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
