64m (by wiring the cores in series). This gives us 6.4Ω.

Required drive:
If we have a look at Planning a Hearing Aid Loop we will see we need about 0.44A to achieve the suggested 100mA/m magnetic field strength. As we have 4 turns this value is divided by 4 making our requirement only 110mA. This translates into a power of 80mW into 6.4Ω.

What the TV can deliver:
There are very few TVs with speaker outputs less than 5 watts. Most are typically 10 to 15 watts. This would apparently make the audio output amp more than suitable for driving the loop.

The front socket:
This socket on a TV or similar home entertainment device appears to offer a convenient connection to the output of the internal audio amp. Unfortunately these sockets are most times limited with respect to the amount of power output they can deliver. This is normally done with a resistor, usually in the ground leg. This is done by the manufacturers, in a vain attempt, to have the user protect their ears. However, the power output is still likely to be in excess of 100mW per channel else most headphones would not yeild sufficient sound level.

A small experiment can be conducted to ascertain if this is the case by wiring up a plug to a spare speaker, taking special note that such sockets are usually stereo. A typical PC multimedia speaker usually works very well for this type of test. Please note that this must be a non-amplified speaker as using an amplified speaker will defeat the object of the exercise. Use two 3.9Ω, 1 watt resistors and wire up a plug to a speaker as per the following picture.

wiring of plug for loop

The resistors have two purposes. The first is should you have a stereo TV then connecting to the two channels together can destroy the amplifiers. The resistors stop this from happening. Secondly, audio amplifiers do not like having inductors (which is what the loop will be) on their outputs. Using the resistors ensures the audio amp(s) will not suffer instability. The test will soon indicate whether sufficient output is available from the front socket.

If the tests prove there is not much drive from this socket, all is not lost. You'll just have to locate the offending resistor(s) in the TV and reduce their values. I strongly advise that you seek help in doing this. The voltages inside a TV are not to be treated lightly (to ensure you are really put off see Why a TV Chassis "Bites").

If your tests, however, reveal a reasonable sound level then your luck is in. All that will need to be done is to remove the speaker and connect the loop in its place after having taken note of the warnings and adjustments below.

A few warnings:
For safety reasons manufacturers should have this socket isolated from the chassis by means of a transformer, but not all do. Although highly unlikely that the headphone socket will be attached to the chassis, it is still well worth testing before attaching a loop to it. The reason is fairly simple. A loop is likely to be put under a carpet and should the loop, by some fluke, cut through to ground then the TV will suffer damage as well as posing a potential fire hazard.

There is one issue that must be borne in mind. When the loop is powered by the TV's audio amp no-one else can hear what's going on (unless they too wear a hearing aid). Should the listener, however, live alone then this is of no concern. In actual fact, the loop will not only be a blessing to the listener, but to the neighbours as well. It is surprising hospitals have not picked up on this idea years ago!

This is fairly simple. Have the listener adjust the volume on his hearing aid to where normal speech is at a comfortable level. Then have them switch to the T setting and adjust the TV to a point where the sound is at the same level as that of speech.

If there is no headphone socket:
All is not lost should there be no front headphone socket on the TV. Most are now fitted with sockets for connecting to a Hi-Fi system. These, if fed through a suitable amplifier, are ideal for driving a hearing aid loop. This is covered in the article "An Economical Hearing Aid Loop".

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© 11.06.01