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Bridge adaptor for stereo power amplifiers 4x The Power Of A Single Module Would you like to connect a stereo amplifier in “bridge mode” in order to deliver double the power to a single loudspeaker system? This simple adaptor allows you to do it, without any modifications being necessary to the power amplifiers themselves. W By LEO SIMPSON E REGULARLY GET requests from readers asking how to connect a stereo power amplifier in “bridge mode” but up until now we have not had a specific project article to suit the application. Then recently we received an email from a reader asking how to run the SC480 ampli- fier modules (SILICON CHIP, January & February 2003) in bridge mode, just as we were proof-reading last month’s article on the “Balanced/Unbalanced Converter for Audio Work”. We immediately realised that half of that project would provide the needed adaptor. Before going into the details, let’s BRIDGE ADAPTOR Out-of-phase signals The two power amplifiers are driven with signals that are out-of-phase by 180°. If we consider a sinewave signal (or any other audio signal for that matter), when one power amplifier is delivering the positive half cycle of the waveform, the other amplifier will be delivering the negative half-cycle. The amplifiers drive a single loudspeaker and the result is that the two amplifier voltage waveforms are added, ie, we SPEAKER CONNECTED BETWEEN +VE L & R SPEAKER TERMINALS STEREO AMPLIFIER LEFT INPUT OUTPUT 1 AUDIO INPUT SIGNAL briefly describe how a pair of power amplifiers can be run in bridge mode to extract more power. Fig.1 shows the set-up. For a start, you must have two identical power amplifiers and this is why this arrangement is often convenient with a stereo amplifier. LEFT AMP INPUT OUTPUT 2 (STANDARD RCA-RCA STEREO CABLES) RIGHT INPUT RIGHT AMP + – + + – – Fig.1: this diagram shows how the Bridge Adaptor is connected to two power amplifiers to drive a single loud­speaker. Note that only the active output terminals of the power amplifiers are connected to the loudspeaker while the ground terminals are not connected. 82  Silicon Chip siliconchip.com.au V+ 10 F NP INPUT 2 100k 10k 100nF 8 3 22 F NP 1 IC3a 150 OUT1 4 100pF 100k V– 100pF 10k IC3: LM833 TO AMPLIFIER GND INPUTS 10k 6 4.7k 5 IC3b 7 22 F NP 150 OUT2 100k + R2 R1 V+ K – SC  2008 ZD1 15V A 1W 470 F 25V DC INPUT RAILS 0V FROM AMPLIFIER K 470 F 25V R4 R3 A Resistors R1-R4: Values for Various Supply Voltages ZD2 15V 1W V– BRIDGE adaptor for power amplifiers INPUT VOLTAGES (FROM AMPLIFIER) R1– R3 VALUES, POWER RATING R2– R4 VALUES, POWER RATING ±15V DC ±30V DC ±40V DC ±50V DC ±60V DC 33 , 0.5W 1k, 1W 1.8k, 1W 1.2k, 1W 1.5k, 1W (WIRE LINK) (WIRE LINK) (WIRE LINK) 1.2k, 1W 1.5k, 1W ZD1, ZD2 A K Fig.2: the Bridge Adaptor uses a dual op amp to provide “in-phase” and “out-of-phase” signals to drive two power amplifiers and a single loudspeaker. get double the output voltage of one amplifier across the loudspeaker. Since power is “voltage squared” times current, the resultant power in the loudspeaker is four times the power that could be obtained with one power amplifier driving that same loudspeaker. Well, that’s the theory anyway. In practice, the results may not be quite as good but it is still a worthwhile exercise if you have two amplifier modules and a single loudspeaker that you want to drive with a lot of power. What if you use the SC480s? Let’s now consider a real case, as suggested for the SC480 modules in the email mentioned above. As originally published and using the specified power supply circuit, the SC480 module is rated to deliver 50W into an 8-ohm load and 70W into a 4-ohm load. Furthermore, its music power was 77W into an 8-ohm load and 105W into a 4-ohm load. Hence, under music power conditions and depending on the regulation siliconchip.com.au of the power supply, two SC480 modules in bridge mode could be expected to deliver over 200W into an 8-ohm load. In fact, that is four times the rated power from a single module into an 8-ohm load, so our general rule of “four times the power” is not far off. Note that the continuous power would only be about 150W or twice the rated power into a 4-ohm load. Do not use a 4-ohm speaker So could we go even further and use a 4-ohm loudspeaker instead of an 8-ohm model. Well sorry, but that is not possible because it would overload the amplifier modules. The reason for this is that each amplifier in a bridge set-up actually “sees” half the real load impedance. So, for the 8-ohm example we have just talked about, each SC480 amplifier module sees or behaves as if was driving a 4-ohm load and it can only deliver the power it would deliver if it was driving a 4-ohm load. Why is that? Consider two modules driving a single 8-ohm loudspeaker, with each amplifier delivering a sinewave of 8V. Since the voltages across the speaker are added, the resultant current flowing in it is 16/8 or 2A. So as far as each amplifier module is concerned, it is delivering 8V and 2A is flowing, therefore as far as the amplifier is concerned, it is driving a 4-ohm loudspeaker. Now you know as well as we do that amplifiers are not “animate” and they cannot think or see but you get the picture. To repeat the concept: each amplifier in a bridge set-up “sees” half the real load impedance. Therefore, if you are going to use a 4-ohm load in a bridge set-up, each amplifier must be able to drive a 2-ohm load. The SC480 is not rated to drive 2-ohm loads and that is the end of the story as far as that module is concerned. Now let us describe the circuit of the Bridge Adaptor. Before doing so, we should note that if you want to drive a subwoofer in bridge mode, then the Subwoofer Controller featured in the August 2007 issue is your answer, July 2008  83 CUT PC BOARD HERE (OPTIONAL) Parts List OUTPUTS TO AMPLIFIER INPUTS 150 + 0V – R4 ZD2 ZD1 10k R3 R1 10k IC3 LM833 NP 100nF NP R2 22 F 100k 22 F 100pF 100k 150 DC POWER FROM AMPLIFIER OUT2 GND OUT1 GND 4.7k 100pF NP SIG 100k 10 F 10k ADD THESE WIRE LINKS UNDER PC BOARD 470 F 470 F /DE C NALA B DE C NALA B NU RETREV N O C 18060110 GND AUDIO INPUT Fig.3: use this diagram to populate the PC board. Only one half of the board is used and the unused section can be cut off if you wish. Fig.4: repeated from our August 2007 issue, this scope shot shows the principle of bridged power amplifier operation. The two upper traces show the in-phase (yellow) and out-of-phase (purple) signals. The red trace shows the expected signal across the loudspeaker and this is the “sum” of the two amplifier drive signals which will result in four times the power being delivered into the loudspeaker. In practice, depending on the amplifier output configuration and the power supply regulation, the results may not be quite as good. 1 PC board, code 01106081, 103 x 85mm 1 3-way screw terminal blocks (5.08mm or 5mm spacing) 3 2-way screw terminal blocks (5.08mm or 5mm spacing) 3 M3 x 6.3mm tapped standoffs 3 M3 x 6mm screws 1 60mm length of 0.8mm tinned copper wire (for links) Semiconductors 1 LM833 dual op amp (IC3) 2 15V 1W zener diodes (ZD1, ZD2) Capacitors 2 470mF 25V PC electrolytic 2 22mF NP electrolytic 1 10mF NP electrolytic 1 100nF MKT polyester 2 100pF ceramic Resistors (0.25W, 1%) 3 100kW 1 4.7kW 3 10kW 2 150W R1-R4: see table in Fig.2 adaptor and in fact, it is identical to the Unbalanced to Balanced Output Converter shown on page 70 of the June 2008 issue. We show it as using one LM833 low-noise dual op amp which is labelled as IC3. IC1 & IC2 on the same circuit are deleted. The input signal is fed to op amp IC3a which is connected as a unitygain buffer by virtue of the fact that its output (pin 1) is connected directly to its inverting input (pin 2). The output of IC3a is fed via a 22mF non-polarised (NP) capacitor and a 150W resistor to become the “in-phase” output signal to one of the power amplifier modules. IC3a also drives op amp IC3b which is connected as an inverting amplifier with a gain of -1, due to the 10kW resistors connected to pins 6 & 7. IC3b’s output is fed via a 22mF non-polarised (NP) capacitor and a 150W resistor to become the “out-of-phase” output signal to the second power amplifier module. Power supply since it already has the bridge drive facility. The same comment can be made on the Sub Bass Processor featured in the September 1999 issue of Electronics Australia, as it also has 84  Silicon Chip out-of-phase signals to drive a subwoofer loudspeaker. Bridge adaptor circuit Fig.2 shows the circuit of the bridge The power supply for the Bridge Adaptor assumes that the power amplifier modules will be run from balanced positive and negative supply rails. These supply rails are fed in via siliconchip.com.au Still Confused As To How It Works? Separate Boards Some readers may still be confused about how feeding out-of-phase signals to a single loudspeaker can result in double the drive voltage (and four times the power). After all, out-of-phase signals cancel, don’t they? They may be further confused if they look closely at the scope screen grab (Fig.4) and see that the MATHematical operation used to produce the large amplitude red trace is minus (-). So let us explain. Normally, if you add two out-of-phase signals using an oscilloscope, they do cancel. The sum would be written as: V1 + (-V1) = 0 However, when you have out-of-phase signals delivered to a loudspeaker (or any other load, for that matter), the loudspeaker always responds to the voltage difference between the two signals. So if one side of the loudspeaker is at +6V (say) and the other side is at -6V, the total voltage across the speaker will be 12V. Once you have built the Bridge Adaptor and hooked it up to a pair of amplifiers, you can confirm this with a digital multimeter set to a low-voltage AC range. That is why we set the scope to subtract the signals to portray the correct result. The sum would be written as: V1 - (-V1) = 2V1 Bob Barnes at RCS Radio (Phone (02) 9738 0330) has produced separate boards for the two sections of the “Balanced/Unbalanced Converter” project published in June 2008. The “Unbalanced To Balanced Converter” board is coded 01106082.PCB, while the “Balanced To Unbalanced Converter” board is coded 01106083.PCB. For this Bridge Adapator project, you can use the 01106082.PCB board. Note, however, that you will still have to install a wire link under the board between the junction of ZD1 & ZD2 and the junction of the two 470mF capacitors. This is necessary because of the different power supply arrangement for the Bridge Adaptor. The two wire links to the left of the input terminal block in Fig.3 are taken care of by the new board design. series resistors (R1-R4) and regulated using two 15V 1W zener diodes (ZD1 & ZD2) which are each shunted by 470mF 25V capacitors to ensure low hum and noise. A table on the circuit shows the values for various supply combinations. In particular, if you are using the power supply board for the SC480 amplifier modules, they already have provision to provide ±15V supply rails. In that case, you can simply install wire links in place of R2 & R4 and 33W resistors for R1 & R3 and omit zener diodes ZD1 & ZD2. The 33W resistors are included to improve the supply filtering (bypassing) in conjunction with the 470mF capacitors. To illustrate another case, if your amplifier modules use ±50V supply rails, you should install four 1.2kW 1W resistors in the R1-R4 positions. Construction As already noted, the Bridge Adaptor uses the same PC board as the Balanced/Unbalanced Converter except that one half of the board is unused. The parts layout is shown in Fig.3 and includes three links which must be installed underneath the board. If you want, you can cut off the unused section of the board to make it smaller but then you should also provide a third plastic pillar and mounting screw. Installation is simply a matter of deciding how you want to mount the board in conjunction with your power amplifiers which may or may not be in a common chassis. 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Order by phoning (02) 9939 3295 & quoting your credit card number; or fax the details to (02) 9939 2648; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. siliconchip.com.au July 2008  85