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Amplified neckloop circuit design

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Good day.

Has anyone come across a circuit design for an amplified neckloop. There are plenty of large scale schematics for room or even desk loops. But I would like a better quality amplified neckloop with which to listen to audiobooks on my smartphone - I have an unamplified neckloop and it’s not too bad but being able to clean up and amplify the input signal would be very useful. I’d have thought that with the micro-circuits around these days, it ought not to be too hard (and I do have experience with building basic circuits) but I can’t find a schematic/parts list.

Can I digress (kind of) by saying that I’ve seen very small, very cheap amplified neckloops on the Internet being sold by small Chinese companies as part of a “spy listening” secret service type of setup where you can put a tiny mike in your ear and listen to stuff coming over your phone, But as soon as something becomes a “medical” or “health” item it jumps in price by 400 - 500%. It irritates hell out of me being ripped off.

Regards and thanks for any help you can offer.

Kevin

Here is a neckloop amplifier schematic:

http://www.siliconchip.com.au/cms/A_112366/article.html223

Microphone To Neck Loop Coupler For Hearing Aids
Hearing-aid wearers often have difficulty understanding conversations in noisy environments, especially if they have switched their personal hearing aid from microphone mode to mode. This projects solves that problem.
by John Clarke
In order to listen to a via a hearing aid fitted with a T-coil, the wearer needs to switch off the inbuilt microphone receiver. Instead, the hearing aid is switched to T-coil mode so that it can receive and process signals from the hearing loop. Such loops are often installed in public buildings, churches and halls.
However, while this allows signals from the hearing loop to be heard, it prevents the user from hearing ambient sounds. It also prevents them from hearing other people around them, making conversation difficult.
This Microphone to Neck Loop Coupler is the answer to this problem. It comes in two parts: (1) a small battery-powered unit that can be slipped into a shirt pocket; and (2) a wire neck loop coil that the user wears around
. . . well, yes . . . their neck. This neck loop plugs into the battery-powered unit via a 3.5mm mono jack socket.
The battery-powered unit has an inbuilt electret microphone, a microphone preamplifier and an amplifier to drive the neck loop. In use, the microphone picks up local sounds (or conversations) and sends them to a neck loop. The neck loop then couples the signal into the hearing aid via its T-coil.
A volume control allows the level to be adjusted to suit the listener’s requirements, or it can be turned right down (or the unit switched off) to eliminate ambient sound.
In summary, this unit can be thought of as a personal version of the much larger inductive loop systems installed in public places. It can operate in parallel with such systems or on its own.

I did a little more investigation. It looks like the common solution is to use a typical headphone amp IC like an LM386 or LM486 (HWD2171). Then you put the output loop in series with a few ohms in order to meet the min resistance that the headphone amp can supply. The loop is either one or more turns of wire (not sure why you’d really go for more turns as that might have other undesirable effects).

Here is another thread on the topic: http://health.groups.yahoo.com/group/Loops_and_Telecoils/message/3242123

And another project link: http://www.jameco.com/Jameco/workshop/diy/magnetic.html183

So it might be a worthwhile experience to buy a good quality headphone amp and seeing what replacing the headphone with a loop (and small series R) and seeing what happens.

Anyway, my first venture reminds me of AM transistor radio quality sound … perhaps even shortwave (in that as you move around the signal fades in a and out).

@grantb5 said:

I did a little more investigation. It looks like the common solution is to use a typical headphone amp IC like an LM386 or LM486 (HWD2171). Then you put the output loop in series with a few ohms in order to meet the min resistance that the headphone amp can supply. The loop is either one or more turns of wire (not sure why you’d really go for more turns as that might have other undesirable effects).

So it might be a worthwhile experience to buy a good quality headphone amp and seeing what replacing the headphone with a loop (and small series R) and seeing what happens.

Anyway, my first venture reminds me of AM transistor radio quality sound … perhaps even shortwave (in that as you move around the signal fades in a and out).

thanks for this useful input and links. this is exactly what i was looking for.

low volume pcb assembly56

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