Speaker protection circuit
United States Patent [191
[11]
4,034,268
Klauck
[45]
July 5, 1977
154] SPEAKER PROTECTION CIRCUIT
1751 Inventor: David E. Klauck, St. Joseph, Mich.
[731 Assignee: Heath Company, Benton Harbor,
Mich.
-
Nov. 10, 1975
[22] Filed:
[21] Appl. No.: 630,598
[521 US. Cl. ......................... ..
361/111; 330/207 P;
Int. Cl.2 ........................................ .. H0211 3/00
Field of Search .............. .. 330/207 P; 179/1 A;
317/33 R, 36 TD, 31, 50
References Cited
UNITED STATES PATENTS
[561
3,691,427
3,835,412
3,965,295
9/1972
9/1974
Honda et a1. ............ .. 330/207 P X
Honda et a1. ................ .. 330/207 P
6/1976
Evans et al. ............... .. 317/33 R X
FOREIGN PATENTS OR APPLICATIONS
20,601
7/1966
McMahon; Joseph J. Kaliko
[57]
ABSTRACT
A circuit for protecting speakers connected to the out
put of an audio ampli?er. The output of the ampli?er is
connected to a ?lter which blocks normal audio fre
361/88
[511
[531
Attorney, Agent, or Firm-William R. Sherman; Kevin
quency signals but passes both high and low frequency
signals outside the normal audio range. The positive
and negative ?lter output signals are connected respec
tively to the inverting and non-inverting inputs of an
operational ampli?er whose output is connected via a
turn-on delay circuit to a speaker relay, which, when
closed, connects the speaker to the ampli?er. The tum
on delay circuit prevents speaker relay closure until
turn-on transients have ended. If a DC voltage or a high
frequency signal is present at the ampli?er output, the
operational ampli?er generates a signal to open the
relay thereby disconnecting the speaker from the am
pli?er output. The speaker relay quickly opens when
power is turned off so that ampli?er turn-off transients
are not connected. to the speaker either.
Japan ........................... .. 330/207 P
Primary Examiner-Harry E. Moose, Jr.
58
26 Claims, 2 Drawing Figures
U.‘S. Patent
4,034,268
July 5, 1977
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4,034,268
1
2
damage due to DC, high frequency or transient signals
at any connected ampli?er output.
In accordance with these and other objects of the
BACKGROUND OF THE INVENTION
invention, a protection circuit is provided for protect
This invention relates generally to audio ampli?ers 5 ing a speaker directly connected to an audio ampli?er
by disconnecting the speaker whenever a signal outside
and, more particularly, to circuits for protecting speak
the normal audio range is present at the vampli?er out
ers from damage caused by DC voltage imbalance of
put.
the ampli?er, high frequency signals, turn-on or turn»
SPEAKER PROTECTION CIRCUIT
off transients, and the like.
The circuit includes a ?lter connected to the ampli
.
?er output which passes signals outside the normal
audio range. The passed signals connect to the input of
The advent of transistor technology has permitted
the design of audio ampli?ers having speakers which
are DC connected to the ampli?er output. Ampli?ers
an operational ampli?er whose output connects via a
of this type have many advantages, however, they also
have many problems which heretofor have not been
turn-on delay circuit to a speaker relay. The speaker
relay opens when signals outside the normal audio
solved satisfactorily. For example, ampli?er turn-on
range are present at the ampli?er output thereby dis
and turn-off transients, if connected to the speaker,
produce an annoying thump that can damage a
speaker. In addition, component failure or aging can
cause a non~zero quiescent output voltage to develop
which produce a direct current through the connected
connecting the speaker.
DESCRIPTION OF THE DRAWINGS
The foregoing objects, features and advantages of the
present invention will become more fully apparent
from the following detailed description of a preferred
embodiment and the drawings which forms a part of
speaker coil which, if suf?ciently large, will cause dam
age to or even destroy it. Some ampli?er failures pro
duce high frequency oscillations which may damage
both the ampli?er and speaker.
In attempting to overcome some of the above men
the original disclosure wherein;
.
FIG. 1 is a schematic diagram of an embodiment of
25
tioned problems, protective circuits have been de
signed which, on detecting a DC voltage at the ampli
?er output terminal, shorts the ampli?er output to
the speaker protection circuit.
FIG. 2 is a schematic diagram of an alternative cur
rent source used in the protective circuit.
ground thereby by-passing the speaker. This approach,
GENERAL DESCRIPTION
however, can cause a large current to flow in the ampli
?er output transistors which could cause them to burn
Referring ?rst to FIG. 1, the ?nal output stage 10 of
a typical directly coupled audio ampli?er includes two
out.
transistors 11 and 12 connected in a push-pull con?gu
'
Another frequently employed approach is to provide
ration with the ampli?er output terminal 13 being dis
posed between the emitter of transistor 11 and the
a circuit which'disconnects the speaker from the ampli
?er output if a DC output voltage of suf?cient magni 35 collector of transistor 12. For ampli?ers of this type,
the output terminal 13 is usually connected directly to
tude is present. Known circuits of this type, however,
one terminal of a speaker while the other speaker ter
have not provided isolation between different channels
minal is connected to the ampli?er. ground. The
of an ampli?er system so that, under ‘certain circum
speaker 14 is connected via a normally open relay
stances, it is possible that a failure in more than one
switch 16 and a wire 17 to the ampli?er output 13. The
channel could occur simultaneously and the protection
other terminal of the speaker 14 is connected to
circuit would fail to protect the speakers on these chan~
ground. The speaker 14 is connected to the ampli?er
output 13 whenever suf?cient current flows through
the relay coil 18 to cause the relay switch 16 to close.
age resulting from turn-on or turn-off ampli?er tran
sients nor does it address the problem of spurious high 45 In accordance with the invention, the ampli?er out
put 13 is also connected to the input of a ?lter 20 which
frequency output signals.
'
nels. Additionally, a protection circuit of this type does
not prevent either the thumping noise or possible dam
passes ampli?er output signals outside the normal
OBJECTS AND SUMMARY OF THE INVENTION
In view of the foregoing dif?culties associated with
prior art speaker protection circuits, it is a primary
objective of the invention to provide a speaker protec
(
audio band but blocks signals within it. As used in this
patent application, the term normal audio band com~
V
prises signals having frequencies lying between approx
imately 20 Hz and 20,000 Hz. Signals lying signi?cantly
tion circuit for audio ampli?ers which prevents speaker
outside the normal audio band are more easily passed
damage resulting from the presence of either DC or
by the ?lter 20 while signals outside but close to the
high frequency signals at the ampli?er output.
normal audio band are attenuated by the ?lter 20.
The output 21 of the ?lter is connected to an isola
It is another objective of the invention to provide a 55
speaker protection circuit which prevents speaker
tion circuit 22 which separates the positive ?lter output
damage due to DC or high frequency signals at the
ampli?er output and also prevents transient currents
voltages from the negative ?lter output voltages and
connects them to the inverting and non-inverting inputs
from passing through the speaker during ampli?er turn
27 and 25 respectively of an opertional ampli?er 26
on or turn-off.
which functions as a detector. The operation ampli?er
26 provides a low voltage level at‘ its output 28 when
ever the voltage at either input 25 or 27 has a magni
'
It is yet a further objective of the invention to provide
a speaker protection circuit for an audio ampli?er
which is inexpensive but reliable and prevents speaker
damage due to DC, high frequency or transient signals
at the ampli?er output.
>
It is still a further objective of the invention to pro
vide a speaker protection circuit connectable'to a plu
rality of audio ampli?ers which will prevent speaker
65
tude greater than a given magnitude.
The operational ampli?er output 28 is connected to a
turn-on delay circuit 29. After the ampli?er 10 is
turned on, the turn-on delay circuit 29 keeps its output
30 at a low voltage for a delay period which prevents
the relay 18 from closing to connect the speaker 14 to
3
4,034,268
lected and selection of different circuit component
values will cause a shift in the threshold with the value
connected to the amplifier output 13..Negative going
inputs from the operational ampli?er 26 after the delay
for the feedback resistor R10 having the greatest effect
on the threshold level.
period also cause the output of the delay circuit 29 to
The output 28 of the operational ampli?er 26 is con
nected to the turn-on delay circuit 29 which prevents
the speaker switch circuit 31 from closing switch 16
until after turn-on transient signals should have sub-‘
sided at the ampli?er output 13. When the ampli?er is
turned on by closing switch 33 the voltage of the power
supply 32 terminals labeled B+,‘B—, +15, —l5 and +12
quickly builds up to their quiescent values. At the same
go low thereby causing the relay switch 16 to open.
The output 30 of the turn-on delay circuit 29 is con
nected to a speaker switch circuit 31 which is respon
sive to a highlevel input voltage to close the relay
switch 16 thereby connecting the speaker 14 to the
ampli?er output 13.
4
exceeds 7 volts. This 7 volt threshold is arbitrarily se
I the audio ampli?er output 13. The delay period is long
enough so that power-on transients at the audio ampli
?er output 13 will subside before the speaker 14 is
_
The speaker protection circuit of the illustrated em
bodiment of the invention and the ampli?er 10 to
which is connected are both powered by a power sup 5 time, the voltage across capacitor C7 gradually builds
up from zero volts to about 30 volts. Consequently, the
ply 32 which is turned on and off by a switch 33. When
voltage at the connection point between capacitor C7
the switch 33 is turned to its off position, the supply
and the diode 37 builds up from —15 volts to about +15
voltage to the protective circuit falls quickly compared
volts. When the voltage at the connection point be
to the voltage supplied to the ampli?er 10 so that the
relay switch 16 will open to disconnect the speaker 14 20 tween C7 and diode 37 reaches about +1.8 volts, the
diode 37 becomes back biased and the diode 38, the
from the ampli?er output 13 before output transients
base-emitter junction of Q4_and the base-emitter junc
are generated by the ampli?er 10 due to the falling
tion of Q5 become forward biased causing Q4 and O5
voltage from the power supply 32 to the ampli?er 10.
DETAILED DESCRIPTION
to conduct.
25
Before the diode 37' becomes reverse biased, the
capacitor C7 is charged through R11, R12, and R13.
Filter Network
The ?lter network 20 comprises a pair of cascaded
low pass ?lters and a high pass ?lter. The two low pass
?lters include the resistors R1 and R2 and the capaci
tors C1 and C2. The high pass ?lter includes the resis
tor R3 and the capacitor C3. The electrical values for
the resistors Rl-and R2 and the capacitors C1 and C2
are selected so that DC and AC signals signi?cantly
below 20Hz in frequency, i.e. below 2112, are passed to
the ?lter output line 21. The electrical values'for R3
and C3 are Selected so that signals signi?cantly above
20,000 Hz, i.e. above 200,000 Hz are also passed to the
output line 21. As such, the ?lter 20 blocks signals in
the normal audio band between 201-12 and 20,000 Hz
After diode 37 becomes reverse ‘biased, however, the
capacitor C7 is charged through R12 so that the diode
37 will not conduct until afterrthe capacitor C7 is dis
charged following the turn off of power supply 32. This
circuit action causes the voltage at 40 to remain below
+1.8 volts for a period of time following the turn on of
power supply 32 determined by the RC time constant
of R11, R12, R13 and C7, which, for the circuit values
set forth in the table below, is approximately?lO sec
onds. Thisdelay of about‘ 10 seconds assures that the
switch 16 closes after turn-on transient signals —at the
ampli?er output 13 have subsided. Thereafter, the
delay circuit 29 has no effect on the overall operation
while passing signals signi?cantly outside the normal 40 of the system as the voltage across C7 reverse biases
audio band to the ?lter output line 21.
diode 37 and the voltage at 40 remains above +1.8
The output 21 of the ?lter network 20 is connected
volts provided that the signals at the amplifier output
to an isolator circuit 22. This circuit comprises two
13 outside the normal audio range have a magnitude ‘
diodes 35 and 36 which connect respectively between
less than the threshold voltage of the operational ampli
the ?lter network output 21 and the non-inverting and 45 ?er 26. Should the magnitude of the signals at the am
the inverting inputs 25 and 27 of the operational ampli
pli?er output 13 outside the normal audio band rise to
?er 26 to conduct negative signals at the ?lter output
21 to the non-inverting input 25 and to conduct posi
the threshold voltage, then the output voltage at 28
would fall suf?ciently to cause the voltage at 40 to. go
tive signals 21 to the inverting input 27.
The operational amplifier 26 includes three transis
50
tors Q1, Q2 and 03 which are connected in combina
tion with several resistors and capacitors in a well
below +1.8 volts.
'
>
‘
'
‘
The speaker switch circuit 31, in the illustrated em
bodiment of the invention, comprises two transistors
Q4 and ()5 whose collector terminals are' each con
known operational amplifier con?guration. The com
nected to the relay coil 18. vThese two transistors 04
ponent values for the operational amplifier 26 are se
and 05 will conduct so' longas the voltage at the base
lected so that Q1, Q2 and Q3 are normally conducting 55
‘of
O4 is above +1.2 volts which occurs when the volt
and the voltage at the'collector of O3 is approximately
age at point 40 is above +1.8 volts. Transistors Q4 and
0 volts. As the voltage at the non-inverting input de
05 are selected to draw suf?cient current through the
creases or as the voltage at the inverting input in
creases, the voltage at the collector of Q3 decreases.
Eventually, the Q3 collector voltage falls suf?ciently so
that the voltage at 40 is less than 1.8 volts and transis
tors Q4 and OS are turned off thereby causing the
speaker 14 to be disconnected from the ampli?er 10
60
relay coil 18to close theswitch 16 and can be replaced,
if desired, by a single transistor with suf?cientccurrent
carrying capacity to accomplish the same result. The
illustrated two transistor embodiment, however, re
quires less base drive from the delay circuit 29' than an
equivalent one transistor embodiment.
_'
The collector terminal of the transistors 04 and OS
because the relay coil 18 is de-energized, opening the
relay switch 16. For the circuit component values listed 65
are shunted to ground by va ,capacitorCS and a diode
below, the operational ampli?er 26 causes the relay
39. These shunting elements areyprovided to protect
coil 18 to de~energize when the absolute magnitude of
the transitors Q4 and OS from damage which might be
the signal outside the normal audio range at point 13
4,034,268
5
caused by a high voltage developed across the relay coil
18 when the transistor 04 and OS are turned off.
While the illustrated embodiment of the speaker
switch circuit 31 employs two transistors and a relay, it
will be recognized by those skilled in the art that other
switching arrangements can be employed to connect or
disconnect the speaker 14 from the ampli?er output 13
in response to the signals at point 40 in the delay cur
rent 20.
'
-
6
which is principally controlled by the charge time of C7
through resistors R11 and R13.
The circuit according to the invention is capable of
protecting a plurality of speakers connected to aplural
ity of audio ampli?er channels. For example, the out
put stage 50 of a second audio ampli?er includes an
output terminal 51 connected via a wire 52 through a
relay switch 53 to a second speaker 54. The relay
switch 53, as indicated by the dotted line 55, is magnet
ically coupled to the relay coil 18 and is closed when
The speaker switch circuit 31 also disconnects the
ampli?er 10. In other’words, the ?lter capacitor C3’
ever the switch 16 is also closed.
' The second amplifier output 51 is connected to a
second ?lter network 56 which is identical to and func
tions the same as the ?lter 20.
The output of the ?lter network 56 is connected to an
isolator 57'“ identical to and functions the same as the
connected to the +12 ‘volt output terminal is much
isolator 22. The negative outputof the isolator 57 is
speaker 14 from the ampli?er output 13 when power is
turned off by opening switch 33. Preferably, the +12
volt output of the‘ power supply 32 which powers the
switch circuit 31 is lightly ?ltered in comparison to the
heavily ?ltered 8+ and B- outputs which power the
connected via a wire 59 to the non-inverting input of
smaller than the ?lter capacitor C1’ and C2’ at the 8+
the operatinal ampli?er 26 while the positive output is
and B- outputs of the power supply 32. Thus, when the
power supply 32 is turned off, the +12 volt power sup 20 connected via a wire 58 to the inverting input. The
protectionv circuit operates to protect the speaker 54
ply output drops to zero more quickly than the 8+ and
from transient or spurious signals present at the ampli
B— outputs thereby’ causing the relay 18 to become
?er output 51 in the same manner as it protects speaker
deactivated before turn-off transients develop'at the
.14 from such signals at the ampli?er output 13.
output 13. Consequently, the circuit shown in H6. 1
It is clear that protection of additional speakers cou
prevents turn-off transients from reaching the speaker
pled to other audio ampli?er channels can be provided
14 thereby preventing possible‘ speaker damage or
thumping.
,
‘i
g
merely by adding'an additional ?lter network, such as
"
56, an isolator circuit, such as 57, and a relay switch,
such as 53, which is actuated by the relay coil 18. With
Typical values for the circuit components of the illus
trated embodiment are shown below in Table. 1.
TABLE]
Rl
R2
68K
68K
Cl
c2
4ufd.
4ufd.
R3
1200
c3
47pfd.
such a system, however, a spurious signal at the output
of any coupled audio ampli?er, which turns off transis
tors Q4 and Q5, disconnects all the speakers from their
respective audio amplifiers. On the other hand, if indi
vidual control is desirable and the added cost accept
35
able, the entire protection circuit can be duplicated for
R6
lllK
C6 1
47pfd.
each audio channel.
R7,
47k
c7
SOOufd.
R8
lllk
While the invention has been described with refer
R9
lOk
ence to one examplary embodiment, it will be readily
RH).
lOOk
Rll
15k
recognized by those skilled in the art tht many modifi
40
R12
mm;
cations may be made without departing from the spirit
R13
331;
and scope of the invention as de?ned in the claims. For
R14
15k
RIS
4.71;,
instance, other ?lter networks with similar transfer
functions can be substituted for ?lter 20. Also, other
While the foregoing table shows the component val 45 circuit con?gurations can be substituted for the switch
circuit 31 and the threshold voltage at which the switch
ues preferred by applicant, it is clear that variations in
circuit 31 disconnects the speaker from the ampli?er
the values of particular components may be appropri
output can be modi?ed.
"
ate to optimize the particular circuit according to the
What is claimed is:
invention for use in a particular audio system.
1'. In a system including an- audio ampli?er with an
In one alternative embodiment of the invention, the
output, a power supply for powering the audio ampli
resistor R12 in FIG. 1 can be replaced by the current
?er, the power supply including means to turn the
source of FIG. 2. The output of this current source is
power supply on or off, and a speaker, a protection
connected between the diode 37 and the capacitor C7
circuit to prevent connection of the speaker to the
while the other components are connected as ‘shown.
R4
R5
10K
4700
C4
cs
.OSufd.
.USufd.
Both R12 of the current source of FIG. 2 are provided 55 ampli?er output whenever turn-on or turn-off tran
sients or signals outside the normal audio band are
in the circuit in order to maintain the voltage across the
present at the ampli?er output comprising, in combina
capacitor C7 high enough to back bias diode 37 once
tion:
the delay time has elapsed. For capacitors with a low
a ?lter network connected to the amplifier output,
leakage current, a resistor such as R12 is adequate.
However, for capacitors with rather high leakage cur 60 ' said filter network being operative to block signals
rent but relatively low cost, a current source like‘that of
FIG. 2 is more desirable because diode 37 will remain
within the normal audio band and pass signals both
above and below the normal audio frequency band;
back biased due to maintaining a charge on capacitor
C7. To accomplish this result with a; single resistor, the
value of R12 must be reduced which affects the delay
an isolation circuit responsive to the signals passed by
time. The current source of FIG. 2 has a high imped
ance as compared to resistor R12 and, consequently,
the current source has ‘little effect on’the delay time
said'filter network for’ separating the passed signals
into a positive and a negative component;
detector means responsive to both said positive and
65
'
said negative component for producing a control
signal whenever the magnitude of either said posi
4,034,268
7
tive or said negative component is below a given
8
6. The protection circuit of claim 5 wherein said ?lter
network includes a low pass ?lter for passing very low
frequency signals to the ?lter network output and a
magnitude;
delay circuit means powered by the power supply for
passing said control signal following a delay time
high pass ?lter for passing very high frequency signals
after the power supply is turned on; andv
a switching circuit powered by the power supply and
responsive to said passed control signal from said
to the ?lter network output, said ?lter network being
operative to ‘prevent feedthrough of signals within the
normal audio frequency range.
7. The protection circuit of claim 5 wherein said
isolation circuit comprises at least two diodes wired to
delay circuit for connecting the speaker to the
amplifier output, said switching circuit and power
supply, in combination, being operative to discon
nect the speaker from the ampli?er output before
turn off transients appear at the ampli?er output
when the power supply is turned off.
2. The protection circuit of claim 1 wherein said ?lter
network includes a low pass ?lter for passing very low 15
frequency signals to the ?lter network output and a
separate the positive and the negative component of
the ?lter network output signal.
8. The protection circuit of claim 5 wherein said
detector means comprises an operational amplifier
with its non-inverting input connected to receive said
negative component and its inverting input connected
to receive said positive component.
high pass ?lter for passing very high frequency signals
9. in a system including at least two audio ampli?ers
to the ?lter network output, said ?lter network being
operative to prevent feedthrough of signals within the
with each ampli?er having an output power supply
powering each of the audio ampli?ers including means
normal audio frequency range.
to simultaneously turn the power on or off at each
3. The protection circuit of claim I‘ wherein said
isolation circuit comprises at least two diodes wired to
a protection circuit to prevent connection of each
separate the positive and the negative components of
the ?lter network output signal.
speaker with the associated ampli?er whenever turn-on
transients, turn-off transients, or signals outside the
ampli?er, and a speaker associated with each ampli?er,
4. The protection circuit of claim 1 wherein said 25 normal audio frequency range are present at least at
detector means comprises an operational ampli?er
one ampli?er‘ output comprising,‘in combination: '
with its non-inverting input connected to receive said
a ?lter network connected to each ampli?er output,
negative component and its inverting input connected
each said ?lter network being operative to block
to receive said positive component.
signals within the normal audio frequency range
5. In a system including an audio ampli?er with an 30
and pass signals both above and below the normal
output, a power supply for powering the audio ampli
?er, the power supply including means to turn the
power supply on or off, and a speaker, a protection
circuit to prevent connection of the speaker to the
ampli?er output whenever turn-on or turn-off tran 35
sients’ or signals outside the normal audio frequency
range are present at the ampli?er output comprising, in
combination:
a ?lter network connected to the ampli?er output,
said ?lter network being operative to block signals
40
within the normal audio frequency range and pass
signals both above and below the normal audio
frequency range;
work output and responsive to the signals passed
thereby for separating the passed signals into a
positive and a negative component;
detector means responsive to both said positive and
said negative component from each said isolation
circuit for producing a control signal whenever the
magnitude of said positive or said negative compo
nent from any said isolation circuit exceeds a se
lectable magnitdue;
‘
delay circit means powered by the power supply for
producing, after a predetermined delay time after
.
an isolation circuit responsive to the signals passed by
said ?lter network for separating the passed signals
audio band to a ?lter network output;
an isolation circuit connected to each said ?lter net
45
into a positive and a negative component;
detector means responsive to both said positive and
the power supply is turned on, a switching signal,
said delay circuit responding to said control signal
to block said switching signal;
_
a switch circuit powered by the power supply and
said negative component for producing a control
signal whenever the magnitude of either said posi
responsive to said switching signal for connecting
tive or said negative component exceeds a select 50
each speaker to its associated ampli?er output, said
switch circuit and the power supply, in combina
tion, being operative to disconnect each speaker
' able magnitude;
delay circuit means powered by the power supply for
from its associated amplifier output before turn-off
producing a switching signal below one level for a
transients occur at any ampli?er output when the
predetermined delay time after the power supply is
power supply is turned off.
_
turned on and producing a switching signal at least 55
10. The protection circuit of claim 9 wherein each
at said one level after said delay time has elapsed,
said ?lter network comprises a low pass ?lter for pass
said delay circuit responding to said control signal
to produce a switching signal below said one level;
ing very low frequency signals outside the normal audio
frequency range and a high pass ?lter for passing sig
a switching circuit powered by the power supply and
responsive to said switching signal for connecting
the speaker to vthe ampli?er output whenever said
?lter network being operative to prevent feedthrough
switching signal is at least at said one level and for
disconnecting the speaker from the ampli?er out
put whenever said switching signal is below said
one level, said switching circuit and power supply,
in combination, being operative to quickly discon
nect the speaker from the ampli?er output when
the power supply is turned off.
i
.
nals above the normal audio frequency range, each said
of signals in the normal audio frequency range.
11. The protection circuit of claim 9 wherein each
said isolation circuit includes at least two diodes wired
to separate the signals received from the connected
65 ?lter network into its positive and its negative compo
nent,
.
,
l2.aThe protection circuit of claim 9 wherein said
detector means comprises an operational ampli?er
9
4,034,268
nent.
l7. The protection circuit of claim 14 wherein said
detector means comprises an operational ampli?er
with the non-inverting input electrically connected to
from each said isolation circuit.
receive the negative component from each isolation
13. The protection circuit of claim 9 wherein the
power supply includes a lightly filtered power output
network and the inverting input of said operational
ampli?er being wired to receive the positive compo
for powering said switch circuit, said lightly ?ltered
nent from each said isolation circuit.
output being turned on and turned off ‘simultaneously
with the audio ampli?er, said lightly filtered output
being operative to quickly drop to zero when the audio
ampli?er is turned off so that said switch circuit will
quickly disconnect the speaker from the audio ampli
?er to prevent turn-off transients at the ampli?er out
put from being connected to the speaker.
l4. In a system including at least two audio ampli?ers
10
?lter network into a positive and a negative compo
with the non~inverting input electrically connected to
receive‘the negative component from each isolation
circuit and the inverting input of said-operational‘ am
pli?er being wired to receive the positive component
18. The protection circuit of claim 14 wherein the
power supply includes a lightly ?ltered power output
for powering said switch circuit, saidlightly filtered
output being turned on and turned off simultaneously
with the audio ampli?er, said lightly ?ltered output
15 being operative to quickly drop to zero when the audio
ampli?er is turned off so that said switch circuit will
with each ampli?er having an output, power supply
powering each of the audio ampli?ers including means
quickly disconnect the speaker from the audio ampli
simultaneously turn the power on or off at each ampli
turn-off transients at the ampli?er output are not con
?er before the ampli?er itself begins to turn off-so that
-,
?er, and a speaker associated with each ampli?er, a 20 nected to the speaker.
19. The protection-circuit of claim 1 whereinthe
protection circuit to prevent connection of each
speaker to the associated ampli?er whenever turn-on ~ power supply includes a lightly ?ltered power output
for powering said switch circuit and a heavily ?ltered
transients, turn-off transients or signals outside the
output for powering the ampli?er, said lightly ?ltered
normal audio frequency range are present at least at
25 output being turned on and turned off simultaneously
one ampli?er output comprising, in combination:
with the audio ampli?er, said lightly ?ltered‘. output
a ?lter network connected to each ampli?er output,
being operative to, quickly drop to zero when the power
each said ?lter network being operable to block
supply is turned off and before the ampli?er turns off so
signals inside the normal audio frequency range
that said switch circuit will quickly disconnect the
and pass signals both above and below the normal
30 speaker from the audio ampli?er output and prevent
audio band to a ?lter network output;
turn-off transients at the ampli?er output from being
an isolation circuit connected to each said ?lter net
connected to the speaker.
20. In a system including an audio ampli?er with an
work output and responsive to the signals passed
thereby for separating the passed signals into a
positive and a negative component;
output, a power supply for powering the audio ampli
detector means responsive to both said positive and 35 ?er, the power supply including means to turn the
power supply on or off and a speaker, a protection
said negative component at each said isolation
circuit to prevent connection of the speaker to the
circuit output for producing a control signal when
ampli?er output whenever turn-on or turn-off tran
ever the magnitude of said positive or said negative
sients or signals outside the normal audio frequency
component from any said isolation circuit exceeds
vrange are present at the ampli?er output comprising, in
a selectable magnitude;
combination:
delay circuit means powered by the power supply for
a ?lter network connected to the ampli?er output,
producing a switching signal below one level for a
said ?lter network including a low pass section and
predetermined delay time after the power supply is
a high pass section to block signals within the nor
turned on and producing a switching signal at least
mal audio frequency range and pass signals outside
at said one level after said delay time has elapsed, 45
the normal audio frequency range;
said delay circuit responding to said control signal
an isolation circuit including at least two unidirec
to produce a switching signal below said one level;
tional current conducting elements responsive to
a switch circuit powered by the power supply and
responsive to said switching signal for connecting
each speaker to its associated ampli?er output
50
whenever said switching signal is at least at said one
level and for disconnecting each speaker from its
associated ampli?er output whenever said switch
ing signal is below said one level, said switch circuit
and said power supply means, in combination, 55
being operative to quickly disconnect each said
speaker from its associated ampli?er output before
turn-off transients develop at any ampli?er output
when the power supply means removes power from
each ampli?er.
60
15. The protection circuit of claim 14 wherein each
of either said positive or said negative component
exceeds a selectable magnitude;
’
delay circuit means responsive to the power supply
being turned on for producing a switching signal y'
below one level for a predeterined delay time after
the power supply is turned on and producing a ‘E
switching signal at least at said one level after said
said ?lter network comprises a low pass ?lter for pass
ing very low frequency signals below the normal audio
frequency range and a high pass ?lter for passing sig
nals above the normal audio frequency range.
the signals passed by said ?lter network for separat
ing the passed signals into a positive and a negative
component;
operational ampli?er means having its non-inverting
input wired to receive said negative component
and its inverting input wired to receive said positive
component, said operational ampli?er producing a
control signal at its output whenever the magnitude
delay time has elapsed, said delay circuit respond
said isolation circuit includes at least two diodes wired
ing to said control signal to produce a switching‘
signal below said one level; and
a switch circuit powered by the power supply and
to separate the signals received from the connected
responsive to said switching signal for connecting
16. The protection circuit of claim 14 wherein each
65
4,034,268
11
12
the speaker to the ampli?er output whenever said
above the normal audio band, said ?lter means opera
switching signal is at least at said one level and for
tive to block passing of signals in the normal audio
disconnecting the speaker from the ampli?er out
put whenever said switching signal is below said
band.
'
"
l,
23. The protective circuit of claim 21' wherein said
one level, said switch circuit and the power supply,
means sensitive to passed signals includes means for ‘
in combination, being operative to disconnect the
speaker from the amplifier output before turn-off
opening said switch means prior to the production of
turn-off transients at the amplifier output following
transients are generated by the ampli?er when the
removal of power from the ampli?er.
power supply is turned off.
24. The protective circuit of claim 21 additionally
including ‘a power ‘supply with a plurality of power
.
'1
21. A speaker protection circuit for protecting a
speaker from audio ampli?er output signals above and
"
outlets, the power supply outlet for said means sensitive
below the normal audio band comprising, in combina
to passed signals being lightly ?ltered compared to the
tion:
switch means connected in series between the output
power outlet for the audio ampli?er, the voltage at the
?rst mentioned power' outlet falling faster than the"
of the ampli?er and the speaker;
voltage at the second mentioned power outlet so that
?lter means connected to the amplifier output for
said switch means is opened before turn-off transients
are developed at the ampli?er output.
25. The protective circuit of claim 21 wherein said
passing only said output signals;
means sensitive to passed signals above a given mag
nitude for opening said switch means to disconnect
20 means sensitive to‘ passed signals includes an isolator
the speaker from the amplifier ; and
circuit for separating the passed signals into both a
turn-on delay means for holding said switch means
positive and a negative component.
open for a delay time following the application of
26. The protective circuit of claim 21 wherein said
power to the ampli?er until turn-on transients at
22. The protection circuit of claim 21 wherein said
means sensitive to passed signals additionally includes
an operational ampli?er with its inverting input wired
?lter means includes a low pass ?lter for passing very
to receive said positive component and said noninvert~
the ampli?er output subside.
ing input wired to receive said negative component.’
low frequency signals below the normal audio band and
a high pass ?lter for passing very high frequency signals
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