Tuesday, September 9, 2014
Overload protection for speakers
Although the protection circuit is fairly simple, it forms an effective guard against overload of the input of amplifiers and loudspeakers. Why these inputs may need protection now that line levels have been standardized is because there are signal sources on the market that generate outputs of several volts instead of the standardized 1 V r.m.s. Also, in some applications, the loudspeaker signal is applied to the line output of a separate amplifier via a voltage divider, in which case the levels may be well above 1 V r.m.s. The diagram shows a schematic that resembles the familiar series resistor and zener diode. Here, however, the zener is constructed from a small rectifier and a transistor, since commercial zeners appear to start conducting way below their rated values, which gives rise to unwanted distortion. The constructed zener makes a well-defined limitation possible and does not affect signals below the critical level. Configuring T1 as a diode reduces the number of components needed to a minimum: not even a voltage divider or potentiometer is required. Measurements on the prototype show that the input signal remains virtually undistorted at levels up to 700 mV r.m.s. At the threshold of 1 V r.m.s., the distortion is about 0.02%. Above this level, limiting is welldefined. The peak output voltage of the circuit is about 3 V with an input voltage of about 13 V r.m.s. If the limiting level is required to be slightly higher, consideration should be given to replacing T1 by three or four cascaded diodes.
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