Speech Amplifiers

Author: J.B. Hoag

The purpose of a speech amplifier is to increase the strength of microphone voltages to a sufficient value to operate a terminal amplifier which has a power output of the desired magnitude. The overall voltage amplification is equal to the product of the amplifications of the individual stages. A typical three-stage amplifier would have a first-stage voltage gain of 100, a second of 20 and a third of 15, or a total of 30,000, corresponding to 89.5 decibels. With carbon microphones, two triode stages and a power output stage are usually sufficient. Such a circuit is shown in Fig. 25 O.

Fig. 25 O. A speech amplifier suitable for carbon-button microphones

With crystal microphones, the gain must be greater; the first tube is usually a pentode, as in Fig. 25 P.

Fig. 25 P. A speech amplifier suitable for crystal microphones

For reasonable fidelity of speech, the gain should not vary by more than one decibel over the range from 100 to 4,000 cycles per second. For music, the presence of the higher harmonics requires that the response be flat to much higher frequencies, say 8,000 or 12,000 cycles per second.