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A 50 watt per channel stereo amplifier module Want to build a stereo power module which requires few components & no adjustments? This module is the answer. It deliv­ers 50 watts per channel into 8-ohm loads or, with a reduced supply rail, 60 watts per channel into 4-ohm loads, using the LM3886 mono­lithic power IC. By LEO SIMPSON & BOB FLYNN This stereo module came into being for several reasons. First, it supersedes the 50 watt per channel amplifier board which we published in the February 1992 issue of SILICON CHIP. This board was based on the plastic Darlington transistors TIP142 & TIP147 but these are no longer available. Hence, this new module is a drop-in replacement for the 50W/ channel board and has the advantages that it requires no quiescent current adjustment, has better thermal stability and is short-circuit proof. Second, while the 50 watt module based on the LM3876 and featured in the March 1994 issue has been very popular, we wanted to feature the later version of this monolithic IC, the LM3886. This version has the advan18  Silicon Chip tage of being able to deliver slightly higher power into 4Ω loads, provided the supply rails are re­duced. We’ll talk more on this point later. Third, while the abovementioned module was quite popular, it was clear that there was a need for a stereo version, prefer­ably also with provision for ±15V supply rails for a preampli­fier. Performance A look at the performance panel shows that this new module is a very respectable performer, roughly equivalent overall to the now superseded module featured in our February 1992 issue, and subsequently in the March & April 1992 issues, as the Studio Twin 50 amplifier. We are also featuring performance graphs taken with our recently acquired Audio Precision audio test set. Working under the control of a computer, this instrument can take stereo per­formance measurements in a fraction of the time it takes using the old methods. Fig.1 shows the harmonic distortion versus frequency for the module, at 25 watts into an 8Ω load. As you can see, the distortion is below .01% for frequencies below 3kHz. In fact, for much lower powers which is where the amplifier would operate for most of the time on normal program material, the distortion would be around .005% or less. Fig.2 looks to be very similar to Fig.1 but in this case it shows distortion versus frequency at 30 watts into a 4Ω load. As you might expect, the distortion is a little worse over the whole spectrum but is still pretty respectable. Fig.3 depicts the THD (total harmonic distortion plus noise) versus power into an 8Ω load at a frequency of 1kHz. The slow rise in harmonic distortion as the power is reduced below 10 watts is a natural consequence of the increasing noise in the measurement. However, you should not conclude that the amplifier is noisy; far from it. It is very quiet, with an unweighted signal to noise ratio of -107dB with respect to 50 watts. In fact, a few calculations on that noise level will reveal that the THD shown at 100 milliwatts is virtually all noise – the true harmonic distortion would be under .003%. Fig.3 also shows the very rapid rise in THD as the amplifi­er reaches and exceeds the clipping point, just below 50 watts. Fig.4 depicts the separation between channels of the modu­le and, as you can see, this exceeds -80dB over most of the audible spectrum. We should note that this high degree of separation can be easily degraded if the signal connections to the module are not made correctly. In general, you must avoid earth loops at any cost. To do this, the earth of the signal source driving the module must only connect at one point, preferably the earth for the preamp supply. The shielded cables for the stereo signal source must only be earthed at the source, not at the module PC board, even though we have provided earth connections. However, we are getting a little ahead of ourselves. AUDIO PRECISION AMP-THD THD+N(%) vs FREQ(Hz) 5 20 DEC 94 20:43:19 1 0.1 0.010 0.001 20 100 1k 10k 20k Fig.1: harmonic distortion versus frequency at 25 watts into an 8Ω load. AUDIO PRECISION AMP-THD THD+N(%) & THD+N(%) vs FREQ(Hz) 5 21 DEC 94 01:15:02 1 Circuit details Fig.5 shows the circuit details of the module, with just one channel shown. The circuit of each power amplifier is very similar to that of the LM3876 50W module published in the March 1994 issue of SILICON CHIP. The main difference is that the LM3886 has a positive supply connection to pin 5 (O/C on the LM3876). Now let’s just briefly describe the main points of the circuit. The input signal is coupled via a 1µF MKT polyester capacitor and then via an RC network consisting of a series 1kΩ resistor and shunt 220pF capacitor. This is an RF suppression network. The voltage gain of the amplifier is set to 23 by the 22kΩ nega­tive feedback resistor from pin 3 to pin 9, in conjunction with the 1kΩ resistor and 47µF capacitor. The latter capacitor and the 1µF input capacitor sets the low frequency roll-off to about -1dB at 15Hz. The output drives the loudspeaker via an RL network con­sisting of a 10Ω resistor in parallel with an inductance of 0.7µH. This acts in conjunction with the Zobel network 0.1 0.010 0.001 20 100 1k 10k 20k Fig.2: harmonic distortion versus frequency at 30 watts into a 4Ω load. comprising the series 5.6Ω resistor and 0.1µF capacitor to ensure that the amplifier is stable under varying load conditions. Muting We’ve included the optional mute function at pin 8. This is connected via link LK1 and a 39kΩ resistor to the negative supply rail and this disables the muting. To mute the amplifier, a switch should be connected in series with LK1 and when the switch is open the amplifier will mute the signal by 110dB which will be below the noise level. The 22µF capacitor provides a slow turn-on for this feature. Power supply The power supply uses a 50V centre-tapped transformer feed­ing a bridge rectifier and two 4700µF 63VW capacitors. The trans­ former should be rated at 160VA as a minimum; February 1995  19 AUDIO PRECISION THDVSLVL THD+N(%) vs measured LEVEL(W) 20 20 DEC 94 20:00:57 10 1 0.1 0.010 0.001 0.1 1 10 PARTS LIST 1 PC board, code 01102951, 248 x 58mm 2 single sided heatsinks, 72mm high (Altronics Cat. H-0522) 2 TA11B IC mounting kits 8 20mm fuse clips 4 2A M205 20mm fuses (use 3A for 4Ω loads) 2 3-way PC terminal blocks (Altronics Cat P-2035) 13 PC pins 2 15mm tapped standoffs 2 3 x 10mm machine screws 1 1-metre length 0.5mm enamelled copper wire 3 0.25-metre lengths 32 x 0.2mm hook-up wire (three different colours) 100 Fig.3: THD (total harmonic distortion plus noise) versus power into an 8Ω load at a frequency of 1kHz. Semiconductors 2 LM3886 audio power amplifiers (IC1,IC2) 1 KBPC10-04 bridge rectifier 1 LM7815T 3-terminal regulator (REG1) 1 LM7915T 3-terminal regulator (REG2) Capacitors 2 4700µF 50VW electrolytics 4 100µF 63VW electrolytics 2 100µF 16VW electrolytics 2 47µF 63VW electrolytics 4 22µF 16VW electrolytics 2 1µF 63V MKT polyester 6 0.1µF 63V MKT polyester 2 220pF 50V ceramic Resistors (0.25W, 1%) 2 39kΩ 4 330Ω 1W 4 22kΩ 2 10Ω 1W 4 1kΩ 2 5.6Ω 1W Fig.4: separation between channels of the module between 20Hz and 20kHz. anything less and the power output will be degraded. If you plan to drive 4Ω speakers, the transformer should be a 40V centre-tapped unit, again rated at 160VA. Positive and negative 3-terminal regulators have been included to provide ±15V supply rails to a preamplifier board. If you will not be using this feature, these regulators and their asso20  Silicon Chip ciated components should be deleted. If the 3-terminal regulators are not loaded with at least 470Ω each (ie, to draw about 30mA), their input voltage ratings of 35V may be exceeded when the AC mains voltage is high. Construction All of the components for the stereo module except the heatsinks are in- stalled on a PC board measuring 248 x 58mm and coded 01102951. Fig.6 is the component overlay diagram. You will notice that there is a vacant portion of board between the two power amplifiers. This might seem like a mistake at first sight but was in fact necessary to accommodate the mounting centres of the specified heatsinks. The amplifier channel closest to the power supply compon­ents has its supply rails directly connected via the PC board tracks. The other amplifier has its connections made via heavy duty hook-up wire which is twisted to minimise radiation of signal compo- GND INPUT 47uF 22uF 22k SPEAKER 1 100uF SPEAKER GND IC1 3886 100uF +35V 22uF SPEAKER 330  1W +15V -15V 0V 100uF 100uF REG2 47uF 25VAC CT 25VAC the copper pattern thoroughly for any shorts or breaks in the copper tracks. If you find any, they should be fixed, using either a sharp utility knife for shorts or a soldering iron and solder to bridge open circuits. This done, install the PC pins and BR1 REG1 47uF 330  1W nents. These measures are necessary to maximise the separa­tion between channels and also to minimise harmonic distortion when both channels are being driven. Before you begin assembling any components onto the board, check February 1995  21 24 x 0.2 INSULATED WIRE ON COPPER SIDE OF BOARD -35V 4700uF 0V 4700uF F3 Stability �������������������������������������� unconditional +35V F2 Output power ����������������������������� 48 watts per channel into 8Ω; up to 60 watts into 4Ω (see text) Frequency response at 1W ������� 16Hz to 200kHz ±1dB Input sensitivity �������������������������� 870mV RMS (for full power into 8Ω) Harmonic distortion ������������������� <.05% from 20Hz to 20kHz; typically <.005% Signal-to-noise ratio ������������������ 107dB unweighted (20Hz - 20kHz); 109dB A-weighted. Protection ���������������������������������� 2A fuses plus SPiKe(TM); 3A fuses, if driving 4Ω loads. Damping factor �������������������������� >150 (for 8Ω loads) SPEAKER GND 100uF 100uF 0.1 Performance Measurements GND INPUT -35V 47uF 22k 1 Fig.5: the module is based on two LM3886 audio amplifier ICs, although only one channel is shown here. IC1 3886 -15V Fig.6 (below): follow this overlay diagram when installing the parts on the PC board. Note the supply connections via twisted hook-up wire to one channel. OUT 2x33 0  IN REG2 1W -35V 330  1W GND 330  1W CASE 100 16VW 0.1 GND E 0.1 47 63VW +15V 100 16VW 5.6 1W 4700 50VW GND 10 / L1 47 63VW 39k N 4700 50VW 220pF 25V 1k 25V 240VAC 1k T1 L1 : 16T 0.5mm DIAMETER ENAMELLED COPPER WIRE WOUND ON 10  1W RESISTOR BR1 KBPC10-4 +35V REG1 2x33 0  7815 1W OUT IN 22k F1 1A 0.1 -35V 100 63VW 1uF A 11 22 16VW F3 1 F3 2A LK1 F2 0.1 0.1 47 16VW 39k 8W 0.1 5.6 1W 0.1 4 22k 1k 10  1W 3 5.6 1W 7 8 5 10 / L1 9 IC1 LM3886 39k 220pF 1 220pF 22k 10 1k INPUT 1k L1 0.7uH 1k 1 +35V 22k 0.1 100 63VW 1uF F2 2A Fig.7 (left) shows an actual size artwork for the PC board, while Fig.8 (right) shows how the LM3386 IC is insulated from the heatsink using a mica washer and insulating bush. Smear the mating surfaces lightly with heatsink compound before bolting the assembly to­gether. HEATSINK 3mm SCREW links, followed by the resis­tors and capacitors. Make sure that you install the electrolytic capacitors with correct polarity. Next, install the fuse clips and note that there is a trick to this task. The clips have little lugs at one end which stop the fuse from moving longitudinally. If you install the clips the wrong way around, you won’t be able to fit the fuses. Note that the four 330Ω resistors which supply the 3-terminal regulators should not be fitted until after the module has been tested and is to be connected with a preamp, otherwise the input voltage on the regulators could exceed their ratings, as noted above. L1 consists of 16 turns of 0.5mm enamelled copper wire wound onto a 10Ω 1W resistor and soldered at both ends. To wind it, scrape the enamel off the start of the copper wire and solder it to one end of the resistor. Then neatly wind 16 turns onto the resistor body, scrape the enamel off the end of the wire and solder to the other end of the resistor. Finally, install and solder the assembly into the PC board. The positive and negative power supply connections to the second channel should be made with heavy duty hook-up wire (32 x 0.2mm or better) which should be twisted as shown on Fig.6. The 0V connections should be made via the same sort of hook-up wire but underneath the board. Finally, you can install the power ICs. Make sure that the tabs of the devices line up precisely with the back edge of the PC board so that they can be properly secured to the heatsinks. This done, fit 15mm metal standoffs to the board and line up the heat­sinks against the ICs so that the positions of the mounting screws can be marked. After drilling these holes, use standard TO-3P mounting kits to secure 22  Silicon Chip DEVICE MICA WASHER INSULATING BUSH 3mm WASHER 3mm NUT the ICs to the heatsinks – see Fig.8 for the details. Use your multimeter (switched to a high “Ohms” range) to make sure that the IC mounting tabs are isolated from the heat­sinks. The heatsinks we used are supplied by Altronics (Cat H-0522). To mount them into the chassis you could use small L-shaped brackets or, as we did, blind-tap holes into the edge to secure them directly. Testing To test the module, connect the power transformer and apply power. The supply rails will normally be around ±37V depending on the value of the AC mains voltage. Now check the quiescent current in each channel. This can be done in one of two ways. The first is to remove one fuse (while the power is off) and connect your multimeter, switched to an “Amps” range, across the fuse clips. With no input signal and no load, the quiescent current should typically be around 30mA but may range up to 70mA. Alternatively, you can connect a 100Ω 1W resistor across the fuse clips and measure the voltage across it. For a current of 30mA, the voltage across the 100Ω resistor would be 3V DC. The DC voltage at the output of each channel should be within ±15mV of 0V DC. Next connect suitably rated loudspeakers and check that you can get an output. With no signal, both channels should be very quiet. If you touch the input pin on the PC board you should get an audible “blurt” from the loudspeaker. If the circuit isn’t working, check all the audio paths from the input through to the output for continuity. You should also check that the PC pins are well soldered into position, as is link LK1. If LK1 is open circuit, the SC amplifier will be muted.