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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
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