Noisy environments such as restaurants and parties are always challenging for hearing aid uses. Phonak do several wireless microphones under the Roger trademark designed to help in such situations. These couple via Bluetooth either direct to Bluetooth-enable hearing aids or to a telecoil neckloop. But they're very expensive. I've thought for a while that it should be possible to do something much more cheaply, and have been trying to make my own neckloop, with limited success. (I'll think about the wireless-coupled microphone later, prbably using a 2.4GHz audio sender and receiver.)

Connecting a single loop of wire around my neck to a headphone socket gave poor results. A length of 26 way cable with all strands in series was hardly better. I decided I needed a step-down transformer, so I got some ferrite cores, 10mm od, 6mm id, 4mm height, N30 grade. With 2 turns secondary connected to a single loop around my neck, and the primary connected to a headphone socket gave the same barely acceptable result, either with 10 or 20 turns primary.

I also tried a miniature mains transformer - 240v primary to the headphone socket and 3V secondary to a single loop, but got nothing.

So I tried driving the primary of the ferrite core with a PAM8203A Class D amplifier, but it started distorting badly as the ferrite saturated, before it reached the level I wanted.

Maybe I should wind the windings around several of the cores stacked (or get some bigger cores). But I'm wondering if I'm even on the right track. I wonder if anyone round here knows how a commercial neckloop is constructed. Perhaps I should be driving it with a current source rather than a voltage source - but how?

Commercial induction loops for churches and halls are designed to produce a peak fields strength of 400mA/m so I guess if I could pump a couple of hundred mA through my loop I should get a strong result. It should only take milliwatts to do that - I shouldn't need an amplifier.

 

I know very little about telecoil neckloops but I believe the audio signal from the microphone is modulated on a carrier. I have no idea what the frequency is. It is demodulated at the neckloop and sent to the headphones.
That would explain the poor reception if you are just trying to couple to the audio signal.

 

I inherited a telecoil and it is purely a passive device, that works with the telephone pickup coils in some hearing aids. So it must be fed the exact signal you want to hear.
The problem is that it is half of a transformer, and the receiver is a small coil inside the HA device. So having the proper position between the two is rather vital. It is trying to substitute for the coil in a telephone handset, so you can understand how it is supposed to function.

 

Induction loop amplifiers are current driven. There is no modulation. Obviously, for a large building the impedance is approaching that of a loudspeaker, so the amplifier circuits are similar.
For a small loop you will have a very low impedance:
I think you will need an iron core transformer, something like the LT700 which was originally designed for the output stage of (germanium) transistor radios, and I would go for several strands of wire in series. Have you tried ribbon cable?

 

Induction loop amplifiers are current driven. There is no modulation. Obviously, for a large building the impedance is approaching that of a loudspeaker, so the amplifier circuits are similar.
For a small loop you will have a very low impedance:
I think you will need an iron core transformer, something like the LT700 which was originally designed for the output stage of (germanium) transistor radios, and I would go for several strands of wire in series. Have you tried ribbon cable?

My first thoughts were to use an iron core transformer but couldn't find a suitable one. An LT700 I found on eBay is 1.2KΩ primary and 3.2Ω secondary. I think I'd do better to to completely rewind some such small transformer, But they tend to be plastered in shellac so getting the laminations out could be tricky. (Now what did I do with my 1960's Bernards Radio Coil Construction Manual that I nearly threw out a few months ago? ... I didn't, did I?...)

So what is this LT transformer designation? eBay is also offering LT44, LT72, LT100 and maybe one or two more. Does that indicate the core size?

And yes, I tried 26 way telephone cable will all strands in series, remarkably with barely if any improvement.

 

So what is this LT transformer designation? eBay is also offering LT44, LT72, LT100 and maybe one or two more. Does that indicate the core size?

Unfortunately not. I think it’s just a catalogue number. LT44 and LT700 are both the same size.

 

The "Phonac" brand induction loop that I inherited was intended to connect to an audio source that would also power headphones. That is the whole reason for the neck coil existence. So the audio power provided is not much, and the signal is just audio, no carrier or other complex modulation scheme. But the coupling is wildly variable given the variable positioning of the neck loop relative to the HA pickup located at the ear.
Consider that the ear pickup is optimized for use with ADA compliant telephone handsets, and the magnetic field they produce.
The magnetic "telecoil" scheme is a creation that works better in theory than in reality, as my father discovered. The variable coupling is the challenge.
I will see if I can locate that loop and measure the resistance. That should provide some valid information.

 

The "Phonac" brand induction loop that I inherited was intended to connect to an audio source that would also power headphones. That is the whole reason for the neck coil existence. So the audio power provided is not much, and the signal is just audio, no carrier or other complex modulation scheme. But the coupling is wildly variable given the variable positioning of the neck loop relative to the HA pickup located at the ear.
Consider that the ear pickup is optimized for use with ADA compliant telephone handsets, and the magnetic field they produce.
The magnetic "telecoil" scheme is a creation that works better in theory than in reality, as my father discovered. The variable coupling is the challenge.
I will see if I can locate that loop and measure the resistance. That should provide some valid information.

Thank you, that would be most useful. If you could crack it open and see what's inside, that would be incredibly helpful. Most things just have a 2-part plastic shell, if you're lucky, screwed together, or maybe with plastic clips you can release with a thin spatual, if you're unlucky, glued. Does it contain an iron or ferrite transformer? Any indication of the turns ratio? Is the loop a singl wire or multiple?

In our church, the telecoil works tolerably well wherever you are provided the coils in my hearing aids aren't pointing at the wire. With a neckloop, the main coupling will be with that part of the loop going round the back of the neck, so should be fairly constant. But I gather there are active neckloops which are better, and thats effectively what I'll be aiming for.

 

Here is an homemade hearing aid Loop design using a psu also, but the transmitter is what you're looking for the loop is current driven, and there's a receiver too.

https://www.nutsvolts.com/magazine/article/build-an-audio-loop-system

 

I just attempted to measure the resistance of the induction loop that I inherited, and I find that it reads open on my good analog ohm meter. So I am guessing that either it is damaged or it is capacitor coupled. But there was no slight meter pointer twitch when I connected the meter leads, nor when I reversed them.. And unfortunately the junction block between the coil and the connection lead is a solid molded block. Made to Never be repaired. The circumference of the coil is about two feet, if that is of any use.

 

I just attempted to measure the resistance of the induction loop that I inherited, and I find that it reads open on my good analog ohm meter. So I am guessing that either it is damaged or it is capacitor coupled. But there was no slight meter pointer twitch when I connected the meter leads, nor when I reversed them.. And unfortunately the junction block between the coil and the connection lead is a solid molded block. Made to Never be repaired. The circumference of the coil is about two feet, if that is of any use.

That's a shame. (I don't think it'd be capacitor coupled - it'd need a massive capacitor to couple into a loop of fraction of an ohm, unless the wearer had hearing extending well into the MHz region!) Presumably the neckloop has a break-away anti-strangulation connector which enabled you to break the loop and measure its resistance. What is the resistance measured at the 3.5mm plug?

Are you in th UK? Would you consider sending it to me for the cost of postage? I might take a dremel to it. (I'm seeing my dentist shortly. He's a jolly sort of chap, but I don't think I'd quite have the nerve to ask him to x-ray it for me.)

 

Many years ago I experimented with a hearing aid loop. For a single loop round the perimeter of the room I used a TDA2050 with a + and - 21 volt supply and a 5 ohm resistor in series ith the loop. I think it worked reasonably OK. For the loop round the neck type system I used an 80 cm length of 8 core cable with the ends of the wires linked so it formed an 8 turn loop. I can't remember what t I used to drive this but in my notes I had the schematic from an article from the "Silicon chip" magazine March 2011 which used an LM386 with a 9 volt supply and a 10 ohm resistor in series with the loop. I think I probably used a simailr rating amplifier.
I hope these notes may be of some help.

Les.

 

That's a shame. (I don't think it'd be capacitor coupled - it'd need a massive capacitor to couple into a loop of fraction of an ohm, unless the wearer had hearing extending well into the MHz region!) Presumably the neckloop has a break-away anti-strangulation connector which enabled you to break the loop and measure its resistance. What is the resistance measured at the 3.5mm plug?

Are you in th UK? Would you consider sending it to me for the cost of postage? I might take a dremel to it. (I'm seeing my dentist shortly. He's a jolly sort of chap, but I don't think I'd quite have the nerve to ask him to x-ray it for me.)

No, I am not in the UK, but rather about 5 miles north of the Detroit, Michigan city limits.
I just investigated that loop for any possible break-away feature, none found. And the line around the junction block does not respond to efforts of prying. But as it registers open circuit perhaps I can "take a bash at it", an expression I did hear when I was over there a few years back.
And oof course, a magnetic field loop is driven by current, but without voltage to push it, current stays rather still. As for the required current, consider that the earpiece of a compliant telephone handset, at least the ones I have seen, had a coil about an inch in diameter that looked like 20 to 30 turns of rather fine wire, perhaps #36 or so. The pickup coil of a clamp-to-the-earpiece amplifier from Radio Shack was much smaller, perhaps 5/8 inch OD, less than 1/8 inch ID, and about 1/16 inch thick, wound very solid with closer to #32 or 334 wire, varnished into a solid coil..

 

Thank you, that would be most useful. If you could crack it open and see what's inside, that would be incredibly helpful. Most things just have a 2-part plastic shell, if you're lucky, screwed together, or maybe with plastic clips you can release with a thin spatual, if you're unlucky, glued. Does it contain an iron or ferrite transformer? Any indication of the turns ratio? Is the loop a singl wire or multiple?

In our church, the telecoil works tolerably well wherever you are provided the coils in my hearing aids aren't pointing at the wire. With a neckloop, the main coupling will be with that part of the loop going round the back of the neck, so should be fairly constant. But I gather there are active neckloops which are better, and thats effectively what I'll be aiming for.

I just attempted to measure the resistance of the induction loop that I inherited, and I find that it reads open on my good analog ohm meter. So I am guessing that either it is damaged or it is capacitor coupled. But there was no slight meter pointer twitch when I connected the meter leads, nor when I reversed them.. And unfortunately the junction block between the coil and the connection lead is a solid molded block. Made to Never be repaired. The circumference of the coil is about two feet, if that is of any use.

I use a Sennheiser ETZ3011 neck loop. It measures 13 ohms on both the left and right side with a common ground. Hope this helps.