Sound Powered Phones for Crystal Sets
Introduction




First a few definitions:

Balanced Armature: A technology used to reproduce sound and is used in speakers, headphones and telephone units. First patented in 1918 by Henry Egerton and based on the 1882 balanced armature telephone patent of Thomas Watson.

    Magnetic forces from a permanent magnet(s) and coil(s) work on a "balanced" or centered  "Armature" or plate. This Armature is connected to a diaphragm. Much more efficient than the old vintage magnetic headsets because they are "bi polar" meaning they use both side of the magnetic forces. Also the most sensitive spot on a diaphragm is the very center. The closer you get to center, the more sensitive the unit. This is why magnetic units try to get their coils as close as possible to each other and to the center. Balanced Armature can achieve this easily by sending the energy from the "Balanced Armature down a "connecting rod" to the exact center of the diaphragm.

Sound Powered unit: A unit in which the operating power is derived from the speech input only. There are no "outside" batteries or power source . The power is generated when "sound energy" is converted to "electrical energy" by the "Mic" unit. These unit were made for the Navy in W.W.II and continue to be made today.

Sound Powered units might use Balanced Armature technology to achieve their goal, but they might also use magnetic elements or carbon "mics" or other technologies also.

Units like the early 1900's Baldwin's (vintage headphones) are usually considered "Balanced Armature" but not Sound Powered, because they were never designed for "Sound Powered" applications.

Elements that were designed for "Sound Powered" application generaly are more sensitive than magnetic type because they had to be. They had to work with very low signal levels compared to magnetic sets. Also from the early 1900's to the 1940's technology just got better.

“Sound Powered” elements, as they are called by crystal radio designers and builders, are the rage for use on Crystal radios. They have improved performance over the “vintage” style magnetic headphones of the early 1900’s or any other device made since. Even Baldwins which are the considered some of the best of the vintage headsets do not come close to a good set of Sound Powered headsets for Dxing on a crystal radio. Not always the best for music(some units), they are much more sensitive than any other type of unit for voice, which makes them invaluable for picking out that very faint station. Please remember that Sound Powered elements reach their "glory" while trying to pick up a very faint signal. This is where they will excell. On signals that are very strong you might see a very small difference compared to vintage magnetic headsets. In fact in some cases vintage might sound better such as music.

   Most Sound Powered headsets and handsets that are used for this purpose were made for the military during WWII. These units were made for the Navy ships where it was critical that loss of electrical power onboard did not knock out communications between “plane spotters” and the “gunners”, or the bridge and steering or engine rooms. Balanced Armature or Sound Powered elements were developed that used no outside power source. The sound from a person’s voice travel to the “Mic” unit, which generated a small amount of electrical signal which could be picked up by headset or “ear” unit. No outside power was needed, because it generated its own power. If a section of the ship was knocked out by a bomb blast and the wires were destroyed between the spotters and the gunners, a cord could be rolled out on the deck, and the two units could be reconnected up in an instant. In this way the ship was not left defenseless for very long. Two basic types of units were used by the Navy, “Deck Talkers” which were a headphone and chest mic unit for spotters and gunners, and handsets which resembled a telephone handset.
    The very efficient design of these units, due to the small signals that were used, is what makes them perfect for crystal radios. The main problem with these units is their very low impedance. They must be matched to the radio using matching transformers to obtain high sensitivity, without “loading” the radio’s circuit. For more info on matching transformers go to “Matching sound powered headsets to crystal sets using transformers” link below or to Ben Tongue’s web page at http://uweb.superlink.net/bhtongue/5hpXform/5hpXform.html.

    The “mic” unit can be used as “ear” piece also. On the main page of this web site are examples of units as they look before being converted to use on crystal sets. Some are WWII era and some are post-war era. There are a few units still being made for use today. The fire departments use sound powered units today because they can be used in areas which may contain explosive gases without the fear of a spark setting the gases off. Also they don’t use batteries, which might go dead at the wrong moment.

I would like to try and help clear up a few misconceptions about Sound Powered headphones.

Misconception #1
Sound Powered elements will be louder then my vintage magnetic sets so I will get a pair.

Please remember that Sound Powered elements reach their "glory" while trying to pick up a “very faint” signal. This is where they will excel. Because of their efficiency they will pick up stations that magnetic can not. On signals that are very strong you might see a very small difference compared to vintage magnetic headsets. In fact in some cases, vintage might sound better such as music. Sound Powered Elements will continue to pick up faint stations where vintage or magnet headsets stop. In this one case only are Sound Powered elements “louder” than vintage. They will perform quit nicely on local stations also, but they are at their best at the lower level signals because of their efficiency.

Misconception #2
I want to get a set of Sound Powered headsets so I can start receiving DX.

If you don't already know, adding headphones of any kind will not always get you DX. You must optimize the radio first to gain its full potential. More can be done to the radio to increase efficiency than you can gain by adding SP headphones. If you are already there then that is another story. Sound Powered headphones will not instantly bring in “DX”. They might, but probably not.
 

Misconception #3
I’ll buy a set of Sound Powered and just put them on my set and they will work better than my vintage.

You must match the SP headphones to the radio. If you do not use a matching transformer, you might as well not use the headphones. The impedance is low on Sound Powered elements and must be raised to more closely match the radio. For example, a set of Vintage or magnetic headphone are around 12k to 23k ohms AC. Not a perfect match to a good radio, but not bad. But a set of Sound Powered elements wired in series will get you about 1200 ohms AC. That is way too low for a radio. By using a transformer to raise that to a higher level such as 50k to 100k (or higher) you can “match” the impedance of the radio. Any loss from the transformer is more than made up in the matching. Even a vintage headset can be helped with a matching transformer. Please see the "Matching" section which is off the "Main" page for further information.
 
 

Now I would like to make a few comments on comparing Sound Powered units. I know there is a huge rush to the king of Sound Powered elements, RCA, but you must please keep some very important things in mind when trying to buy a set of Sound Powered units.

When comparing Sound Powered units there are at least three things to take into consideration first.
Condition, Condition, Condition. Sounds like real estate’s  location, location, location! Here is what I mean by “ Condition, Condition, Condition".

Contition #1-- It depends on the condition in which they came out of the factory. Did they meet the high end of the factory specs. or did they just barely pass minimum standards? If they are RCA's and met minimum standards, (which a lot of them did because there was a war going on and they needed them) then they might not be the "best" but are still very good.

Condition #2 -- It depends on the condition they are in now. A lot has happened to these units since W.W. II. Salt water, bomb shells going off next to them, sinking ships, laying in someone's back yard or garage for 50 years and so. They are very susceptible to rust, corrosions, sharp blows that can knock the armature out of center, and so on.

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Introduction Part 2
What makes a "Sound Powered" element more sensitive than
a old style "Balanced Armature" element or a set of "Magnetic Headphones"?

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