This is only a preview of the June 1994 issue of Silicon Chip. You can view 35 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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Need another pair of audio inputs?
Convert the phono
inputs on your amplifier
Do you need another pair of line inputs on
your stereo amplifier? And do you no longer
use the phono inputs? Well, here’s the answer
– convert the phono inputs to normal line
inputs & presto, another pair of inputs.
By LEO SIMPSON
Let’s face it, most people these days
do not listen to vinyl records and have
long since dispensed with their turntable, if indeed they ever had one. So
the phono inputs on their stereo system are unused and useless. But these
same people often also need another
pair of inputs so they can hook in an
extra tape deck, VCR or other program
source with line outputs. What to do?
Until now, there wasn’t much of an
answer. If you knew your way inside
an amplifier or stereo receiver, you
could whip out the phono preamplifier and rewire the inputs as straight
line inputs but that is an option few
people would care to take. Some
people may also still have a turntable
which they want to able to use from
time to time but want the option of
using the phono inputs for line level
inputs.
The solution is to convert the phono
inputs to line inputs via an external
adaptor box and for this idea we are
indebted to Gary Johnston of Jaycar
Electronics.
Essentially, all phono inputs provide a great deal of am
plification
(about 34dB or 50 times, at 1kHz),
as well as RIAA equalisation for the
signal from magnetic cartridges. The
RIAA equalisation curve provides lots
of bass boost at very low bass frequencies (approaching +20dB at 20Hz),
tapering down to zero at 1kHz. Beyond
1kHz, the circuit applies treble cut and
this increases as the frequency rises so
that at 20kHz, the cut is almost 20dB.
The general slope of the attenuation is
-6dB octave right across the frequency
range but there are two inflections in
the curve at 500Hz and 2kHz (2122Hz,
to be precise).
So to convert the phono input to a
line input with a sensitivity of around
100mV at 1kHz, you need a circuit
which will provide about 34dB attenuation overall and a filter characteris
tic which is exactly the reverse of the
RIAA equalisation – see Fig.1. This
may sound as though a fairly complicated circuit is required but in practice
it turns out to be quite simple.
Circuit details
Our prototype inverse RIAA network was built into a relatively large metal
diecast case & fitted with a Dynamark self-adhesive label to dress it up. It
enables the phono inputs on your amplifier to be used with line level sources
such as tape decks or a VCR.
54 Silicon Chip
The suggested circuit is shown in
Fig.2. The circuit is passive; ie, it has
no active components such as transistors or integrated circuits and no
power supply is needed.
Actually, while the circuit is quite
simple, a good deal of design work
has gone into it to make sure that
it does the job. As you can see, it
involves just three resistors and two
Performance
We used our prototype converter
with a number of commercial amplifiers and also with our latest Studio
series remote control preamplifier as
described in the September, October
+20
+10
DECIBELS
capacitors for each channel. So how
did we go about designing this little
network?
First of all, we had to consider the
type of source which would feed the
circuit. Now all program sources,
whether they are a CD player, tape
deck, AM/FM tuner or whatever, use
operational amplifier ICs in their output stages. This is important because it
means that the inverse RIAA network
can have quite a low impedance and
still not upset the performance of the
program source.
Second, the phono inputs of the
amplifier have three re
quirements
if they are to give optimum performance. They must be presented with
a source which has the same frequency
characteris
tic as a magnetic phono
cartridge and the signal level must be
about the same. But most important,
the source impedance “seen” by the
phono inputs should be as low as possible so that we obtain the optimum
signal to noise ratio.
If we were to use a high impedance
filter and attenuation network, it
would give the right overall frequency response and signal levels but the
resulting sound quality would be
unsatisfactory because the background
hiss and noise would be too loud. So
we have designed a network which
provides a low impedance source for
the phono inputs and the result is very
low background noise. In fact, when
you have the device installed properly,
it will be difficult to hear the difference
between your regular line inputs (ie,
CD, tuner etc) and your converted
phono inputs.
As you can see from the circuit
of Fig.2, the inverse filter network,
consisting of a 200kΩ and 16kΩ
resistors shunted by capacitors, is
virtually identical to the feedback
components of the RIAA version of
the universal preamplifier published
in the April 1994 issue of SILICON
CHIP. Surprised? You should not be,
since in an RIAA preamplifier the
feedback network is an attenuator and
filter, exactly what is needed here.
We tried several inverse RIAA filters
before settling on this one.
0
-10
-20
20
100
HERTZ
1k
10k
20k
Fig.1: this graph shows the frequency characteristic of the filter circuit.
RIGHT
INPUT
.015
.0047
200k
16k
RIGHT
OUTPUT
560
LEFT
INPUT
.015
.0047
200k
16k
LEFT
OUTPUT
560
TO AMPLIFIER
CASE
INVERSE RIAA NETWORK
Fig.2: the circuit of the inverse RIAA
network is an attenuator & filter which
exactly compensates for the magnetic
cartridge equalisation in the amplifier. It
provides a low impedance source to the
phono inputs to ensure low background
noise.
LEFT INPUT
PARTS LIST
1 diecast case to suit PC
board
1 PC board, code 01105941,
39 x 46mm
2 2-way RCA socket panels
(Jaycar Cat. PS-0263 or
equivalent)
1 solder lug
4 6mm untapped spacers
Capacitors
2 .015µF MKT polyester
2 .0047µF MKT polyester
Resistors (0.25W, 1%)
2 200kΩ
2 560Ω
2 16kΩ
Miscellaneous
Screws, nuts, lockwashers,
hookup wire, solder.
.015
.0047
200k
16k
LEFT OUTPUT
560
RIGHT INPUT
200k
560
16k
.015
.0047
RIGHT OUTPUT
Fig.3: the component overlay of the PC board. Note that the shields of
the RCA phono sockets are not connected to the case of the device.
June 1994 55
This photo shows the internal
details of the inverse RIAA
network prototype. No
shielded cable is required for
wiring the RCA sockets since
the metal box provides a total
shield for the circuit.
& November 1993 issues
of SILICON CHIP. With the
latter control unit, we were
able to obtain an overall
frequency response from
the converted phono inputs
within ±0.3dB from 20Hz
to 20kHz and a signal to noise ratio
of 78dB unweighted with respect to
1kHz and 200mV. These are excellent
figures. Just how well the inverse
RIAA network performs will depend
on the quality of the amplifier it is
teamed with but as you can see, the
results are more than adequate for
program sources such as tape decks,
VCRs and tuners.
For our listening tests, we used a
Yamaha CDX-1110 CD player which
has two sets of outputs. One set we
connected to the CD inputs on the
amplifier in the normal way while
the other set of outputs were connected via the inverse RIAA network
and then to the phono inputs of the
amplifier. We were then able to make
direct comparisons between the CD
LEFT
SILICON
CHIP
and phono inputs. In practice, unless
the CD player was in pause mode, it
was very difficult to tell the difference.
Construction
For this project you can go as basic
Fig.4: actual size artwork for
the PC board.
OUTPUT
RIGHT
GROUND
+20
DECIBELS
+10
0
-10
-20
20
100
HERTZ
1k
10k
20k
INVERSE RIAA FILTER
LEFT
INPUT
RIGHT
Fig.5: this the front panel artwork used for our prototype. It shows the inverse
RIAA filter characteristic provided by the circuit.
56 Silicon Chip
or as deluxe as you want. You could
assemble the circuit on a small piece
of Veroboard or you could go for the
deluxe approach as we did – put it on
a small PC board which is then wired
in a diecast metal case. Whichever
approach you use, the finished circuit
must be mounted in a metal case which
is earthed back to the amplifier’s case.
If this is not done, you will have problems with hum pickup.
We wired up our prototype circuit
on a small PC board measuring 39 x
46mm and coded 01105941. After
the six resistors and four capacitors
are mounted, the board was mounted
in the diecast box. This needs to be
drilled for the four RCA phono sockets and four screws to mount the PC
board. Finally, a solder lug and wire
lead must be fitted to the case for
earthing.
When installing the unit, keep
the unit away from the power transformer in your amplifier and make
sure that the input and output leads
do not drape across mains power
cords otherwise hum pickup may
be a problem.
Finally, while we optimised the
circuit to suit CD player signal levels
and typical audio amplifiers, you may
need to increase or reduce the signal
level to suit your application. This is
easily done. If you want more signal
level, increase the 560Ω resistors at
the output to say, 1kΩ. Alternatively, if you need less signal level, try
reducing the 560Ω resistors to 330Ω
SC
or 270Ω.
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