Ultra-low-distortion
Preamplifier with
Tone Controls Part 3
This high-performance Audio Selector can expand the number of inputs to
the Ultra-low-distortion Preamplifier. It can also be used to ‘upgrade’ just
about any piece of audio equipment with stereo line level inputs or simply
operate as a stand-alone device.
Here we discuss both options.
by John Clarke
Six-input Stereo Audio
Selector – no more swapping cables
every time you want to change audio sources!
I
f you’re one of those people
who enjoys listening to music from
a variety of sources, you’ll know just
how much of a pain unplugging and
replugging cables can be.
For example, you might want to listen to CDs or DVDs one day, an MP3
player another, and a turntable on another. Other times there’s the audio
from your TV... but most of the time
it’s a DAB, FM or AM tuner. That’s five
but there are many more.
Our current Preamplifier project
was designed to switch between three
stereo sources, using either a remote
control or front panel pushbuttons.
And while three inputs are enough
for many people, inevitably, some
need more!
This Preamplifier is a very high-performance stereo unit, with vanishingly
low noise and distortion. It has remotecontrolled volume and input switching, with stereo and bass tone controls.
32
While it is possible to add an external
input switcher to expand the number of
available inputs (eg, our January 2013
Three Input Switcher), that’s an unsatisfying solution. After all, who wants an
extra box and an extra remote control?
This project expands the number of
stereo inputs to the Preamplifier (or indeed any other preamp or all-in-one) to
six, which should satisfy most people.
Yes, we know that there will be people
who need seven or eight, but you have
to stop somewhere!
It’s an easy upgrade. Simply build
the two new boards, wire them up to
the existing Preamplifier main board
and reprogram the Preamplifier’s microcontroller and you’ll have more inputs! You can still use the same remote
control to adjust the volume and switch
between the six input pairs.
So that you can use it with other
preamp designs, or other equipment
entirely, we have designed it so that it
can be used as a standalone unit. All you
need to do is build the boards, put them
in a box and connect a small 9-15V DC
power supply and you have a remotecontrolled Six-input Selector with front
panel pushbuttons and LED indicators.
You can control it with just about any
universal remote.
Overall design
The Audio Selector consists of two
PCBs. The main one has the six stereo
inputs, one pair of stereo output sockets and the relays used for switching
between the inputs.
The control PCB has the six pushbutton switches to select each input, with
integral LEDs and mounts on the front
panel of the unit.
The two PCBs are connected by a 14way ribbon cable with IDC connectors
at each end. When used as a standalone
unit without the Preamplifier, an infrared receiver can be included on the
Practical Electronics | June | 2020
control PCB. The main PCB also has a
5V regulator to power the whole circuit
from a 9-15V DC source.
When used with the Preamplifier, the
Audio Selector is connected to the main
Preamplifier board via a 10-way ribbon
cable with IDC connectors. In this case,
the Audio Selector is powered from the
Preamplifier over this cable. The infrared receiver on the Preamplifier is
then used to control the Audio Selector as well as adjusting the volume on
the Preamplifier.
This requires revised firmware to be
loaded onto the Preamplifier microcontroller. If you have a PIC programmer,
you can download the software from the
June 2020 page of the PE website and
reprogram the chip yourself.
If you are buying your Preamplifier parts as a kit from Altronics (code
K5171) then the microcontroller will be
supplied ready for the Six-input Audio
Selecctor – no reprogramming needed.
Circuit description
Fig.17 shows the circuit of the main
(switching) board, while Fig.18 is the
circuit diagram of the front panel control board.
Let’s start with the main circuit,
Fig.17. It’s based around microcontroller
IC1, which drives the DPDT input selection relays (RLY1-RLY6) via NPN transistors Q1-Q6 and monitors the switches
and infrared receiver via CON10.
When the circuit is powered up, the
coil of one of six relays RLY1-RLY6 is
energised at any given time. Each relay’s pair of COM terminals is connected to its corresponding pair of RCA input sockets, CON1-CON6. So when its
coil is energised, those signals are fed
through a pair of 100Ω series resistors
and ferrite beads FB1 and FB2 to the
output sockets, CON7 and CON8.
The series resistors, ferrite bead and
470pF capacitors heavily attenuate any
ultrasonic signals which may be picked
up by the Preamplifier inputs and wiring.
Such signals often come from electromagnetic emissions from nearby equipment or broadcast radio stations (the
wires act like antennas). However, we
only want to feed audio (20Hz-20kHz)
signals to the following equipment.
One end of each relay coil is permanently connected to the +5V supply,
Features
n Six stereo inputs
n Input selection via pushbutton or infrared remote control
n LED indicators to show currently selected channel
n Remembers currently selected input even when powered off
n Build as a standalone unit or incorporated into one of two high-performance preamplifiers
n No mains wiring required; can run off low-voltage DC
n Retrofit to suitable existing preamplifiers
Constructors – please see note
n Negligible noise and distortion
in the April 2020 issue Parts List
n Easy construction
before purchasing components.
n Uses common parts
while the other end is connected to
ground by one of six NPN transistors,
Q1-Q6. Each of these transistors has a
2.2kΩ base-current limiting resistor,
which is driven by one of the digital
outputs of IC1; RA2 (pin 1) for Q1, RA3
(pin 2) for Q2 etc.
So when one of these outputs goes
high, the base-emitter junction of the
corresponding transistor is forward biased, switching on that transistor and
pulling current through the connected
relay coil, energising it.
When that output goes low, the transistor switches off and the connected
diode (one of D1-D6) prevents the coil
from generating a high-voltage spike
as its magnetic field collapses, which
could damage the connected transistor.
When used as a standalone unit, an
external source of DC power is applied
to terminal block CON11, and this is
regulated to 5V by REG1 to power the
relays and IC1. Diode D7 provides reverse polarity protection while 100µF
capacitors are used for input bypassing
and output filtering of REG1. JP1 is fitted in the upper position.
When used as part of a Preamplifier, 5V power comes from pins 7 and
8 of CON9, with the ground connection made at pins 9 and 10. In this
case, JP1 is fitted in the lower position.
IC1 has a 100nF bypass capacitor and
10kΩ reset pull-up resistor to ensure
correct operation.
Control board circuitry
As shown, CON10 on the main board
connects to CON12 on the control board
(Fig.18). This allows microcontroller IC1
to detect when one of the front panel
pushbuttons is pressed and also illuminate the LED in one of the buttons, to indicate the currently selected input.
Looking at the rear of the input PCB with its six stereo RCA sockets,
hiding the low-profile relays behind. At left foreground is the
connector which has the cable connecting to the Preamplifier board.
Practical Electronics | June | 2020
LED1-LED6 are housed within
pushbuttons S1-S6. Their cathodes
are joined together and to a 2.2kΩ
resistor to ground, setting the maximum LED current to around 0.8mA
([5V − 3.3V] ÷ 2.2kΩ). One LED anode
is driven to +5V to light it up and the
others are left low at 0V, turning off
the other LEDs.
This is done via pins 5, 7, 9, 11, 13
and 14 of CON12, which connect back
to the same pins on IC1 as are used to
drive the relays via the six transistors
(see Fig.17).
Hence, whenever a relay is activated by that output going high, the corresponding LED on the front panel
lights up.
The pushbutton switches are connected in a ‘matrix’ manner to pins 3, 4,
6, 8 and 10 of CON12. This reduces the
number of pins needed to sense a press
of one of the six buttons by one (to five).
Pins 3 and 4 of CON12/CON10 connect to the RB3 and RB4 outputs of
IC1, while pins 6, 8 and 10 of these
connectors go to the RB5, RB6 and RB7
inputs of IC1. These inputs are typically held at 5V via pull-up currents
which are internal to IC1.
Switches S1, S3 and S5 have one
side connected to the RB4 output,
while switches S2, S4 and S6 have one
side connected to the RB3 output. The
other sides of the switches are monitored by the RB5, RB6 and RB7 inputs.
Periodically, outputs RB3 and RB4
are briefly brought low in turn, and if
one of the three inputs (RB5, RB6 or
RB7) goes low at the same time, that
means one of the three switches connected to that output has been pressed.
The micro figures out which one has
been pressed based on which combination of these five pins is low and
switches to the selected input.
The current input can also be
changed by infrared remote control.
Infrared receiver IRD1 is a complete
infrared detector and processor; its 5V
supply is filtered by a 100Ω resistor
and 100µF capacitor. It receives the
38kHz signal from the remote control, amplifies, filters and demodulates it. The result is a serial data
burst at its pin 1 output. This is fed
33
Reproduced by arrangement with
SILICON CHIP magazine 2020.
www.siliconchip.com.au
Six-input Stereo Audio Switcher
Fig.17: the circuit of the main Audio Selector board. Microcontroller IC1 switches on one of the six relays, to connect
the appropriate pair of input sockets to the output, using NPN transistors Q1-Q6. It connects to the front panel
pushbutton/LED board via CON10. That front panel board also hosts the infrared receiver, if built as a standalone
unit. If part of a Preamplifier, the IR receiver is on the Preamplifier board, which is connected via CON9.
to the RA6 digital input of IC1via pin
12 of CON12.
Software within IC1 determines
whether the received code is valid and if
so, which button on the remote control
34
has been pressed and whether that corresponds to one of the six inputs. If it does,
it switches to the new input.
Regardless of which method is used
to select an input, as well as changing
over the relays as needed, IC1 stores
the current input selection in its permanent EEPROM memory so that the
same input will be selected the next
time the unit is powered up.
Practical Electronics | June | 2020
Six-input Stereo Audio Switcher
Fig.18: the circuit of the front panel control board is quite simple; it mainly hosts
pushbuttons S1-S6, which have integral LEDs, plus the infrared receiver and its
supply filter, which are only fitted if building the Audio Selector as a standalone
unit. Otherwise, these parts will already exist on the Preamplifier board.
If the Audio Selector circuit is built
as part of the Preamplifier, IRD1 and its
supply filter components are not fitted.
The infrared receiver on the preamplifier board is used instead. This controls
the volume on the Preamplifier directly. If an input change is required, the
Preamplifier board sends a coded signal
over pins 1-6 of CON9.
These signals are fed to the RA1, RA0
and RA7 inputs of IC1 (pins 18, 17 and
16). The signals carry serial data indicating which input has been selected.
If you built the 3-input Preamplifier
and you are now upgrading to the 6-input version, the microcontroller on the
Preamplifier must be reprogrammed to
send these signals, as the earlier designs
did not have this capability. (The revised firmware, coded 0111111M.HEX,
can be downloaded from the June 2020
page of the PE website.) Once IC1 receives valid serial data from that micro,
it switches inputs as required.
Construction
The components for the circuit shown
in Fig.17 are fitted to a double-sided
PCB coded 01110191, and measurPractical Electronics | June | 2020
ing 165 × 85mm. The separate control
section components are mounted on
a double-sided PCB, coded 01110192,
and measuring 106 × 36mm. Both
boards are available from the PE PCB
Service.The overlay diagrams for these
boards, which indicate where the components go, are shown in Fig.19 and
Fig.20. Start by building the main board.
Fit the resistors first, and be sure to
check values with a DMM set to measure resistance.
Follow with diodes D1 to D6, and
install D7 if building the standalone
unit. Ensure that their cathode stripes
face as shown, then feed resistor lead
off-cuts through the ferrite beads and
solder them in place.
We recommend that IC1 is installed
using a socket. Make sure its pin 1 dot/
notch faces toward CON9, as shown.
Fit the two 470pF MKT/MKP/ceramic
capacitors next. Any of these types can
be used, but if you use ceramics, they
must use the NP0 or COG dielectrics for
excellent low-distortion performance.
If building the standalone version,
you can now bend REG1’s leads to fit
the pads, attach it to the board using the
specified machine screw and nut and
solder and trim its three leads.
Mount the remaining capacitors
such as the 100nF MKT polyester or
ceramic and the 100µF electrolytic
capacitors. Electrolytic capacitors are
polarised so the longer positive leads
must go through the holes marked ‘+’.
Note that only one 100µF capacitor is
needed when the Audio Selector is
used as part of a Preamplifier.
Fit the six transistors next. You may
need to gently bend their leads out
(eg, using small pliers) to fit the PCB
footprints. Ensure the flat sides face
as shown.
Construction continues with the installation of the 3-way pin header for
JP1 and the 10-way and 14-way box
headers, CON9 and CON10. These sockets must be installed with their slotted
keyways oriented as shown. Remember
that you don’t need CON9 for the standalone version, but you do need CON11,
so now is a good time to fit it.
Finally, complete the assembly by installing the six relays, the stereo RCA input sockets and the two vertical RCA output sockets. The red vertical RCA socket goes on the left and the white socket
on the right. These colours then match
those for the CON1-CON6 stereo sockets.
Once you’ve finished soldering the
parts to the board, plug the programmed
microcontroller (IC1) into its socket, ensuring that it is oriented correctly.
Front panel control board assembly
There are only a few parts on the control board but be careful to install the
parts on the correct side of the PCB.
The component footprints are screen
printed on the side they should be installed. Pushbutton switches S1-S6 and
IRD1 are on one side (the underside, as
shown in Fig.20), and the 14-way IDC
header CON12, the resistors and 100µF
capacitor are on the other (top side).
IRD1, the 100µF capacitor and 100Ω
resistor are not required when the Audio Selector is part of a Preamplifier.
Fit the pushbuttons first but note
that they must be installed the right
way around. These have kinked pins at
each corner plus two straight pins for
the integral blue LED. The anode pin
is the longer of the two, and this must
go in the hole marked ‘A’ on the PCB
(towards CON12).
Once the pins are in, push the buttons all the way down so that they sit
flush against the PCB before soldering
their leads. IDC header CON12 can
then be installed on the other side of
the board, with its keyway notch towards the bottom.
IRD1, the 100Ω resistor and the
100µF capacitor should now be fitted,
if building the standalone version. The
35
Parts list – Six Input Audio Selector
Main board and Control board
1 double-sided PCB, code 01110191, 165 x 85mm
1 double-sided PCB, code 01110192, 106 x 36mm
Both PCBs are available from the PE PCB Servce
6 PCB-mounting DPDT relays with 5V DC coil (RLY1-RLY6)
[Altronics S4147]
6 PCB-mounting dual vertical RCA sockets (CON1-CON6)
[Altronics P0212]
1 white vertical PCB-mount RCA socket (CON7)
[Altronics P0131]
1 red vertical PCB-mount RCA socket (CON8)
[Altronics P0132]
2 14-pin PCB-mount vertical IDC headers (CON10,CON12)
[Altronics P5014]
6 PCB-mount pushbutton switches with blue LEDs (S1-S6)
[Jaycar SP0622, Altronics S1173]
2 ferrite beads (FB1,FB2)
[Jaycar LF1250, Altronics L5250A]
1 3-way pin header, 2.54mm spacing (JP1)
1 jumper shunt/shorting block (JP1)
1 18-pin DIL IC socket (for IC1)
4 M3 x 12mm nylon tapped spacers
4 M3 x 6.3mm nylon tapped spacers
16 M3 x 6mm panhead machine screws
2 14-pin IDC line sockets [Altronics P5314]
1 350mm length of 14-way ribbon cable
Semiconductors
1 PIC16F88-I/P microcontroller programmed with 0111019A.
HEX (IC1)
6 BC337 NPN transistors (Q1-Q6)
6 1N4004 1A diodes (D1-D6)
Capacitors
1 100µF 16V PC electrolytic
1 100nF MKT polyester or multi-layer ceramic
2 470pF NP0/C0G ceramic or MKT polyester or MKP
polypropylene
[eg, element14 Cat 1005988]
Resistors (all 0.25W, 1% metal film)
1 10kΩ
6 2.2kΩ
12 100Ω
Extra parts for standalone version
1 3-pin infrared receiver; eg TSOP4138, TSOP4136 (IRD1)
1 7805 5V regulator (REG1)
1 1N4004 1A diode (D7)
3 100µF 16V PC electrolytic capacitors
1 2.2kΩ 0.25W 1% resistor
1 100Ω 0.25W 1% resistor
1 2-way screw terminal, 5.08mm spacing (CON11)
1 M3 x 6mm panhead machine screw and hex nut (for REG1)
Extra parts for connecting to Preamplifier
1 PIC16F88-I/P microcontroller programmed with 0111111M.
HEX*
1 10-pin PCB-mount vertical IDC header (CON9)
[Jaycar PP1100, Altronics P5010]
2 10-pin IDC line sockets
[Jaycar PS0984, Altronics P5310]**
1 250mm length of 10-way ribbon cable**
* replaces IC5 in the Preamp described in April and May 2020
** not required if already part of pre-existing preamp
You will also need a ‘Universal’ Remote Control (see text) – eg
Altronics A012 or Jaycar AR1954 or AR1955
36
100Ω resistor and 100µF capacitor are mounted on the same
side as CON12, while IRD1 is mounted on the pushbutton
side, with its lens in line with the switches. The leads are
bent at right angles, and it is mounted so that IRD1 is at the
same height as the buttons.
Making the cables
You need to make the interconnecting cables before you can
test the Audio Selector. If you’re building a standalone unit,
you only need to make the 14-way cable which connects
the two boards, shown at the bottom of Fig.21. Otherwise,
make both the cables, including the 10-way cable that will
connect back to the Preamplifier board.
If you’re building this unit as an upgrade to an existing
Preamplifier which already has the three-way input switcher, you should already have those cables.
Pin 1 is indicated on each socket by a small triangle
moulded into the plastic, while wire 1 in each section of
ribbon cable should be red. The red stripe of the cable must
go to pin 1.
The best way to crimp the IDC connectors onto the cables is by using a dedicated IDC crimping tool, such as the
Altronics T1540.
Alternatively, you can crimp them in a vice or using large
pliers that have jaw protectors, or a woodworker’s screw-adjust G clamp with the IDC connector sandwiched between
two strips of timber.
Don’t forget to fit the locking bars to the headers after
crimping, to secure the cable in place. Having completed
the cables, it’s a good idea to check that they have been correctly terminated. The best way to do this is to plug them
into the matching sockets on the PCB assemblies and then
check for continuity between the corresponding pins at either end using a multimeter.
When complete, plug the 14-way cable into CON10
and CON12. The 10-way IDC cable (if used) connects between CON9 of the 6-Input Audio Selector and CON7 on
the Preamplifier.
Now place the shorting block on JP1 in the correct position, ie, to the left if you are building this as part of a Preamplifier, or to the right if it is a standalone unit.
Initial testing
Before programming the remote, it’s worthwhile to power the
unit up and check that the pushbutton, relays and LED indicators work as expected. If you’re building it as a standalone
unit, this is easily done by feeding 9-15V DC into CON11.
Otherwise, you will need to plug the unit into the Preamplifier board and power it in the usual way.
You can run the Preamplifier off an AC plugpack for testing, if you have one, via a rectifier and regulator board (see
the last issue for options). You can switch to using a mainsbased power supply once testing is complete.
Apply power and check that one LED lights up and you
should hear a relay click on when power is applied. Press
all the buttons and verify that you hear a click and that the
LED in that button lights up, with all the others off.
If you want, you can feed an audio signal into each input
in turn and check that it’s only fed through to the output
connectors when that input is selected.
Setting up the remote control
The remote control functions can now be tested using a suitable universal remote, as described below. By default, the
Audio Selector expects remote control codes for a Philips
TV. If this conflicts with any other equipment in your possession, you can switch it to use SAT1 or SAT2 instead.
If you have built the Audio Selector as a standalone unit,
all you need to do to change modes is to press and hold S1
Practical Electronics | June | 2020
Fig.19: follow this diagram and the photo below to build the main Audio Selector PCB. Make sure that the header
sockets are correctly oriented, as well as IC1, the diodes and electrolytic capacitors. Note that CON1, D7, the two 100µF
capacitors and REG1 are only installed if you are building it as a standalone unit.
on the pushbutton board during powerup to switch to using the SAT1 code,
or S2 for SAT2. Pressing and holding
S3 at power-up reverts to the default
TV mode.
It’s a bit more tricky if you’re building this as part of a Preamplifier because the Preamplifier board has no
way of reading the switch states. So in
this case, you have to unplug the 10way cable from CON7 on the Preamplifier board and then use a femalefemale jumper lead to temporarily
Practical Electronics | June | 2020
short pins 1 and 9. Apply power, and
wait a few seconds, then switch off,
remove the jumper cable and plug
the ribbon cable back in. That selects the SAT1 mode. If you need to
select SAT2 mode, bridge pins 3 and
9 instead. To go back to the TV code,
bridge pins 5 and 9.
Pin 1 is the one in the upper righthand corner of CON7, nearest to the
microcontroller, while pin 9 is in the
upper left-hand corner. Pin 3 is immediately to the left of pin 1, and so on.
Programming the remote itself
Once you’ve chosen the mode, the correct code must be programmed into the
remote control. This involves selecting TV, SAT1 or SAT2 on the remote
(to agree with the microcontroller setup) and then programming in a three
or four-digit number to tell the remote
control to send the codes that the unit
is expecting to receive.
Most universal remote controls can
be used, such as the Altronics A1012
and the Jaycar AR1955 or AR1954. For
the Altronics A1012, use a code of 023
or 089 for TV mode, 242 for SAT1 or
245 for SAT2. Similarly, for the Jaycar
37
Fig.20: the six pushbutton switches and infrared receiver IRD1 (for the standalone
version) are mounted on the back of the pushbutton board (which faces towards
the front of the unit when installed) while the header socket, resistors and
capacitor go on the top (with CON12’s keyway towards S3 and S4). Make sure that
the longer straight lead of each pushbutton goes to the pad marked ‘A’.
remotes, use code 1506 for TV, 0200
for SAT1 or 1100 for SAT2.
In the case of other universal remotes, it’s just a matter of testing the
various codes until you find one that
works. Start with Philips devices as
these are the most likely to work. There
are usually no more than 15 codes (and
usually fewer) listed for each Philips
device, so it shouldn’t take long to find
the correct one.
Note that some codes may only partially work, eg, they might control the
volume on the Preamplifier but not the
input selection. In that case, try a different code. Also, some remotes may only
work in one mode (eg, TV but not SAT).
Once you have set up the remote
control, you can power the unit up
and complete the testing process by
pressing the buttons 1-6 in sequence
and verifying that the corresponding
LED lights up and the relays click over.
Troubleshooting
If you run into any problems, the most
likely causes are improperly crimped
or wired cables, mixed up or reversed
components, bad solder joints or unprogrammed/incorrectly programmed
microcontrollers.
These problems can all cause similar faults, so if it doesn’t work the first
time, go over the boards and compare
38
them to Fig.19 and Fig.20. Ensure that
all components have been installed correctly, then carefully inspect the solder
joints to make sure you haven’t missed
any, you have used sufficient solder and
there are no dry joints or solder bridges.
Presumably, you checked the continuity of your cables earlier, but if not,
do so now. It’s common to have problems with an IDC ribbon cable because
the crimp has not been done with sufficient force for all the blades to cut
through the insulation and make good
contact with the copper inside.
If the unit responds to the 1, 2, 3, 4,
5 and 6 buttons on the remote, but the
button switches don’t work, check that
the IDC ribbon cable to the pushbutton board has been plugged into the
line sockets properly. Similarly, if the
Preamplifier remote volume function
works but not the remote input selection, check the cable from the Preamplifier board to the input selector board.
Since the cable from the Preamplifier
board also supplies power to the other
two boards, it’s worthwhile checking
that there is 5V between pins 5 and 14 of
IC1 on the Audio Selector board. Also,
check that JP1 is in the correct position.
If everything works except the remote control, check that it has fresh
batteries. If it does, most likely it is
not programmed for the code that the
unit is expecting. Re-check that you
have set up the Audio Selector board
to the right code, and programmed the
remote control with the correct corresponding code.
Mounting it in the case
If building a standalone unit, you will
need to choose a case large enough to
mount both boards; ie, at least 200mm
wide and 150mm deep. If powering it
from a plugpack, fit a chassis-mount
concentric DC socket and wire it up
to CON11.
The 12mm tapped spacers can be
used to mount the main board in the
bottom of the box, while the 6.3mm
tapped spacers are used to mount the
front panel board after drilling six
9mm-diameter holes spaced 15.1mm
apart for S1-S6.
Once you’ve made those holes, you
can temporarily fit the front panel
board and mark out the locations of
the four mounting holes, then drill
them to 3mm.
You may want to use black machine
screws to attach the front panel board to
the front of the case if using a black case,
so they are not so visible, and possibly
even use countersink head screws. It
would also be a good idea to attach some
rubber feet to the bottom of the case.
Fig.21: this shows how to make the two ribbon cables. Only the bottom one is
required if building the standalone unit. If upgrading an existing Preamplifier
which already had a 3-input switcher, you should already have both cables.
Practical Electronics | June | 2020
