This is only a preview of the August 1995 issue of Silicon Chip. You can view 31 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. Items relevant to "Vifa JV-60 2-Way Bass Reflex Loudspeaker System":
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M
Powered Loudspeaker
Build the
Revitalise the
sound card in your
computer with the
Mighty-Mite Powered
Loudspeaker. It uses
a miniature surfacemount IC amplifier
which only requires a
5V supply to deliver
a 1W output.
By JOHN CLARKE
Sound cards for computers and
multimedia are all the go at present.
But without suitable loudspeakers,
much of the impact of the sound can
be lost. By building the Mighty-Mite
Powered Loudspeaker, you can obtain
sound quality that’s far superior to that
available from low-cost multimedia
loudspeaker systems.
The system to be described is based
on an LM4860M integrat
ed circuit
(IC) audio amplifier and this drives
a 100mm dual-cone loudspeaker.
Both the amplifier circuitry and the
loudspeaker are housed in a compact
sealed plastic case. The only front
panel control is for volume while at
the rear are the signal input and DC
supply sockets.
The amplifier circuit is powered
from a 5V rail and this can come from
60 Silicon Chip
VDD
CS
0.1
Rf
10k
AUDIO
INPUT
C1
1
Ri
10k
VDD
13 -IN
GAIN-OUT
Vo1 10
14 +IN
40k
AMP1
5 BYPASS
Vo2 15
VDD/2
AMP2
Av = -1
50k
6 HP-IN1
7 HP-IN2
RL
8
40k
50k
LM4860
CB
0.1
PARTS LIST
BIAS
3 HP-SENSE
2 SHUTDOWN
GND
1,4,8,9,16
Fig.1: the internal arrangement of the LM4860M audio amplifier
IC. Amp1 is the main amplifier & is connected in inverting mode.
Its output appears at pin 10 & also drives inverting amplifier stage
Amp2 to derive an out-of-phase output at pin 15.
L1
FX115
or sim.
47
16VW
+5V
6.8pF
Semiconductors
1 LM4860M surface mount 1W
audio amplifier (IC1)
100k
INPUT
VOLUME
VR1
10k
LOG
1
16VW
22k
13
14
12
11
IC1
LM4860
5
2
3
1,4,6,
7,8,9,
16
10
15
4 OR 8
10
16VW
MIGHTY-MITE
Fig.2: this diagram shows the complete circuit details for the
Mighty-Mite Powered Loudspeaker. It operates with a gain of
nine & this provides an input sensitivity of about 320mV for
1W output into 8-ohms.
the computer itself – either from the
games port or from the sound card
input/output port (see Fig.4). Alternatively, the circuit may be powered from
a 9-12VAC plugpack via an optional
5V regulator circuit which is also
described here.
Of course, the Mighty-Mite is not
just suitable for multimedia applications. It can be used anywhere a powered loudspeaker system is required;
eg, as part of a low-cost audio system
or in a workshop. If you do use them
with a computer system though, be
sure to heed the accompanying warn-
1 PC board, code 01305951, 33
x 25mm
1 sealed ABS box, 171 x 121 x
55mm (Jaycar HB-6128 or
equivalent)
1 dual-cone 100mm
loudspeaker, 4Ω or 8Ω (DSE
Cat. A9651 or equivalent)
1 10kΩ 16mm log pot (VR1)
1 FX115 ferrite bead or
equivalent (L1)
3 self-adhesive labels
1 16mm dia. knob
1 DC panel socket
1 chassis mount RCA panel
socket
2 25mm long x 3mm dia. screws
& nuts
2 9mm tapped spacers
2 6mm long x 3mm screws
4 black countersunk 4mm
diameter screws & nuts (to
attach loudspeaker)
9 PC stakes
1 25mm length of 0.8mm tinned
copper wire
ing panel. Unlike most multimedia
loudspeakers, these units are not
magnetically shielded, so don’t place
them too close to the monitor.
The audio amplifier IC
One of our first tasks in designing
this system was to choose a suitable
audio amplifier IC. There are many
such units available, most capable of
providing excellent results. We finally
settled on the LM4860M because of its
excellent specifications (considering
its small surface-mount package), its
1W (RMS) power output and its ability
Capacitors
1 47µF 16VW PC electrolytic
1 10µF 16VW PC electrolytic
1 1µF 16VW PC electrolytic
1 6.8pF ceramic
Resistors (0.25W, 1%)
1 100kΩ
1 22kΩ
Optional 5V Regulator
1 PC board, code 01305952, 59
x 35mm
1 heatsink, 26 x 29 x 13mm
1 7805T 3-terminal regulator
(REG1)
1 B104 bridge rectifier (BR1)
1 470µF 25VW PC electrolytic
capacitor
1 10µF 16VW PC electrolytic
capacitor
4 PC stakes
1 6mm long x 3mm dia. screw
& nut
to operate from a 5V supply.
At first glance, it might seem impossible to obtain a 1W output into
8-ohms from such a low supply rail.
August 1995 61
SPEAKER
DC
INPUT
6.8pF
L1
100k
22k
IC1
INPUT
47uF
1
IuF
VR1
10uF
Fig.3: install the parts on the PC board & run the external wiring
as shown in this diagram. Note that IC1 is a surface mount device
& is mounted on the copper side of the board (see text).
This is because, to obtain 1W, the
amplifier would have to deliver 2.83V
RMS or 4V peak into the load. In other
words, it would have to deliver 8V
peak-to-peak, which is greater than
the supply voltage.
However, the LM3860M is a bridge
amplifier which drives both terminals
of the loudspeaker. Thus, when one
terminal of the loudspeaker is driven
high, the other terminal is driven low
with the opposite phase. As a result,
the effective power output from a
bridge amplifier is four times that
available from a standard amplifier
(P = V2/R).
Fig.1 shows the internal arrangement of the LM4860M and the typical
external connections. Amp1 is the
main amplifier and this is connected
in inverting mode. Its gain is set by the
ratio of the feedback resistor (Rf) to
62 Silicon Chip
the input resistor (Ri), and in this case
is set to -1. The non-inverting input
is set to half-supply by two internal
50kΩ voltage divider resistors and is
decoupled using capacitor CB.
The output of Amp1 appears at pin
Fig.4: a 5V rail
to power the
Mighty-Mite
can be derived
from a games
or sound card
port of a PC.
This diagram
shows the
supply connections. You will
need to make up a suitable
power cable which is fitted at
one end with a matching DB15
connector.
10 of the IC and also drives the inverting input of a second internal amplifier. Designated Amp2, this amplifier is
also connected in inverting mode, with
its gain set to -1 by two internal 40kΩ
resistors. Its signal output appears at
pin 15 and is 180° out of phase with
the signal at pin 10.
This arrangement forms the bridge
amplifier configuration. Compared to
a single-ended amplifier, it effectively doubles the output voltage swing
applied to the loudspeaker and thus
quadruples the power. Note that the
overall gain of the amplifier is 2Rf/Ri,
due to the bridge configuration.
With no signal applied, the outputs of Amp1 and Amp2 will be
at the same voltage because both
amplifiers are biased at half supply.
Consequently, there is no need for an
output coupling capacitor to prevent
DC from flowing in the voice coil.
This not only reduces the component
count but also improves the low fre
quency response.
As well as the internal amplifiers,
the IC also contains a shutdown feature
which can be used to reduce the power
consumption when the amplifier is not
in use. It is activated by connecting
pin 2 to the positive supply rail (or
to some other point above 3V). This
reduces the no-signal supply current
from a nominal 7mA to 500µA.
Alternatively, the shutdown feature
can be activated via an internal OR
gate which has its pin 3 output connected to pin 2. The amplifier is then
shut down by feeding control signals
to the pin 6 and pin 7 OR gate inputs.
When either or both of these inputs
are at a logic high, the amplifier is
disabled.
These control inputs are typically
used to shut down the amplifier in
situations where a set of headphones
is plugged into a preceding stage. In
this case, the control input is derived
by switching in a suitable voltage via
an internal switch in the headphone
socket.
Circuit details
Refer now to Fig.2 for the final
circuit details of the Mighty-Mite
Powered Loudspeaker. In this circuit,
the gain has been set to nine by the
100kΩ feedback and 22kΩ input resistors. This provides an input sensitivity of about 320mV for 1W output
into eight ohms. In addition, a 6.8pF
capacitor has been connected across
the feedback path and this rolls off
the high-frequency response above
230kHz to prevent instability.
The incoming audio signal is applied to IC1 via volume control VR1
and a 1µF coupling capacitor. This
coupling capacitor is necessary to
prevent DC current from flowing
through the 22kΩ input resistor and
VR1. It rolls off the response below
7Hz.
The 10µF capacitor decouples the
half-supply rail at pins 5 & 14 to improve supply rejection and reduce the
distortion below 100Hz. Note that pins
6 & 7 (the OR gate inputs) are tied low,
while the OR gate output at pin 3 is
tied to the shutdown input at pin 2.
Because the OR gate output is always
low in this design, the amplifier is
permanently enabled.
Finally, the power supply to IC1 is
isolated using a ferrite bead and decoupled by a 47µF capacitor. This measure
helps to reduce noise injection into the
amplifier if it is powered from a 5V
computer supply (a computer supply
rail usually has a fair degree of hash
and high frequency noise).
This view shows the completed amplifier module. Note that the volume control
potentiometer (VR1) is mounted by soldering its terminals to three PC stakes at
one end of the board.
Construction
The Mighty-Mite is built onto a PC
board coded 01305951 and measuring
33 x 25mm. Fig.3 shows the wiring
details.
Begin construction by installing PC
stakes at the external wiring points;
ie, at the loudspeaker outputs, the
+5V and 0V supply inputs, and at
the signal inputs. In addition, install
PC stakes at the three wiring points
for VR1.
Once the PC stakes are in, IC1 can
be installed. Because this is a surface-mount component, it is mounted
on the copper side of the board.
Before soldering IC1, the copper
lands should be pretinned using a
fine-tipped soldering iron. This done,
place the IC on the board with the
notch in its plastic body towards the
1µF capacitor position, then carefully
tack solder a couple of pins to the pretinned lands by heating them gently
with the iron. The pins can then all be
carefully soldered.
Be sure to use only small amounts
of solder during this job, to prevent
unwanted shorts between adjacent
pins of the IC. In fact, it is a good idea
to carefully inspect the completed job
under a magnifying glass to ensure that
all is correct.
This close-up view shows the mounting details for the amplifier board. It sits
27mm above the floor of the case on two 9mm-long spacers which are screwed
onto 25mm long x 3mm dia. mounting screws.
Performance of Prototype
Output power ����������������������������� 1.3W into 4٠at onset of clipping; 1W into
8Ω at onset of clipping
Distortion ����������������������������������� <1% see graphs
Signal-to-noise ratio ������������������ 76dB with respect to 1W with 1k٠input
resistor & 20Hz to 20kHz bandwidth; 91dB
A weighted
Frequency response ������������������ -2dB at 10Hz & 100kHz
Sensitivity for 1W out ���������������� 320mV RMS
Supply voltage ��������������������������� 2.7-5.5V
Quiescent current ���������������������� <15mA; typically 7mA
Output offset voltage ����������������� <50mV
August 1995 63
Building The Optional 5V Regulator Board
BR1
B104
9-12VAC
INPUT
IN
470
25VW
REG1
7805
GND
OUT
+5V
10
16VW
0V
+5V REGULATOR
I GO
Fig.5: you will need this simple regulator circuit
if you intend powering the unit from an AC or DC
plugpack supply.
BR1
The regulator board only takes a few minutes to
assemble. Make sure that all parts are correctly
oriented & use PC stakes at external wiring points.
470uF
REG1
7805
9-12V
AC
INPUT
10uF
HEATSINK
GND
5V
OUTPUT
Fig.6(a): here’s how to install the parts
on the regulator board. Note that REG1
is bolted to a small U-shaped heatsink.
If you wish to power the unit from
a 9-12VAC (or 9-12V DC) plugpack,
then you will need to add the 5V
regulator circuit shown in Fig.5. As
shown, the output from the plugpack
is fed to a bridge rectifier (BR1) and
this in turn drives 3-terminal regulator REG1 to derive a regulated 5V
rail. The 470µF and 10µF electrolytic
capacitors provide filtering for the IN
and OUT terminals of the regulator
The LM4860 IC (IC1) is mounted
on the copper side of the PC board
as shown here. Use a fine-tipped
soldering iron for this job & make sure
that the device is correctly oriented.
64 Silicon Chip
Fig.6(b): this is the full-size etching pattern
for the regulator PC board. Check the
board carefully before installing any parts.
respectively.
A PC board (code 01305952) has
been designed to accommodate the
regulator components – see Fig.6(a).
Install the parts on this board exactly
as shown and note that REG1 is
bolted to a small finned heatsink to
ensure adequate heat dissipation.
Apply a thin smear of heatsink
compound to the metal tab of the
regulator before bolting it down.
The remaining components mount
on the top of the PC board. Take care
with the electrolytic capacitors - they
must be oriented with the correct polarity, as shown on Fig.3. The resistors
mount end on, while L1 simply consists a short length of tinned copper
wire fed through the ferrite bead.
Finally, the board assembly can be
completed by soldering VR1’s terminals to the top of the PC stakes.
The completed amplifier, along with
the loudspeaker, is housed in a sealed
ABS box measuring 171 x 121 x 55mm.
In no circumstances should you
power the amplifier from a voltage
greater than 5.5V. The audio amplifier chip could fail if you do.
The 5V regulator board can either be mounted inside the case of
the Mighty-Mite, or mounted in a
separate case and used externally.
Take care to ensure that the supply
connections to the amplifier board
are correct.
This box is fitted with three adhesive
labels – two on the front panel and
one on the rear. Fit these labels to the
locations shown in the photographs,
then drill mounting holes in the rear
panel for the DC power socket and the
RCA input socket.
Similarly, on the front panel, drill
a hole for the volume control shaft.
Note that it’s best to start with a small
pilot hole and then carefully enlarge
the hole to the correct size using a tapered reamer. Once this has been done,
mark out and drill the loudspeaker
YOU CAN
AFFORD
AN INTERNATIONAL
SATELLITE TV
SYSTEM
SATELLITE ENTHUSIASTS
STARTER KIT
Fig.7: this graph shows the distortion as a function of output power into an
8-ohm load. Note that the distortion is less than 1% for output powers up to 1W
& rises steeply beyond this level of output.
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SYSTEM FROM ONLY:
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HERE'S WHAT YOU GET:
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preprogrammed for all viewable satellites
1.8m solid ground mount dish
20°K LNBF
25m coaxial cable
easy set up instructions
regular customer newsletters
BEWARE OF IMITATORS
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Fig.8: distortion vs. output power into a 4-ohm load. The distortion is less than
for 8-ohm loads, while slightly more output power (1.3W) can also be obtained.
mounting holes plus a circular pattern
of holes in front of the cone position
to let the sound escape.
Next, carefully measure out, mark
and drill the mounting holes for the
PC board in the base of the case. The
PC board assembly is then installed
in the case as shown in one of the
photos.
To do this, first fit two 25mm long
x 3mm dia. screws to the mounting
holes and secure them with nuts.
A 9mm spacer is then fitted to each
screw. Screw these spacers down until
their top surfaces are 27mm above the
base of the case, then fit the PC board
and secure it to the spacers with 6mm
long screws.
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information on international band
satellite systems.
Name: __________________________________
Address: ________________________________
____________________P'code:
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Phone: (_______) ________________________
ACN 002 174 478
August 1995 65
Below: the input & DC power
sockets are mounted on the
rear panel. At left is the view
inside the prototype.
All that remains now is to run the
small amount of internal wiring – see
Fig.3. This consists of: (1) connecting
the supply leads from the DC socket
to the PC board; (2) running a short
length of shielded cable from the RCA
socket to the input termi
nals; and
(3) running a length of figure-8 cable
from the board to the loudspeaker
terminals.
Warning
Unlike most multimedia
loudspeakers, the MightyMite design does not include
magnetic shielding. As a result,
the strong magnetic field around
the loudspeaker can cause colour distortion if placed too close
to a monitor screen, due to magnetisation of the shadow mask.
Usually, this problem will be
cured by the internal degaussing
circuitry of the monitor each time
it is switched on. Severe cases,
however, will require the use of a
degaussing wand, which means
a trip a professional service organisation.
To avoid this problem, do not
place the Mighty-Mite Powered
Loudspeaker any closer than
about 300mm from a monitor or
TV set.
Testing
To test the unit, first connect a 5V
DC supply to the DC socket, taking care
to ensure that the polarity is correct.
This done, switch on and check that
the wire link through the ferrite bead
is at +5V with respect to ground. If this
is correct, check that the accessible
lead of the 22kΩ resistor is at 2.5V
(ie, half supply). The two loudspeaker
terminals should also each be at 2.5V,
give or take 50mV.
If all checks out, then you are ready
to try the Mighty-Mite out. This simply
involves completing the case assem-
VOLUME
Fig.9: here are full-size
artworks for the three labels
plus a full-size etching
pattern for the Mighty-Mite
amplifier board.
66 Silicon Chip
+
MIN
MAX
Power supply
As mentioned previously, the
Mighty-Mite can be powered directly from a games or sound card port.
Fig.4 shows the +5V and GND (0V)
connections for these ports. You will
need to make up a suitable power
cable which is fitted at one end with
a matching DB15 connector. Note that
the +5V rail can be derived from pin
1, pin 8 or pin 9.
Alternatively, you can assemble the
optional 5V regulator board & power
the unit from an AC or DC plugpack
SC
supply.
5VDC MAX.
+
MIGHTY-MITE
bly, fitting the knob and feeding in a
suitable signal from your computer’s
sound card, or from some other suitable audio source (eg, a tuner or tape
deck).
+
SIGNAL IN
-
+
|