Fullscreen HTML5Med Res (??MB)HTML4

Getting lots of power from an amplifier when you only have 12V to play with Powerful, 12V Mini Stereo Amplifier by JOHN CLARKE Many commercial 12V amplifiers can’t deliver much power, despite sometimes amazing claims to the contrary. This little stereo amplifier can: up to 18W per channel into 4Ω speakers and with the added bonus of volume, bass and treble controls. I (wryly!) is when we see consumer t’s small and compact and can de- the output devices with a 12V supply liver quite a punch to your loud- is 6V in the positive direction and 6V “hifi” claiming 50W output or more – and they take six “C” cell batteries. speakers. Controls are simple, with in the negative direction. This equals bass and treble controls which can about 4.25V RMS (6/1.4142). Oh yeah? be used to brighten up your listening Power output equals the square of So how can this stereo amplifier pleasure and a volume control to set the RMS voltage divided by the load produce any more output power? you rocking. resistance (P=I2/R), so we get 4.25 x The trick is to use two amplifiIt makes a great little amplifier for your 4.25 / 4, or about 4.5W. ers, one to push current one direcWalkman, persontion through the al CD or mini-disc speaker and the SPECIFICATIONS player, etc. And it other to push curPower output : see graphs can be operated from rent the opposite see graphs a 12V battery or good Distortion: direction. Tone controls: see graphs 12V power supply. When one amW h e t h e r y o u Frequency response: plifier drives the -3dB <at> 10Hz and 100kHz want an amplifier Sensitivity for full power output: 50mV RMS loudspeaker in for your car, boat, Signal to noise ratio: a positive volt-69dB with respect to full output power caravan or for some age direction, the (20Hz to 20kHz filter, -78dB A weighted) other 12V appli- second amplifier -46dB at 100Hz, -36dB at 1kHz and 10kHz cation, it is often Channel separation: drives the louddifficult to find a speaker in a negdesign which will ative voltage direction. produce very much power output. However, even this 4.5W is a theThis means that the voltage across oretical maximum and the output the loudspeaker is effectively double It is just an unfortunate fact of life that at 12V the very absolute maximum power is more likely to be closer to 3W that of a single amplifier driver. power that can be delivered into a 4Ω due to losses in the output devices of Now from the formula above, we can the amplifier. load is 4.5W. see that doubling the voltage swing efAs an aside, one of those little The reason for this is that the maxfectively quadruples the output power. imum voltage “swing” possible from mysteries of life which make us smile So if we use two typical amplifiers 28  Silicon Chip which on their own can only deliver 4.5W into 4Ω we can expect to obtain about 18W into the same load (but with a 14.4V supply). Again, there are a few limiting factors which mean we cannot get this theoretical maximum without a fair bit of distortion but this little amp delivers about 14-15W before it “hits the hump” and, if you’re prepared to put up with distortion, up to about 18W – a lot better than 4.5W, you would agree! (Having listened to many, many car stereos and ghetto blasters being driven into massive distortion, it’s not unreasonable to suggest that many users don’t care. As long as it’s LOUD!) You might have noticed that we call this a “12V” amplifier yet our tests were done using a 14.4V supply. The 12V is a “nominal” figure. This is quite legitimate because virtually all vehicles run with more than 12V DC when the motor is running. That extra couple of volts is also handy in giving us an extra few watts! and treble controls to the front of the amplifier to improve its versatility. Most components mount onto a single PC board. It’s just the right size for mounting in a small plastic instrument case so you can really dress up your project if you want to. The circuit The circuit for the stereo amplifier is shown in Fig.1. Only the left channel is shown, based on IC2, with the right channel (IC3) pin numbering shown in brackets. Both channels “share” IC1, each using two of its four op amps. Signal is applied to the left channel via the 10µF bipolar capacitor and 10kΩ log volume potentiometer, VR1. Output from the wiper of VR1 is AC-coupled to the non-inverting input of one of IC1’s op amps (pin 10) via a 0.22µF capacitor in series with a 1kΩ AUDIO PRECISION SCTHD-W THD+N(%) vs measured LEVEL(W) 10 30 APR 100 06:34:48 1 Special IC The 12V Stereo Amplifier uses a hybrid IC package which incorporates all the amplifier circuitry into the one unit. All we need to make it fully operational is to add a few extra components, connect up a speaker and input signal, apply power and we have a ready-built amplifier. It could be that simple – but we have added a volume control and bass 0.1 0.1 1 10 20 Here’s proof: with a 14.4V supply and a 4Ω load, the amplifier does indeed produce up to 18W, albeit with a fair bit of distortion. The horizontal scale is output power (in watts); the vertical is total harmonic distortion (THD). MAY 2001  29 Believe it or not, this photo of the inside of the amplifier is actually larger-than-life, so you can get a good idea of just how tiny it is! The photo also shows all the component locations in glorious living colour. stopper resistor. This resistor helps prevent RF pickup. The 100kΩ resistor connects to the half-supply rail, which biases op amp IC1a at mid-supply voltage. This sets up the op amp to provide symmetrical voltage swing about the mid voltage. The tone controls IC1a is connected as a unity gain buffer to provide a low impedance drive to the following tone control circuitry. The tone controls are based on op amp IC1b and potentiometers VR2 & VR3. These pots and their as30  Silicon Chip sociated resistors and capacitors form the feedback between the op amp’s inverting input and output. Each of the bass and treble stages can be considered separately since they are connected in parallel between the signal input following IC1a and the inverting input (pin 2) to IC2 which is a virtual ground. Operation of the bass control is as follows: with VR2 centred, the same value of resistance is connected between the input from IC1a and the inverting input to IC1b as is between IC1b’s output and inverting input. Thus the gain is set at -1 (the minus symbol doesn’t mean less than zero in this case, it means that the output is inverted with respect to the input). The .01µF capacitor has no effect since it is equally balanced across the potentiometer. However, if we move the wiper of VR2 fully toward the input side (toward the IC1a output), the resistance becomes unbalanced and there is a 22kΩ resistance between input and the inverting input to IC1b and 122kΩ (100kΩ + 22kΩ) between the inverting input and output. (We can ignore the other 22kΩ resistor in the wiper as its job is simply to isolate the two pots). Also the .01µF capacitor is across the 100kΩ resistance in the feedback between IC1b’s output and inverting input. Without the capacitor the gain would be -122kΩ/ 22kΩ or -5.5 at all frequencies. The .001µF capacitor and 100kΩ resistance forms a rolloff above 100Hz so that below this frequency the gain remains at -5.5 or 14.8dB but above 100Hz the gain reduces towards -1 as the frequency increases. Thus we have boost at and below 100Hz. When the wiper is brought to the IC1b output side, the resistive gain becomes 22kΩ/122kΩ or -0.18 or -14.8dB. The capacitor is now on the input side and provides less gain at frequencies below 100Hz but with gain increasing to -1 at frequencies above 100Hz. Thus we have bass cut. Various settings of VR2 between these two extremes will provide less boost or cut. The treble section works in a similar manner except that there is now a .0047µF capacitor in series with the input and output. This produces a high frequency boost or cut at 10kHz. The 10pF capacitor between IC1b’s inverting input and output provides high frequency rolloff, preventing instability. The amplifier(s) The High Power 12V Amplifier is based on IC2 (or IC3), a Philips TDA1519A car radio power amplifier module. This incorporates all the complexity found in much higher power amplifiers. It has output protection against short circuits, good supply ripple rejection, overheating protection, reverse polarity protection, overvolt-age shutdown and is protected against static discharge. The best part is that it is virtually indestructible within its limits. The TDA1519A contains the two power amplifiers. The first power amplifier is non-inverting with its input at pin 1 and output at pin 4. The second amplifier is inverting with its input at pin 9 while its output is at pin 6. When both these amplifiers are fed Fig. 1: not much to it, is there? The amplifier modules (one for each channel) do most of the work. Only the left channel is shown here as the right channel is identical. MAY 2001  31 Fig. 2: all the components, including the three potentiometers, mount on the one PC board, so once you’ve finished the PC board off you’re about 90% of the way to sitting back and listening to your handywork! The final wiring diagram is overleaf. the same signal, their outputs have amplified signals which are effectively “mirror images” of each other, or 180° out of phase. This is described as “bridge mode” operation. The pin 3 input is for decoupling of a half-supply rail internal to the amplifier module. Pin 7 is the positive supply input while pins 2 and 5 are the signal and power grounds. Pin 8 is a mute and standby input which selects the amplifier to be ac- tive or on when connected to the pin 7 supply. When pin 8 is open circuit, the amplifier is effectively turned off and the quiescent current drawn by the circuit is around 100µA. This input is best used to provide the on and off switching since pin 8 draws a low current and we can use a lowcost switch. If we were to switch the 12V supply on and off with a switch, then we would need a switch rated at 4A or more. Signal input to the amplifier is applied to both the non-inverting amplifier input and the inverting amplifier input via a 1µF coupling capacitor. Input impedance is 25kΩ for this bridged mode of operation and so the low frequency rolloff is at 6Hz. The series 10Ω resistor provides some protection against RF pickup which could otherwise be amplified by IC2. The 12V supply is decoupled by a 2200µF and 0.1µF capacitor for each amplifier IC. The outputs of IC2 and IC3 appear at pins 4 and 6 and are connected to Zobel networks comprising 10Ω resistors and 0.1µF capacitors. These help prevent instability in the power amplifiers. Construction The front panel components of the amplifier, taken from the rear. Note the green earth wire which solders to the shield, the three pots and back to the PC board. 32  Silicon Chip The 12V Stereo Amplifier is constructed on a PC board coded 01105011 and measuring 117 x 100mm. It is housed in a small plastic instrument case measuring 140 x 110 x 38mm. Begin by installing the wire links and the resistors on the PC board. Use the accompanying resistor colour code Parts List – 12V Amplifier Both the amplifier modules mount on the heatsink/rear panel but must be insulated from it. Note also the insulation on the wires which go through the rear panel to the speaker connectors. You don’t want a short here! table as a guide to selecting the correct value or use a digital multimeter to measure each one. Now insert the PC stakes for the three input terminals, the power inputs and for the switch. Capacitors can be inserted next –take care to correctly orient the electro-lytics with the polarity as shown. Diode D1 and IC1 are also polarised and inserted as shown. The amplifier ICs can be mounted by firstly bending the leads at 90° about 12mm away from the body of the package. These can be inserted into the PC board holes. Do not solder the amplifiers in position yet. The potentiometer shafts are first cut to length, suitable for the knobs used, their distance from the front panel and the type of box (if any) you will be mounting the amplifier in. Before you insert and solder the pots directly into the PC board holes, scrape away a small portion of the coating on the top of each potent-iometer. You will shortly need to solder an earth wire to the pots and it can be very hard to solder to the passivated metal surface. The heatsink We don’t use the rear panel of the case; instead, it is replaced by an integral panel/heatsink made from 1.5mm aluminium. Its dimensions are shown in Fig.5. Before folding the heatsink/panel, you will need to drill holes for the DC socket, the fuseholder, the loudspeaker terminals and mounting screws and for the RCA sockets. Holes are also required for the amplifier ICs. A shield is also required on the base of the case. This can be made from either insulated aluminium foil glued to the case, or from a piece of single- sided PC board. Drill holes to This straight-on view of the rear panel/heatsink gives you a good idea of where the various sockets and the fuseholder are located. By the way, you can connect just about anything from a Walkman or minidisc to a CD/DVD player or tuner into the input sockets. You could even plug an electric guitar in for practice! 1 PC board coded 01105011, 117 x 100mm 1 plastic instrument case, 140 x 110 x 38mm 1 piece of aluminium, 50 x 135 x 1.5mm 1 100 x 115mm single sided PC board or insulated aluminium foil (for shield – see text) 1 10kΩ dual-ganged 16mm log pot (VR1) 1 100kΩ dual-ganged 16mm linear pot (VR2) 1 50kΩ dual-ganged 16mm linear pot (VR3) 1 dual insulated RCA panel socket 1 4-way loudspeaker terminal strip 64 x 17mm 1 SPST rocker switch 1 M203 fuse holder 1 5A M203 fast blow fuse 1 DC power socket 2 TOP3 insulating washers 5 M3 x 6mm screws 6 M3 x 10mm screws 2 M3 x 15 Nylon screws 10 M3 nuts 2 M3 solder or crimp lugs 1 150mm length of 0.8mm tinned copper wire 1 230mm length of green hookup wire 1 80mm length of red hookup wire 1 100mm length of black hookup wire 1 100mm length of blue hookup wire Semiconductors 1 TL074 quad op amp (IC1) 2 TDA1519A or TDA1519C 12V stereo amplifiers (IC2,IC3) 1 IN5404 3A diode (D1) Capacitors 2 2200µF 25VW PC electrolytic 2 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 4 10µF bipolar electrolytic 2 1µF bipolar electrolytic 2 0.22µF MKT polyester 6 0.1µF MKT polyester 2 .01µF MKT polyester 4 .0047µF MKT polyester 2 10pF ceramic Resistors (0.25W 1%) 4 100kΩ 4 22kΩ 2 10kΩ 4 4.7kΩ  4 1kΩ 6 10Ω MAY 2001  33 Fig.3 (left): there’s very little wiring to do as most is taken care of by the PC board. Don’t leave the earth wire out (shown in green) or forget to solder to the pot bodies as your amplifier could be very sensitive to hum and noise. Fig.4 (above): here’s how to mount the power amplifier ICs to both the PC board and the rear panel/heatsink. Fig.5: use this diagram as a template to both cut and fold your rear panel/heatsink but drill the holes for the connectors and fuses first. accommodate the integral standoffs in the base of the case. You will require a securing screw and nut plus a solder lug to make contact with the aluminium foil. For a PC board shield you can simply solder a wire directly to the board. Fit insulation (eg, a sheet of self-adhesive plastic) to the top side to prevent con34  Silicon Chip tact with the amplifier PC board (if you use a PC board for the shield, simply fit it upside down). Drill holes in the front panel for the power switch and pot shafts using the front panel label as a guide to the hole positions. Attach the front panel label in position. Attach the components to the heat- sink making sure that the RCA sockets are insulated from the metal, and stand the loudspeaker terminals off the heatsink with an extra M3 nut. This will allow extra cooling area for the heatsink. Secure the heatsink to the amplifier PC board using the screws and nuts as shown in the amplifier mounting details in Fig.4. The insulating washers are made from TOP3 insulators which are cut to shape. Cut a notch in the side of the washers in the positions required for the securing screws. If you use a mica washer, use heatsink compound between the two mating surfaces of the heatsink and amplifier package. No compound is necessary for silicone washers. Place the shield into the base of the case and the PC board and heatsink into the case. Note which of the inner mounting bushes in the case foul any of the pigtails on the underside of the PC board and cut them off or grind them down. Place the front panel in position. and secure the PC board in place with the corner mounting screws. Wire up the amplifier PC board as shown. We used heatshrink tubing over the loudspeaker terminal wiring to prevent shorting to the case. Also don’t forget to connect the wire which connects to the DC socket, negative Here’s how the heatsink/rear panel looks when folded up and secured to the PC board. The screws which hold the power amps in place also hold the heatsink in place. Drill the holes for these and the input/output sockets before folding. terminal on the PC board, the three pots and the shield. Testing Full testing of the amplifier will require a 12V supply which can deliver This photo shows the scrap of thin aluminium we used to make the shield for the bottom of the plastic case – you could use a scrap of PC board if you wish. Insulate the alumininium with plastic sheet. The doodles are an optional extra. Resistor Colour Codes                             No. Value   4-Band Code (1%)  5-Band Code (1%) 4   100kΩ brown black yellow brown  brown black black orange brown 4   22kΩ red red orange brown  red red black red brown 2   10kΩ brown black orange brown  brown black black red brown 4   4.7kΩ yellow violet red brown  yellow violet black brown brown 4   1kΩ brown black red brown  brown black black brown brown 6   10Ω brown black black brown  brown black black gold up to 4A but a lower current supply can be used for initial testing. Most 12V SLA (sealed lead acid) batteries, even those rated lower than 4A, will deliver 4A for a short time. Also note that most “12V” car battery chargers deliver significantly more than 12V (they have to, to charge!) but more importantly do not include any filtering and so are unsuitable for use as a DC supply. Without a speaker connected at this stage, apply power and check that there is 12V between pins 2 and 7 of IC2 and IC3. This voltage should also be between pins 4 and 11 of IC1. Check for about 6V at pins 3 and 5 of IC1 and at pin 3 of IC2 and IC3. Further testing is done by listening: connect a speaker to the outputs and apply a signal to the input. Turn the volume pot to minimum and apply power. Check that the amplifier can be switched on and off at the power switch and that the amplifier does amplify – ie, the volume control works! Also check that the tone controls operate as expected. Note that if you are using a power supply to drive the amplifier, it may prevent the amplifier delivering full power during transients. If this hap- Capacitor Codes   Value    IEC code    EIA code       0.22uF 220n 224   0.1uF 100n 104   .01uF 10n 103   .0047uF 4n7 472   10pF 10p 10 MAY 2001  35 Figs.6 & 7 (right): Full-size artwork for the PC board and front panel. You can make your own PC board using this artwork (see how in March 2001 SILICON CHIP) or use it to check commercial boards. The front panel artwork can also be used as a drilling template. pens, the signal may go off as the muting voltage threshold is reached when the power supply level drops. This occurs at around 8.5V. Using a 12V battery should allow the amplifier to drive the loudspeakers to full power. The amplifier can be run from slightly higher voltage and will give even more power output if it is. Car electrical systems normally don’t run at 12V, at least healthy ones don’t – most run at 13.8V or even 14.4V when the motor is running. This amplifier is designed to handle that voltage. The absolute maximum voltage rating of the power amplifier ICs is 17.5V so make sure your supply cannot ever exceed this or you may do some permanent damage. Speakers You can use a huge variety of speakers with this little amplifier – in fact, just about anything you can lay your hands on! AUDIO PRECISION FREQRESP AMPL(dBr) vs FREQ(Hz) 20.000 26 APR 100 08:33:40 15.000 10.000 5.0000 0.0 -5.000 -10.00 -15.00 -20.00 20 100 1k 10k 20k This graph shows the tone control responses with full bass boost, full treble boost, full bass cut and no boost. Note the flat response when the controls are set flat. 36  Silicon Chip Even speakers rated at less than the 18W output power can be used, just as long as you don’t wind the wick up too far! The amplifier is designed to use 4Ω speakers and will deliver maximum power into 4Ω. Most car audio speakers are 4Ω for this reason. However, the amplifier will operate quite happily into 8Ω speakers but you will only get half the power output of 4Ω speakers. There is a common misconception that large speakers require more power to drive than small speakers. This is not usually the case – a larger speaker tends to be more efficient than a small one of similar ratings so all else being equal, will sound louder when driven by the same amplifier. If you have an old pair of hifi speakers gathering dust somewhere, try them with this amp – you could be pleasantly surprised at both the volume and the sound quality! SC