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Need Extreme, Earth-shattering Power? Want to unlock immense power from an audio amplifier and speaker? You can combine this easy-to-build unit with a standard stereo amplifier to easily get 400W into a single 8Ω speaker. That’s about three times the power that amp would typically manage. With the right amplifier, you could even get 1000W or more – per channel! Bridge-mode Audio Amplifier Adaptor L et’s say you want to put on a rock concert. The way it works is simple. This Bridge Adaptor (often called a BTL [bridge-tied load] adaptor) splits your audio You’re going to need thousands of watts of power, signal in two. One output signal is virtuand it’s a bit impractical to stack up dozens of ally identical to the input, while the smaller amplifiers and speakers. What you need is other is inverted. something B I G ! So when you connect those Assuming you have the speakers you outputs to two audio amplineed, how would you drive them? fiers (possibly the two chanMany potent audio power amnels within a single stereo plifiers produce their full-rated amplifier), the outputs swing in power output into a 4load. But opposite directions. all too often speakers have an 8Ω That means the voltage between impedance, and so, for example, a the outputs is double that of a single 500W amplifier module would only output. This arrangement is shown in Fig.1. achieve a measly 278W into such a Since the power into a load can be calculated as load; and that’s just not good enough! V2÷R, if you double the voltage but keep the impedance The trick is to drive the speakers in constant, you quadruple the power. bridge mode. If you use two amplifiers, plus Of course, this is assuming your amplifier is capable of this little device, you can drive a single 8 8Ω speaker with delivering that much power. But if you use an 8Ω speaker, two amplifiers, and achieve full amplifier power. since most amplifiers will happily drive a 4Ω load, it should be Thus, because the speaker’s impedance is effectively capable of it. (You must be careful if using halved when being driven in bridge a 6Ω or 4Ω speaker; many amplifiers aren’t mode, you will get that 500W figure from BY NICHOLAS VINEN very happy with a 3Ω or 2effective load.) each amp: 500W + 500W = 1kW. Rock on! 24 Practical Electronics | May | 2020 Performance We ran our prototype through a number of tests using our Audio Precision System Two. We haven’t reproduced any of the resulting graphs here since the results can be summarised in just a couple of paragraphs. We used a 15V AC plugpack as the power source during these tests. The distortion and noise levels are very low. The signalto-noise ratio is 114dB with respect to 2V RMS (a common signal level from a CD/DVD/Blu-ray player), with a measurement bandwidth of 20Hz-22kHz. The frequency response is ruler-flat, being only 0.2dB down at 20Hz and less than 0.1dB down at 20kHz. THD+N is 0.0005% from the non-inverting output and 0.0006% from the inverting output over the 20Hz-20kHz range with an 80kHz bandwidth. Measuring with a 20kHz bandwidth, these figures reduce to 0.0003% and 0.0005% respectively. The distortion across the two outputs (ie, what you would actually hear) measures the same as the inverting output. Updated version We published a Bridge Adaptor in the October 2010 issue. This one is considerably smaller and will fit into a UB5 Jiffy box for convenience. But you also have the option of building it into an amplifier chassis if that’s what you want. This design also has much more flexible power supply options. It will run off AC, DC or split rails. It also uses parts that are easier to get, and cheaper, than our last design. The circuit diagram is shown in Fig.2. The input audio signal is fed in via RCA socket CON1, which has a 100kresistor to bias the input signal to 0V (if it’s floating). That signal then passes through two ‘back-to-back’ electrolytic capacitors. We’ve used this arrangement, rather than a single nonpolarised (NP) electrolytic capacitor because the size and cost of NP capacitors can vary dramatically. By using two small, low-cost regular electrolytics, we get the same effect with a low price and small footprint. The signal is DC-biased to signal ground (more on that later) with a 10kΩ resistor, and RF signals are filtered out by a 100pF ceramic capacitor. The signal is then applied to the non-inverting input, pin 3, of low-noise op amp IC1a. IC1a acts as a buffer, feeding both non-inverted RCA output connector CON2, and the inverting stage, which is based around op amp IC1b. The signal to CON2 passes through another pair of 22µF DC-blocking capacitors and is re-biased to 0V DC via a 100kΩ resistor. The 100Ω series resistor protects IC1a against an output short circuit and also isolates any cable capacitance to prevent oscillation. Features and specifications • Up to four times the power into a single speaker, using a stereo amplifier or two mono amplifiers • Low noise and distortion • Powered from 9-16V AC, 12-40V DC, 18-32V centre-tapped transformer or ±6-20V DC (split rails) • Low current draw – around 10mA • Fits inside a compact, low-cost UB5 Jiffy box • Can also be mounted inside an amplifier chassis • A switch can be used to change amp mode between stereo and bridged mono • Uses low-cost, commonly available parts • Suitable project for beginners IC1b is configured as a classic inverter with a gain of −1, set by the ratio of the two 2kΩ resistors. These values were chosen to keep noise to a minimum without unduly loading the output of IC1b. After all, it has to drive whatever is connected to inverting RCA output connector CON3 as well. The signal is coupled from IC1b to CON3 in the same manner as described for CON2 above. IC1b’s non-inverting input, pin 5, is tied to signal ground via a 1kΩ resistor. This means both of its inputs (pins 5 and 6) have the same source impedance, as the two 2kΩ resistors which are effectively in parallel, given that both are driven from op amp outputs, which have an effective impedance close to 0Ω. Power supply options You can power the Bridge Adaptor from a 9-16V transformer, standalone or plugpack, or you can use a 12-40V DC supply, a ±6-20V DC split supply, or an 18-32V centretapped transformer. That last option is most useful if you’re building this project into an amplifier chassis. Later on, we’ll show you how to wire up a switch so that an amplifier can be easily configured as either stereo or bridged mono. When the unit is powered from AC or a split rail DC supply, signal ground is tied to power supply ground by a 0Ω resistor (ie, wire link), as shown in Fig.3. But if the unit is powered from a single DC supply (eg, 12V DC) then signal ground needs to be around 6V DC, so that the AC signals have a symmetrical swing. The power supply section is therefore reconfigured, as shown in Fig.4, by omitting some components and replacing others with wire links. Fig.1: the Bridge Adaptor is connected to two power amplifiers (separate mono amps, or two channels in a stereo or multi-channel amp) to drive a single loudspeaker. This doubles the maximum voltage across the speaker, and increases the output power to up to four times the original. Only the active terminals of the amplifiers are connected to the loudspeaker, while the ground terminals are not connected. Note that this will not work with an amplifier that’s already internally bridged – ie, where both the positive and negative outputs are actively driven! Practical Electronics | May | 2020 25 Amplifier Bridge Adaptor Fig.2: the circuit of the Bridge Adaptor, (also known as a BTL, or bridge-tied load adaptor) without the power supply (shown in Fig.3 and Fig.4) . The incoming audio signal is AC-coupled to non-inverting input pin 3 of IC1a, which acts as a buffer. The buffered signal is fed to CON2 and also IC1b, which inverts the signal and then feeds this inverted version to CON3. By connecting CON2 and CON3 to two separate single-ended power amplifiers (or left and right channels in a stereo amplifier), you can almost quadruple the power delivered to a single speaker. In this case, the 0Ω resistor is instead 10kΩ, and another 10kΩ resistor forms a voltage divider across the DC supply rail, to generate a half-supply rail for signal ground. A 220µF capacitor between signal ground and power supply ground prevents any ripple or noise on the supply rail from getting into the signal ground, and thus affecting the audio signal. It also presents a low AC impedance to the op amp feedback divider, so that the unit’s frequency response is not affected by the resistors used to generate the signal ground rail. Before construction Before you start assembling the board, if you’re going to be fitting it in a UB5 Jiffy box, place the board in the bottom of the box and use a marker to place dots in each location where a mounting hole is required. We’ve provided four different PCB overlay diagrams, to show you what components you need for each of the supply configurations. Fig.5 shows the components required for a transformer with a single secondary (including most AC plugpacks) and Fig.6 for a transformer with two secondaries connected in series, or a single centre-tapped secondary. Fig.6 also applies if you have a transformer with individual secondaries (eg, two 9V or two 12V secondaries). In this case, the phases of the windings need to be correct: the end of one secondary is connected to the start of the other secondary to effectively form a centre-tapped winding. You need to be careful with this connection – measure across the two windings (ignoring the centre tap) to ensure you have twice the individual winding voltage. If you get 0V (or close to it) you have connected the two windings incorrectly. Where a split DC supply is called for (eg, +15-0-15V) use the overlay shown in Fig.7, whereas a single DC supply uses the overlay shown in Fig.8. So let’s get building! Start by fitting the smaller resistors. While these have colourcoded bands indicating their values (as shown in the parts list), because certain colours can look similar depending on your lighting, it’s much safer to measure their values with a multimeter before installing them. Use the appropriate overlay diagram as a guide as to which resistors go where. Next, fit whichever of diodes D1-D4 are required for your particular configuration, followed by zener diodes ZD1 and (if needed) ZD2. In each case, ensure that the cathode stripe faces in the direction shown on the overlay diagrams, ie, towards the top or left edge of the board. Now install the 1W resistor(s) and a socket for IC1, assuming you’re using one. You can then mount the two 100pF capacitors and single 100nF capacitor. None of these are polarised. Follow with the terminal block, if you’re going to be using it, ensuring that it is pushed down flat onto the board and that its wire entry holes face the nearest edge. Universal Power Supply (Single AC, CT AC or Split DC Input) Fig.3: this shows a ‘universal’ power supply, suitable for a single ended DC input, a split DC input (ie, +V/0V/–V) or an AC transformer with or without a centre tap. CON4 is used for single-ended DC or AC since it only has two contacts. CON5 is used for split DC or an AC transformer with centre tap. 26 Practical Electronics | May | 2020 This photo of the PCB actually has all the power supply components shown in the four overlays. Some are obviously not necessary, depending on the version you build. (Use the component overlay for your version). You can use a 3-way terminal block for all four configurations; however, with two of the configurations, only a 2-way block is required as shown in Fig.5 and Fig.8. If using a two-way block, make sure to solder it to the right pair of pads. Now fit the electrolytic capacitors. These are polarised and must be oriented correctly. The longer (+) wires go into the pads marked with a ‘+’ on the PCB, towards the top edge of the board. The striped side of the can shows the negative terminal, so the stripes should all face towards the bottom edge. There are three different types of electrolytic capacitors used, so don’t get them mixed up. Finally, fit the RCA terminals and DC socket (if required). In each case, make sure the connector is pushed down fully onto the PCB and lined up nicely with the PCB edge before soldering its pins. Be generous with the solder, as these pins are quite large. Testing Before mounting it, it’s a good idea to test the unit. If you’ve fitted a socket for IC1, you can leave IC1 out until you have verified that the power supply is working OK. It’s best to test the unit with the same type of supply that you will eventually be using; however, if you intend to use a centre-tapped transformer or split supply, you could use a 9-16V AC plugpack for initial testing. Apply power and measure the voltage between GND (eg, the RCA connector shells) and pin 8 of IC1 (or its socket). If using an AC supply, you should get a reading of around +16V DC, or perhaps slightly lower if your AC supply is below 12V. Similarly, pin 4 of IC1 should be at around –16V DC. Pin 5 should be close to 0V. If you’re using a DC split supply, you should measure voltages at pins 8 and 4 of IC1 that are around 0.7V less than the applied voltages, while pin 5 should be close to Fig.5: the overlay for a single AC supply, from either a 9-16V transformer or plugpack, plugged into CON5. Fig.6: if you have a transformer with a centre-tapped secondary, use this overlay. Fig.7 if you have a split DC supply (eg, ±15V and 0V) you will connect it to CON5 and omit some components. Fig.8: this overlay is for the single-ended DC supply, as shown in the circuit diagram at left. Power Supply (Single-ended DC) Fig.4: the power supply can be much simpler when the unit is only to be operated from a single-ended DC supply. Some components are omitted while others have their values changed. The negative supply rail for dual op amp IC1 is connected to 0V via a wire link, while the signal ground is biased to half supply by a pair of resistors and a capacitor. Practical Electronics | May | 2020 27 Fig.9: if you’re mounting the Bridge-mode Adaptor in a UB5 Jiffy box, here’s where to drill the holes required in the sides and base. The PCB is attached to the base using untapped spacers and machine screws (see parts list and text). 0V. And if using a single-ended DC supply, pin 8 should be around 0.7V less than your applied DC voltage, while pin 4 should read 0V and pin 5 should be almost exactly half the reading at pin 8. If you get readings that are significantly different from those described above, switch off power and check your board carefully. Things to look out for are bad solder joints, incorrectly oriented components or components that are in the wrong location. If the power supply checks out, switch off power, short out pins 4 and 8 of IC1’s socket briefly (to discharge the capacitors) and then insert IC1 in its socket. Re-apply power and apply a signal to CON1, from a Blu-ray player, iPod, mobile phone, PC or whatever’s convenient. Connect CON2 to the input of an amplifier with its volume set to minimum, then slowly ramp its volume up. You should hear the input signal being reproduced cleanly. Disconnect CON2 from the amplifier and connect CON3 instead. You should again hear the input signal being reproduced cleanly (the fact that its phase is inverted will not be audible). You can now do a final test, with CON2 hooked up to one amplifier input and CON3 to another, and the speakers connected across the amplifier outputs, as shown in Fig.1. Again, you should hear the signal loud and clear. Only this time, the maximum output power of the combined amplifiers will be much higher! Mounting it in a box As mentioned above, you will need to drill four 3mm holes in the base of the Jiffy box. If you forgot to do that earlier 28 (using the PCB as a template) you could instead make the holes where shown in the drilling diagram, Fig.9. You will also need to drill three 9mm holes in one side of the box for the RCA sockets, plus a 7mm-diameter hole in the opposite side to access the barrel socket. The positions of these holes are also shown in Fig.9. You can copy (or download from the May 2020 page of the PE website) this diagram, cut out and stick the copy onto the box and then mark and drill the holes. Ensure the template is aligned accurately with the top, bottom and sides and drill the holes accurately, starting with a smaller pilot drill and then enlarging to size with larger drill bits, a stepped drill or a tapered reamer. This ensures that the unit will fit nicely in the box and look neat. Once you’ve made the holes, deburred them and removed any debris from the box, feed the four 16mm machine screws up through the bottom of the box and set it on a flat surface so they won’t fall out. Next, drop the 6-7mm untapped spacers over the screw shafts and lower the board into the case. To do this, first insert the RCA socket barrels through the holes in the case, then drop the opposite edge down into the box. You may have to push a little, getting the box to flex, to get it into place. Once all four screw shafts are through the holes on the board, use thin-nosed pliers to hold a nut on top and do up each screw one at a time. This is a bit tricky since initially, the other three screws will be loose, so you can’t just lift up the box, or they will fall out. We did it by sliding one corner of the box over the edge of the desk while holding that screw so it couldn’t fall out, then carefully rotating it so it threaded onto the nut, then moving on to another corner. Each nut you thread will make it easier to do the next one. Alternatively, you could use Blu-tack, silicone sealant or some other type of glue to temporarily hold the untapped spacers over the holes in the box while you insert and do up the screws. Do them all up tight, then put the lid on the box and affix the label. The artwork for the label can be downloaded from the May 2020 page of the PE website in PDF format and then printed out. Mounting it in an amplifier If you want to integrate it into an amplifier, this is quite easy. You can use longer tapped nylon spacers and mount it to the bottom of the chassis using eight short M3 machine screws. It’s then just a matter of wiring up the AC or DC power supply connections to CON5 and connecting the audio signals using cables terminated with RCA plugs. You could make these with two RCA-to-RCA plug cables and then cutting them in half and stripping off the insulation. Fig.10: same-size label which fits the UB5 Jiffy box. You can download it from the May 2020 page of the PE website. Practical Electronics | May | 2020 Here’s what it looks like mounted in the UB5 Jiffy box, drilled as shown in Fig.9. If CON5 is used, access holes would also be needed on that side. Fig.11 shows how you can use a standard SPDT switch (toggle, latching pushbutton, rotary or slide) to allow the amplifier to be reconfigured as either stereo or bridged mono at any time. Using it If you’ve built the unit into an amplifier with the switch as described above, you can apply a stereo signal to the amplifier’s left and right input channels, with the switch in the STEREO position, and it will operate normally as a stereo amplifier. Or apply a single signal to the left input channel and put the switch in the MONO position, then connect a speaker wired as in Fig.7 for the bridged mono mode. Or if you’ve built the unit into a Jiffy box, connect it to a stereo amplifier or pair of mono amplifiers as shown in Fig.1, for mono mode. If you want to use the stereo amplifier in stereo mode, merely disconnect the unit and wire up the inputs and speakers as you usually would. Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au And here is the finished device, complete with a panel (see text). The beauty of this design is that no extra holes are required in the panel itself – they’re all in the box sides. Parts list – Bridge-mode Adaptor for Amplifiers 1 double-sided PCB, code 01105191, 79 x 44.5mm 1 UB5 Jiffy box (optional) 3 PCB-mount right-angle switched RCA sockets (CON1-CON3) 1 PCB-mount right-angle barrel power socket (CON4) AND/OR 1 3-way (or 2-way) mini terminal block (CON5) All of these components 1 8-pin DIL IC socket (for IC1) are commonly 4 3mm inner diameter, 6.3mm-long untapped spacers available from your 4 M3 x 16mm machine screws normal parts suppliers. 4 M3 hex nuts The PCB (01105191) is Semiconductors available from the 1 LM833 or NE5532 dual low-noise op amp (IC1) PE PCB Service. 2 16V 1W zener diodes (ZD1,ZD2)* 4 1N4004 1A diodes (D1-D4)** * only one required for single-ended DC supply Capacitors ** only two needed for single AC/split 2 470µF 25V electrolytic* DC supply 1 220µF 10V electrolytic*** ***only required for single-ended 2 100µF 25V electrolytic* DC supply 6 22µF 50V electrolytic 1 100nF 50V multi-layer ceramic or MKT 2 100pF 50V NP0 ceramic (code 100n, 104 or 0.1) (code 100p or 100) Resistors (all 0.25W, 1% metal film unless otherwise stated) (brown black yellow brown or brown black black orange brown) 3 100kΩ 3 10kΩ (brown black orange brown or brown black black red brown) 2 2kΩ (red black red brown or red black black brown brown) 1 1kΩ (brown black red brown or brown black black brown brown) 2 100Ω (brown black brown brown or brown black black black brown) 2 100Ω 1W 5%* (brown black brown brown or brown black black black brown) Fig.11: this is how you can use an SPDT switch to allow an amplifier to be easily reconfigured for either stereo or bridged mono. This makes a lot of sense when building the unit into an amplifier; when building it separately into a box, you can easily achieve the same result by re-plugging cables. The switch is shown here as a toggle type, but it could be a push-on/ push-off, slide or even a rotary switch. Practical Electronics | May | 2020 29