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Build the PreChamp – a tiny, versatile preamplifier to mate with the CHAMP! If you’ve built the Champ amplifier from the February 1994 issue then you will probably have a use for this tiny pream­plifier. It uses two common transistors, provides up to 40dB of gain, runs from a 6-12V supply & has provision for an electret microphone. By DARREN YATES The CHAMP amplifier has been a great success with kits available from most of the kit retailers, with lots of interest coming from schools and colleges. However, as versatile as the CHAMP is, unless you have a signal of sufficient amplitude, it will not provide its maximum power output. And if you need to use the CHAMP’s maximum gain of 200 (46dB), the sound quality is not as good as it would be when the circuit has less gain. So we thought, “Why not produce a simple preamp to go with it?” The PreChamp is the answer. It’s not much bigger than a 9V battery yet it has a gain in excess of 40dB, which is more than enough for most applications. You can also vary the gain by changing a single resistor. Furthermore, we have made provision on the circuit board for an electret mic insert. The circuit Let’s take a look at the circuit diagram – see Fig.1. As you can see, the circuit consists of just two transistors – a BC548 NPN type and a BC558 PNP type. These make up a DC feedback pair, with the negative feedback coupled from the collector of Q2 to the emitter of Q1. The input signal is applied via a 0.1µF capacitor to the base of transistor Q1. The bias voltage for this transistor is set up by the 2.2kΩ, 100kΩ and 150kΩ resistors. A lowpass filter consisting of the 2.2kΩ resistor and a 10µF capacitor removes This tiny preamplifier board was specifically built to match the CHAMP power amplifier featured in the February 1994 issue of SILICON CHIP. However, it can be used anywhere you need a preamp with a gain of up to 100 times. unwanted hum and noise from the DC bias voltage. This is known as “supply decoupling” and is usually necessary in preamp circuits to ensure that the output signal is free from hum and unnecessary noise. The output from the first stage is taken from the collector of Q1 and its 22kΩ load resistor. Although this 22kΩ resistor is not strictly necessary, it helps to linearise the output and significantly reduces distortion. Q1’s output is fed to the base of Q2 (the BC558 PNP transistor) and the final output signal appears at its collector. Negative feedback is applied by the 2.2kΩ resistor between the collector of Q2 and the emitter of Q1. The 1500pF capacitor across this resistor ensures that the circuit’s response to radio frequency (RF) is greatly reduced by rolling off frequencies above 48kHz. The overall gain is set by the ratio of the 2.2kΩ resistor and the 100Ω resistor also connected to the emitter of Q1. The full gain equation is: Gain = 1 + (2200/100) = 23 which is equivalent to 27dB. The 22µF electrolytic capacitor in series with the 100Ω resistor sets the lower frequency response to 72Hz. The output is taken from across the 2.2kΩ collector load resistor of Q2 via a 10µF electrolytic capacitor. Power is supplied from any DC source of 6-12V. At 12V the current drain of the preamp is 3mA, dropping to 2mA at 9V. Optional electret microphone We mentioned at the start that the preamp has provision for an electret microphone. This is simply the 10kΩ resistor connect­ing the input side of the 0.1µF capacitor to the decoupled supply rail. This resistor provides bias current to the electret micro­phone’s internal FET. To use the electret all July 1994  43 2.2k 10 16VW 10k Q1 BC548 B 0.1 INPUT Q2 100 BC558 E 16VW B 22k 100k C GND 0V C E 2.2k 150k Fig.1: this is the circuit of the PreChamp. Just two transistors are employed & it can run from a 6-12V supply. Current drain at 12V is 3mA. The 10kΩ resistor at the input makes provision for an electret mic capsule. If the electret is not used, the 10kΩ resistor should be omitted. +6-12V 10 16VW .0015 OUTPUT 100k GND B 100  E C VIEWED FROM BELOW 22 16VW 2.2k Construction All of the components for the PreChamp are installed on a PC board which measures 46 x 36mm and is coded 01107941. Before you begin any soldering, check the board carefully for any shorts or breaks in the copper tracks. These should be repaired with a small artwork knife or a touch of the soldering iron where appropriate. That done, you can start by installing the resistors, fol­lowed by the capacitors. Make sure that you install the electro­ lytic capacitors correctly otherwise reverse polarity will damage them – use the overlay wiring diagram to be sure. Next, install the two transistors and finally the six PC stakes. As noted above, if you are not using the electret mic capsule, then don’t install the 10kΩ resistor at the input. Testing You can test the circuit by just connecting it up to the CHAMP amplifier Q2 GND Q1 .0015 100 2.2k 100k INPUT OUTPUT 2.2k 150k 100uF +6-12V 10uF 10uF 0.1 you need do is to connect it between the INPUT and GND. If you are not going to use the preamplifier with an electret microphone, the 10kΩ resistor must be omitted from the circuit. 10k 100k 2.2k 22k LOW-COST PREAMP FOR THE "CHAMP" 22uF GND 0V Fig.2: the PreChamp board is easy to assemble. If you want to use an electret mic, install the 10kΩ resistor shown dotted & connect the mic between the INPUT and GND terminals. PARTS LIST 1 PC board, 01107941, 46 x 36mm 4 PC stakes Semiconductors 1 BC548 NPN transistor 1 BC558 PNP transistor Capacitors 1 100µF 16VW electrolytic capacitor 1 22µF 16VW electrolytic capacitor 2 10µF 16VW electrolytic capacitors 1 0.1µF MKT polyester 1 1500pF MKT polyester Resistors (0.25W, 5%) 1 150kΩ 1 10kΩ 2 100kΩ 3 2.2kΩ 1 22kΩ 1 100Ω Miscellaneous Solder, shielded audio cable etc. anti-clockwise. When you do this, you should hear a “blurt” from the speaker. If you don’t, check that all the connections between the two PC boards are correct and compare the PreChamp board with the overlay wiring diagram (Fig.2) to double-check for any possible mistakes. You should also inspect the back of the PC board for missed solder joints. Bench amplifier Fig.3: here is the full size PC artwork for the PreChamp board. and doing the “blurt” test. This consists of simply touching the two input PC stakes with your finger with the input pot of the CHAMP wound fully Because of their size, you could quite easily mount the two PC boards and the battery inside a small zippy box and use the completed unit as a bench amplifier for other projects. Be sure to use shielded audio cable for the input signal wiring and for the signal wiring between the PreChamp SC and CHAMP. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 1 1 3 1 44  Silicon Chip Value 150kΩ 100kΩ 22kΩ 10kΩ 2.2kΩ 100Ω 4-Band Code (1%) brown green yellow brown brown black yellow brown red red orange brown brown black orange brown red red red brown brown black brown brown 5-Band Code (1%) brown green black orange brown brown black black orange brown red red black red brown brown black black red brown red red black brown brown brown black black black brown