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At last, a worthy adversary to vanquish the CHAMP and Pre-CHAMP! The Champion . . . a tiny audio amplifier module that can deliver 7W peak power! By Nicholas Vinen Have you built one of our CHAMP or PreCHAMP modules? They have been extremely popular but we have to admit that their audio performance is (was?) pretty ordinary. Now there’s a new kid (module) on the block and we have dubbed him the Champion because his performance is far superior. In fact, he can deliver up to 7W of peak power whereas the now-deposed CHAMP would struggle to deliver more than half a watt. B OTH THE CHAMP and Pre-CHAMP have had a very long run since they hit the scene in the February & July 1994 issues of SILICON CHIP. In fact, many thousands of each have been built. That’s because they were cheap, easy to build and did the basic job required of them – to give just about anything the ability to drive a speaker and make a sound, be it a radio, sound effects generator, music player, communications receiver – whatever. But while they will no doubt continue to be popular, they are now over-shadowed by our new module, the “Champion”. 24  Silicon Chip It dances all around the CHAMP, evading all of its jabs and delivering a knock-out combination of convenient connectors, higher power, lower minimal operating voltage, much lower distortion and noise, mute and standby features and input mixing. Actually, the Champion doesn’t have it all over the CHAMP. Sometimes the old guys have a few tricks up their sleeves. The young and energetic Champion is a bit hungry, with a quiescent current of around 30mA while the older and cunning CHAMP picks at its meals with a quiescent current of just 4mA. Still, the Champion again wins out because it has a logic-level standby control pin to shut it down to a negligible 1µA! New amplifier IC The heart of the Champion is the AN7511 audio amplifier IC from Panasonic. The CHAMP’s LM386 was born in the mid-1970s and that makes it an old geezer by now. By comparison, the AN7511 isn’t even a teenager yet, having been released in late 2001. One of the main advantages of the AN7511 over the LM386 is the fact that it drives the speaker in a bridge-tied load (BTL) configuration. This allows siliconchip.com.au the IC to deliver twice the RMS voltage to the speaker, for up to four times the power. Thus, as already noted, the Champion punches well above its class, giving around 7W peak power into an 8Ω load from a 12V supply. Mind you, the Champion can’t deliver that sort of power continuously. The small DIP chip package simply can’t deal with the dissipation under those conditions continuously and thermal limiting quickly kicks in, even if a heatsink is fitted. The continuous power available (depending on supply voltage) is around 2W. That’s still quite a bit better than the LM386. The LM386 also needs more external components than the AN7511, despite having fewer features. The LM386 needs a “Zobel network” at its output (resistor and capacitor) for stability whereas the AN7511 doesn’t. The LM386 also needs a large DC-blocking capacitor between its output and the speaker but because the AN7511 drives the speaker in bridge mode, no DC blocking capacitor is required. All we really need to build a working circuit around the AN7511 is a bypass capacitor, AC coupling for the signal input and some RC filters for the mute and standby control pins. In standby mode, the AN7511’s current consumption drops to just 1µA so if used in combination with (say) a microcontroller, the AN7511 won’t draw any power unless you are actually using it. The mute and standby Features & Specifications Features • • • • • • • • Wide operating voltage range Bridged output gives high power at low supply voltages Low parts count Low distortion Preamplifier compatible with microphones & electric guitars Preamplifier has two inputs, mixed 1:1 Mute and standby control Over-temperature protection (auto-limiting) Specifications Operating voltage range: 4-13.5V Output power: up to 4W continuous (see Fig.3); 7W peak Music power: 3W <at> 9-12V Signal-to-noise ratio: ~65dB Frequency response: -2.5dB <at> 20Hz, -0.3dB <at> 20kHz (see Fig.5) THD+N, 1kHz: ~0.25% (see Fig.4) Gain: 34dB for Champion, up to 58dB with Pre-Champion Input sensitivity, Champion only: 52mV RMS <at> 5V, 125mV RMS <at> 9-12V Input sensitivity, Pre-Champion + Champion: 2mV RMS <at> 5V, 5mV RMS <at> 9-12V Quiescent current: 2mA (Pre-Champion) + 30-60mA (Champion) Standby current: 2mA (Pre-Champion) + 40-120µA (Champion) features are designed to avoid clicks and pops when the unit goes into and out of standby, too. The Pre-Champion As good as the Champion is, we know that many readers will want a companion preamplifier to go with it, just as the CHAMP had the Pre-CHAMP. But whereas the Pre-CHAMP was a very basic 2-transistor circuit, the preamplifier for the Champion is a special low-voltage op amp IC that has considerably better performance. This will enable you to use the Champion with a microphone or many musical instruments, such as electric guitars. We have designed a small PCB to Australia’s Lowest Priced DSOs Shop On-Line at emona.com.au Now you’ve got no excuse ... update your old analogue scopes! Whether you’re a hobbyist, TAFE/University, workshop or service technician, the Rigol DS-1000E guarantee Australia’s best price. RIGOL DS-1052E 50MHz RIGOL DS-1102E 100MHz 50MHz Bandwidth, 2 Ch 1GS/s Real Time Sampling 512k Memory Per Channel USB Device & Host Support 100MHz Bandwidth, 2 Ch 1GS/s Real Time Sampling 512k Memory Per Channel USB Device & Host Support ONLY $ Sydney Melbourne Tel 02 9519 3933 Tel 03 9889 0427 Fax 02 9550 1378 Fax 03 9889 0715 email testinst<at>emona.com.au siliconchip.com.au Brisbane Tel 07 3275 2183 Fax 07 3275 2196 362 Adelaide Tel 08 8363 5733 Fax 08 8363 5799 inc GST Perth ONLY $ Tel 08 9361 4200 Fax 08 9361 4300 web www.emona.com.au 439 inc GST EMONA January 2013  25 Parts List Pre-Champion 1 PCB, code 01109121, 57 x 41mm 1 10kΩ log PCB-mount 16mm potentiometer (VR1) OR 1 10kΩ mini horizontal trimpot (VR2) 4 mini 2-way terminal blocks (CON1-CON4)* 1 8-pin DIL socket 4 M3 x 10mm tapped Nylon spacers 4 M3 x 6mm machine screws Semiconductors 1 LMC6482 dual op amp (IC1) (Jaycar ZL3482) 1 LP2950-5 5V LDO regulator (REG1) (Jaycar ZV1645) 1 1N5819 schottky diode* Capacitors 2 100µF 16V electrolytic 2 10µF 16V electrolytic accommodate the Champion and its companion preamp. If you don’t need the preamp, you can cut off that section to make the PCB quite a bit smaller. Circuit description Fig.1 shows the complete circuit of both the Pre-Champion (left) and Champion (right). The signal is applied to either CON2 or CON3. If you apply a signal to both, they will be mixed together with a 1:1 ratio, ie, the apparent volume of both signals will be the same. This could be useful, for example, if you want to down-mix stereo to mono or if you want to combine music and voice. The two signal paths are identical until they are mixed. Each signal passes through a lowpass filter consisting of a 100Ω resistor and 100pF capacitor, designed to attenuate RF signals. There is also a 2.2MΩ bias resistor to pull the input signal to ground. If you are going to feed the unit with an iPod or similar player you may need to reduce the value of that 2.2MΩ resistor dramatically, to say 1kΩ, to provide it with sufficient load current. However, as presented, the high-impedance inputs will suit microphones and some musical instruments, as well as general line-level signals. The signals are then AC-coupled with 100nF capacitors and 2.2MΩ bias 26  Silicon Chip 3 100nF MMC 2 100pF ceramic 2 10pF ceramic Resistors (0.25W, 1%) 4 2.2MΩ 2 2.2kΩ 2 22kΩ 2 1kΩ 2 10kΩ 2 100Ω Champion Amplifier 1 PCB, code 01109122, 42 x 41mm 4 mini 2-way terminal blocks (CON5-8)* 1 micro-U TO-220 heatsink, 12.7 x 19mm (Futurlec Cat. TO220S, Rockby Cat. 36255, Altronics Cat. H0628 [with pin cut off]) 1 TO-220 heatsink pad or thermal transfer compound 4 tapped nylon spacers 4 M3 x 6mm machine screws 1 M3 x 10mm machine screw 2 M3 nuts resistors which go to a 2.5V half-supply rail. This biases the incoming signal so that it has a symmetrical swing within the supply rails of dual op amp IC1, running off a 5V rail. The two 2.2MΩ bias resistors for each channel, on either side of the 100nF AC-coupling capacitors, are in parallel as far as the signal source is concerned, setting the unit’s input impedance to around 1MΩ. IC1a buffers and amplifies the signal from CON2 while IC1b does the same for the signal from CON3. Gain is set at 23 times (27dB) by the 22kΩ and 1kΩ feedback resistors. The 10pF capacitors reduce the gain for high-frequency signals, giving a little extra stability and noise filtering. Note that this high gain suits relatively low level signals such as those from microphones or musical instruments. To feed the unit with line-level signals, you will either need to knock back the gain for that channel by reducing the value of the 22kΩ feedback resistor or else connect the signal to its respective input via a potentiometer. The latter solution is probably the best one. It not only provides for a wide range of input signal levels but also lets you adjust the ratio by which the two audio input signals are mixed (eg, by using a similar arrangement to that shown in Fig.6). 1 M3 split washer 2 M3 shakeproof washers Semiconductors 1 AN7511 bridge output amplifier (IC2) (Jaycar ZL3388) 1 BC557 PNP transistor (Q1) 1 1N5819 Schottky diode* Capacitors 1 470µF 16V electrolytic 1 10µF 16V electrolytic 1 1µF 16V electrolytic 1 470nF MMC 1 100pF ceramic Resistors (0.25W, 1%) 1 1MΩ 3 10kΩ 2 100kΩ 1 100Ω * If building both the Pre-Champion and Champion on a single PCB, omit one 1N5819 diode and four 2-way terminal blocks The outputs of the two op amp stages are mixed using a pair of 2.2kΩ resistors and then AC-coupled to potentiometer VR1 or VR2, depending on which is installed. One is a trimpot and the other is a full-size pot. Regardless of which is installed, they do the same job, allowing the output level to be adjusted. The 100µF coupling capacitor is specified for good low-frequency performance as this capacitor forms a high-pass filter, in combination with the pot’s track resistance (10kΩ). The LMC6482 dual op amp was chosen for this application because it can run off low voltages and has a rail-to-rail output swing. For example, when running from 5V, its output can be over 1.5V RMS while a standard op amp would be limited to about 0.5V RMS if it could operate from 5V at all. The aforementioned 2.5V rail, which effectively acts as the signal ground in this circuit, is derived from the 5V supply rail by a pair of 10kΩ resistors acting as a 1:1 voltage divider. This rail is filtered with a 100µF capacitor, to reduce noise and keep its impedance low so that the feedback dividers can work effectively. IC1 is powered via an LP2950 5V low-dropout regulator (REG1). This regulator is fed from either CON1 or CON8 via Schottky diode D1 or D2 siliconchip.com.au siliconchip.com.au OUT K 1N5819 A CON5 * ONLY ONE OF VR1 (16mm POT) OR VR2 (TRIMPOT) TO BE INSTALLED VR1* 10k LOG VR2* 10k 100 F CON4 PREAMP OUT AMP IN 1 2 100 1M 470nF 100pF 100k C Q1 BC557 2.2k 2.2k C B E 10 F 1 F 10k B E GREEN DASHED LINE INDICATES WHERE BOARD CAN BE CUT TO SEPARATE AMPLIFIER AND PREAMPLIFIER SECTIONS. NOTE THAT IF BOARD IS NOT CUT APART CON1, D1, CON4 AND CON5 CAN BE OMITTED. 100 F SC 2013 2.2M 100pF 2.2M CON3 CHAMPION AMPLIFIER & PREAMP +2.5V 10pF 1k IC1b 22k 7 IC1: LMC6482 5 6 2 2.2M 100nF 100 1 PREAMP IN2 10pF 1k 2.2M 2 1 100 PREAMP IN1 CON1 4 2 100pF 100nF 10 F 2 CON2 8 3 10 F IC1a 22k 1 100nF +2.5V 10k OUT GND IN + 9-12V DC – Fig.1: complete circuit diagram for the Pre-Champion (preamplifier) and Champion (amplifier). Dual op amp IC1 provides some gain for microphones and musical instruments connected to inputs CON2 and/or CON3. The signals are then mixed and either potentiometer VR1 or VR2 is used to adjust the volume. The signal then passes to the amplifier section at right, where IC2 provides a further 34dB of gain and drives the speaker in bridge mode. 3 Mute Gnd 4 IN 2 5 100k 1 SBY Vcc OG BC557 7 8 6 Out+ IC2 AN7511 Out– 10k 470 F 16V 10k IN GND LP2950 CON7 1 2 SPEAKER OUT MUTE 2 CON6 STANDBY 1 AMP CONTROL 1 CON8 9-12V DC – + AMP PWR A K V+ 10k +5V REG1 LP2950 V+ K D1 1N5819 1 A The signal from the volume control pot is fed via CON5, an RF filter network (100Ω/100pF) and a 470nF capacitor to IC2, the AN7511 chip input. This time, the input bias resistor is 1MΩ and there is no bias resistor at input pin 2 of IC2 since it has internal biasing (30kΩ to ground). The combination of the 470nF coupling capacitor and a 30kΩ input impedance gives a low-frequency roll-off of -3dB at around 11Hz. The balanced outputs from IC2 are at pins 6 and 8. The pin 6 output signal is in-phase with the input signal, while the pin 8 output is inverted. The overall gain is typically 34dB, so a 30mV input will give an output of around 1V RMS or 125mW into 8Ω. Note that due to this bridged output configuration, the recommended minimum speaker impedance is 8Ω. Pin 1 of IC2 is the standby input (SBY) which, if pulled low, shuts down the amplifier and puts IC2 into a low-power mode where it consumes around 1µA rather than the typical quiescent current of 30-60mA. This can be controlled using an SPST switch or by a microcontroller. The 10µF capacitor from pin 1 of IC2 to ground, combined with the associated 100kΩ resistor, forms a “soft start” circuit which prevents clicks and pops from the speaker when power is first applied. The 10µF capacitor is initially discharged and so pin 1 is held at ground, enabling the standby feature. This capacitor charges through the 100kΩ resistor and so IC2 comes out of standby a short time after power is applied, when the circuit voltages have had time to settle. Similarly, the 10kΩ resistor from pin 1 of CON6 to pin 1 of IC2 limits the rate at which the shutdown feature is enabled, preventing a sudden transition which would cause the output to also generate a transient, resulting in a loud sound from the speaker. Note that these resistors consume some additional current in standby mode (VCC ÷ 110kΩ), giving a total standby current of up to 120μA at maximum supply voltage. There is also a separate mute input at pin 4 of IC2. This allows the output PREAMP POWER Amplifier D2 1N5819 2 which protect against reversed supply polarity (note: D1 is not installed if the preamp is built on a single PCB with the amplifier). January 2013  27 5819 * FIT EITHER VR1 OR VR2, NOT BOTH Q1 10k 1 F CON7 470 F BC557 + + Speaker Power CON8 IC2 AN7511 10k Mute + + 5819 + + 100k REG1 LP2950 100pF Input Standby 100k 10k + CON5 Out CON4 22k 10 F + + Power Champion 01109122 D1 10pF 470nF D2 100 F VR2* 1k 10k + CON6 100nF 10k + VR1* 10pF tor plus a Schottky diode for reverse polarity protection. If the two units are built on a single PCB, power can be applied to CON8 for both the Champion and Pre-Champion. In this case, CON1 and D1 may be omitted. CON4 and CON5 can also be left out as the output tracks from the Pre-Champion feed straight into the input of the Champion. (OPTIONAL HEATSINK) 1M 100 + CON1 1k 2.2M 2.2M 100 F CON2 IC1 100 100pF + 100nF 22k CON3 2.2M 2.2M 2.2k 100nF LMC6482 2.2k + 100pF100 In 2 In 1 2013 10 F 01109121 Construction The PCB measures 100 x 41mm and is coded 01109121/2. If you wish to build the Champion and its preamplifier separately (or build just one of these), cut the board between the dashed lines using a hacksaw. The following instructions apply whether you are building one or both of the PCBs; simply repeat for each separate board. Fig.2 shows the parts layout on the PCB. Start by fitting all the resistors. A colour code table is provided for convenience but you should check each one with a DMM before fitting it as some colours can be difficult to distinguish. Follow with D2 but note that D1 will also have to be fitted if you build the preamp separately. Make sure that the diode(s) are orientated as shown. Next, fit the ICs with the pin 1 dot or notch in the direction shown, ie, towards the top of the PCB. You can use a socket for the op amp but for best heat dissipation, the AN7511 should be soldered directly into circuit. Make sure that it’s sitting all the way down CUT BOARDS APART HERE IF REQUIRED Fig.2: PCB overlay diagram for the Pre-Champion (left) and Champion (right). Potentiometer VR1 can be used for an externally accessible volume control or trimpot VR2 can be fitted instead for a one-time adjustment. A small heatsink is normally fitted to amplifier IC2 as it can dissipate quite a bit of power at higher supply voltages and output power levels. Below: the PCB should only take about an hour to assemble. Take care with component orientation and don’t get the ICs mixed up. to be shut off while leaving the amplifier running, in case you just want to temporarily shut off the sound. This, however, is an active high function, ie, pin 4 is pulled up to VCC to enable the muting. For convenience, we have arranged the circuit so that the two control inputs at CON6 are both active-low and can be driven in the same manner. The capacitor from pin 4 (mute) to ground is a lower value than for standby, at 1µF, but the 100kΩ pulldown resistor is the same value as the 100kΩ pull-up resistor for the standby pin. This ensures that when power is removed, the mute function engages before the amplifier goes into standby, preventing switch-off thumps. IC2 has its own 100µF bypass capaci- Table 2: Capacitor Codes Value µF Value IEC Code EIA Code 470nF 0.47µF 470n 474 100nF 0.1µF 100n 104 100pF NA 100p 101 10pF NA   10p   10 Table 1: Resistor Colour Codes o o o o o o o o o No.   4   1   2   2   2   2   2   2 28  Silicon Chip Value 2.2MΩ 1MΩ 100kΩ 22kΩ 10kΩ 2.2kΩ 1kΩ 100Ω 4-Band Code (1%) red red green brown brown black green brown brown black yellow brown red red orange brown brown black orange brown red red red brown brown black red brown brown black brown brown 5-Band Code (1%) red red black yellow brown brown black black yellow brown brown black black orange brown red red black red brown brown black black red brown red red black brown brown brown black black brown brown brown black black black brown siliconchip.com.au THD vs Power, 1kHz, 8Ω, 22kHz BW 11/27/12 10:21:06 10 5 5 2 2 1 1 THD+N % THD+N % with Pre-champion 10 0.5 0.2 0.5 0.2 0.1 0.1 4V supply 5V supply 6V supply 0.05 7.5V supply 9V supply 12V supply 4V supply 5V supply 4V no preamp 5V no preamp 0.05 0.02 0.02 .2 .5 1 Power (Watts) 2 Fig.3: distortion vs power for the Pre-Champion/ Champion combination for various supply voltages. With a higher voltage supply, the power output increases and distortion drops except for 12V. This curve is unusual because the increased dissipation resulting from the higher supply voltage causes thermal overload and the chip’s self-limiting kicks in, reducing the power output to prevent damage. Maximum continuous power is therefore at a lower voltage, ie, around 9V. on the PCB before soldering its leads, otherwise the heatsink won’t mate properly when it is fitted later on. Next fit the LP2950 regulator (REG1) and the BC557 transistor (Q1). You may need to bend the leads with small pliers to match the pad spacing on the PCB. Follow with all the ceramic and monolithic ceramic (MMC) capacitors. The 2-way terminal blocks are next. These must be installed with their wire entry holes towards the adjacent outside edge of the PCB. There are four per board and this holds true even if you are building the two sections as a single unit. In other words, if you are building a single unit, leave out the terminal blocks in the middle of the combined PCB (ie, CON1, CON4, CON5 & CON6). The next step is to decide whether you want to fit potentiometer VR1 or trimpot VR2 to adjust the volume from the Pre-Champion (you can fit one or the other but not both). If you intend using trimpot VR2 to set the volume, solder it in now. You can then fit all the electrolytic capacitors, except for the 470µF unit. In each case, the longer (positive) lead goes into the hole marked with a “+” sign. The Pre-Champion section of the board can now be completed by fitting potentiometer VR1 (if this is to siliconchip.com.au 0.01 5 20 50 100 200 500 1k 2k Frequency (Hz) 6V supply 7.5V supply 9V supply 12V supply 5k 10k 20k Fig.4: distortion vs frequency with a number of different supply voltages. As is typical, distortion increases with frequency. The output power level is 500mW in each case except with the 4V supply, where the output power is 200mW. Note that the THD performance vs frequency is much the same for the various supply voltages. Refer to the panel on the following page for an explanation as to why distortion increases at low frequencies when the preamp is used. Fig.5: the frequency response for the Pre-Champion and Champion combination. It’s quite flat from 20Hz20kHz, being down by just 0.3dB at 20kHz and around 2.6dB at 20Hz. +3 Frequency Response, 0.5W, 80k BW 11/27/12 09:57:12 +2 +1 0 Relative Power (dBr) 0.01 .1 THD vs Frequency, 10W, 80kHz BW 09/28/12 12:37:20 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 10 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) be used instead of VR2), plugging op amp IC1 into its socket and attaching M3 x 10mm tapped Nylon spacers to the corner-mounting positions using M3 x 6mm machine screws. Attaching the heatsink For the Champion, the next step is to attach the heatsink. This is not strictly necessary but allows for a higher average output power level before the chip goes into thermal limiting. A small TO-220 heatsink is specified and this is clamped on top of the DIP package. To do this, start by passing an M3 x 10mm machine screw up from 100k the underside of the board, through the hole next to IC2. Fit a nut to hold this screw place, then place a split washer on top of this nut and then a shakeproof washer. Next, spread a little thermal transfer compound on top of the IC. Alternatively, you can use a TO-220 thermal pad to ensure efficient heat transfer. This thermal pad is simply fitted over the screw shaft and pushed down so that it sits on top of the DIP chip. That done, place the heatsink on top of the chip and thread a second nut onto the end of the screw. Do it up tightly with small pliers, while January 2013  29 5819 1 F 470 F BC557 CON7 + + Speaker Power Champion 01109122 10 F Q1 10k + D2 Mute + + CON8 10k 100pF Input Standby 100k 10k 10k LOG POT IC2 AN7511 470nF 100k + CON6 LINE LEVEL SIGNAL SOURCE (OPTIONAL HEATSINK) 1M 100 CON5 1 F MKT CAPACITOR Fig.6: for higher level signals (eg, line level), the Champion can be used on its own. In this case, you will normally still need a volume control, wired as shown here. Even if the volume will be fixed, you will still usually need a pot otherwise higher level signals will overload the input. holding the heatsink so it can’t rotate. Check that the heatsink sits flat against the IC when the nut is fully tightened. If it’s sitting proud, then remove the star washer. Now solder the 470µF capacitor in place. This will be too large to fit right down onto the PCB, especially with the heatsink alongside. If so, it can just sit on top of the adjacent diode. Simply angle its leads down through their holes, then push it down as far as it will go before soldering it in place. Finally, fit the tapped spacers to this PCB using M3 x 6mm machine screws and the PCB assembly is complete. Wiring it up For the speaker, simply connect its two wires to the corresponding terminals on CON7. The polarity doesn’t matter if you are building a single Champion. If using two modules for stereo, ensure that the speaker polarity is the same for each. The DC power is fed in via CON8; ie, if you have a combined Pre-Champion/ Champion then both units get power from the same connector. If you are using the Champion by itself, you will probably need to fit some kind of volume control/input attenuator. This can be arranged using a 10kΩ log pot and a 1µF capacitor as shown in Fig.6. Even if you don’t need an externally adjustable volume, it’s still a good idea to have this pot in order to match the input signal level to the input sensitivity of the Champion. For a stereo amplifier, you can use a dual 10kΩ log pot. The mute and standby pins of CON6 can be left open, in which case the amplifier will run while ever power is applied. If you do want to use either or both of these control inputs, simply pull that pin to ground to activate the associated feature. Remember that both pins normally sit near VCC so if you want to drive them with a micro and it’s running off a different supply, then you will need to drive these pins using NPN transistors. Be sure to use shielded cable to the signal input(s), especially for the Pre-Champion as its inputs are very sensitive and will otherwise pick up noise and possibly also mains hum. If using the Pre-Champion separately, you will also need to apply power to Smear thermal grease over the top of the audio amplifier IC (or use a TO-220 thermal pad) before fitting the heatsink. Make sure the heatsink sits flat against the IC when the mounting nut is tightened (see text). CON1 and then run the output from CON4 to the Champion’s input, again using shielded cable. Note that if you’re using only one input on the Pre-Champion, it’s a good idea to short the other one out with a wire link or low-value resistor, as this reduces the output noise. If the gain of the Pre-Champion is too high, it can be reduced by reducing the two 22kΩ feedback resistors. The gain is calculated as R ÷ 1kΩ + 1 (where R is the feedback resistor value), so if you use say 2.2kΩ resistors, then you get a gain of 2.2kΩ ÷ 1kΩ + 1 = 3.2. Microphone bias current The Pre-Champion is a relatively simple design and doesn’t have onboard support for balanced microphones, electret bias current and so on. However, most mics have a built-in power supply or require no bias, in which case you can just connect them straight to one of the inputs If you do want to use an electret, you could wire a 10kΩ resistor between the regulator’s +5V output pin and terminal 1 (the upper terminal) of SC either CON2 or CON3. Low-Voltage Performance If you look at the graph of distortion vs frequency (Fig.4), you will see that for supply voltages below 6V, there is a large increase in distortion at signal frequencies below 1kHz. This is only an issue if you are using the Pre-Champion; if you look at the low supply voltage distortion figures without the preamp (orange and lavender lines), it is actually quite good. The reason for this is that when the supply is below about 5.3V, REG1 enters dropout and this allows ripple on the supply 30  Silicon Chip line, due to the current demand of amplifier IC2, to affect the operation of op amp IC1. It only has a limited amount of supply rejection and so a small amount of the supply ripple makes it though to its output. This is further amplified by IC2, producing the relatively large amount of distortion. The easy solution, if you are going to use the Champion and Pre-Champion at 5V or below, is to change REG1 to a 3.3V LDO regulator such as the LM2936-3.3 (Jaycar Cat. ZV1650). The pin-out is identical so it’s just a matter of substituting one for the other. The supply rail for IC1 should then remain in regulation down to the minimum supply of 4V. This will reduce the maximum signal handling of the Pre-Champion but you are only likely to be using this combination with low-level signal sources anyway (eg, microphones) so it should not be an issue. It will not affect the amount of power the amplifier can generate, nor should any other circuit changes be required. siliconchip.com.au