Summary: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 Revisi
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.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.
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.
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.
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.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.
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 ...
VR7 = Vsupply - 24 (where 24 is the zener voltage)
R7 can then be calculated, based on a zener current of 10mA ...
R7 = VR7 / 0.01 (Ohms)
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