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2-Way Stereo Headphone Adaptor By Mauro Grassi Do you have a stereo amplifier without a headphone socket but want to listen to your music via headphones? If so, this versatile Stereo Headphone Adaptor will do the job. It connects between your amplifier and loudspeakers, has several operating modes and features two output sockets with individual volume controls. I F YOU BUILT our 20W Class-A Stereo Amplifier described last year, you will be aware that it lacks a headphone socket. Similarly, many hifi valve amplifiers also lack a headphone socket, the assumption being that a true hifi enthusiast will want to listen via good-quality loudspeakers. 70  Silicon Chip A headphone output was not included in the Class-A Stereo Amplifier because it would degrade its superb audio performance. Both the wiring paths and the general circuit layout are critical factors in the design and any changes, however slight, can cause big changes in the signal-to-noise ratio and harmonic distortion figures of the amplifier. If you do want to listen via headphones, a far better option is to build the simple Stereo Headphone Adaptor presented here. It connects directly to the amplifier’s speaker terminals and switches the loudspeakers and stereo headphone sockets using two DPDT (double-pole, double-throw) relays, so there’s no chance of it degrading the audio performance. As mentioned in the introduction, you can connect up to two sets of stereo headphones. These can be switched on or off at the touch of a button and the volume of each can be individually controlled. In addition, the loudspeakers can be switched on or off and there’s also a Mute switch which turns everything off. This means that you can operate siliconchip.com.au the system in one of four modes: (1) loudspeakers only; (2) headphones only; (3) headphones and loudspeakers operating together; and (4) mute (all off). It’s also possible to mute the system by individually turning the headphones and the speakers off. Perhaps we should clarify the operation of the Mute switch, as it doesn’t function quite like a traditional mute switch. Pressing it once certainly mutes the headphones and/or loudspeakers but pressing it a second time doesn’t “unmute” the system. Instead, you have to press either the “Phones” button or the “Headphone” button (or both) to restore the sound. DPDT relays Because it uses two DPDT relays to do the switching, the Stereo Headphone Adaptor can be used with amplifiers with quite high power outputs. In fact, it’s good for use with amplifiers with outputs up to about 100W RMS or more, provided you’re sensible with the volume control setting on the amplifier. We’ve also designed the unit to not only work with solid-state amplifiers but with valve amplifiers as well. The latter must be taken into account separately because unlike solid-state amplifiers, operating a valve amplifier without a load (ie, a loudspeaker) can cause problems. The reason why most valve amplifiers should not be operated without a load is that they can sometimes oscillate supersonically. Worse still, they can then produce very high AC Fig.1: the Stereo Headphone Adaptor connects between your stereo amplifier and the loudspeakers and can drive two pairs of headphones. STEREO PHONES 1 STEREO AMPLIFIER LEFT OUT STEREO PHONES 2 LEFT SPKR HEADPHONE ADAPTOR RIGHT SPKR RIGHT OUT voltages in the primary windings of the output transformers. These voltages can be so high that they can cause flashover across the valves sockets or even within the valves themselves. This can not only damage the valves but other components as well. As shown in the photos, the unit is housed in a low-profile instrument case with the volume controls, headphone sockets and pushbutton switches neatly laid out on the front panel. The miniature pushbutton switches incorporate integral LEDs which indicate the settings – red for “power on”, green for “phones on” and blue for “speakers on”. On the rear panel is a DC power socket and nine gold-plated binding post terminals. Eight of these terminals are used to connect the amplifier input and loudspeaker output leads, while the ninth terminal connects to the amplifier’s chassis and is the earth return for the headphone sockets. Default setting The default mode setting was an important consideration in designing this circuit. We opted to have both the loudspeakers and the headphones on when the circuit is unpowered and this is done using the normally-closed (NC) contacts of the relays. The advantage of this scheme is that the loudspeakers (and the headphone outlets for that matter) will operate normally when the unit is switched off (ie, zero power consumption). This is also the default setting when power is first applied to the unit. After that, it’s just a matter of using the pushbutton switches to toggle the loudspeakers and the headphones off and on. The leftmost switch is the Mute switch and, as mentioned, this turns both the loudspeakers and the headphones off (but not on again). This switch carries a red LED which is permanently lit while ever power is applied – ie, this LED simply serves as a power indicator. The next switch controls the two headphone sockets and its green LED lights when the headphones are on. The rear panel carries gold-plated binding post terminals for the loudspeaker and amplifier connections plus a DC power socket. Power comes from a 12V DC 400mA plugpack. siliconchip.com.au April 2008  71 72  Silicon Chip siliconchip.com.au 10nF 10k A K D4 D5 10 F 16V A K 39k 6 2 1 10 F 16V 8 12 13 39k 100nF K Tr2 Th2 D2 Tr1 Th1 7 GND IC1 556 CV2 O2 CV1 11 9 3 4 14 10 R1 Vcc R2 D1 5 O1 A 10nF 10nF 470 D4 – D5: 1N4148 STEREO HEADPHONE ADAPTOR 10k 10nF 10k K  A 14 Q2 Q2 Q1 Q1 1 7 A K 13 R1 GND R2 D2 IN 10 S2 IC2 74HC74 CK2 D1 CK1 S1 Vcc 4 D1 – D3: 1N4004 12 11 2 3 100nF +5V 10 F 16V GND OUT 8 9 6 5 REG1 7805 E K A B C B K B LED2 (IN S2) 100 2.2k K BC337 100 A  2.2k 1 F 16V 470 220 F 25V LED3  (IN S3) 100nF +11.4V D2 A OUT Q2 BC337 D3 Q1 BC337 IN E C E C GND A K A K D1 7805 GND RLY2 RLY1 220 F 25V VR1a 1k VR2a 1k VR2b 1k VR1b 1k 4x 270 22  10W 22  10W RIGHT 12V DC INPUT SPKR SPKR STEREO PHONES SOCKET 2 STEREO PHONES SOCKET 1 – + LEFT – + RIGHT AMP CHASSIS – OUT AMP + LEFT OUT – AMP + – + Fig.2: the circuit uses two DPDT (double-pole double-throw) relays to toggle the loudspeakers and/or stereo headphone outputs when switches S2 & S3 are pressed. IC1, a 556 dual timer, debounces these two switches and its outputs at pins 5 & 9 each clock one section of dual D-type flipflop IC2 whenever a switch is pressed. IC2’s Q-bar outputs (pin 6 & 8) in turn drive transistors Q1 & Q2 which then control the relays. 2008 SC  S1 MUTE (RESET) PHONES ON/OFF S2 SPKRS ON/OFF S3 10k POWER LED1 (IN S1) 470 +5V Finally, the third switch controls the speakers and its blue LED lights when the speakers are on. These latter two switches toggle their respective outputs on or off each time they are pressed. Note that when the loudspeaker switch is toggled to the off position, it places dummy 22W loads across the amplifier’s left and right channel outputs – ie, these loads appear in place of the speakers. This is done to accommodate valve amplifiers, as these should be loaded at all times as explained previously. Circuit details Refer now to Fig.2 for the circuit details. It’s based on two ICs (IC1 & IC2), a couple of transistors (Q1 & Q2) and the aforementioned DPDT relays (RLY1 & RLY2). IC2, a 74HC74 dual D-type flipflop, forms the heart of the circuit. This is wired in toggle mode, with its D1 & D2 inputs directly connected to their corresponding Q1-bar and Q2-bar outputs. The two set inputs (S1-bar & S2-bar) are connected to a poweron reset circuit consisting of a 470W resistor and a 1mF capacitor, while the two resets (R1-bar & R2-bar) are connected to ground via the Mute (reset) switch. In operation, the D-type flipflop toggles its outputs on the rising edges of the clock signal pulses. When that happens the state of the D input (either a logic high or low) is transferred to the Q output and Q-bar toggles to the opposite state. For example, let’s assume that IC2’s Q1 output (pin 5) is low. This means that Q1-bar (pin 6) and D1 (pin 2) will both be high. When the next clock pulse arrives, the high on D1 is transferred to Q1 and Q1-bar and D1 toggle low. Similarly, on the next clock pulse, the low on D1 is transferred to the Q1 output and Q1-bar and D1 then toggle high again. When power is first applied, the two set inputs (pins 4 & 10) are pulled low via the 1mF capacitor. This sets IC2’s Q outputs high and so Q1-bar and Q2bar are both low and transistors Q1 & Q2 are off. As a result, the relays also remain off and the loudspeakers and headphone outputs are switched on via the NC contacts. In addition, LEDs 2 & 3 both light (since the two Q outputs are high) siliconchip.com.au Parts List 1 PC board, code 01104081, 172 x 104mm 1 ABS instrument case, 190 x 140 x 50mm (Altronics H-0374 or equivalent) 1 12V 400mA DC plugpack 2 1kW dual 16mm log pots 2 knobs to suit 2 12V DPDT relays with 10A 240VAC contacts (Jaycar SY4065, Altronics S-4310) 2 6.35mm PC-mount stereo jack sockets (Jaycar PS-0195, Altronics P-0073) 1 SPST horizontal PC-mount tactile switch with green LED (Jaycar SP-0616) 1 SPST horizontal PC-mount tactile switch with red LED (Jaycar SP-0615) 1 SPST horizontal PC-mount tactile switch with blue LED (Jaycar SP-0617) 3 3-way heavy-duty PC-mount screw terminal blocks (Altronics P-2053) 1 PC-mount 2.5mm DC socket, OR 1 panel-mount 2.5mm DC socket 4 self-adhesive rubber feet 2 M3 x 12mm machine screws 1 M3 x 6mm machine screw 3 M3 nuts to indicate that the speakers and the headphones are on. Dual timer IC1 is a 556 dual timer and is basically two independent 555s timers in one package. Both sections are configured as one-shot monostables with pulse widths of just under 0.5s. They are used to debounce switches S2 (Phones On/Off) and S3 (Speakers On/Off), to provide clean clock pulses for the D inputs of IC2. This debouncing circuitry is necessary because the metal contacts in the switches tend to “bounce” as they close. As a result, we get a series of short pulses from the switches instead of just one pulse. If these pulses were fed directly to the clock (CK) inputs of IC2, there’s no guarantee that the flipflops would toggle as the switches are just as likely to produce an even number of pulses as an odd number. The monostables in IC1 eliminate 2 M3 x 6mm tapped spacers 4 panel-mount gold-plated binding posts, red 5 panel-mount gold-plated binding posts, black 1 1m-length of heavy-duty speaker cable 1 300mm-length tinned copper wire for links Semiconductors 1 NE556 dual timer IC (IC1) 1 74HC74 dual D-type flipflop (IC2) 2 BC337 NPN transistors (Q1-Q2) 1 7805 +5V regulator (REG1) 3 1N4004 diodes (D1-D3) 2 1N4148 diodes (D4-D5) Capacitors 2 220mF 25V electrolytic 3 10mF 16V electrolytic 1 1mF 16V electrolytic 3 100nF monolithic (code 104 or 100n) 4 10nF ceramic (code 103 or 10n) Resistors (0.25W, 1%) 2 39kW 3 470W 4 10kW 4 270W 1W 2 2.2kW 2 100W 2 22W (10W wirewound) this problem. As shown, S2 & S3 are connected to the trigger inputs (Tr1 & Tr2) of the monostables via 10nF capacitors. When a switch is pressed, its corresponding trigger input is briefly pulled low (via one of the 10nF capacitors) and this triggers the monostable. As a result, the monostable’s output (pin 5 or pin 9) goes high and applies a positive-going clock pulse to the relevant clock input of IC2. This causes the corresponding D-type flipflop to toggle. For example, let’s assume that the circuit is powered up and is in the default state. If S3 is now pressed, pin 5 of IC1 goes high for about 0.5s and applies a clock pulse to pin 3 (CK1) of IC2. As a result, the relevant flipflop toggles and sends its Q1-bar output high. This turns on transistor Q1 and relay RLY1 and so the NC (normally closed) contacts open and disconnect April 2008  73 AMP EARTH 12V DC IN CON1 LEFT SPEAKER LEFT AMP OUT +LS- -LA+ + – + RIGHT SPEAKER + – – +RA- -RS+ + DNG A-< ENOHPDAEH OIDUA ROTPADA PIHC NOCILIS )C( 18040110 220 F RIGHT AMP OUT + – D1 LK3 LK9 LK5 LK6 LK7 D5 8002/10 GM 4148 10k 10nF 470 10k 100 LK4 + (LED2) S1 MUTE (RESET) S2 PHONES ON/OFF 270 270 10 F 10nF D4 4148 10k 10nF CON3 CON2 10k VR1 1k (LED1) RLY2 RLY1 LK12 LK13 10nF 2.2k 39k IC1 NE556 100nF + 10 F 10 F + 470 100nF 39k 270 100 270 + IC2 74HC74 LK2 22  10W + 2.2k LK8 D3 BC337 Q1 100nF REG1 7805 LK11 D2 470 LK1 LK10 22  10W 1 F 220 F BC337 Q2 VR2 1k (LED3) S3 SPEAKERS ON/OFF STEREO PHONES 1 VOLUME PHONES 1 VOLUME PHONES 2 STEREO PHONES 2 Fig.3: install the parts on the PC board as shown here. Leave the DC socket out if you intend mounting the board in a case and note that the two 22W 10W resistors must be mounted 3-4mm proud of the board to allow the air to circulate beneath them for cooling. the loudspeakers. At the same time, the relay’s NO contacts switch two 22W 10W resistors across the amplifier outputs to provide the dummy loads. In addition, LED 3 turns off since IC2’s Q1 output is now low. Pressing switch S3 again retriggers the monostable and toggles the flipflop to its opposite state, so that Q1-bar is low again. This turns off transistor Q1 and RLY1 and reconnects the loudspeakers via the relay’s NC contacts. In addition, LED 3 turns on (to indicate that the speakers are on) since IC2’s Q1 output is now high. Switch S3 and its following circuitry work in exactly the same fashion to control transistor Q2 and relay RLY2. This relay, in turn, switches the signals from the left and right channel amplifier outputs to the two headphone sockets (via the volume controls). Both the ring (right channel) and tip (left channel) terminals of the 74  Silicon Chip headphone sockets are driven via 270W 1W resistors and dual 1kW log potentiometers VR1 & VR2, the latter functioning as volume controls. Even with the volume wound right up, the 270W resistors should provide sufficient attenuation to protect the headphones from damage. Note, however, that you should increase these resistors to 680W or more if you have high-impedance (say 600W) headphones. The sleeve (ie, earth) terminal of each headphone socket is connected to the amplifier chassis to provide the ground return. Diodes D4 & D5 are there to ensure that IC1’s trigger inputs (pins 6 & 8) can not go more than 0.6V above the +5V supply rail. What happens is that when a switch is pressed, the relevant 10nF capacitor quickly charges to +5V via a 10kW resistor (ie, one side of the capacitor is pulled to ground and the other side goes to +5V). When the switch is subsequently released, the side that was at ground is immediately pulled to the +5V rail by another 10kW resistor and so the other side of the capacitor would go to +10V if not for the diode – ie, we would get a brief 10V spike. D4 & D5 clip these voltage spikes to +5.6V and thus prevent damage to IC1. Muting Switch S1 is the Mute (or reset) switch and is connected directly to the reset inputs (pins 1 & 13) of both flipflops in IC2. When this switch is pressed, the reset inputs are pulled to ground and the flipflops are both set with their Q outputs low and their Qbar outputs high. As a result, transistors Q1 & Q2 and the relays are on and so the headphones and loudspeakers are off. They siliconchip.com.au This view shows our completed prototype PC board. Note that it differs slightly from the final version shown at left in Fig.3, particularly with regards to the locations of the wire links. can then only be turned back on again by pressing S2 and S3. Diodes D2 & D3 are used to quench the high back-EMF spikes that are generated when the relays switch off. This is necessary to protect the relay driver transistors from damage. Power supply Power is derived from a 12V DC plugpack. This supply is filtered using a 220mF electrolytic capacitor and used to directly power the relay driver transistors and the relays. The rest of the circuit is powered from a +5V rail which is derived via 3-terminal regulator REG1. Diode D1 provides reverse polarity protec- tion and its output is filtered using a second 220mF electrolytic capacitor before being applied to the input of the regulator. A 10mF capacitor decouples the regulator’s output, with additional 100nF capacitors placed close to the supply pins of IC1 & IC2. Finally, the power LED (inside S1) is powered via a 470W current-limiting resistor. This LED is on while ever power is applied. Construction Construction is straightforward with all the parts mounted on a PC board coded 01104081. Our prototype was housed in a plastic case measuring 190 x 140 x 50mm. Note that if you intend using this case, it will be necessary to cut out the front corner pieces from the PC board in order to clear the front case pillars. Fig.3 shows the parts layout on the PC board. Before mounting any parts, check the board carefully for etching defects, then check the hole sizes for the headphone sockets, screw terminal blocks and relays by test fitting these parts into position. Enlarge any holes if necessary. Begin the assembly by installing the 12 wire links in the positions indicated. These links should all be run using tinned copper wire and must be straight. To straighten the link wire, simply Resistor Colour Codes o o o o o o o o siliconchip.com.au No.   2   4   2   3   4   2   2 Value 39kW 10kW 2.2kW 470W 270W 100W 22W 4-Band Code (1%) orange white orange brown brown black orange brown red red red brown yellow violet brown brown red violet brown brown brown black brown brown NA 5-Band Code (1%) orange white black red brown brown black black red brown red red black brown brown yellow violet black black brown red violet black black brown brown black black black brown NA April 2008  75 As shown here, it’s necessary to notch (or cut out) the front corners of the PC board to clear the case pillars. The wiring to the binding post terminals on the rear panel is run using heavy-duty speaker cable. clamp one end in a vice and then stretch the wire slightly by pulling on the other end with a pair of pliers. Each link can then be cut to length and its end bent down at right angles before installing it on the PC board. Note particularly that LK4 goes under IC1, while LK9 runs directly behind the screw terminal blocks. That done, install the resistors and diodes D1-D5 but leave the 22W 10W resistors out for the time being. Table 1 shows the resistor colour codes but you should also check each one using a digital multimeter. Take care with the orientation of the diodes and note that D1-D3 are 1N4004s while D4 & D5 are 1N4148s. Note also that D4 and D5 face in opposite directions. The 7805 3-terminal regulator is 76  Silicon Chip next on the list. As shown, it’s installed with its metal tab flat against the PC board and its leads bent down through 90° to go through their respective holes. To do this, first position the device on the board, then use a pair of needlenose pliers to grip the leads at the appropriate point and bend the leads down by 90°. The device’s metal tab can then be fastened to the board using an M3 x 6mm screw, nut and lockwasher and the leads soldered. Do not solder the leads before bolting down the metal tab. If you do this, you could crack the soldered joints as the tab is bolted down. The capacitors are next on the list. Start with the monolithic and ceramic types, then install the six electrolytics. Make sure that the latter are all in- stalled with the correct polarity. Next, install the two 22W 10W resistors. These should be mounted 3-4mm proud of the PC board to allow the air to circulate beneath them for cooling. A couple of thick pieces of cardboard can be used to achieve an even spacing. Now for the ICs and transistors. Push the transistors down onto the board as far as they will comfortably go before soldering their leads and be sure to use the correct IC at each location. Make sure also that each IC is correctly oriented and be careful not to create unwanted solder bridges when soldering their pins. Finally, the board assembly can be completed by mounting the larger hardware items. These include the two pots, the headphone sockets, siliconchip.com.au 12V DC INPUT LEFT SPEAKER AMPLIFIER EARTH LEFT CHANNEL AMPLIFIER OUT RIGHT SPEAKER RIGHT CHANNEL AMPLIFIER OUT + + + + – – – – (REAR PANEL OF BOX) + REAR EDGE OF PC BOARD ENOHPDAEH OIDUA DNG A-< ROTPADA PIHC NOCILIS )C( 18040110 +LS- -LA+ +RA- -RS+ + + Fig.4: follow this diagram to complete the wiring between the PC board and the rear panel hardware. Note that the leads to the loudspeaker and amplifier terminals should be run using heavy-duty speaker cable. the relays and the three 3-way screw terminals blocks. Cut the pot shafts to about 15mm long before fitting them and make sure that each part is seated correctly against the PC board before soldering its leads. The DC socket should also be installed unless you are mounting the board in a case and intend using a panel-mount DC socket instead. Testing Before applying power, go over the board and carefully check your work. In particular, check that the correct part has been used at each location, that all polarised parts are correctly oriented and that there are no missed solder joints or solder bridges. Once you are satisfied that all is correct, connect a 12V DC plugpack, switch on and check that all three LEDs in switches S1-S3 light. Check also that the relays remain off at switch-on. Now press the Phones switch (S2) and check that relay RLY2 toggles. At the same time, the green LED in S2 should go out. Pressing this switch again should toggle RLY2 off again and turn the green LED back on. Finally, check that RLY1 and the blue LED alternately toggle on and off each time the Speakers switch (S3) is pressed. If the module passes all these tests, siliconchip.com.au then it is working correctly. If not, then you’re in for a spot of troubleshooting. Here’s what to look for if it doesn’t work: (1) Symptom: no LEDs light when power is applied. Do this: check the supply polarity. If that’s correct, check the orientation of diode D1 and check for +5V at the output of regulator REG1. (2) Symptom: all LEDs initially light but one relay refuses to toggle when its switch is pressed Do this: check that the corresponding Q output from IC2 toggles correctly (ie, between 0V and about +4.8V) each time the switch is pressed. If it does, then check the relevant transistor – its collector should toggle high or low each time the switch is pressed. If the transistor is switching correctly but the relay doesn’t operate, check that the diode across the relay coil is correctly oriented. (3) Symptom: a Q output from IC2 does not toggle when the relevant switch is pressed. Do this: check IC2 & IC3 for correct placement and orientation and check that their pins are all soldered correctly. Check also that diodes D4 and D5 are the right way around (note: these two diodes face in opposite directions). If you have a scope, check the rel- evant output (pin 5 or 9) from IC1 – you should see a 0.5s positive-going pulse each time the switch is pressed. Check that this pulse is being applied to the corresponding clock input of IC2. If there are no pulses from IC1, check the parts associated with the switches at the trigger inputs to this IC. The IC itself may also be faulty (unlikely). Final assembly Now for the final assembly. If you are building the unit from a kit, the case will probably be supplied predrilled with screen-printed front and rear panels. If not, then you will have to drill the panels yourself using the front and rear panel artworks (Figs.5 & 6) as templates. The best approach is to first centrepunch the hole locations, then drill each one using a small pilot drill before enlarging it to the correct size. The larger holes (ie, for the headphone sockets, the pots and the DC socket) should be initially drilled to about 5mm, then carefully enlarged to size using a tapered reamer. That done, the panel artworks can be downloaded from the SILICON CHIP website and printed onto photographic paper. They are then attached to the panels using an even smear of silicon sealant and the holes cut out using a very sharp hobby knife. April 2008  77 2-WAY HEADPHONE ADAPTOR volume mute/ reset right speaker phones on/off speakers on/off right amplifier phones 2 phones 1 left amplifier left speaker amplifier earth 12V DC + – SILICON CHIP 2-WAY HEADPHONE ADAPTOR Figs.5 & 6: these full-size artworks can be used as drilling templates for the front and rear panels Next, fit the front panel to the module and secure it by doing up the nuts for the two headphone sockets. Similarly, fit the panel-mount DC socket and the nine gold-plated binding post terminals to the rear panel. Now drop the PC board and front panel assembly into the case and mark out the locations in the base for the rear corner mounting holes. Remove the board and drill these two holes to 3mm, then mount two tapped 6mm spacers in these positions, securing them using M3 x 12mm machine screws which pass up through the bottom of the case. That done, solder a couple of 50mmlong medium-duty hook-up leads to the power supply pads on the PC board. The board assembly can then be secured in place and the wiring to the rear panel completed as shown on Fig.4. Be sure to use extra heavy-duty speaker cable (eg, 32/0.20) for all connections between the speaker binding posts and the screw terminal blocks. The lead to the amplifier earth terminal The completed 2-Way Stereo Headphone Adaptor can be used with both valve and solid-state amplifiers. 78  Silicon Chip can be run using medium-duty hookup wire. You can then complete the assembly by fitting the knobs to the pot shafts and attaching the case lid. Trying it out As already mentioned, this unit connects in series between the amplifier outputs and the loudspeakers, so disconnect the loudspeaker leads from the amplifier and connect them to the Stereo Headphone Adaptor instead. The outputs from the amplifier then connect to the left and right channel input terminals on the adaptor. Finally, connect a lead from the terminal marked “Amp Earth” to the amplifier’s chassis. If your amplifier doesn’t have a ground terminal, then it may be possible to attach a solder lug under one of the case screws. Alternatively, the earth lead can go to the “negative” terminal of one of the amplifier output channels (but not to both, otherwise you’ll get an earth loop and lots of hum). This can be done by connecting an insulated wire link between the “Amp Earth” terminal and the “Left Amp -” terminal on the back of the Stereo Headphone Adaptor. After that, it’s just a matter of switching everything on, plugging in your SC headphones and trying it out. siliconchip.com.au