By RALPH HODGES
GLOSSARY OF TECHNICAL TERMS--13
Feedback is a technique for reducing some of the distortion or other "errors" that occur in electronic circuits by "feeding back" part of the output signal to an earlier part of the circuit. The circuit is arranged so that the fed-back signal is "negative"- that is, it is out of phase with the input signal. For example, say that there is 20 dB of signal feedback applied to the amplifier input. This nega tive signal cancels by 20 dB all areas of the signal where the input and the feed back output waveforms coincide. How ever, when there is a difference (other than in amplitude) between the two waveforms because of distortion in the output, the cancellation cannot take place. The difference between the output and input (in other words, the distortion) is added to the input as a "corrective" signal. The feedback reduces both the distortion and the amplifier gain by 20 dB. However, the lost gain is easily compensated for elsewhere in the system.
The feedback principle has also been applied to speaker systems ("correcting" the amplifier's output signal to compensate for certain types of speaker distortion inherent in the speaker).
The feedback described above is of the negative variety; positive feedback is also found in audio equipment, some times purposefully, sometimes accidentally. The unwanted howling or roar that is heard when a microphone or phono gain is turned up too high for a particular setup is acoustic feedback (from the loudspeakers) of the positive type.
Field-effect transistors (FET's) are usually described as semiconductor de vices that share many of the desirable characteristics of both conventional transistors and vacuum tubes. These advantages include good linearity (and hence more freedom from spurious responses and other distortions), high input impedance, and low noise. FET's were first introduced into audio equipment in the input stages of FM tuners, where they were beneficial in avoiding the overload effects from strong local stations that plagued conventional transistor circuits. Recently they have begun appearing in preamplifiers and the preamplifier sections of amplifiers and receivers. And the introduction of some power amplifiers employing special power FET's has been announced.
Filters are circuits that selectively reduce the level of certain frequencies in an electrical signal. Audiophiles are most familiar with the high-cut ("scratch") and low-cut ("rumble") filters that can be switched in and out manually at the control panels of amplifiers and receivers. These are intended to reduce, respectively, the levels of the extreme high and extreme low frequencies in the program, in the hopes of eliminating unwanted noise at those frequencies without too much effect on the pro gram material. Audio systems usually contain numerous other "fixed" filters (see Equalization) inaccessible to the user which process the program material to remove specific frequency areas of noise or interference.
Filters are usually characterized by the "turnover" frequencies at which they begin acting (the point at which the response is 3 db down from flat response) and the rate (slope) at which their effect takes place. A 6-dB-per-octave slope is rarely as desirable as one of 12 or 18 dB per octave, since the steeper slopes discriminate more sharply against the region of unwanted noise while affecting the music-containing frequencies much less. A special family of filters, the "dynamic" types, are con trolled by the program, which is to say that the signal levels and frequencies themselves automatically regulate the filtering action. Noise filters can be relatively simple or quite complex, but they all attempt to achieve the greatest possible noise elimination together with the least audible effect on the musical material.
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