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Champion Preamp By LEO SIMPSON You can use this simple unit as a general-purpose stereo preamp or as a dual-channel preamp, with a microphone for one channel and guitar in the other. One channel can have fixed gain while the other is variable with an on-board trimpot or external potentiometer. Better still, it gives good performance and will work over a wide range of supply voltages. A RE YOU ONE of the thousands of readers who built our very popular PreChamp preamplifier from the July 1994 issue? This is still very popular and available as a kit but it is only a single-channel unit and its 2-transistor design is quite basic. With the inexorable march of technology, it is now possible to do much more, in a module which is only a little larger and with the bonus of two channels rather than one. Better still, this 2-channel design draws less current than the PreChamp. I should state at the outset that this 2-channel preamp is not a brand-new design. It is based on the preamp section of the Champion amplifier module which was featured in the January 2013 issue. The major feature of that article was the tiny AN7511 monolithic amplifier chip which can deliver up to 7W peak power, depending on load Main Features •  2-channel preamplifier configurable •  •  •  •  for different inputs Low distortion Low current drain: 2mA Signal-to-noise ratio: ~80dB Operating voltage range: 6-12V with LP2950CZ-5.0 5V LDO regulator; 12-20V with 78L09 9V regulator 74  Silicon Chip and supply voltage. The preamp section might have been seen almost as an afterthought but it would be a pity for it to have passed mostly unnoticed. Which is partly why we have decided to devote an article just to the preamp; that and the fact that we have recently had a number of requests for preamps which would be neatly answered by this design. So what is good about it? First, it can use one of two dual rail-to-rail op amps and these have the outstanding feature of maximum output voltage swing. So, for example, if you have a 9V supply rail, the maximum undistorted output voltage can be within a whisker of 9V peak-to-peak; about 8.5V p-p, to be more precise. That is much better than the old PreChamp design and you don’t have to tweak the input bias to obtain it. Another advantage is that the spec­ ified rail-to-rail op amps can be designed into a preamp with a very high input impedance. This is highly desirable if you want a preamplifier to suit a ceramic phono cartridge or a piezoelectric pick-up in a musical instrument such as a violin. In both cases, an input impedance of 5MΩ is desirable for good bass response. Optional electret microphone One of the attractions of the Pre- Champ was that you could install an on-board electret microphone. The only modification required was to add a bias resistor. That feature can also be included in this 2-channel design and you could, in fact, have two electret microphones, although for useful channel separation you would need to install them both on shielded leads. Circuit details Let’s have a look at the circuit which is similar to but not exactly the same as the preamp in the January 2013 article. Fig.1 shows the details. Both channels are shown and the dual op amp is an LMC6482. Since we are employing a single DC supply rail, we need a halfsupply reference from which to bias the inputs of both op amps. This reference is derived from the supply rail via a voltage divider consisting of two 10kΩ resistors bypassed with a 100µF electrolytic capacitor. We can use such high-value resistors for the divider because the bias current drawn by each input of the op amps is a just a fraction of a picoamp. On the other hand, we want that bypassed half-supply to have quite a low impedance, hence the relatively large capacitor value of 100µF. Both op amp circuits are identical although it is possible to have different gains in each channel, depending siliconchip.com.au PREAMP POWER + 12–20V DC D1 1N5819 1 A 2 – K REG1 78L09 1 0 0 µF 25V 10k +9V OUT IN GND 10 µF musical instrument such as a guitar but you can easily increase or reduce the gain to suit by changing the value of R5 and you can change the input impedance as well. For example, if you want to configure it for a dynamic microphone, R2 & R3 are changed to 100kΩ each to give an input impedance of 50kΩ, while R5 is changed to 100kΩ to give a gain of 101 times (41dB). If you want to install an electret microphone insert on the PCB, you would install it in place of 100pF capacitor C101. At the same time, R101 is changed to 10kΩ and it provides the bias current for the electret. The other end of the 10kΩ resistor is connected to the positive supply rail, from REG1. Finally, R102 is omitted, R103 is 220kΩ and the gain is set to 23 (27dB) with R105 being 22kΩ. + 4 .5V 10k 100nF 100 µF CON1 PREAMP IN1 1 2 CON2 R1 100Ω R2 2.2M TO PIN 1 OF CON3 WHEN ELECTRET MIC FITTED 100nF 3 2 C1 100pF 8 100Ω 1 IC1a 4 R5 56k R3 2.2M CUT TRACK 100Ω ADDED ELECTROLYTIC CAPACITOR FERRITE BEAD R4 1k LINK TO +9V RAIL FROM REG1 WHEN ELECTRET MIC FITTED PREAMP IN2 10pF 100 µF + 4 .5V R101 2 CON3 R102 100nF 5 6 C101 100pF VR2* 10k CON4 CUT TRACK 10k R105 R103 OPTIONAL ELECTRET MIC INSTALLED INSTEAD OF C101 7 IC1b PREAMP OUT VR1* 10k LOG IC1: LMC6482 1 100 µF R104 1k ADDED RESISTOR * ONLY ONE OF VR1 (16mm POT) OR VR2 (TRIMPOT) TO BE INSTALLED 10pF + 4 .5V 78L09, LP2950CZ-5.0 SC  20 1 5 GND 1N5819 CHAMPION PREAMP A K IN OUT Ceramic cartridge Fig.1: the preamplifier circuit. It’s based around dual rail-to-rail op amp IC1. The signal from each input is AC-coupled and biased to half supply, then amplified and re-biased to 0V DC before being fed to CON4. on your application. For the moment though, let’s assume that both are identical and we will just describe channel 1, based on op amp IC1a. The input signal from CON2 passes through a low-pass filter consisting of a 100Ω resistor (R1) and a small ferrite bead in series, together with a 100pF capacitor connected to the 0V line (C1). This is to attenuate any RF signals that may be picked up by the input leads. There is also a 2.2MΩ resistor to pull the input signal to ground (R2). If you are going to feed the preamp with an iPod or similar player you will need to use a much lower value of, say, 1kΩ to provide it with sufficient load current. For the moment though, the values we have shown on the circuit for channel 1 are selected to suit the pick-up in an electric guitar. The signal is then AC-coupled via a 100nF capacitor to pin 3 of IC1a and a 2.2MΩ resistor biases the op amp’s input to the half-supply rail. This ensures that the output waveform will swing symmetrically within the supply rails of dual op amp IC1. The two 2.2MΩ siliconchip.com.au resistors on either side of the 100nF AC-coupling capacitor are in parallel as far as the signal source is concerned, setting the unit’s input impedance to around 1.1MΩ. IC1a buffers and amplifies the signal from CON2 while IC1b does the same for the signal from CON3. Gain is set at 57 times (35dB) by the 56kΩ (R5) and 1kΩ (R4) feedback resistors. The 10pF feedback capacitor reduces the gain for high-frequency signals, giving a little extra stability and noise filtering. Changing the gain Note that this high gain suits a Another interesting application is to use the Champion preamp with a stereo ceramic cartridge (don’t laugh; this was a standard fitment on millions of record players and many people are dragging them out to listen to their old record collections). Ceramic cartridges require a high input impedance and this is an easy option with this preamp. Both R2 & R3 are specified at 10MΩ, giving an input impedance of 5MΩ which ensures good bass response. The gain does not need to be high though and so we can set R5 to 2.7kΩ. This gives a gain of 3.7 (11.3dB). The same configuration can be used for a piezo pick-up on musical instrument such as a violin. So to summarise, depending on Table 1: RC Gain Selection Values Input Gain R1/101 C1/101 R2/102 R3/103 R4/104 R5/105 Guitar 57 100Ω 100pF 2.2MΩ 2.2MΩ 1kΩ 56kΩ Microphone 101 100Ω 100pF 100kΩ 100kΩ 1kΩ 100kΩ Electret 23     10kΩ* – – 220kΩ 1kΩ 22kΩ MP3 28 100Ω 100pF 1kΩ 220kΩ 1kΩ 27kΩ Piezo Pick-up 3.7 100Ω – 10MΩ 10MΩ 1kΩ 2.7kΩ * Connect one end of this resistor to the +9V rail from REG1. June 2015  75 100µF TOP VIEW OF PCB 100nF 10pF VR2* 10k + VR1* R104 1k 10k + 100 µF CON2 + 100 µF Out CON4 CUT TRACKS + 10pF 10 µF Power 100 µF 25V + + 10k CON1 100nF R5 + C1 IN 2 100pF R2 IN 1 R3 100Ω 100nF IC1 BEAD R1 R4 1k CON3 R102 R103 UNDERSIDE OF PCB + 01109121 R105 + 100pF BEAD LMC6482 C101* 100Ω R101 100Ω − REG1 78L09 D1 + 5819 * OPTIONAL ELECTRET LINK WHEN ELECTRET MIC FITTED INSTEAD OF C1 * FIT EITHER VR1 OR VR2, NOT BOTH INSTALLED INSTEAD OF C101 Fig.2: follow this layout diagram to assemble the PCB. It’s best to cut the tracks first and then check with a continuity meter before fitting the parts. Table 2: Resistor Colour Codes   o o o o o o o o o o o Value 10MΩ 2.2MΩ 220kΩ 100kΩ 56kΩ 27kΩ 10kΩ  2.7kΩ 1kΩ 100Ω 4-Band Code (1%) brown black blue brown red red green brown red red yellow brown brown black yellow brown green blue orange brown red violet orange brown brown black orange brown red violet red brown brown black red brown brown black brown brown what type of source you are using and the gain required, you can easily obtain the required input impedance and gain. Table 1 shows the values to use. Two outputs In the original Champion preamplifier, the outputs of the two op amp stages are mixed using a pair of resistors and then AC-coupled to potentiometer VR1 or VR2, depending on which is installed. In our application, we want two separate outputs and so if an output level control is to be used, it can only affect one channel. As shown on the circuit of Fig.1, the output of IC1b connects to VR1 (or VR2) via a 100Ω resistor and 100µF DC blocking capacitor. The wiper of VR1 then connects to one terminal on CON4. The output of IC1a is also fed via a 100Ω resistor with a second blocking capacitor and bias resistor added under the board. This output goes to the other terminal on CON4. Note that two track cuts on the PCB need to be made, in order to give this independent two channel operation. IC1 is powered via a 78L09 lowpower 3-terminal 9V regulator, assuming you are using a DC plugpack with 76  Silicon Chip 5-Band Code (1%) brown black black green brown red red black yellow brown red red black orange brown brown black black orange brown green blue black red brown red violet black red brown brown black black red brown red violet black brown brown brown black black brown brown brown black black black brown   Table 3: Capacitor Codes Value 100nF 100pF 10pF µF Value 0.1µF NA NA IEC Code EIA Code 100n 104 100p 101 10p 10 an output of 12V or more (up to 20V DC). This regulator is fed from CON1 via Schottky diode D1 which protects against reversed supply polarity. Note that if you intend using a 9V battery for this project, you may want to employ the LP2950CZ-5.0 5V regulator. No other modifications are required if you make this change but the preamplifier will inevitably have a reduced output voltage swing and therefore a reduced overload margin for strong input signals. Construction You will be using the Champion PCB for this project (code 01109121) and you will need to cut off the section for the AN7511 audio amplifier. Don’t discard it – it’s a handy little amplifier module in its own right and the AN7511 amplifier chip is quite cheap. Parts List 1 PCB, code 01109121, 57 x 41mm (see text) 1 PCB-mount electret microphone insert (Jaycar Cat. AM4011) (optional; see text) 1 10kΩ log PCB-mount 16mm potentiometer (VR1) OR 1 10kΩ mini horizontal trimpot (VR2) 2 ferrite beads, Jaycar LF1250 4 mini 2-way terminal blocks (CON1-CON4) (omit one if electret is installed) 1 8-pin DIL socket 4 M3 x 10mm tapped Nylon spacers 4 M3 x 6mm machine screws 1 short length hookup wire (60mm) Semiconductors 1 LMC6482 or LMC6032 dual op amp (IC1) (eg, Jaycar ZL3482) 1 78L09 or LP2950CZ-5.0 5V LDO regulator (REG1) (eg, Jaycar ZV1645) – see text 1 1N5819 Schottky diode (D1) Capacitors 1 100µF 25V electrolytic 3 100µF 16V electrolytic 1 10µF 16V electrolytic 3 100nF MMC or MKT 2 100pF ceramic (omit one if electret is installed) 2 10pF ceramic Resistors (0.25W, 1%) 3 10kΩ 2 100Ω See Table 1 for R1-R5 Note: Jaycar will be selling a kit of parts for this project – Cat. KC5531. The remaining preamplifier PCB measures just 57 x 41mm. It has provision for mounting pillars at its four corners and four 2-way connector blocks. One of those blocks is used as the terminals for the two preamplifier outputs. Since there are changes to the component layout, this means that you will have to follow the parts layout of Fig.2 and ignore most of the resistor values shown on the screen-printed layout on the PCB itself. You also need to cut the copper tracks of the PCB in two places as shown on Fig.2. Having cut the tracks, start the assembly by installing the resistors. siliconchip.com.au +3 11/05/15 12:00:11 Pre-champion Frequency Response 1.0 +2 11/05/15 12:15:43 Pre-champion THD+N vs Frequency Input signal = 50mV RMS, gain ≈ 27, bandwidth = 80kHz 0.5 Total Harmonic Distortion + Noise (%) +1 Amplitude Variation (dBr) 0 -1 -2 -3 -4 -5 -6 -7 0.2 0.1 0.05 LMC6482 0.02 LMC6032 0.01 .005 -8 .002 -9 -10 10 20 50 100 200 500 1k 2k 5k 10k 20k 50k 100k .001 20 50 100 200 Frequency (Hz) Fig.3: frequency response is within +0,-0.5dB between 30Hz and 20kHz with -3dB points around 8Hz and 55kHz. It’s less than 1dB down at 20Hz. 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.4: the LMC6032 has about half the noise of the LMC­ 6482. The LMC6032 requires slightly more operating current than the LMC6482 but still under 1mA. These two larger-than-life-size views show the completed PCB assembly. Note the wire link on the back of the PCB when an electret mic is used. You will need to refer to Table 1 for the values for R1-R5 and R101-R105. Table 2 shows the colour codes but it is good idea to check each value with a multimeter before fitting it. A ferrite bead should be slipped over one leg of each 100Ω input resistor, if fitted (ie, R1 & R101). Follow with diode D1 and then fit the IC socket with its pin 1 notch orientated as shown. Next, fit the 78L09 or LP2950CZ-5.0 regulator, REG1. Follow with the ceramic and monolithic capacitors. The 2-way terminal blocks are next, each installed with its wire entry holes facing outwards. Note that CON3 is not installed if you have fitted an electret microphone insert for channel 2. The next step is to decide whether to fit potentiometer VR1 or trimpot VR2. It will only control the output signal level from one channel and you may decide to link it out. You can then fit all the electrolytic capacitors. In each siliconchip.com.au case, the longer lead goes into the hole marked with a “+” sign. Once those parts are in, fit the M3 x 10mm tapped spacers to the corner mounting positions using M3 x 6mm machine screws. If you are installing an electret, wire a 10kΩ resistor in the position for R101 and connect the end adjacent to CON3 to the output of the 3-terminal regulator. We show this with a dotted red line on Fig.2. In addition, R102 and C101 are omitted. If you are going to use only one channel of the preamplifier, it’s a good idea to short the unused channel’s input to 0V by using a wire link for resistor R1 (or R101) and by shorting the two terminals of CON2 (or CON3). When you have carefully checked your assembly and soldering against the circuit of Fig.1, Table 1 and the overlay diagram of Fig.2, you are ready to apply power. Check that the output of REG1 is 9V (or close to it) if a 78L09 has been fitted. If an LP2950CZ-5.0 has been fitted, REG1’s output should be close to 5V. Next, turn off the power, insert the op amp (carefully), power back on and then check the DC voltage at pins 1 & 7. In each case, they should be sitting at half supply; 4.5V for a 9V supply and 2.5V for a 5V supply. Performance Figs.3 & 4 show the frequency response and total harmonic distortion curves of the preamplifier. Note that of the two op amps we’ve specified, the LMC6032 gives the best performance but it isn’t as easy to get as the more common LMC6482. To achieve a THD+N this low, the preamp will need to be installed in an earthed metal box. Otherwise, hum and RF pick-up will reduce the signalto-noise ratio and consequently the total harmonic distortion perform­ SC ance. June 2015  77