Teardown: RadioShack Amplified Telephone Earpiece (43-229)

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.

Project Evolution

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.

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Teardown: FiiO E5 Headphone Amplifier

The FiiO E5 was a low-cost but decent performing compact headphone amplifier a few years back. The unit bore a vague resemblance to an iPod Shuffle clip-version, and was relatively compact with integrated rechargeable lithium-polymer battery. Since someone didn’t want theirs, I managed to snag it second hand for cheap, but I never really had that much of a use for it. After a while, the clip broke, the battery wasn’t really lasting and I decided I might as well tear it apart and throw it out. So here’s a few pictures which I took much earlier this year, which I never got the chance to post until now.


The exterior is a nice aluminium curved shell, and the interior unit slides out after some fiddling with the clips and persuasion to break the glue used inside. The PCB sits inside a plastic frame, with its rear littered in test-points which is covered by a self-adhesive layer of plastic. There is a terminal mounted at the bottom, which provides the grounding point to help reduce any RF interference which might be experienced by the unit.

The internal metal has been cleared of paint where the grounding contact would sit, which is necessary to make it effective.

All the magic is on the other side of the frame, where space seems to be very well utilized. The Li-Poly battery can be seen up-front, with the headphone jacks (in/out) being the next highest profile components. There is a mini-B port for charging.

The battery is marked AK402030PL and has a printed capacity of 190mAh.

The main PCB has all its components mounted on the one side, and all of them are surface mount. Quite a few ceramic capacitors, a tantalum, a number of resistors, two LEDs for charging/operation status, three push switches for volume up/down and power, and a slide switch for bass boost. It seems that the unmarked 8-pin IC is the controller that runs the whole unit. The Burr Brown/Ti OPA2338UA dual op-amp is likely to be responsible for buffering the input and doing the bass boost function. Finally, the TPA6130A2 138mW DirectPath Stereo Headphone Amplifier with I2C Volume Control does the actual driving of the output. It seems there’s a 5-pin charge controller/voltage regulator chip for the li-poly cell marked 54b9, although the battery does have its own protection board.


The E5 did indeed contain the promised Texas Instruments solutions in the signal chain, which explains why it was considered to have good performance, but it was made by the Chinese and at a very low price. This was all good for the consumer, as decent performance could be had for cheap, and it seems that the more savvy consumers know to look towards these brands as they develop a positive reputation.

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Failing: Sandisk Ultra microSDXC 128Gb (Class 10, UHS-I, Up to 80MB/s)

The new year just keeps getting better. Sure, the battery in the phone is failing, but the microSDXC card as well?!

The Culprit

The culprit was a Sandisk Ultra microSDXC card of 128Gb capacity, Class 10, UHS-I rated up to 80MB/s. This is not the same card that I previously reviewed, but a newer version of that card that I bought when I purchased my Xiaomi Redmi Note 2. As 128Gb cards (at the time) were not widely available from different manufacturers, and compatibility was problematic for some, I stuck with Sandisk. As I’m aware of the catastrophic consequences of fake flash, I purchased the card from a reputable local seller which sells local stock.

The card was purchased on the 4th February 2016, less than a year ago. To see it run into trouble is rather unexpected.

The Symptoms

The first sign of troubles came about two months ago, when I realized that some photos I took on my Redmi Note 2 came up as “Load Failed!” in Quickpic. It was only one photo, so I dismissed it as probably a glitch such as an interface bus issue – maybe something like a sparking pantograph on a passing train during the photo recording process might have disrupted it.

Later on, it affected three photos in a row sequentially while I was at the conference in Canberra. I thought “gee, something must be wrong with the software” and dismissed it again.

It was only until last week, the phone began to struggle. The camera app started to freeze, taken photos were never recorded, and the phone took forever to reboot. At times, apps stopped starting up and the phone would hang. Removing the microSD card cured everything, so I decided it was the culprit, and seeked to recover as much as I could from the card.

Copying from the card was fast for a few files, and then would stop and stall for a long time before failing with an I/O error.

It wasn’t just one or two files. A large number of files succumbed, including my TWRP backup of the phone. To copy it off and work out exactly which files were affected would take a long while, and because of card misbehaviour, ddrescue was not really as viable as it could be.

The card had a tendency on hitting some error to hang for a while then completely drop off the bus, requiring a cycle of the card (unplug/replug) to regain access. Worse still, some affected sections appear to be directory entries in the FAT, thus even listing certain folders was not reliable.

As I didn’t want to waste a lot of time, I devised a method to copy as much as possible in as short a time. Using Windows 7, I copied all files to a new folder, but when the card struggled, I pulled the card from the reader, provoking this error …

… while at the same time, plugging the card back in, allowing enough time to recognize the card and then hitting skip to bypass the “damaged” file.

In all, I only lost 9 of about 1000 photos, the backup of the phone, and a few random temporary download files. Nothing too major, although if you’re going to yell at me to “use cloud backup”, I’d have to decline since I really don’t want to tie up my limited upload bandwidth with that nonsense. Sharing my 1Mbit/s with another two family members that do use such systems and their friends is enough …

The Curse

Now came the time to consider returning the card under warranty. It had a “limited lifetime” warranty, and the failure occurred in less than a year.

The HDTune Pro error scan failed after 2356 errors, and it seemed the card had fully given up returning data at a point. The errors were real.

But I could not let a card containing my own personal data out to return, so I wiped the card. Unfortunately, as it turned out, post full formatting, it was again able to store and retrieve data across its entire surface correctly.

The fault had cleared, thus making me ineligible to return the card, probably due to internal reallocation or reprogramming of the faulty flash cells. However, it seems highly likely that the issue will re-manifest itself into the future.

After this experience, I cannot recommend any Sandisk Ultra cards. It seems the whole planar TLC memory thing has gotten out of hand, and that their cards of exceedingly poor endurance. This is the third Ultra card I’ve had fail in 2016, and it’s not because I use them harshly at all. In fact, that card had only been fully cycled in commissioning testing, and then was never filled completely. It was used in a phone as photo storage, and never even received a load of video/audio. The other two cards that had failed started similarly, but ended in complete lack of recognition of the cards (a 32Gb and a 64Gb card).

The Final Nail

Unfortunately, it wouldn’t be the end if there wasn’t a little twist. When putting the card back into the phone, I had it formatted as exFAT as per SDXC standards. I didn’t realize (or remember) that the Xiaomi Redmi Note 2 did not support exFAT and required it formatted FAT32. When the message saying “external SD blank or unsupported filesystem” came up, and an offer to format the card correctly was proposed, I leapt at it without thinking twice.

It seems that an interaction between this command and the custom recovery (TeamWin Recovery Project) meant that instead of formatting the external SD card as it should have, it wiped the internal SD leaving the unit back to factory defaults and me without a backup to restore from thanks to the card failure.

So, four hours of my time was spent flashing the newest weekly Xiaomi.eu ROM (why not?) and reinstalling everything … just to get it back to life … with its swollen battery. I’m flying out in four weeks – I really don’t need this.


It’s a shame but it seems the Sandisk brand no longer represents the quality it once did. But that’s not the only brand I’ve had issues with – Lexar is also part of the group too.

As a result, I can’t really recommend the Sandisk Ultra as a good choice anymore – I now look towards Toshiba and Samsung instead, as none have faultered in my hands yet.

However, it seems quite likely that the whole trend towards cheaper flash memory devices and TLC memory means that reliability is on the down-hill. I’d hate to think what people using them for constant data writes (e.g. data-loggers, dash-cams, surveillance-cams) will see in terms of endurance.

Posted in Computing, Flash Memory, Tablet, Telecommunications | Tagged , , , , , , | 1 Comment