After my experimentation in building a T-coil receiver on my own, it seemed that the unit did have some issues which I eventually corrected. While it seems the schematic may have been a victim of a transcription error, I thought it would be nice to share the inspiration behind the whole project and where it’s gone since.
When I was younger, back in 2004, I worked at the Blacktown Tandy store for work experience. It was an interesting time, because already then, the store was in a “shake-down” phase as there was a local Dick Smith nearby and they were headed towards an eventual closure. It eventually happened, although since the Tandy performed well, it starved off its death for a few years. In that time, they cleared out the “InterTAN” RadioShack stock which they imported, and replaced them (mostly) with local Dick Smith products which were cheaper.
One of the good things about this was that there was a number of products which they held stock of, and nobody bought. Things like 100m rolls of 210mm and 216mm wide thermal fax paper, which they cleared out at just 50c a roll. I bagged a number of them (probably 10), and then respooled them into separate ~50m rolls so that the machine could take it, before forming a “continuous feed” system with the roll outside of the machine instead. Of course, I never received many faxes, but instead, I used a 9v battery to form the loop current between the machine and modem, and used the Sharp fax “pick up” number of 5** to trigger a transmission between a computer and the fax machine without any need for ringing current. This worked to turn the fax machine into a slow printer, which I used to print a number of reference documents for my own use. It was good value.
Another thing they got rid of was this amplified earpiece for clipping onto a phone using an adjustable silicone rubber band. Used by the hard of hearing, it would boost the telephone receiver volume, but it had no microphone on the rear. At the time, I thought this was somewhat magical – how could it do such a thing without a microphone. I didn’t realize that if it had a microphone, a feedback loop would be inevitable.
Of course, when it was supplied, it wasn’t yellowed and didn’t have wires poking out of it. But that being said, they got rid of them for about $2 a piece, so I couldn’t resist buying a few, although I managed to lose every other one.
One of the reasons this unit was a poor seller was the use of the “odd” N-size cells which have never been very common around here. They’re also fairly expensive to obtain even where available, so I couldn’t put this to use immediately. Its destiny was modification.
The first thing I did was solder to the battery terminals and hook a regular AA cell to it. I slapped the unit onto a phone and it did work – it managed to receive the audio just fine and amplify it. I started putting it on a number of devices and found it could receive audio or some odd noises as well. By then, I realized it was probably a magnetic coil pick-up with a speaker behind, and I’ve always wanted a magnetic coil pick-up.
Part of the reason for wanting one stems from wanting to be able to record the telephone. I was born too late to be a phone phreak, but I had some modems I really wanted to record. Much better methods were devised, including using a voice modem as a monitor, and eventually just using VoIP packet interception, but at the time, this was what I had. As a result, I cut-out the speaker as well, and soldered a pair of wires to plug into the line-in of a sound card.
Seeing as the unit doesn’t get any real use, but was the inspiration behind the whole induction loop receiver idea, I felt it would be befitting that it would be taken apart for a quick peek at the circuit.
Part of the secret as to how this works is on the left. The green and white wires come from the magnetic coil pickup which sits underneath the shaped plate. The plate is made of a particular sort of metal that resists magnetic fields, similar to those used to shield hard drives. This “breaks” the potential feedback loop between the speaker (removed) and the pick-up coil. The PCB itself is a paper-type single-sided through-hole board with a combination power and volume rotary potentiometer. Judging from the trace shapes, it could have been designed by hand.
There is white silkscreening on the top, and the code suggests the PCB was made in Week 34 of 2000. The circuit is made of all discrete components – no op amps to be found here, but instead four transistors, and an assortment of resistors and capacitors. The black and white wires represent the audio output.
Here’s a look at the PCB from many different angles so you can see the components that were used and their values. I don’t have the time to trace out a schematic, however, I’m not even sure that identical components are available. I suppose it’s probably possible to substitute more modern transistors in their place.
That being said, I remember that the audio wasn’t so clean, instead having some hiss on the output maybe due to the amplification intended to drive a speaker. I’ve never tried operating it outside the house, although I suspect its performance on a train may not be so good, as the shielding plate may shape its reception to be more close-field for a nearby speaker rather than for a hearing loop transmitter coil which could be more remote. However, I have not tried it, so I really don’t know.
It’s interesting to see how often you have these kinds of “weird” ideas pop up early on, but at the time, I was only just starting out in electronics and only had basic soldering skills. It wasn’t until about 10-years later that I could build something of my own that did something similar, if not better.
As I wasn’t entirely satisfied with the quality of the recordings I obtained from the receiver I had built, I wanted to seek out better designs. Unfortunately, there weren’t many, however the one from Silicon Chip Magazine seemed to be the only one out there. These designs often made their way into kits, such as this one from Jaycar or this one from Altronics. Unfortunately, the price was too high for my liking from Altronics, but I still wanted to know exactly what they did. I tried to succumb to ordering from Jaycar, but surprise-surprise, they had absolutely no stock.
As a result, I went on a wild goose chase looking for the September, 2010 issue of the magazine. Sadly, a number of libraries I had checked do not hold stock of the magazine, and ultimately, I had to register with the State Library of NSW to obtain access.
As the publication is copyrighted, I can’t share the schematic with you. However, you can probably go seeking for the issue if you want to know just how they did it. Some more generalized hints as to the main part of the circuit include the use of a number of stages involving a TL072 dual JFET Op Amp amplifying-and-filtering with each stage followed by an LM386 audio amplifier. The pick-up was a xenon trigger transformer.
After seeing their design, I felt it was rather logical, so I ordered some components and set about building an abridged version of it. I felt no need for their LED power indicator which was likely to suck as much power as the circuit itself, and I felt no desire to use a 9V battery. I had no need to drive headphones directly, as I had a fairly sensitive audio recorder I could employ. As a result, I removed the LM386 entirely, and the zener/LED combination. I had no xenon trigger transformer, so I opted for a similar inductance instead. I decided to build it in the Hammond enclosure I received as a sample, to make it a little less fragile.
I opted to make things nice and neat … from the outside.
But from the inside, it was a bit of a patchwork. Note that this wasn’t the final configuration – there was still more wire cutting and jumpering around as I noted the level control didn’t do as much as I had intended and instead used it on the output instead of the second stage. This instead had the side effect of introducing pot-crackle when the level was adjusted, so … I think it’s probably better to omit it altogether and have the level on maximum at all times, but it’s too late for that now. Most of the inside volume is batteries – two AA’s to be precise.
I can’t speak for the performance of the original circuit as designed, however, this one does perform about equally well SNR wise as to the unit I built before, so it was a bit disappointing to see that all that effort didn’t result in any tangible audio quality gains. That being said, it’s good to know that too, since it shows that the EMI is probably limiting the SNR and thus, improvements in noise level or intelligibility are likely only to come about with DSP noise reduction (and its associated issues in terms of audio quality).
It’s been a few years since I did the initial project, but the impetus to revisit it was due to discovering the unit that started the whole idea in the first place. After a quick revisit by building one based on the Silicon Chip Magazine design, it was discovered that the audio quality limitations are likely to be inherent to the transmission system, rather than of my own design. It also seems I’ve made some probable transcription error or drawn an earlier revision of my design prior to a number of fixes, as pointed out by a reader. Unfortunately, since the unit is under a mountain of stuff now (including a pile of hot melt glue), I probably won’t get around to revising it – however, if you’re in Sydney, you can probably hop down to the State Library and grab the Silicon Chip design yourself.