Wireless Headphones 1957

[Bob Scott]

http://books.google.com/books?id=pykDAA ... ues_anchor

Check out the link. I was amazed at an article in the October 1957 issue of Popular Science for constructing wireless headphones. They used a method where audio is sent direct to a 2 turn coil wound around the perimeter of a room. No RF! No IR! The article also shows 3 kinds of headphone circuits to convert the transmitted AF direct onto headphones. Two are passive and the third is transistorized. The transistor circuit uses Micky-Mouse base biasing on the PNP Ge transistor, just a 470K resistor from base to B-.

The circuit would not work properly these days as constructed. The 2 turn loop in the room can be made of one single turn of 2 conductor 18 gauge lamp cord. But, the load this loop presents to an amplifier would be only about 0.6 Ohms resistive and about 60uH inductance, way to low for a solid state amp. They get away with attaching the loop to the secondary of the AF output transformer of the typical 1957 tube-type amp. Tubes were kinder to experimenters than instantly-blowing transistors. Tubes don't blow so easily when overloaded and the cheap transformer stage adds its own resistance.

I just mentioned this because I have not seen audio tranmitted directly to headphones in any other magazine in my career. It is definitely an experimenter-grade design though. It hardly works and would probably damage modern amplifiers.

It's funny reading these ancient issues. I get the urge to write back to the editor, then realize this happened half a century ago. No! No! Don't buy that new Edsel you are gushing praise for!

[Dean Huster]

Oh, no, Bob!! That was a supurb idea and design. It worked really well!

One of the electronics hobbyist mags of the 1960s had an article like that. I made one and it worked great! The loop around the room (mine was my basement shop, so looped maybe a 10 x 20 foot rectangle on the floor joists overhead) had enough turns to match the speaker impedance of the driving amplifier (I drove it with my Knight-Kit R100A SW receiver) so made it 8 ohms. The mag article indicated to use smaller wire gauge for a small room so that you didn't end up having to make 100 wraps around the perimenter. I think I had 4 or 5 with some pretty smallish wire. You measured the loop resistance until you matched the output impedance of your driver. The headphones I used were those older 1940-1960 Bakelite headphones (2000 ohms in series) and were uncomfortable as all heck, but I had a set and the newer audiophile headphones were too pricey for a kid back then. On my end was the receiver. If you had one of those battery-powered 3-inch reel-to-reel recorders back then that was junked out but had a good amplifier, it was a perfect receiver amplifier. You wound a bunch of turns around a 5" ferrite loopstick core and fed that coil to the tape head input of the amp. I wish I'd had half dozen of those little Japanese amps. The ferrite loop was mounted to the phones on one side and the amp was mounted of the other earphone. In 1967, those wireless headphones were a miracle -- monophonic miracle, but a miracle nonetheless!

I don't think that a solid state amp would complain since you're matching the impedance rather than trying to drive into a dead short. These days, you can design a voltage-to-current converter to drive the loop and match the amp impedance and end up with just one wrap around the room. And by diddling with the V/I converter, you can hit the loop with enough current to drive anything -- including tape heads, mag mics and mag phono cartridges! MRI in miniature!

[Bob Scott]

Oh, no, Bob!! That was a supurb idea and design. It worked really well!

Yes. I was attracted to the article because the method is so simple. (Maybe my post didn't convey my enthusiasm well.) I am surprised that the idea was not repeated often in those electronic mags. So many ideas were recycled over and over, like "big-ear" microphones and intercom circuits appeared many times over the years.

I don't think that a solid state amp would complain since you're matching the impedance rather than trying to drive into a dead short. These days, you can design a voltage-to-current converter to drive the loop and match the amp impedance and end up with just one wrap around the room. And by diddling with the V/I converter, you can hit the loop with enough current to drive anything -- including tape heads, mag mics and mag phono cartridges! MRI in miniature!

All you would need to do with the original loop in the '57 article would be to add an 8 ohm resistor. There's your V/I converter. It would have the same results as choosing the right small resistive wire gauge.

I'd like to see an article about rolling your own MRI!

[CeaSaR]

Hey Bob,

I remember a similar article a number of years ago, possibly in Poptronics. It used an audio
amp (min. 50 W, solid state, IIRC) to drive a VLF loop to send the signals to a specially
prepped set of headphones. That's all I can recall.

CeaSaR

[k.anderson3454]

Nice article. Is this really possible? It would be nice to have a wireless headphones.

[plk]

Check out the April 2004 issue of N & V for a similar project.

[plk]

Sorry, that's the January 2004 issue of Nuts & Voolts

[CeaSaR]

I think that's the article I remember. Thanks PLK.

CeaSaR

[Bob Scott]

HEY DEAN!

How did they work for you? There is no carrier frequency, only baseband. I imagine they would pick up HUM, especially if there is a loop of AC house wiring with the neutral return not precisely next to the supplying hot wire, as in a typical 2 way switch circuit (two switches on one hall light light fixture.) How about BUZZ from fluorescent lamps.

Caesar: I guess I was wrong about this article not being repeated.
"It's all been done before".

k.anderson3454: It's MONO, and I bet it picks up short range noise really well too, like AM radio.

[CeaSaR]

...
Caesar: I guess I was wrong about this article not being repeated.
"It's all been done before".
...

Link that is oh so appropriate...

CeaSaR

[Einar M]

I remember doing this back around 1968. I strung a very thin wire ( 36-40 Ga ) once around the room. The pickup coil was a couple hundred turns on a few metal plates from a medium sized audio transformer which I connected to a tape amp like Dean. It's been a while, but I recall it as working very well with no problem of hum or noise. My thought now is that there is no RF involved, you are just making a big transformer.

[CeaSaR]

Wow,

Embedding links in the smilies. Sneaky spam. All about plastic surgery and
something I cannot make heads nor tails of...

CeaSaR

PS - Delete this when you delete the spam. Thank you.

[Bob Scott]

A Wireless headphone is a cool thing for sure. It gives numerous benefits. But the cost still is a bit high...

There must be spam in there somewhere. This message has no other worth.

EDIT: The SPAM IS ON HIS PROFILE PAGE.

[dacflyer]

i have done somethng simillar...a cheap way to detect where our traffic loops are located.
we have attached a transistor radio speaker output to the loop in question, then use a telephone pickup connected to a simple battery amp. and we can hear the radio play thru the wire. thus tracing the path of the loop. it comes in handy .. sometimes faster then using the bulky "PIPE HORN"

[Dean Huster]

The loop method doesn't pick up hum from power lines since those magnetic fields are inherently cancelled since the two wires in the Romex or zip cord have opposite fields.

Adding an 8 ohm resistor in series matches the impedance but reduces the current in the primary, hence the amount of excitation of the secondary coil at the headphones. It's a voltage divider. Better to make up the impedance with extra wraps of wire and use no resistor. If you have a long length of ribbon cable or several multiconductor (unshielded) cables, you can do multiple wraps with just one or two wraps, splicing the wires in series at the open end of the cable.

One disadvantage of the system is that it is monophonic. Can't do stereo with a transformer.