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Now you can practice your guitar at any time by using this low cost Guitar Headphone Amplifier. It provides ample power for headphones so that you can play to your heart’s content without disturbing the rest of the household or your neighbours. By JOHN CLARKE Build this guitar headphone amplifier Guitarists usually have their own amplifier and loudspeaker system but it is not always convenient to set it up when you want to practice on your own. Alternatively, you may not wish to disturb other memb­ers of your household. This is where this Guitar Head­phone Amplifier comes into play (pun intended). Capable of providing an output of one watt, it will deliver ear-splitting sound without disturbing anyone nearby. The Guitar Headphone Amplifier is housed in a small plastic case which clips onto your belt. It is powered from a 12VAC plugpack, so that you can play for as long as you like without fear of flattening batteries. The controls are simple and consist of an on/off switch and a volume control – just plug in your guitar and head­phones, adjust the volume and play. Note that while the amplifier is specified for use with 8-ohm stereo headphones, higher impedance types will also be quite satisfactory. How it works The circuit for the amplifier is shown in Fig.1. It uses the well-proven National Semiconductor LM386 amplifier (IC1) to drive the headphones directly. A 3-terminal regulator sets the supply voltage at 13.5V DC. Signal from the guitar is directly coupled to the 10kΩ volume control (VR1) and then AC-coupled to the non-inverting input of IC1 at pin 3. Additionally, the AC coupling minimises any offset at the output of the amplifier which will reduce the useable peak to peak Spec ificatio Power ns Output 0.9W in to a 16 -ohm lo Freque ad ncy Re sp -3dB po ints at 3 onse 0Hz an Total H d 70kH a z Typicall rmonic Disto rtion y less than .0 graph) 4% (se e Signalto-Nois 78dB e Ratio u 20kHz) n w e ig h t e d ( 20Hz to output; with respect 81dB A to the sam -weighte rated e cond d unde itions r Input s ensitiv ity 50mV; 10kΩ in put imp edance May 1995  41 +13.5V PARTS LIST 1 PC board, code 01305951, 60 x 70mm 1 front panel label, 125 x 65mm 1 plastic utility box, 130 x 68 x 41mm 1 belt clip 1 12VAC 5W plugpack with 2.5mm DC plug 1 stereo 6.5mm unswitched panel socket 1 mono 6.5mm unswitched panel socket 1 DC panel socket with 2.5mm pin 1 SPDT toggle switch (S1) 1 10kΩ log pot (VR1) 1 500Ω miniature horizontal trimpot (VR2) 1 knob 1 heatsink, 19 x 19 x 9.5mm 1 3mm screw & nut 1 40mm length of shielded cable 1 100mm length of yellow hookup wire 1 50mm length of red hookup wire 1 50mm length of blue hookup wire 1 20mm length of 0.8mm tinned copper wire 13 PC pins or stakes Semiconductors 1 LM386N-1 power amplifier (IC1) 1 LM317T 3-terminal adjustable regulator (REG1) 1 W04 1A 400V bridge rectifier (BR1) 1 1N4004 1A diode (D1) 100 GUITAR INPUT 0.22 VOLUME VR1 10k LOG 22 16VW 3 6 12VAC INPUT 1 7 47 16VW 4 0.1 330  IC1 LM386 2 POWER S1 8 5 470 16VW 16  L OAD OUTPUT .047 10  D1 1N4004 BR1 W04 REG1 LM317T IN 1000 25VW OUT ADJ 120  +13.5V 10 16VW 1k VR2 500W AO I GUITAR HEADPHONE AMPLIFIER Fig.1: the circuit is based on the well-proven LM386 power ampli­fier IC and drives the stereo headphones in series (16-ohm load) for optimum power and distortion performance. voltage. The pin 3 input impedance is 50kΩ and so the 0.22µF capacitor rolls off signals below 15Hz. The 0.1µF capacitor at pin 2 grounds this inverting input. The AC gain of the amplifier is set to 60 by the 330Ω resistor between pins 1 and 8 while the 22µF capacitor ensures that the DC gain is zero. The amplifier is internally biased so that its output at Capacitors 1 1000µF 25VW PC electrolytic 1 470µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 0.22µF MKT polyester 1 0.1µF MKT polyester 1 .047µF MKT polyester Resistors (0,25W 1%) 1 1kΩ 1 120Ω 1 330Ω 1 10Ω Miscellaneous Pair of 8-ohm stereo headphones, heatsink compound, solder, etc. Fig.2: the power versus distortion characteristic of the head­phone amplifier. Distortion is typically below .04% at 1kHz. 42  Silicon Chip HEADPHONES POWER S1 VR2 10uF .047 10 REG1 LM317 1k 470uF 100uF 22uF 47uF 25VW IC1 LM386 330  120  1000uF 1 VR1 0.22 0.1 D1 BR1 POWER SOCKET GUITAR INPUT Fig.3: this is the component overlay for the PC board. Note that the 10kΩ volume control pot is secured to the board & not to the front panel. pin 5 sits at half the supply voltage for maximum output swing. Hence, a DC blocking capacitor of 470µF is required to couple the output signal to the 16-ohm load. This ca­pacitor rolls off frequencies below 21Hz. Power supply ripple rejection is vastly improved by including the 47µF capacitor between pin 7 and ground, while a 100µF capacitor is used to decouple the power supply pin (pin 6) to ground. A Zobel network comprising a .047µF capacitor and a 10Ω resis­tor connects to the amplifier output to prevent high frequency oscillation. Note that the stereo headphone socket is wired with the normal common ground connection open circuit. That is, the tip connection goes the amplifier output, the ring connection goes to ground and the sleeve connections is open-circuit. This effec­tively connects both 8Ω headphones in series to give a 16Ω load which enables much better performance in terms of power output and distortion than the LM386 is capable of into 8Ω or 4Ω loads. This series connection of the headphones also means that they are correctly in phase to give a centred sound image. As noted above, power for the circuit is derived from a 12V 300mA AC plugpack. Switch S1 applies power while the bridge (BR1) rectifies the AC. The resulting DC is filtered using a 1000µF capacitor. REG1 is an adjustable 3-terminal regulator which is set by trimpot VR2 to provide 13.5V DC. Construction Most of the components are mount­ed on a PC board coded 01305951 and measuring 60 x 70mm. The PC board mounts in the integral side clips of a plastic box measuring 130 x 68 x 41mm. We designed a front panel label measuring 125 x 65mm for the lid. Begin construction by inspecting the PC board for any de­fects in the copper pattern and, if necessary, make any repairs before fitting components. Start by installing PC pins The PC board clips into the bottom of the case. Note the small heatsink for the LM317 regulator. The unit is powered from a 12VAC plugpack. at the external wiring points, then fit the wire link and the resistors. This done, fit trimpot VR2, IC1, BR1 and diode D1. Be sure to orient IC1, D1 and BR1 correctly as shown on the overlay diagram. Install the capacitors next, taking care with the electrolytic capacitors which must be inserted with the correct polarity. Heatsinking The LM317 (REG1) is mounted onto a small heatsink using a nut and bolt to secure them to the PC board. Apply a smear of heatsink compound between the mating surfaces before mounting. VR1 mounts directly onto the PC board with the terminals solder­ing to the PC RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 Value 1kΩ 330Ω 120Ω 10Ω 4-Band Code (1%) brown black red brown orange orange brown brown brown red brown brown brown black black brown 5-Band Code (1%) brown black black brown brown orange orange black black brown brown red black black brown brown black black silver brown May 1995  43 on the case and clip the PC board into the box. Wire up the input socket to the PC board using shielded cable and use hook-up wire for the remaining wiring. Now attach the adhesive label to the front panel and drill the hole for the volume pot. You may also need to cut the shaft to length so that the knob will sit flush with the lid. Testing Apply power to the circuit and measure the voltage between a GND PC pin and the metal tab of REG1. Adjust Fig.4: this is the full-size etching pattern VR2 for a reading of +13.5V for the PC board. Check your board DC. Final testing can be done carefully for defects before installing with the guitar and head­ any of the parts. phones connected. Connect the lid to the case using the pins. Also solder the body of the pot four self-tapping screws and to the two adjacent PC pins located attach the knob. near the edge of the board to earth Now you can practice your the pot body and to aid in anchoring guitar without disturbing others, it in position. no matter what time it is. Note Drill holes in the plastic case for the that while any stereo headphones power switch (S1), the headphone and can be used, the best bass will be input sockets, and the power input obtained with those that fully socket. Holes will also be required for enclose the ears with well-fitting SC the belt clip. Mount these components surrounds. GUITAR INPUT 12VAC INPUT + MIN MAX VOLUME GUITAR HEADPHONE AMPLIFIER HEADPHONE POWER ON OFF Fig.5: the full-size front panel artwork. 20 Electronic Projects For Cars On sale now at selected newsagents or order your copy from Silicon Chip Yes! Please send me ___ copies of 20 Electronic Projects For Cars Enclosed is my cheque/money order for $­________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Price: $8.95 (plus $3 for postage if ordering from Silicon Chip). Order by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or mail the coupon to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 44  Silicon Chip Signature­­­­­­­­­­­­_________________________ Card expiry date_____/______ Name _________________________Phone No (_____)_____________ PLEASE PRINT Street ___________________________________________________ Suburb/town _____________________________ Postcode_________