The advantages of direct-coupled amplifiers have been known for a great many years, but the inability to attain a simple and practical direct-coupled inverter, in order to achieve push-pull output.

Here is a complete PA system that can be built at comparatively low cost. It is ideal for PA work at dances, for orchestras, and singing, and all general ballyhoo work.

In recording music (particularly symphonic music), some passages may be so low as to be barely audible, while other portions of the same selection may be loud enough to vibrate the walls of a building.

A handy nomograph is one relating decibels to voltage or power ratio. The three variables are input, output and decibels.

Amplifiers are rated according to the number of watts output they can handle without distortion.

In the reproduction of any record we must take into account the various recording characteristics and compensate the pickup accordingly.

The human ear responds differently to sounds of different frequencies and intensities, and attempts to compensate for losses in high-and low-frequency gain which are common in most types of audio amplifiers.

Push-pull stages are often used in a speech amplifier either to supply the necessary voltage to drive an output stage or to reduce certain types of distortion.

The degree to which the falling off at low frequencies takes place in a resistance-coupled amplifier may be seen by feeding a voltage of fixed amplitude but varying frequency through the coupling condenser.

Let us attempt to throw some light on the theory behind the operation of the lowly speech amplifier found in so many types of electronic equipment: radios, electric phonographs, public address apparatus, electronic pianos and sound-on-film projectors.