THE FIRST AUDIO AMPLIFIER
An Early Modulated Rectifier

We have always had natural rectifiers, crystals and other touching/conducting things that pass current mostly in one direction. This includes carbon granules. A working carbon microphone can be built with only one granule. I have seen commercial carbon mikes with only six to eight small spheres of carbon. Bell may have experimented with several lab models. Many telephone mikes had had a shallow-thimble-sized cup of carbon granules like black table salt held against the conducting diaphragm. Two or three "dry" cells in a closed circuit supplied current through the varying resistance of the granules and on through an inductor [choke or transformer] coupled to the telephone line.

While in a steady state, the direct current in the mike circuit remained constant and no signal was heard on the line. When sound pressure waves moved the diaphragm, the current (varying dc) produced ac in the coil and onto the line at voice frequencies. The modulated power on the line followed the voice faithfully enough to be recognizable.

Something else was happening. The energy of the voice was controlling the added power of the dry cells! The voice heard on the line was louder. The audio level from the hand-held earpiece was enough to modulate the mike! The two had to be kept apart (not acoustically coupled). Of course this was done for the fun of it, and to the disgust of the called party. This electro-mechanical device was an audio amplifier.

When you have an audio amplifier you can have an audio oscillator. Such a direct-coupled combination of mike/ receiver was manufactured during WWI and used in early Signal Corps transmitters. The keyed, tone-modulated, continuous wave signal could be heard, cutting through the battlefield interference. (Radio Telephony was waiting for 1916 and De Forest! He changed the tone modulation to voice modulation.)

Note: A hand-cranked generator powered this field equipment. When the oscillator was switched on, the load on the hand crank increased so as to quickly tire the operator. I have had opportunity to find this out for myself.

Morse, about 100 years earlier, had used a hand key to send a square wave. Bell closed the circuit and modulated the current at an audio frequency by shaking carbon granules in a small box that Edison had invented—the carbon microphone!

WW2 SCR-268 Radar replaced an acoustic predecessor used in WWI. At Wright Field I saw an old gimbaled-like frame that operators could elevate and rotate to maximize the sound of approaching aircraft. Huge trumpet-shaped stereo horns reached for sounds as the operators rode the device, cranking to the sound source, peaking the sound by cranking in the elevation. These were pictured as being strictly acoustic i.e. the operators were wearing stethoscopes connected to the horns. (It may have used telephonic amplification instead.) U.S. propaganda pictured this device while we actually used radar, a code word that was not spoken aloud! Instead, we said "electronics."