Fullscreen HTML5Med Res (??MB)HTML4

Jam along with your favourite CD with the: Want to play your guitar in accompaniment to your fa­vourite CDs? Want to do it without disturbing anyone? Then build the Guitar Jammer, a neat little headphone amplifier with mixing facilities for guitar and CD inputs. It sounds great and won’t cost a bomb. Guitar Jammer By LEO SIMPSON & PETER SMITH O K, WE KNOW THAT most guitar amplifiers have a headphone socket that you can use for a quiet practice session at night but who wants to have to switch on a hulking big amplifier just to listen to headphones? Also playing guitar via the headphone socket on many amplifiers is not that great. Often there is quite a lot of hum and buzz and it often doesn’t sound particularly clean either. Nor do guitar amplifiers perform all that well with the high level signals from a CD player. So we have come up with a low cost and compact headphone amplifier with mixing for the signals from a guitar and a CD player. For economy the headphone drive is mono, from a single LM386 IC amplifier. It can be pow­ered from a 9V DC plugpack or a 9V battery. Either way, the sound quality is surprisingly 22  Silicon Chip good considering the simplicity of the circuit and it is certainly better than the sound from the head­phone socket of most guitar amplifiers. Performance Power Output 120mW into 8Ω headphones Frequency response -3dB at 30Hz and 70kHz Total Harmonic Distortion Typically less than 0.2% (see graph) Signal-to-Noise Ratio 71dB unweighted (20Hz to 20kHz) with respect to full output; 76dB A-weighted under the same conditions Input sensitivity Guitar input 45mV; CD input 550mV Actually, the idea is not new. We picked up the idea from an article on a “Guitar Jammer” in the July 1998 issue of “Popu­lar Electronics”. This was also based on an LM386 but we have refined the circuit in a few aspects and produced a new PC board with all the components, including the pots and jack sockets, on the board. The circuit is also similar to a headphone guitar amplifier we published in the May 1995 issue but that circuit did not include mixing facilities. Input facilities The Guitar Jammer is housed in a compact plastic box and has two potentiometers for setting the input levels for the CD player and guitar. It has two 3.5mm stereo jack sockets, one from the CD inputs and the other for the headphone output. The 6.5mm Fig.1: based on an LM386 power amplifier IC, the circuit has mixing facilities for a guitar (or other electronic musical instrument) and both channels from a CD player. jack socket is for the guitar lead. The circuit will drive virtu­ally any stereo headphones, whether they are 400Ω, 32Ω or 8Ω, although the best bass will come from headphones with full ear-enclosing muffs. Now let’s have a look at the circuit of Fig.1. It shows a stereo input socket (CON2) for the CD left and right channel inputs. These are mixed together and reduced in level via sepa­ rate 220kΩ resistors and fed to VR1, a 10kΩ logarithmic poten­tiometer. The guitar input (CON3) is coupled via a 100Ω resistor and .0039µF shunt capacitor to VR2, a 50kΩ logarithmic potentiom­eter. The signals from the wipers of VR1 and VR2 are then mixed together with 10kΩ resistors and fed via a 0.22µF capacitor to pin 3 of IC1, the LM386 power amplifier. The inverting input, pin 2, is grounded via a 0.1µF capacitor. The AC gain of the amplifier is set to 33 by the 220Ω resistor between pins 1 & 8, while the 22µF capacitor ensures that the DC gain is zero. The amplifier is biased so that the voltage at pin 5 sits at half the supply voltage, for maximum output swing. A DC blocking capacitor of 470µF is used to couple the output signal to the headphone socket. This socket is wired so that both channels of the headphones are connected in parallel and this gives a nominal load of 4Ω, if 8Ω headphones are used. However, 4Ω is not an optimum load for the LM386 as it results in higher distortion. Hence, the headphone socket is connected in series with a 10Ω resistor to give a nominal load of 14Ω. We are not worried about the power loss in the 10Ω resistor because the signal level to the headphones is more than adequate. The 470µF capacitor rolls off signals below 24Hz, assuming 8Ω headphones are in use. For higher impedance headphones the rolloff will be at a much lower frequency. We have specified a 9V DC plugpack as one of the power supply options so we have included a 47µF capacitor There are relatively few parts, so the PC board should only take a few minutes to assemble. Make sure that the polarised parts are installed correctly. October 2000  23 at pin 7. This improves the power supply ripple rejection of the circuit while a 470µF capacitor on the 9V supply provides extra filtering. A Zobel network consisting of a .047µF capacitor and 10Ω resistor is included at the output to ensure high frequency stability. As already noted, the power can come from a 9V DC plugpack or a 9V battery although the 9V battery will not last long. By the way, don’t be tempted to use a 12V DC plugpack because its unloaded DC output of around 17V or higher will probably blow the LM386. As it is, a typical 9V DC plugpack is likely to deliver close to 12V. Diode D1 is included to provide reverse polarity protection for the circuit. Construction All of the components, including the two pots and the vari­ous sockets, are mounted on the PC board which measures 63 x 106mm (code No. 01110001). It is designed to be a snug fit in a standard plastic case measuring 130mm x 67mm x 44mm (Jaycar HB6023). The component layout diagram is shown in Fig.2. The suggested order of assembly is as follows. First, mount the toggle switch S1, the three jack sockets and the two pots on one side of the board. Then mount the DC socket but before you do, make sure that it matches the plug on the DC plugpack. There is nothing quite so frustrating as trying to connect DC power when the plug and socket don’t match! Fig.2: this is the component layout for the PC board. Note that JP1 is not used; just wire in the link. Table 1: Capacitor Codes      The bodies of the two pots are earthed by connecting them to the PC board using tinned copper wire. Note the mounting details for LED1 (see text). Value IEC Code EIA Code 0.22µF   220n   224 0.1µF   100n   104 .047µF   47n  473 .0039µF   3n9  392 Table 2: Resistor Colour Codes  No.   2   2   1   1   1   2 24  Silicon Chip Value 220kΩ 10kΩ 2.7kΩ 220Ω 100Ω 10Ω 4-Band Code (1%) red red yellow brown brown black orange brown red violet red brown red red brown brown brown black brown brown brown black black brown 5-Band Code (1%) red red black orange brown brown black black red brown red violet black brown brown red red black black brown brown black black black brown brown black black gold brown If you are going to use a DC plug­ pack, you don’t have to install the 9V battery snap connector. Alternatively, you can install both the battery connector and the DC socket, since the DC socket automatically disconnects the battery when the plugpack is plugged in. Note that the two pots must have their shafts cut to suit the knobs; ie, about 10cm long, Make sure that both pots have a milled flat on their shafts so that the push-on knobs will fit properly. When installed on the PC board, the bodies of the pots must be earthed to the copper pattern. Solder a short length of the tinned copper wire to the body of each pot; you may need to scrape away the plating on the body to do this. Then solder both wires to the board, via the hole between the two pots. Next, fit the diode and the resistors, followed by the capacitors. The diode and electrolytic capacitors must be con­nected in the right way around otherwise the circuit won’t work and damage may occur. Note that the LED needs to be stood off the board on two stiff lengths of tinned copper wire and then bent over so that it will poke through the side of the case (see photo). Finally, the LM386 can be installed, making sure that it goes in the correct way – see Fig.2. If you look at Fig.2, you will notice that there is provi­sion for JP1 (near the CD input) but this is not fitted. Instead, fit a short link between points 1 & 2. Drilling the case We’re rather proud of the mounting arrangement of the PC board in the Fig.3: use this diagram as a template when drilling the case. case. The board is just wide enough to sit upside down on the integral board slots inside the case. Then, when the lid is fitted, the lid becomes the base and the rubber bungs which fit in the screw holes become the feet for the case. All that means that the case must be drilled exactly as shown in the diagram of Fig.3. Also, on the side with most of the holes drilled, the lower section of the integral board slots must be removed with a sharp chisel. This will allow the pots and input sockets to sit close to the side of the case. Note that there is one hole on the opposite side of the case to take the DC input socket. When you fit the board into the case you will need to slightly pull one side of the case out to allow the DC socket to pop into place. On the other hand, if you are not using a battery and do not have the DC socket fitted, you will not need to drill a hole for it and the board will slip easily into place, to be retained by the lid when it is fitted. Before you fit the board into the case, do a voltage check. Apply power and check that 9V (or thereabouts, depending on your DC plugpack) is The board sits on top of integral slots inside the case and secured by the lid and by fitting nuts to the pots and headphone socket. The inset picture shows how the lower sections of the board slots on one side of the case are removed. October 2000  25 Parts List Fig.4: this graph shows the power output versus harmonic distor­tion at 1kHz when 8Ω headphones are connected. 1 PC board, code 01110001, 63mm x 106mm 1 130mm x 67mm x 44mm plastic case (Jaycar HB-6023) 1 SPDT miniature toggle switch, PC mount (Altronics S-1320) 2 20mm knobs (Jaycar HK-7711) 1 9V battery connector 1 2.5mm PC mount DC jack socket (CON1; Jaycar PS0520) 1 6.5mm mono switched PC mount jack socket (CON3; Jaycar PS-0160) 2 3.5mm stereo switched PC mount jack socket (CON2, CON4; Jaycar PS-0133) 2 PC stakes 1 16mm PC mount 10kΩ log pot (VR1) (Jaycar RP-7610) 1 16mm PC mount 50k log pot (VR2) (Jaycar RP-7616) Semiconductors 1 LM386N-1 audio amplifier (IC1) 1 1N4001 1A 100V diode (D1) 1 5mm high brightness red LED (LED1) (Jaycar ZD-1792) Resistors (0.25W, 1%) 2 220kΩ 1 220Ω 2 10kΩ 1 100Ω 1 2.7kΩ 2 10Ω Fig.5: this is the full-size etching pattern for the PC board. Capacitors 1 470µF 25VW PC electrolytic 1 470µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 1 0.22µF MKT polyester 1 0.1µF 50VW MKT polyester 1 .047µF MKT polyester 1 .0039µF MKT polyester Miscellaneous 9V battery or 9V DC 150mA plugpack, small cable tie, 15cm 22AWG (0.71mm) tinned copper wire, solder, etc Fig.6 actual size artwork for the control panel which goes on the bottom of the case. 26  Silicon Chip present at pin 6 of the LM386. Half this figure should be present at pin 5 and the LED should be alight. OK? Then go ahead and fit the board into the case, fit the label to the bottom of the case, screw on the lid on and you’re ready to play. Note that a kit for this project will be available shortly after this issue SC goes on sale.