原创 Loudspeaker Protection and Muting

Introduction

Please note that the PCB version is different from the circuit shown
in this article. It is actually simpler, but achieves the same
functions. Full details are available when you purchase the board. The
latest boards are Revision-A, and are slightly different from the
previous version.

Many hi-fi amplifiers and professional power amps (and loudspeaker
systems) provide some of protection, either to protect the speakers
from an amp fault, and/or vice versa. Some of these are implemented at
a very basic level - for example the use of a 'poly-switch'. The
poly-switch is a non-linear resistor, having a low resistance at normal
temperatures and a much higher resistance at some designated
temperature. Unlike 'ordinary' thermistors whose characteristics are
more or less linear, the poly switch has a rapid transition once the
limit has been reached.

I don't like poly-switches, because I know that the introduction of
a non-linear element is going to add some degree of distortion, and
because of a finite resistance, will degrade damping. This (i.e.
damping) is not an issue IMHO, but to many audiophiles it is of prime
importance. (I shall not pursue this argument here, however - see Impedance for more info.)

The basic requirement of a speaker protector requires that any
potentially dangerous DC flow to the speakers should be interrupted as
quickly as possible. There are a few issues that need to be solved to
ensure that this will happen fast enough to stop the loudspeaker
drivers from being damaged, and this becomes more critical if a biamped
(and even more so with triamped) system is being used.

Naturally, one can simply rely on fuses. Although these also have
finite resistance it is small, and use of fast blow fuses can be quite
effective. The rating becomes quite critical, and fast blow types are
essential. The problem with this approach is that if the fuse is of a
suitable value to provide good protection, it will be subjected to
considerable thermal stress since it is operating at close to its
limits. Metal fatigue will create the problem of nuisance blowing,
where the fuse blows simply because it is 'tired' of the constant
flexing caused by temperature variations.

This project explains the principles, and shows a suitable detection
method that may be applied. The speed of the relay used is another
critical factor, and we shall see that the conventional method of
preventing the relay's back-emf from destroying the drive transistor
also slows down the response to an unacceptable degree.

The circuit also includes a mute function, which leaves the speakers
disconnected until the amplifier has settled, and disconnects the
speakers as quickly as possible after power is removed to prevent the
turn-off noises that some amps generate. These can range from a low
level thump 5 to 10 seconds after power is turned off, to whistles,
squeaks and other strange noises that I have heard from amps over the
years.

Please Note: While the circuit shown here and the PCB version can both be made to work just fine with high supply voltages (such as ±70V as used with P101), be aware that the majority of relays will be totally incapable of breaking that voltage and the resulting current under fault conditions. The DC causes a significant arc, and this is more than capable of simply burning off the relay contacts. If you are lucky, the fuse(s) will blow before the relay is destroyed, but I wouldn't count on it. While relays capable of breaking perhaps 10A or more at 70V DC are available, they will be expensive, and probably hard to get. Unfortunately, there are few options for an alternative method.

Using the relays as shown below (with the normally open contact
connected to ground), the arc will be diverted from the speaker and
will be to ground, but will almost certainly be destroyed unless a
specialised component is used. Despite their apparent simplicity,
relays are actually rather complex devices. A great deal of engineering
goes into the development of the contacts, but operating them in excess
of the manufacturer's ratings means that nothing is certain.

Please make sure that you understand the limitations of any
such circuit (not just mine - the same applies to all loudspeaker
protection circuits). The circuits themselves are not limited, but the
relays most certainly are.

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The Circuit

It is important to identify the lowest frequency likely to be passed
to a speaker, because this determines the delay that must be introduced
to prevent low frequencies from triggering the protection circuit
(nuisance tripping). For practical purposes, a low frequency limit of
20Hz is satisfactory for a full range system, and this means that a
minimum 25ms delay is essential. In reality, due to the combination of
low frequencies, and asymmetrical waveforms at higher frequencies, a
greater delay will normally be required. Unfortunately, the greater the
delay, the greater the risk of drivers being damaged. In a full range
system (i.e. using passive crossovers), midrange and tweeters will be
offered some protection by the capacitors used in the crossover
network, but these are missing in a biamped or triamped system. For
this reason, it is important that the circuit can be easily modified to
change the initial time delay before the system detects the DC and
disconnects the speakers.

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The Detector

This is the most important of the functions. It must be capable of
detecting a DC offset of either polarity, and be immune to the effects
of asymmetrical waveforms and low frequencies. This is a common
requirement, and it is most expedient to use a simple (single pole)
filter to keep the complexity to a minimum. With this arrangement, a
low frequency cut-off of about 1Hz is about right. Without boring you
with the mathematics behind this, it works out (eventually) that a
filter having a time constant of 1.0s will still provide the ability to
detect high level DC reasonably quickly, but allow low frequencies
through without triggering. With this, the relay could have its supply
removed within about 50ms from the time the output voltage reaches the
supply rail (this is supply voltage dependent) - due typically to a
shorted transistor in the output stage. By changing the time constant
of the filter, we can adapt the circuit for operation at other higher
frequencies to suit a biamped (or triamped) system.

The detector can be built using an opamp, and will work very well,
but this introduces the need for low voltage supplies within the power
amp. This is not always possible (or desirable), so the design uses
discrete transistors throughout to allow for the different supply
voltages found in typical power amplifiers.

The detector circuit shown in Figure 1 (1)
is simple and works well, and as shown will not trigger with a 30V RMS
signal at 5Hz, but operates in 60ms with 30V DC applied, and in 50mS
with a 45V DC supply. This should be sufficient for most applications,
and allows the use of a non-polarised electrolytic capacitor in the
filter. These are cheap, small and quite adequate for this purpose.

NOTE:  The power supplies (+ve and -ve)
shown in these diagrams will normally be the power amp supply rails. Do
not try to substitute different supplies unless you know exactly what
you are doing, or the circuit may not work properly. This is especially
true of the muting circuit, but incorrect supplies will (may) also
affect the DC detection circuit. Like most of my projects, this is
intended for experienced constructors.

Figure 1 - Basic DC Detector Circuit

The input filter is a simple single pole (6dB/ octave) version, and
although it would seem that a 'better' filter would be preferable, a
two pole (or more) filter will actually degrade the DC detection. This
basic circuit is not new (see reference), and has actually existed in
one form or another for some time. It is ideally suited for our
requirements, as it is symmetrical, and with the input diodes as shown,
a single detector can be used with multiple amps and different input
time constants for each individual filter. The unit itself can operate
on a separate supply if desired, so the complete protection circuit can
be in a separate enclosure. Regulated supplies are not needed, and no
hum or other artefacts are introduced into the speaker lines. (Please
see NOTE above.)

The table (below) shows some suggested values for the filter, for
use in bi- and tri-amped systems. You will need one filter and two
diodes for each amplifier channel connected, and a suitable number of
relay contacts to handle them all. In some cases, this will mean
multiple relays.

Frequency (Hz)	C1 Value
Full Range	10 uF (non-polarised)
100 Hz	1 uF
300 Hz	330 nF
1 kHz	100 nF
3 kHz	33 nF

The resistor should be left at 100k for all frequencies. Do not use
a conventional electrolytic capacitor for C1, because any small reverse
bias will eventually ruin it. You may discover that with some types of
music (especially if at high volume) may cause the circuit to false
trigger. If this happens, increase the value of C1, up to a maximum of
47uF. Anything higher than this will slow down the response
unacceptably.

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Relay Specifications

The relays should be easy enough to obtain. At least one of the
Australian component suppliers has relays that are quite suitable, but
they are not particularly cheap. The current rating is very important,
and assuming a supply voltage of +/- 40V, this will cause a current of
about 6A in an 8 ohm speaker if a transistor shorts.
Although 6A may not sound like much, it is at DC, and because there are
no periods of 0V as with AC, the arc is longer, fatter, and far more
destructive of contacts than the same current using AC.

Do not be tempted to use miniature relays, because if the normal AC
speaker signal is too far in excess of the relay contact rating, the
contacts may become welded together - this will almost certainly happen
if the DC rating is too low. You also need to consider that contact
resistance is additional resistance in the speaker lead and may affect
damping (albeit very marginally) and will introduce some small power
loss, and the miniature types will not be suitable in this regard.

I had a look in the catalogue of one Australian supplier, and they
have several relays with a 10A contact rating. I would suggest that
anything lower is unwise for long term reliability. Most of the
commonly available relays will have a 12V coil, and this will cause
problems if the supply voltage is 30V or more. Power relays
often draw significant current (typically > 60mA), and it will
usually be best to connect the coils in series.

Be aware that in some areas there is significant sulphur content in
the air, and this causes heavy tarnishing of silver contacts. If you
live in such an area, it would be advisable to obtain hermetically
sealed relays if possible, to prevent the contacts from tarnishing.

It is well known that the current required to activate a relay is
far greater than that needed to keep the contacts closed, and a common
trick is to use an 'efficiency' circuit to minimise the relay holding
current. I do not feel that the additional complexity is warranted, and
have not included this facility. If you really
want to do this properly, see reference 1
(below). It has been claimed that an efficiency circuit also speeds up
relay drop-out time because of the lower stored magnetic field. I
conducted some tests, and the savings are marginal at best, although
this could be different with different relays.

Figure 2 shows the relay activation circuit, and includes the
connection for the mute and protection signals. No components are
critical, but some will need to be modified based on the relays used. I
have assumed that a minimum of two relays will be needed (one for each
channel), and this increases the total relay coil voltage
to 24V. If you are going to use more than two (for example, four single
pole relays are needed for a biamped system), then if the supply
voltage is 48V or more, all 4 relays can be connected in series. In
most cases you will need to work out the value of a suitable dropping
resistor from the formula below.

The terminal labelled "Off" is common to all three modules, and
these points are simply joined together, as are the +ve and -ve supply
connections. A positive current into the Off terminal will de-energise
the relays, by turning on Q1. This steals all the base current for Q2,
which then turns off, as does Q3.

Figure 2 - Relay Activation Circuit

R7 and D6 are optional. A reader used this circuit on a P68
subwoofer amplifier, and found that the circuit occasionally
false-triggered. It was finally discovered that with some signals, the
supply collapsed enough to re-start the mute timer. By adding the
resistor and zener, this is avoided. R7 and D6 won't normally be
needed, but if you get false triggering they will have to be added. To
leave this section out simply means that D6 is not installed, and R7 is
replaced by a link.

The value for R7 (if needed) is determined by the supply voltage.
The mute circuit draws very little current, so R7 can be calculated by
...

V_R7 = V_supply - 24     (where 24 is the zener voltage)

R7 can then be calculated, based on a zener current of 10mA ...

R7 = V_R7 / 0.01 (Ohms)

P = V_R72 / R7 (Watts)

For example, with a 56V supply, R7 would be 3.2k, and will dissipate 0.32W (a 1W resistor is recommended).

The relays must be turned off in the shortest possible time, so the
use of the normal protection diode across the coil should not be used,
as it slows the response considerably. Instead, the arrangement shown
still protects the driver transistor, but allows the relay magnetic
field to collapse without generating a current
in the coil (this the what slows the relay's release). I cannot predict
the exact delay you will achieve, since the choice of a suitable relay
is outside my control. You will have to pester and annoy your local
suppliers to find a relay that has suitable characteristics, and be
prepared to pay what will seem like an obscene amount of money for a
simple electro-mechanical device.

D5 discharges C1 as the supply collapses. It will not help much in
the case where someone switches the power off then straight back on
(not that anyone would do that !), but will reset the circuit much
faster than would otherwise be the case.

The DC arc can (and does) destroy even 10A relays under some
circumstances. To provide greater speaker protection, the relay wiring
in Figure 2 is designed to short the speaker to earth in case of a
fault. This way, even if the contacts do arc it will be directly to
earth. This is much safer (for the speakers), and the arc to earth will
blow the fuse a lot faster than if an 8 ohm load is a part of the
circuit. It is strongly recommended that this scheme is used as a
matter of course. It is worth noting that any DC protection system that
does not use this method will almost certainly fail to protect the speakers with a medium to high powered amplifier. (My thanks to Phil Allison for the information.)

Note also that this circuit cannot be used as shown with the 12V
relays in series if the supply voltage is less than +/-24V (but you
knew that already )

In order to work out the value of R6, subtract the combined relay
voltage from the supply voltage (you must know the relay coil
current!). To calculate the coil current from its resistance, use the
following (I have assumed a 40V supply for the examples):

I = V / R     Where V = coil voltage and R = coil resistance

So for a 180 ohm coil (fairly typical) this works out to

I = 12 / 180 = 67mA

The resistor value is worked out with:

R = V / I     Where V = the 'left over' voltage from the subtraction and I = coil current

You will also need to work out the power rating for the resistor:

P = V2 / R    Where V is the voltage and R is the resistance

Again, for the above example, this works out to

R = ( 40 - 24 ) / 67mA = 16 / 0.067 = 239 Ohms (220R should be fine)

P = ( 16 x 16 ) / 220 = 1.16W

So for an adequate safety margin, a 2 Watt resistor should be considered the minimum (5W would be better).

To determine the transistor for Q3, add the supply voltage and the
zener voltages to give the maximum collector to emitter voltage. In
this case it is 40 + 48 = 88 Volts, and I would suggest that a
transistor with a breakdown voltage of at least 100V be used to give
some safety margin. The MJ350 (300V rated) will be
suitable in nearly (if not) all applications, or you can use a MPSA92 -
lower current, but still has a 300V rating.

Figure 2A - Alternative Back-EMF protection

Figure 2A shows an alternative method you can use to damp the
back-emf from the relay, but to implement it properly, access to an
oscilloscope is helpful (if not essential). If the resistors have
approximately the same resistance as the relay coils, the back-emf
should (!) be limited to about the normal relay voltage, give or take
50% or so. In the tests I carried out (see Tests, below) using a 24V relay, the back-emf was limited to about -30V, which would be fine in most cases.

This method is slightly cheaper than using zeners, but is less
predictable. An additional alternative is to use a catch diode to the
-ve power supply. A 1N4004 between the top of the relay string and the
-ve amp supply will limit the back-emf to the voltage of the -ve
supply, so for the example case this would be -40V.
I expect that this would be quite acceptable, but have not tried it.
Make sure that the diode is connected the right way around - the
cathode goes to the top of the relays, and the anode to the negative
supply.

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Muting

Since we have all this new circuitry, it is most worthwhile to
incorporate a muting function, so that when power is removed from the
system, the relay will open to stop turn-off transients from being
heard. Likewise, we will normally want to mute the system for about 2
seconds after power is applied to stop the turn-on transients as well.
C1 and R1 in the circuit of Figure 2 provide the turn-on delay, by
supplying current to the "Off" terminal as C1 charges. Once charged,
the current falls to zero, and Q1 turns off, allowing Q2 and Q3 to turn
on, thus energising the relays. (Note that this timer will not be reset
if the power is turned off and back on again quickly, but since this is
a procedure that should be avoided anyway, no provision has been made
for it. )

To be able to do this effectively, we must have access to the AC
from the power amp's transformer, or have the external unit controlled
by the main power switch in the system. In some hi-fi installations,
there will be a multiplicity of different units to turn on (and off)
each time the system is used. I will leave it to the reader to decide
which unit to use as the control, but would suggest that where a
separate preamp is used, this could be an ideal controller for the
entire system. It is unfortunate that hi-fi has not followed the
sensible approach of a lot of computers, with a switched IEC connector
on the back of the preamp to control power amps and other outboard
devices. (I did this on my VP-103 valve preamp, and it is most useful
:-)

Figure 3 - Loss of AC Detector

The power detector cannot rely on the DC supply, as this may take a
considerable time to collapse. The common approach is to use a
rectified but unsmoothed output from the transformer secondary. Because
it is not smoothed, this disappears instantly when power is removed,
and is ideal. Figure 3 shows the basic circuit,
and this will remove relay drive within about 50ms of the power being
turned off. We could make it faster than this, but there is little
point.

The circuit simply uses the current pulses to keep a capacitor
discharged via Q1. When the pulses stop, the cap charges until the
threshold voltage of the "Off" terminal is reached (0.65V), and the
relays are turned off. After power is first applied, the timer circuit
will activate the relays after about 4 seconds (typical). This can be
increased if desired, by increasing the value of C1 in Figure 2.

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Tests

I carried out some tests to see just how quickly the relays could be
operated. The results were something of an eye-opener (and I knew
about the added delay caused by a diode!). The relay I used was a small
24V coil unit, having a 730 Ohm coil and with substantial contacts (at
least 10 Amps). With no back-emf protection, the relay opened the
contacts in 1.2ms - this is much faster than I expected, but the
back-emf went straight off the scale on my oscilloscope, and I
would guess that the voltage was in excess of 500V. When a diode was
added, the drop-out time dragged out to 7.2ms, which is a considerable
increase, and of course there was no back-emf (Ok, there was 0.65V, but
we can ignore that). Using the diode / resistor method described above,
release time was 3.5ms, and the maximum back-emf was -30V, so this
seems to be a suitable compromise.

I was not able to test the zener method prior to publication, since
I did not have the 24V zeners needed on hand. I would expect this
scheme to be as good or better than the diode / resistor combination.
The graphs below show the behaviour of the circuit with and without the
resistor and diode. The estimated 500V or more is quite typical of all
relays, which is why the diode is always included. This sort of voltage
will destroy most transistors instantly. It is exactly the same process
used in the standard "Kettering" ignition system used in cars, but
without the secondary winding, or the "flyback" transformer used in the
horizontal output section of a TV set.

Figure 4 - Relay Voltages

The trace labelled 'Contacts' is representative only, and is not to
scale. The peak relay voltage (above left) exceeded my oscilloscope's
input range (and I was too lazy to set up an external attenuator), and
as shown is cut off at my measurement limit. I estimate that the
voltage is greater than 500V.

Note that the kink in the relay voltage curve is caused by the
armature (the bit that moves) coming away from the relay pole piece,
and reducing the inductance. This causes the stored magnetic charge to
try to increase the voltage again, but it is absorbed by the resistance
and dissipated quickly. The contacts open at the
point where the previously closed magnetic field is opened as the
armature moves away from the pole piece. As can be seen, this is 3.5ms
after the relay supply is disconnected.

These graphs are representative only, as different relays will have
different characteristics. As noted above, I cannot predict what sort
of relay you will be able to obtain, but the behaviour can be expected
to be similar to that shown. All tests were conducted using a 24V
relay, having 10A contacts. Upon contact closure, I also measured 2.5ms
of contact bounce. Provided your amplifier is stable by the time the
contacts close, this will be completely inaudible.

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References

1.   D. Self - Muting Relays, Electronics World, Jul 1999