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Protects up to six amplifier modules (six single-ended or three bridged outputs) Very simple, small in size and low in cost Can operate from the same power supply as the amplifiers (up to ±40V DC) Disconnects the speaker(s) in 100ms for full rail DC fault <at> 30V Provides a 1-2 second turn-on delay, allowing amplifier outputs to settle Insensitive to low-frequency AC signals Uses DPDT relays with contacts rated to break 10A <at> 28V DC (repetitive) Multi-Channel Speaker Protector If you’re driving a lot of speakers, you will need a matching compact speaker protector to prevent driver destruction, should something go wrong! Our Speaker Protector, when combined with our Hummingbird Amplifier module (published last month), is excellent when driving stereo loudspeakers with an active crossover or for surround sound systems where you have many speakers to drive. A re your expensive speaker drivers protected if the worst happens, and an amplifier module failure results in them having direct current applied? This very simple and effective board matches our Hummingbird amplifier modules, protecting between one and six channels with a switch-on delay in a PCB measuring just 67 x 120mm for up to six channels, or 67 x 91mm for the four-channel version. Over a few years of building hifi and PA equipment, it would be fair to say that this author has not destroyed that many speaker drivers. But when I have, it has always been expensive, painful and inconvenient. The experience of watching a 60W amplifier deliver 40V DC to the voice coil of a very expensive driver that represented months of savings is burnt in my memory. This was a 250W driver but it was no match for 40V DC! In a matter of seconds, the voice coil turned into smoke, much faster than a human being could turn the power off—all for the sake of a $1 insulator. 46 Silicon Chip There have been two main destructors of my drivers: 1. Over-excursion of drivers, particularly in vented enclosures below resonance without appropriate subsonic filtering. This is a surefire way of killing a bass driver. That was addressed by the Active Crossover presented in the October & November 2021 issues (siliconchip.com.au/Series/371), which includes a subsonic filter. 2. By DC from the output of an amplifier, either due to a failure in the amplifier or finger trouble by the builder. (Have you ever left a fuse out or forgotten to connect a wire?) This project solves #2. You might ask: what about over-­ powering a speaker? Won’t it blow up that way too? In my experience, that takes a heroic effort if your crossover is set up correctly, so we leave the volume control to your discretion. By Phil Prosser Australia's electronics magazine The impetus Building an Active Crossover combined with six Hummingbird amplifier modules, I found myself running out of room. To fit this lot with power supplies into a 330mm-deep 2RU case, I had to move from beer mat sketches to CAD and ‘the computer said’ that I needed to make the speaker protector small. So I did. This device will protect your speaker from most amplifier failures. The modest investment will pay itself off the first time it activates, but we all hope this is one project that you never see ‘work’. There are many ways of approaching a speaker protector. This design aims to keep it simple and small by keeping the parts list to a minimum. Circuit details The circuit used is straightforward, as shown in Fig.1, with three duplicated stereo sections providing the six protection channels. The main part of the Protector siliconchip.com.au Fig.1: the Speaker Protector has three identical sub-circuits handling two channels each. Each input signal passes through a simple RC lowpass filter and is applied to three transistors. If a large DC voltage is detected, those transistors switch off the associated DPDT relay, disconnecting the speaker from the amplifier. A basic linear power supply provides around 24V to drive the relay coils and incorporates a switchon delay of around one second, to avoid thumps. circuit is elegant, but it might not be obvious how it works at first glance. Its first job is to detect the presence of DC at an amplifier output, connected to one of the AMP x OUT terminals at right. This is done by the 100kW input resistor and 47μF bipolar capacitor, which form an RC low-pass filter with a -3dB point (corner frequency) of 0.25Hz. The output of this filter feeds a DC detector that triggers at the Vbe voltage of a transistor, around 0.6V. So for regular operation, the amplifier must generate less than 0.6V at the output of this filter. Choosing 10Hz as a ‘safe’ low frequency limit and assuming an amplifier that can deliver 100W into 4W, we can calculate that only 135mV would appear on the output of the filter. So it won’t trigger during regular amplifier use. But say an amplifier goes faulty and delivers its rail voltage of 40V DC (of either polarity) to the output instead of an AC waveform. In that case, after 100ms (0.1s), the low-pass filter output will reach 0.84V, which will definitely trigger the DC detection circuit that follows. This filter operates identically for both positive and negative voltages. With 40V across 8W for 100ms, 20J of energy will be delivered to the voice coil (the impedance will drop over time, approaching its DC resistance value, but this is a good enough siliconchip.com.au Australia's electronics magazine January 2022  47 Parts List – Multi-Channel Speaker Protector 1 double-sided plated-through PCB coded 01101221, 67 x 121.5mm 3 (2) 30V DC 10A contact, 24V DC coil DPDT PCB-mount/cradle relays (RLY1-RLY3) [Altronics S4313, Jaycar SY4007] 8 (6) 2-way 5.08mm pitch mini terminal blocks (CON1-CON8) 1 44mm-tall, 16.5 x 10mm PCB-mount finned heatsink (HS1; for Q16) [Altronics H0645] 1 TO-126 or TO-220 silicone insulating washer and insulating bush [Altronics H7230, Jaycar HP1176] 1 M3 x 10mm panhead machine screw 1 M3 shakeproof washer 1 M3 hex nut 4 tapped spacers & 8 machine screws (to suit installation) Semiconductors 16 (11) BC547B/C 50V 100mA NPN transistors, TO-92 (Q1-Q3, Q5, Q6, Q8-Q10, Q12, Q13, Q15, Q17-Q19, Q21, Q22) 6 (4) BC557B/C 50V 100mA PNP transistors, TO-92 [BC558-9B/C will also work] (Q4, Q7, Q11, Q14, Q20, Q23) 1 BD139 80V 1.5A NPN transistor, TO-126 (Q16) 1 27V 1W zener diode (ZD1) [1N4750] 3 (2) 1N4004 400V 1A diodes (D1-D3) Capacitors 6 (4) 47μF bipolar/non-polarised electrolytic [Jaycar RY6820] 3 47μF 50V electrolytic [Altronics R4807 or Jaycar RE6344] Resistors (all 1/4W 1% metal film axial) 6 (4) 33k-100kW (see text; if unsure, use 100kW) 1 47kW 3 (2) 4.7kW (n) for the four-channel version (PCB code 01101222, 67 x 91mm), the quantities required are listed in red. approximation). This will make a solid thump and probably make you jump, but it won’t cause anything to catch on fire. Even better, if the fault exists from switch-on, the speaker will simply never be connected. The DC Detector comprises a total of three transistors. For the top-most section in Fig.1, these are Q2, Q3 and Q4. Positive DC detection is handled by Q2, which has its collector tied directly to the 4.7kW load resistor. A positive voltage from the filter of more than about 0.6V will switch this transistor on and consequently pull the base of Q1, an emitter follower, low and thus turn off the relay. Q3 and Q4 detect negative voltages. NPN transistor Q3 is connected in a common-base configuration; its base is tied to ground, and its emitter is the input. A negative input voltage will pull current from 0V via the base-emitter junction, causing its collector to sink current. Because the current it sinks at the collector goes to the emitter, this current must be kept low. Hence, this tiny current is buffered by Q4, a PNP device connected as an 48 Silicon Chip emitter-follower. The emitter of Q4 connects to the same resistor as the collector of Q1. So a negative DC voltage from the filter similarly pulls the base of Q1 low, switching the relay off. There is a balance in this circuit between setting a low cut-off frequency and the minimum DC voltage at which the circuit will switch the relay off. 47μF is a reasonable maximum for the filter capacitor, so any tweaking is best done by varying the value of the input resistor(s). We chose the 100kW value to guarantee no problems with false triggering for very high power, very low frequency applications. But if you are not protecting a subwoofer, any value greater than 33kW should be fine and, as a bonus, lower values will provide faster turn-off for fault conditions. The 100kW resistors also affect the lowest DC voltage that will cause the detector to trigger. A fault in the amplifier front-end could cause a few volts DC to be present at the output, and if applied to a driver for long enough, it could overheat and be damaged. So ideally, we want to detect that Australia's electronics magazine condition too, not just a fault where it immediately pegs to one of the supply rails. Assuming a minimum transistor hFE of 120, and that the relay will switch off with 20V across the 4.7kW resistor (leaving just a few volts across the relay coil), the transistor base current must be at least 20V ÷ 4.7kW ÷ 120 = 35μA (or thereabouts) to switch the relay off. This means the DC from the amplifier must be at least 3.5V (35μA x 100kW) to trip the relay off. But this is with a worst-case hFE value. We recommend using BC547B or BC547C transistors, which have higher guaranteed hFE figures and will switch the relay off with about 1.5V DC on the input. Lowering the input resistors would reduce that trip voltage further. The DC Speaker Protector disconnects the speaker any time that DC is detected. The relays used are robust and should be able to interrupt the fault current that can be expected from a Hummingbird amplifier module or similar. However, there is the possibility that upon disconnection, the voltage and current will be high enough to form an arc between the relay contacts. The normally closed contact of the relay is used to shunt this current to ground when the speaker is disconnected. So if an arc forms and current continues to flow, the amplifier’s DC fuse for that rail will blow, and the arc will extinguish. You likely have a failed output transistor already, so a blown fuse won’t exactly be high on your list of concerns. We have put three sets of this circuitry on one board, allowing six standard amplifier channels to be monitored and protected. The relay selected has a standard pinout and is available from many suppliers. Make sure that you get the correct version, though; we are specifying 24V DC coils, though you could use 12V provided you adjust the DC regulator, and the BD139 can handle its heat load (see below). The circuit uses the power ground pin as the ground reference. This connects to the Earth of the power amplifiers being protected. Since the inputs are already paired up, this Protector would work well for DC protection in a bridged amplifier. The power supply The power supply is a basic seriespass regulator generating about 25V siliconchip.com.au DC. The relays need 24V on their coils, and this suits amplifiers with various rail voltages. It can be adapted for supplies below ±25V or above ±40V (see below). The power supply provides a turn-on delay of about one second. This is because the 47kW resistor delays the charging of the 47μF capacitor at the base of Q18. This applies to all channels protected by the board. As you increase the supply voltage, the turn-on delay decreases slightly because the capacitor will charge faster. You could compensate for that by increasing the resistor value if needed. If you have an amplifier with rails below ±25V, you have the option of swapping the relays for 12V DC coil versions and make necessary adjustments in the regulator (we expect a 15V zener would work well for ZD4). Similarly, if you have higher rail voltages, this should be fine; just watch the sizing of the heatsink. The specified Altronics H0655 heatsink should be fine for any normal rail voltage. Construction Construction is straightforward. There are two PCBs available; a six-channel version (coded 01101221, 67 x 120mm) and a four-channel version (coded 01101222, 67 x 91mm). We have described the six-channel version here; the four-channel version is identical except that one relay and its associated components are omitted, so the PCB is smaller. Refer to the appropriate overlay diagram, Fig.2 or Fig.3, during assembly. Start with the resistors and diodes. Make sure you get the diodes in the right way around. Then mount the two-way terminals for power, each input/output pair and the Earth terminal to prevent arcing in the relays (CON8). Now is time to solder in the BC5XX transistors. Try to mount them at the same height so it looks neat. Next come the capacitors. The three 47μF polarised capacitors need to be rated at 50V DC, and all go in the same way, with the longer positive leads to the pads marked +. The six 47μF bipolar/non-polarised electrolytic capacitors do not need a high voltage rating as they will never see more than 0.6V – they mount to the PCB in any orientation. Now fit the BD139 to the heatsink siliconchip.com.au Figs.2 & 3: build the smaller board to protect up to four channels, or the slightly larger board for five or six channels. Assembly is straightforward; all components are through-hole types and can be fitted in order from shortest to tallest (the latter being the relays and heatsink for Q16). The finished Speaker Protector board will look something like this. Note the holes drilled into the board under the heatsink to allow convection to pull fresh air up from underneath. The CON8 (GROUND) terminal block is missing on this prototype version; you could leave it off, but it provides better protection for the speakers if you wire it up to the amplifier Earth. Australia's electronics magazine January 2022  49 The Multi-Channel Speaker Protector comes in a six-channel (pictured) and smaller four-channel version. The four-channel version would be suited to a two-way stereo speaker system with an active crossover or a bridged stereo amplifier. using an insulating washer, 10mm M3 machine screw, locking washer and nut. Solder the heatsink to the PCB, but remember it can be hard to get enough heat into it to solder those big pins. Finally, mount the relays. The PCB has 1.5mm holes which are the minimum that this family of relay recommends – the devices from Altronics and Jaycar leave a fair bit of room in the holes. Solder them in well. Testing Now that you have all the parts mounted, it is time to test it out. During the initial tests, leave the amplifier terminals (CON1-CON6) disconnected. First apply power and check for the 25V output of the regulator The end of the closest 4.7kW resistor right near Q11 is a convenient place to probe; you can use the anode of any of diodes D1-D3 as a ground reference. The reading should be between 24V and 26V for an input above 32V DC. If this is not present or correct, check that ZD4 has about 27V across it; if not, look to the 47kW resistor and transistors Q16 & Q18. Check that these two transistors are the right types and soldered in correctly. Also check for short circuits – is the BD139 getting hot? After a second or two, the relays ought to click in. If this does not happen, check the voltage on the bases of Q1, Q8 and Q15 (the driver transistors for the relays). Are these within a volt 50 Silicon Chip or two of the 25V rail? If not, check that they are the proper devices and soldered in correctly. Check the voltage at the output of the RC filters. The voltage at both ends of the 100kW resistors (or other value you might have changed them to) should be close to 0V. If not, check that the BC547 and BC557 parts are in the right places and orientated correctly. Assuming the relays do switch on, let’s check that they will trip off correctly. The easiest way to test it is to take a 9V battery and connect the negative end of the battery to the ground terminal on the DC protector. Then touch a wire from the positive pin of the battery to the “AMP” terminal of each installed channel of the DC protector. The associated relay should switch out almost instantly. Repeat this for all channels, checking that the relays switch quickly. Then repeat the test with the battery the other way around (ie, positive terminal to GND and negative to the AMP terminals). If a channel does not switch as expected, measure the voltage at both ends of the 100kW resistor. One should be ±9V, the other ±0.6V. If the ±9V end is not correct, there is a short or open circuit somewhere in that area. If the other end is not close to +0.6V, check the two NPN transistors on the DC detector (eg, Q9 and Q10), especially their orientations. Check the associated PNP transistor (eg, Q11) for a fault where you aren’t registering -0.6V. We don’t suggest you do this, but to verify that the Protector does indeed protect the driver, we connected a 4W subwoofer to the DC Speaker protector along with a 6A limited, -34V power supply to the “AMP” input of the Protector. There was a solid thump and click as the relay saved the sub from Scope 1: the blue trace is the voltage across a 4W loudspeaker driver (zero volts at top), while the yellow trace is -34V applied to the DC Speaker Protector from a bench supply. The speaker is disconnected in less than 80ms. The AC voltage generated by the cone movement due to back-EMF after the relay disconnected the driver will not occur in this final version as long as CON8 is connected to Earth, as that will brake the cone movement. Australia's electronics magazine siliconchip.com.au Silicon Chip Binders REAL VALUE AT $19.50 * PLU S P&P Remember to fit an insulating washer to the BD139 transistor (visible above) to prevent it from shorting out on the heatsink. Also note the use of shakeproof washers on all screws so they won’t loosen due to vibration or movement. inevitable destruction. We monitored this with an oscilloscope, and the result is shown in Scope 1. We noted a flash of arcing as the speaker was disconnected, which is no surprise when breaking the very high direct current flow. Please don’t try this at home, as a speaker protector does not make this sort of thing safe for your speaker. Application The DC Protector needs to be connected to the power amplifier ground/ Earth via the provided terminal (CON8) and supplied with 30-40V DC to the power connection (CON7). If your amplifier has a higher positive rail voltage than this, you can use a 5W wirewound resistor to drop the supply voltage to the Protector. The six-­ channel Protector draws about 100mA, so a 100W 5W resistor will drop 10V and dissipate about a watt. Connect the terminals marked AMP to the amplifier and the corresponding SPKR terminal to your speaker outputs. It’s OK to leave some channels unused; for example, if you have a 3or 5-channel amplifier. Once installed in your amplifier, let’s hope that you never hear those relays click unexpectedly. But if you do, you will be glad SC they are there! Are your copies of SILICON CHIP getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? Keep your copies safe, secure and always available with these handy binders These binders will protect your copies of S ILICON C HIP. They feature heavy-board covers, hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Scope 2: this scope grab shows the response time of the Protector to a 20V DC fault. The input voltage step is at t=0 and the output starts to drop before t=60ms. It reaches 0V before t=80ms. The 80ms delay is due to the RC time constant of the filter reaching 0.6V. siliconchip.com.au Australia's electronics magazine Order online from www. siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *See website for delivery prices. January 2022  51