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HEADPHONE L FOR CD PLAYERS Does your compact disc player have a headphone socket? It doesn't? How sod. Now you con correct that situation and listen to the music direct, without degrading the sound quality. 28 SILICON Cl-fll' Many CD players do not have a headphone socket which is a pity. There is a great attraction in listening to your favourite discs directly via headphones. By the same token, there is something faintly ridiculous about listening to headphones via a stereo amplifier which might have a power capability of 100 watts per channel - all that amplifier circuitry just to provide a few milliwatts. In fact, a number of smaller CD players do have the internal circuitry necessary to provide headphone drive but the headphone socket and volume control have been omitted. For most of us, the idea of any modification to a CD player, no matter how slight, will be anathema. The alternative of an external headphone amplifier is much more attractive. The headphone amplifier presented here is just about as simple as you can get without in any way prejudicing or degrading the signal quality from the CD player. It is housed in a plastic box and is powered by a 9 or 12V AC mains plugpack. Mounted on the top panel of the box is the on/off switch, a volume control knob and a standard 6.5mm stereo jack socket. On the side of the box is a 4-way RCA socket panel. This allows you to connect the headphone amplifier to the CD player and also connect the CD player signal to your existing stereo amplifier, if need be. Even if your CD player already has a headphone socket, you may want to build the headphone amplifier presented here. Some CD players do not have a level control on their headphone socket, or perhaps the maximum output level IFIER RIGHT~-:;i OUTPUT I I +17V II I RIGHT(} INPUT "1, __ - 22pF r-- HEADPHONES ~ .,. 220k .,. By JOHN CLARKE & LEO SIMPSON .,.. LEFT ~ - ; . OUTPUT -]iI +17V II Main Features • Adds headphone outputs to any CD player • Volume control • Simple design uses only two low noise op amps • Suitable for all headphones, low or high impedance • Simple power supply without 3-terminal regulators • Powered by 9V to 1 2V AC plugpack • Signal-to-noise ratio better than 95d8 . • Harmonic distortion better than LE F T e INPUT --- 22pF VOLUME VR1b 50k LOG 220k .,. -:- POWER 0--0 12VAC PLUG-PACK 01 1N4002 ~-,-..,1-Y,4-....,--------+17V 81 1000 25VW + 0 .001 % • Housed in plastic zippy box -------------11v 02 1N4002 is not sufficient to drive your favourite headphones. Either way, the headphone amplifier presented here could be a big improvement as it gives more than adequate drive signal. How it works The circuit is based on two Signetics 5534 low noise bipolar op amps, one for each channel, and these drive the phones directly. Most op amps could not do this job unaided but the 5534 is unusual in that, besides being a very low noise op amp, it can also drive 6000 loads at full output swing. This means that it is capable of delivering more output current than most op amps and this is the characteristic we are depending on to make the circuit work. Each op amp is connected as a non-inverting amplifier stage with a gain of approximately 3.7, as set by the 22k0 and 82k0 feedback HEADPHONE AMPLIFIER SC1-1-688 Fig.1: the circuit uses two 5534 low noise op amps to give excellent headphone reproduction. The 5534s allow a very simple power supply to be used. resistors. Output signal to the headphones is then fed via a 2200 resistor in each channel. This resistor limits the current in the event of a short-circuit or overdrive to a value which is safe for both headphones and the op amps. As such, the headphone amplifier can drive just about all headphones to ear-shattering volume. This applies whether the phones have a low impedance of 80 or 320 or a high impedance such as 6000 or 2k0. The input signal to each amplifier channel is fed via a dual ganged pot and then coupled via a O. lµF capacitor to the non-inverting input of each op amp. The 22pF compensation capacitor across pins 5 and 8 keep the op amps stable at the selected level of gain. The power supply is crude but effective. It consists of a 9 to 12V AC plugpack as already mentioned. This feeds a positive and negative halfwave rectifier to produce positive and negative supply rails of about 17 volts, depending on the AC input voltage. The resulting DC voltages are filtered by 1000µF electrolytic capacitors. Normally, an op amp circuit such as this would use 3-terminal regulators to provide balanced supply rails of ± 15 volts. But the 5534 op amps are rated for operation up to ± 22V volts which means that we can dispense with 3-terminal regulators. The op amps also have excellent power supply rejection APRIL 1988 29 PARTS LIST 1 PCB code, SC1-1 -488, 84 x 44mm; or Veroboard, 84 x 48mm 1 plastic case, 130 x 68 x 43mm (Altronics H-0203 or Jaycar HB-6013) 1 front panel artwork 1 rear panel artwork 1 4-way RCA panel socket 1 dual ganged 50k0 log pot 1 SPOT switch 1 knob 1 stereo 6.5mm headphone panel socket ~ 8 28 Semiconductors 2 NE5534 op amps 2 1 N4002 diodes 1 red LED ( VAC PLUG-PACK Capacitors 2 1 OOOµF 25VW PC electrolytic 2 0.1 µF metallised polyester 2 22pF ceramic B A \ ~ LEFT RIGHT INPUT LEFT RIGHT OUTPUT Fig.2: here is the wiring diagram for the PCB version. Use shielded cable for the connections to the volume control and the input and output sockets. ' , 1 FROM PLUG-PACK VIA S1 LEFT INPUT 01 K .,.....,_..._,-;-~A GND R L TD HEADPHONE SOCKET Fig.3: parts layout for the alternative Veroboard version. Fig.4: actual size artwork for the printed circuit board. 30 SILICON CHIP Miscellaneous Hookup wire, shielded cable, solder, screws, nuts, etc . . ' ,·. \- Resistors (0 .25W, 5%) 2 x 220k0, 2 X 22k0, 2 X 8 .2k0, 1 X 6.8k0, 2 X 2200 which means that any variations in the unregulated 17V supplies are ignored. Result - extremely quiet and hum-free amplifiers. A light emitting diode connected in series with a 6.8k0 resistor across the ± 17V rails acts as power indicator. Construction We built our headphone amplifier into a plastic case measuring 130 x 69 x 45mm (Altronics Cat No H-0203). The circuitry can be assembled onto a printed circuit board (PCB) measuring 84 x 44mm (code SCl-1-488) or on a piece of Veroboard measuring 84 x 48mm. Whether you use a PCB or Veroboard for the project, it will not take long to assemble. Make sure you follow the relevant wiring diagram carefully. For the Veroboard version, the tracks can be cut using a sharp drill bit. Use screened cable for the wiring between the input sockets and the volume control and to the input on This view view clearly shows the shielded cable and other wiring connections to the PCB. It may be compact but it can drive most stereo headphones to ear-splitting volume. r:- 0 ~ ,- I w 0 ffi :ii: 3 ~ • u::: ::J Q. If your headphones are fitted with a 3.5mm plug you will need a 6.5mm adaptor as shown with these MHD3A phones from Arista. :i <( w the PCB. Note that the shields of all cables are connected, either to the sockets, dual pot or at the printed board. We used a 6.5mm stereo socket for the headphone jack. If you have headphones with 3.5mm plugs you may prefer to use a 3.5mm stereo socket. But be warned - most 3.5mm sockets that are available are flimsy affairs which do not stand up well to prolonged use and are difficult to solder. In those instances, we suggest you install the 6.5mm socket specified and then use a 6.5mmto-3.5mm adaptor plug. (Some headphones are supplied with these adaptor plugs.} We suggest that the AC plugpack be permanently wired to the headphone amplifier as jack sockets for power supplies can give problems. :::c: • z 0 Q. C 0 ~ :::c: ffi z 0 0 0 J: 25 ~ :c L:. 0 0 ~..:..1L _J Fig.5: full-size artworks for the front and rear panels. When the headphone amplifier is finished, apply power and plug in your phones. There should be no sound from the phones, even with the volume control flat out. Now wind down the volume control. connect your CD player and sit back to enjoy the music. ~ 1\l'llll. /!lll/l 31