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One-Chip Mini 2 x 5W Stereo Amplifier It doesn’t get much simpler than this. With one IC and not much else, you get a stereo 5W amplifier that can operate over a wide range of voltages, from 3.5V to 15V. That means it can run off a single or multiple Li-Po cells, USB power or a lead-acid battery. It’s a small and inexpensive module that can do a range of audio amplification jobs. By NICHOLAS VINEN O UR VARIOUS “chip amplifier” or “champ” projects have been very popular because they’re easy to build and in many cases, are all you need to drive a small (or possibly even large) speaker. The Champion (SILICON CHIP, January 2013) was no exception. Unfortunately, the AN7511 IC we used in that project has been discontinued by Panasonic and has become difficult to obtain. The TDA7266D used in this little module is one of the most common small Class-B audio amplifier ICs available today and is made by ST Microelectronics. While there is a through-hole version, the SMD package variant is easier to get and smaller too. It has large pins so it isn’t too difficult to solder; the only tricky part is the large thermal pad as this can suck away quite a bit of heat while soldering. That does mean it’s fairly effective at taking away heat during operation though, so for most applications, the PCB makes a quite adequate heatsink. This single package provides a ster­ eo bridge-tied load (BTL) amplifier, which is ideal for getting maximum Class-B power into a pair of speakers. For example, it will deliver nearly 1W per channel into 8Ω speakers with a supply of just 4V, as is typically avail82  Silicon Chip able from a single Li-Ion or Li-Po cell. Its performance is pretty respectable too, as you can see from the specs and performance graphs. This IC also has over-temperature protection, output short circuit protection and shut-down/mute functions. We’ve added RCA socket inputs, a volume control and supply reverse polarity protection to complete the package. In comparison to the Mini-D switching amplifier from the September 2014 issue, this amplifier is smaller, simpler and costs less to build. It can also run off lower supply voltages. Of course, the Mini-D does offer substantially more power with higher efficiency and can run off higher voltage supplies. It’s basically a case of “horses for courses”; build the one that best suits your requirements. Circuit description The full circuit is shown in Fig.1. The input signal for each channel, from RCA sockets CON1 & CON2, passes through low-pass filters comprising 100Ω series resistors and 1nF ceramic capacitors to ground. These filters reduce audible hash and noise which may be picked up by the input leads. 10Ω resistors in series with each input ground improve channel isolation. The audio signals are then ACcoupled to the 10kΩ volume control potentiometer(s) with 4.7µF ceramic capacitors, giving a -3dB bass roll-off point around 7Hz. You can either use two trimpots (VR2/VR3) for preset volume or a dual log pot (VR1) for adjustable volume. The bottom end of each pot is AC-coupled to ground to prevent DC current passing through the pots, which would cause audible crackling during volume adjustment. The signals then go straight into IC1’s input pins, pin 7 for the left channel and pin 14 for the right channel. An internal half-supply bias voltage provides the correct DC levels. The internal negative feedback sets the gain to 26dB (20 times). The output signals go straight to terminal blocks CON4 and CON5. The outputs are bridge-tied so neither speaker has a ground connection. In other words, both ends of each speaker is actively driven with anti-phase signals, for maximum power delivery. The components connected to pin 8 (mute) and pin 9 (standby) prevent clicks and pops at power-up. When power is applied, both pins are held low so the outputs are off. Over time, the 10µF capacitor charges and IC1 first comes out of standby mode and siliconchip.com.au CON3 + 3.5-14.4V 4.7 µF 1 470 µF 4.7 µF X7R 16V L/ESR X7R 100Ω 4.7 µF X7R 6 Vcc VR1a/VR2 LED1 Q1 IRFML8244 15 7 IN1 2 CON7 K 10k LOG/10k 1nF COG 1 λ K G S Vcc − POWER A 100k D LEFT INPUT CON1 2 ZD1 15V OUT1+ 2 10k A 4.7 µF 10Ω X7R LEFT SPKR CON4 22k 1 RIGHT INPUT CON2 100Ω OUT1– 5 4.7 µF X7R VR1b/VR3 IC1 IC 1 10k LOG/10k 1nF COG OUT2+ 19 1 2 4.7 µF 88 MUTE X7R 9 9 STBY STANDBY CON6 OUT2– 16 1 2 RIGHT SPKR CON5 TDA7266D TDA7 266D 14 IN2 10Ω 2 Vref Vref 1k 10 µF 16V 47k SIG GND PAD 13 0 PWR PWR PWR PWR GND GND GND GND 1 10 11 20 TDA7266D SC 20 1 4 ZD1 LED1 ONE-CHIP MINI STEREO AMPLIFIER A K D K A 11 IRFML8244 G 10 20 S 1 Fig.1: the circuit of the One-Chip Mini Stereo Amplifier. It’s based on TDA7266D amplifier chip and can deliver in excess of 5W per channel. VR1 is the volume control, while Mosfet Q1 provides supply reverse polarity protection. into active mode, and then once other voltages have stabilised, the outputs are un-muted. If the two pins of CON6 are shorted, eg, by a switch, or if pin 1 is pulled low, the 10µF capacitor will discharge and the unit will mute its outputs and then go into standby mode. In standby mode, the quiescent current drops to a low level. If this short is then removed, the unit powers back up and operates as normal. So CON6 can thus be used to save power when the amplifier isn’t being used and it may be under control of a microcontroller. If you don’t need this feature, you can leave the connector off and the amplifier will simply operate whenever power is available. Each power supply pin of IC1 (pins 6 & 15) has an adjacent 4.7µF ceramic bypass capacitor plus there is a 470µF electrolytic reservoir capacitor. LED1 siliconchip.com.au Features & Specifications • • • • • • • • • • • • • Supply voltage: 3.5-15V • Standby muting: 110dB Quiescent current: 40-60mA Standby current: ~0.25mA Load impedance: 4Ω or higher (see Table 1) Output power: in excess of 5W per channel, thermally limited Gain: adjustable, up to 26dB (20x) Signal-to-noise ratio: ~96dB Channel separation: ~66dB Input impedance: 10kΩ Power supply rejection ratio: ~56dB THD+N, 2 x 1W: typically 0.03% <at> 1kHz, <0.1% 100Hz-5kHz (see Figs.2 & 5) Frequency response: 20Hz-20kHz, ±1dB, typically ±0.1dB (see FIg.3) Other features: power indicator LED, standby, short circuit protection, overtemperature shut-down November 2014  83 Parts List 1 double-sided through-plated PCB, code 01109141, 39 x 63.5mm 2 switched right-angle PCB-mount RCA sockets, white & red (CON1,CON2) OR 2 2-way pin headers or polarised headers, 2.54mm pitch (CON1,CON2) 3 2-way mini terminal blocks, 5.08mm pitch (CON3-CON5) 1 10kΩ 9mm dual gang PCB-mount log pot (VR1) OR 2 10kΩ mini horizontal trimpots (VR2,VR3) 1 2-pin polarised header (optional, for standby function) 6 M3 x 6mm machine screws (for mounting) 3 M3 tapped Nylon spacers (for mounting) Semiconductors 1 TDA7266D 2x5W amplifier IC, PowerSO-20 (IC1) 1 IRFML8244 N-channel Mosfet or equivalent, SOT-23 (Q1) (element14 1857298) 1 15V 500mW zener diode, SOT23 (ZD1) (eg, BZX84-C15; element14 1826097) 1 high-brightness SMD LED, size 3216/1206* (eg, element14 2290350) OR 1 2-way pin header, 2.54mm pitch plus off-board high-brightness LED Capacitors (SMD 3216/1206* ceramic unless stated) 1 470µF 16V low-ESR electrolytic 1 10µF 16V electrolytic 6 4.7µF 25V X7R (element14 1828835) 2 1nF 50V X7R or C0G/NP0 (element14 1414658) Resistors (SMD 3216/1206*, 1/8W, 1%) 1 100kΩ (element14 9241060) 1 47kΩ (element14 9336583) 1 22kΩ (element14 9241027) 1 10kΩ (element14 513222) 1 1kΩ (element14 9240942) 2 100Ω (element14 1632521) 2 10Ω (element14 9335790) * 2012/0805 size also suitable (or an external LED connected to CON7) lights to indicate when power is applied. Mosfet Q1 provides reverse polar84  Silicon Chip Fig.2: distortion versus power with both channels driven, at four different supply voltages. This is valid for brief bursts; at higher supply voltages, the maximum power available will drop due to thermal limiting. Note that a useful amount of power is available even with a supply below 5V DC. Fig.3: the frequency response is very flat above 100Hz. Below that, there is a small amount of bass cut or boost depending on the position of the volume pot(s). This is due to their varying source impedance interacting with IC1’s somewhat reactive input impedance. The effect is not very noticeable. ity protection. The supply voltage is fed in via CON3 and if the polarity is correct, Q1’s gate is pulled up via the 100kΩ resistor, switching it on and making the connection between IC1’s ground and pin 1 of CON3. 15V zener diode ZD1 protects Q1’s gate from high-voltage supply spikes. If the supply is connected the wrong way around though, Q1’s gate is pulled below its source and thus Q1 is off and no supply current can flow. Again, ZD1 protects Q1’s gate from going too far negative. Speaker load & power The Mini Stereo Amplifier can drive speakers of 4-16Ω. However, power is limited at higher supply voltages with lower speaker impedances due to the lower efficiency under these conditions. Table 1 shows the maximum voltage for each typical speaker impedance before there is a risk of thermal shutdown at higher power levels. Maximum contiuous power is available at the upper voltage specified, ie, best power into 8Ω is available at around 9.5V. Under these conditions, it will deliver 2 x 5W at 10% THD+N or 2 x 4W at 1% THD+N. Note that there is nothing stopping you from running the unit from a higher voltage than shown in Table 1 but if you drive it hard, it could oversiliconchip.com.au LEFT IN 10 µF 4.7 µF 1k CON6 22k + A 4.7 µF 100k k LED1 4.7 µF CON3 POWER CON2 (VR3) 4.7 µF Q1 470 µF + RIGHT IN RIGHT SPKR + 10Ω LEFT SPKR IC1 TDA7266D – 1nF (VR2) 4.7 µF10k 100Ω STANDBY CON4 + – 10Ω 100Ω 1nF 1 VR1 + 47k 4.7 µF CON1 CON5 ZD1 15V Fig.4: install the parts on the PCB as shown here. Most of the parts are SMDs and the procedure for installing them is described in the text. Be sure to fit IC1 and the two electrolytic capacitors with the correct polarity. Table 1 Load Impedance 4Ω 6Ω 8Ω 16Ω Supply Voltage 3.6-6.5V 3.6-8.5V 4-9.5V 6-14V heat and shut down briefly. Normal operation will resume once the chip has cooled. The IC can typically dissipate 5W total before its die reaches 150°C and it shuts down. Construction The amplifier is built on a doublesided PCB coded 01109141 (39 x 63.5mm). Most of the components are SMDs but there are a few through-hole parts involved too. Fig.4 shows the assembly details. Start by fitting IC1, the amplifier chip. Because it’s relatively large and the board is its heatsink, you will need a relatively powerful (and/or hot) iron to do this. The simplest technique involves little more than a typical soldering iron and some flux paste. First, tin the large mounting pad on the PCB with a thin layer of solder. Spreading a little flux paste on this pad before adding solder will help spread it out. Keep the solder thin and even; if you add too much, remove the excess using some solder wick (again, flux paste helps). Once you’re happy with that, tin the underside of the IC in the same manner. You may need to hold it in some sort of clamp or vice while doing so. Then spread a little flux paste on the tinned PCB pad and place the IC on top, ensuring that its orientation is correct, ie, the notched corner (pin 1) is in the upper-right corner, near the siliconchip.com.au Fig.5: distortion versus frequency at 1W with both channels driven into 8-ohm loads. This is similar to the curve shown in the TDA7266D data sheet. The distortion is mostly due to crossover artefacts. dot on the PCB overlay. Push it down to make sure it’s in intimate contact with the board. Line the IC up with its pads, place a dab of flux paste on one of the pins, then put a little solder on the tip of your soldering iron and touch the pin gently, without disturbing the IC. The flux paste should help “suck” the solder onto that pin and pad. Check the IC alignment and if necessary, reheat the joint and gently nudge it into place. Once the alignment is good, use the same technique to solder the diagonally opposite pin. It’s then simply a matter of heating the main tab under the IC until the two layers of solder melt and solidify into a single mass. Make sure the board is on a heat-resistant surface, then place the soldering iron tip on one of the exposed pads at either end of the IC and melt some solder wire in, to “wet” the joint and help transfer heat. Hold the iron in place until you see a puff of vapour from the liquefying flux under the IC and the solder at the opposite end of the thermal pad re-flows. Note that this procedure could take continued on page 101 November 2014  85 drive need only be 100mA to switch the transistor on. Alternatively, we recommend our High Energy Ignition that uses an IGBT as the switching transistor, as published in November and December 2012 issues. This IGBT could also be used as a replacement transistor and the gate drive need only be via a 1kΩ resistor (that has little dissipation). The IGBT is an ISL9V5036P3-F085. The MJ10012 is available from Jaycar Electronics (Cat ZT-2222) although the H version may suit your 3-leg requirements better. The IGBT is available from SILICON CHIP – see www.siliconchip.com.au/Shop/7 and search for the High Energy Ignition. Circuit confusion with switching regulator I recently purchased a Jaycar kit for the simple 1.5A Switching Regulator, but have yet to construct it (SILICON CHIP, February 2012). On checking the circuit diagram, I was confused by the orientation of the IRF9333 Mosfet (Q1) used to switch the regulator on. I recently received the EPE article Notes & Errata Burp Charger for Nicad and NiMH Batteries, March 2014: disconnecting power using switch S1 allows current flow from the supply into IC3 and Mosfet Q2 via the 0.1Ω and 1kΩ resistors. This causes Q2 to switch on and so battery current flows through the 0.1Ω resistor. This problem can be solved by removing S1 and bridging the two switch contact points on the PCB. Power is then switched either via the DC plug or at the input power source. It is also recommended to connect a 100kΩ resistor between pins 6 & 7 of IC3b. This prevents a possible partial conduction of Q2 in an especially low switch-on threshold Mosfet. This resistor can be placed photocopy, which is identical to the SILICON CHIP article. My question is that being a P-channel Mosfet, the supply voltage at the drain connection should be negative with respect to the source connec- Mini 2 x 5W Stereo Amplifier . . . from p85 a while (up to about 30 seconds), simply because of the mass which needs to be heated. If it takes longer than 30 seconds then your iron may not have enough power. By the way, don’t touch the IC or board during this procedure or immediately afterwards as it will be stinking hot! It’s then just a matter of soldering the remaining IC pins and cleaning up any bridges using solder wick. Refresh the joints on the first two pins you soldered, too. Adding flux paste is recommended for both procedures; when soldering the pins, it reduces the chance of bad joints. Clean off any excess flux using an appropriate solvent (metho will do in a pinch) and check the joints under magnification to ensure that solder has flowed properly onto every pin and pad. Remaining SMDs You can now proceed to fit all the smaller SMDs. Start with the two 3-pin SOT-23 packages. It’s simply a matter of flowing a little solder onto one of the pads, heating that solder while sliding the device into place (using siliconchip.com.au tweezers, for example), soldering the two remaining pads and then refreshing the solder on that first pad. Don’t get the two devices mixed up. If using the on-board power indicator LED, fit it now. First, you will need to check its orientation. Unfortunately, markings for SMD LEDs are not consistent. Some DMMs will light a LED in diode test mode, with the red lead indicating the anode but if your DMM won’t light it in either direction, you may need to connect the LED to a 9V battery via a 1-10kΩ series resistor. Orientate the LED so that the anode (positive) terminal goes to the pad marked “A” and then use a similar method as above to solder it in place. Try not to overheat it but do make sure that the solder has flowed properly onto the ends of the LED and the pads. Once again, flux paste is your friend. Now proceed to fit all the surfacemount resistors and capacitors. Use the same method as described above. The resistors will have their values printed on top although you may need a magnifying glass to see them. The capacitors are unmarked, however the under the PCB across the terminals of the 100nF capacitor that also connects between pins 6 & 7 of IC3b. Bistro Paging System, Circuit Notebook, October 2014: the column connections to the keypad are reversed, ie, the “star” key should be column 1 and pin 3 while the “hash” key should be column 3 and pin 5. The part number above the keypad is for the piezo buzzer not the keypad; the correct keypad number is Jaycar SP0770. The part number (Jaycar AB3452) for the 400Hz buzzer was also omitted; this specifies the correct electromechanical or solid state style buzzer compatible with a micro output. tion, should it not? In this circuit, it is positive, ie, it is illustrated the wrong way round. The correct orientation is shown in the May 2012 Solar Panel Lighting Controller circuit diagram, where an IRF9540 P-channel power 10nF capacitors will be much slimmer than the 4.7µF types. Through-hole parts Dovetail two of the terminal blocks and fit these for CON4/CON5, with the wire entry holes facing the righthand edge of the board. Also fit terminal block CON3 with the wire entry at the bottom. If using the dual-gang log pot, it can go in now, otherwise solder the two mini horizontal trimpots in place. You can now complete the assembly by fitting the two electrolytic cap­ acitors (longer leads towards the “+” symbols at right), the RCA sockets and the pin header(s), if you require them. Testing & use There isn’t much to testing the board. First, hook up an appropriate power supply with current metering (or connect a DMM in amps mode in series with one of the supply leads). Check that the quiescent current is less than 75mA and that LED1 lights. It’s then just a matter of turning down the volume control, hooking up a signal source and speakers and checking that the audio from both channels is clear and undistorted. SC November 2014  101