Position: Index > Protect Circuit >

Loudspeaker Protection and Muting (MC1496BDG)

2016-09-19 21:48  
Declaration:We aim to transmit more information by carrying articles . We will delete it soon, if we are involved in the problems of article content ,copyright or other problems.

 This article describes the Loudspeaker Protection and Muting (MC1496BDG). The content is very simple, very helpful. Components in this article can help you understand better understanding of this article. For example, in this article, you can go to find and buy these components:MC1496BDG.

 

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.

NotePlease 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.


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.


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.

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 Iknewabout 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.