Sound Powered Phones for Crystal Sets
Introduction
Balanced Armature elements or “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 “Gold Standard” of the vintage headsets do not come close to 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.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 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. Below 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.