I was on one of Shanna Groves’ pages last night, and I got into an interesting discussion with Dan Schwartz. Like me, he’s a blogger and an audio engineer, so we had something in common to chew the fat over.
If you’re interested in hearing aid technology, Dan’s a font of information. I mentioned that among my many interests, is a love for antique technology, and he turned me on to an awesome site. The Hearing Aid Museum. I combed through there and came up with this:
Amplivox “Radovox” Vacuum Tube Hearing Aid
The site gives the approximate date of manufacture as 1938. This was a 2 piece unit. The box you see above, and an earpiece that stowed away in a small compartment. The user would hold the earpiece up to her ear, and a microphone inside the box unit would pick up the sound.
Here’s an inside shot:
Now, looking at this picture – I can give you some idea of how this thing worked. Again, this is just my observations from looking at a picture. In order to properly analyze it, I’d need to see an actual schematic diagram.
Anyway, you can see that there are 3 vacuum tubes. The one furthest from view has an anode cap and I imagine that that’s the output stage. Since only one output tube exists, it’s safe to say that this is a class A amplifier. The two tubes closest to us would be the two gain stages of a preamp. The first stage steps up the low level output of the microphone, and the second stage drives the output.
As is standard with tube type amplifiers, the output stage is coupled to the earpiece via the transformer which can be seen in the lower right hand corner.
Volume is controlled by another transformer – of a type called an autotransformer also known as a variac. I find that odd, since a simple variable resistor type potentiometer would have worked as well, and been much cheaper to produce. If this is a potentiometer, it’s really quite large. Beneath that volume control, is the microphone, which I assume to be a standard magnet – voice coil type – similar to that used in early telephones. The possibility exists that it’s a crystal mic., but it doesn’t really look like any I’ve seen.
This battery was actually made by inserting a carbon rod into a jellied acid solution, surrounded by a lead shield. That was covered in paper. The battery itself weighed over a pound, and the device would have need two of them. The one seen above is called a “B” battery. It supplied a 4.5v bias voltage and a 52 volt B+ voltage. The second battery – not shown was called an “A” or “#6” battery and it would have provided another 4.5 volts to heat the filaments of the tubes. These batteries would only have supplied power to the unit for a few hours at most.
The Radovox hearing aid measured 4⅞” high by 6⅞” wide and 3⅞” deep. It weighed 1 lb 7 oz without the batteries.
Add to that about 3 lbs for the batteries, and you’re looking at a hearing aid that weighs in at just under 5 pounds.
In as far as I can discern – again, from looking at the picture – this was strictly a microphone and amplifier system. No filtering, phase shift, frequency shift or any other signal processing associated with modern hearing aid technology.
So this is a far cry from the micro-transmitter, Bluetooth type hearing aids we have now. And Shanna would be appalled at how bling-less it was. Chances are good however, at least in some cases, that it may have helped some people hear.
Here’s the link to the actual page:
- Lyric Hearing Aids (deafinprison.wordpress.com)
- What You Need To Know About Hearing Aids, (celtichearing.wordpress.com)
- Lipreading Mom’s Show Me Your Ears Campaign (deafinprison.wordpress.com)
- Why Do People Who Need Hearing Aids Not Wear Them? (lipreadingmom.com)