Loudspeaker overload circuit

An overload circuit for a loudspeaker system which indicates the overload and also automatically reduces the load on the speaker system to prevent damage in which a rectifier followed by a smoothing circuit is connected in parallel with the speaker and supplies an output to a magnetic coil for energizing a magnetic switch on overload so as to place a load dropping means in series with the speaker during overload. The load dropping means might be a lamp which visually indicates the overload condition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to overload protection circuits and in 
particlar to an overload protection circuit for a speaker system. 
2. Description of the Prior Art 
Loudspeakers are subject to two principle types of overload. The first type 
is thermal overload during which continuous power is applied for 
sufficient time to allow excessive heat to build up in the voice coil of 
the loudspeaker causing it to be distorted and either to short itself or 
to rub against the side of the magnetic gap and, thus, become opened or 
shorted. 
The second form of overload is caused when a signal produces excessive cone 
excursions distorting the motor assembly in a way which prevents it from 
returning to its normal position. The secondary result can be shorting or 
opening of the voice coil or simply freezing of the motion of the cone. 
Failure in a speaker can also be caused by fatigue of the suspension 
members but this is normally the result of long term usage rather than 
accidental overload. 
Since thermal overload is related to energy input rather than power input, 
it is desirable that any sensing circuit integrate the input power so that 
it is energized only when the energy input exceeds the energy output by a 
reference amount. 
However, since excursion failure occurs at low frequencies and is a 
function of instantaneous power input, the sensing circuit must also be 
energized by excess intantaneous poor at low frequencies. 
SUMMARY OF THE INVENTION 
In the present invention, an overload circuit for a loudspeaker system 
provides a magnetic responsive switch in series with the speaker and a 
rectifying circuit connected across the input to the speaker with the 
rectified output supplied to a smoothing circuit which integrates the 
input power and supplies the integrated power to a magnetic coil which 
during overload produces sufficient magnetic flux to open the magnetic 
switch. When the magnetic switch opens, a dropping device such as a lamp 
is connected in series with the speaker to the input so as to drop the 
load on the speaker and simultaneously the lamp will be illuminated so as 
to visually indicate the overload condition such that such condition can 
be limited. Upon overload exceeding a predetermined time as determined by 
the life of the lamp at the applied volatage, the lamp filament will burn 
out thus opening the circuit to the speaker so as to protect the speaker 
from damage. 
A modification of the invention replaces the magnetically operated switch 
with a solid state switching circuit that is energized by the output of 
the smoothing circuit connected to the output of the rectifer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a pair of input terminals 10 and 11 and terminal 11 is 
connected by lead 13 to one side of a loudspeaker 14 which has its other 
side connected to a lead 21 and through a normally closed magnetically 
operated switch 22 to a lead 19 that is connected to terminal 10. A 
permanent magnet 23 holds the normally closed switch 22 in the closed 
position and a magnetic coil 18 procuces a magnetic flux when energized at 
a certain level which will open the switch contact 22. A capacitor C is 
connected in parallel with the coil 18. A diode rectifier comprising 
diodes D1, D2, D3 and D4 are connected across terminals 10 and 11 and 
supplies a negative output to lead 15 which is connected to the capacitor 
C and coil 18. The positive output of the rectifier is supplied to lead 20 
which is connected to a resistor R1 which has its other side connected to 
the second side of the capacitor C and the coil 18. 
A lead 12 is connected to input terminal 10 and to one side of a lamp 16 
which has its other side connected to lead 21 and one side of the speaker 
14. A fuse 17 may, if desired, also be in series with the lamp 16. 
The lamp 16 should be mounted in the speaker enclosure which holds the 
speaker 14 such that it is visible through the grill cloth. Care should be 
taken that the method of mounting the lamp does not produce an audio leak 
in the cabinet which would deteriorate the audio characteristics of the 
speaker system. 
In operation, if an overload condition is applied at terminals 10 and 11 
and through the switch 22 to the speaker 14 for a sufficient time, the 
output of the rectifier appearing on terminals 15 and 20 will be 
integrated by the resistor R1 and the capacitor C and the coil 18 would 
receive sufficient current to produce a magnetic flux so as to open 
magnetic switch 22 thus disconnecting leads 19 and 21 such that the 
current path between terminals 10 and 11 passes through the lamp 16 and 
the speaker 14. This immediately drops the load on the speaker 14 since 
part of the input will be dissipated in the lamp 16. If such load 
continues for any finite time, the lamp 16 will be illuminated and will 
visually indicate such overload condition which can be observed so that 
corrective action can be taken, as for example, by turning down the gain 
of the amplifier supplying input terminals 10 and 11. Furthermore, the 
lamp 16 can be chosen such that it will carry an overload for only a 
predetermined time and can thus act as a fuse such that if the overload 
continues beyond said predetermined time, the filament lamp will burn out 
thus opening the circuit to the speaker 14 since an open circuit will 
exist between leads 12 and 13 as well as between leads 19 and 13. It is to 
be realized, of course, that a fuse 17 can also be placed in series with 
the speaker 14 and lamp 16 between leads 12 and 13 but if the lamp 16 is 
relatively inexpensive and is readily available the fuse would be 
unnecessary since the lamp itself can serve as a fuse. 
In a particular circuit according to the invention, the coil 18 was formed 
of 1168 turns of AWG 40 copper wire wound on a bobbin and the lamp could 
be a type 93, type 94 or a type 1939. 
During a test, the maximum lamp current was 1.8 amperes for the speaker 
when driven continuously from a 300 watt amplifier and at this current the 
type 93 and 94 lamp is rated to last 15 seconds with the volts in at 
terminals 10 and 11 being 50 volts. When the voltage was set at 40 volts 
input to the speaker system, the input current was about 1.6 amperes and 
the lamp life is 280 seconds and if a fuse 17 were to be used in the 
circuit a 1.6 ampere fuse would be suitable. 
The capacitor C was chosen to be an electrolitic type which had a 
capacitive value between 50 and 85 microfarads. The series resistor R1 was 
about 50 ohms. The resistor R1 may also comprise an adustable 
potentiometer to allow the operating point of the coil 18 to be set to a 
desired value. The coil 18, contact 22 and a magnet 23 comprise a reed 
relay. 
The attached data was taken and illustrates in column 1 the voltage applied 
to terminals 10 and 11. Column 2 lists the voltage across the speaker 
terminals, column 3 lists the power dissipated in the speaker 14, column 4 
lists the DB drop and column 5 lists the current through the speaker. 
______________________________________ 
Column 1 Column 2 Column 3 Column 4 
Column 5 
Lamp Type 94 
At Volts In 
Speaker E Speaker P DB Drop Current 
______________________________________ 
5.0 3.0 1.1 -2.2 0.37 
10. 5.0 3.1 -3.0 0.62 
15. 6.6 5.4 -3.6 0.83 
20. 8.0 8.1 -4.0 1.0 
25. 9.3 11. -4.3 1.2 
30. 11. 14. -4.6 1.3 
35. 12. 17. -4.8 1.5 
40. 13. 20. -5.0 1.6 
45. 14. 23. -5.2 1.7 
50. 15. 27. -5.3 1.8 
______________________________________ 
______________________________________ 
Column 1 Column 2 Column 3 Column 4 
Column 5 
Lamp Type 1939 
At Volts In 
Speaker E Speaker P DB Drop Current 
______________________________________ 
5.0 3.2 1.3 -1.9 0.39 
10. 5.3 3.5 -2.7 0.67 
15. 7.2 6.4 -3.2 0.88 
20. 8.7 9.4 -3.6 1.1 
25. 10. 13. -3.9 1.2 
30. 11. 16. -4.2 1.4 
35. 13. 20. -4.4 1.6 
40. 14. 24. -4.6 1.7 
45. 15. 28. -4.8 1.9 
50. 16. 32. -4.9 2.0 
______________________________________ 
In the present invention, I assume essentially resistive impedance in the 
loudspeaker system which assumption assures the worse case is used and by 
integrating the rectified input voltage and operating a triggering device 
such as the reed switch which operates at a chosen level. 
Because cone excursion decreases with frequency, excessive excursions could 
occur at voltages which would not be activated by the thermal limit 
trigger, however the circuit of the invention, the capacitor C was chosen 
to be too small to integrate at low frequencies so that the current in the 
coil substantially follows the instantaneous voltage input at such low 
frequencies. Since the instantaneous current can exceed the average 
current by a factor of .pi./2, 4dB higher sensitivity can be obtained at 
low frequencies than at high frequencies provided those frequencies are 
low enough that the triggering device such as a reed relay can allow the 
instantaneous voltage. This condition is obtained when Reed relays having 
operating times of 2 or 3 milliseconds, are used. At frequencies below 100 
Hz where excursion failures can occur, the relay will lag the 
instantaneous voltage by less than 10% of the wave length. 
Thus, the same circuit provides separate control of thermal and excursion 
overload and 21/2 times the power may be applied at high frequencies. When 
the reed relay opens, the overload is relieved and the shunning of the 
switch with a compression device such as an incandescent lamp prevents, 
audible distortion as the switch opens and closes. The resistance of the 
incandescent lamp element increases as the voltage across it increases 
thereby providing current regulation as well as visual indication of the 
overload. 
FIG. 2 is a modification of the invention wherein the reed relay coil 18, 
the magnetic switch 22 and the magnet 23 are replaced with a semiconductor 
switching circuit 26 which receives the output across the capacitor C and 
has input and output terminals connected to leads 19 and 21. The 
semiconductor switch 26 is normally closed until an overload is detected 
across the capacitor C at which time the switching circuit 26 opens the 
circuit between leads 19 and 21 thus putting the lamp 16 in series with 
the speaker 14 so as to protect the speaker. 
Although the invention has been described with respect to preferred 
embodiments thereof, it is not to be so limited as changes and 
modifications may be made which are within the full intended scope as 
defined by the appended claims.