This is only a preview of the April 2011 issue of Silicon Chip. You can view 35 of the 104 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 "Portable Headphone Amplifier For MP3 Players":
Items relevant to "Fixing Transformer Buzz In The Class-A Amplifier":
Items relevant to "Cheap’n’Simple 100V Speaker/Line Checker":
Items relevant to "A Speed Controller For Film Projectors":
Items relevant to "The Maximite Computer, Pt.2":
Purchase a printed copy of this issue for $10.00. |
Cheap’n’Simple
100V Speaker/Line
Checker
By Ross Tester
This Speaker/Line Checker will be a boon to
anyone setting up 100V PA systems, especially for temporary
installations at sporting events, when you need to do everything quickly
before the event and be sure that it is all working. With this tester,
you can immediately check each PA speaker and line as it is run.
N
ecessity, as they say, is the
mother of invention. My necessity was something to check
both PA speakers and the lines feeding
them as they were temporarily placed
in position for surf lifesaving carnivals.
For many years, I’ve erected temporary PA systems – up in the early
morning, down that afternoon. Usually,
that’s been a matter of placing perhaps
eight horn speakers over a distance of
44 Silicon Chip
maybe 600-700m, all fed from a central
PA amplifier located where the carnival
announcer sits.
With eight 30W speakers, a 250W
amplifier handles the whole thing quite
nicely. But feeding those eight speakers
over such a distance demands they not
be your usual low impedance (ie, 4, 8
or 16Ω) speakers; to minimise losses
they must be so-called “100V” types.
What this means is that the out-
put from the amplifier is (internally)
stepped up by a transformer so that the
lines to the speakers are fed by a 100V
signal. At the speaker itself, the reverse
happens – the 100V is stepped back
down again by a similar transformer so
that the low impedance speaker driver
is presented with just the right level.
Why go to all that trouble?
The answer is simple: to minimise
siliconchip.com.au
Fig.1: as circuits go, it’s pretty simple: a 555 timer creates a square wave which is amplified, then fed into a 100V
speaker transformer and on to the speaker. It’s capable of delivering a little over 1W but it’s not exactly hifi!
losses in the speaker cables.
While copper cable is a very good
conductor, it does have some resistance. Typically, I use lightweight
(14x0.14) Fig.8 cable, which according
to the reference books has a resistance
of about 16Ω÷100m (ie, 8Ω per side).
In a home hifi situation with only a
few metres of cable between amplifier
and speaker that resistance doesn’t
matter too much but when your speakers are up to several hundred metres
away from the amplifier, resistance of
the cable has a major impact.
If, for example, I was to drive an 8Ω
speaker 300m away from the amplifier,
the speaker line itself is going to act like
quite a large resistor in series – about
48 (3 x 16Ω) – and I am going to
lose 48÷56 (ie, line resistance divided
by line + speaker resistance) or 85% of
the signal before it gets to the speaker.
In fact, it’s even worse than that
because inevitable corrosion in the
connectors etc means I’d be lucky to
have even 5-10% of my original signal
left at the far end. And the further away
your speakers are, the worse it gets.
With a 100V PA system, the losses
are much, much lower. The impedance of a 30W 8 ohm “tap” on a 100V
audio transformer is calculated as
(100V2÷30) or 333 ohms. So now
we have a 48Ω speaker line in series
with a 330Ω load. Therefore the loss
is reduced to (48÷(48+330)) or about
12% – much more manageable.
No impedance problem, either!
There is another huge advantage:
with multiple speakers, you don’t have
siliconchip.com.au
to worry about impedance matching.
With a 100V line system, all speakers
are connected in parallel/in phase and
all you need to do is add up the wattage which each speaker is running at
(and that simply depends on the tap
you use on the speaker’s transformer)
and make sure the total doesn’t exceed
the rated output of the amplifier.
For example, I mentioned before I
normally use eight 30W horn speakers (or more correctly, eight speakers
connected to their 30W taps). 8 x 30
= 240, nicely inside the rating of my
250W amplifier.
Incidentally, if you need to add another speaker or so to fill in a “sound
hole” in a 100V system and you’re running close to the amplifier’s maximum
power rating, lower the tap on one or
more speakers so that when you add
the extra(s), you stay within the overall
power limit. Simple, eh?
Back to the checker
As I’ve installed the PA systems,
many’s the time I’ve wished for some
method of ensuring that the lines and
speakers were working properly as I
go. “Easy,” you’re thinking. “Just get
Inside the box: everything except the transformer, output terminals and
batteries mounts on a single PCB. The batteries in their holders can just be
seen underneath the board.
April 2011 45
S1
A
D1 K
1N4004
VR1
10k
LOG
K
100uF
+
+
A
LED1
68k
100k
2.2k
V0
100nF
10nF
1
1
220nF
someone on the microphone to talk
as you go.”
Not so easy, especially when the
system is installed at a beach at 5AM
– with people living all around! For a
start, that requires two people to do
the installation and I normally do the
job by myself.
Second, nothing gets residents
offside quicker than someone saying
“testing 1-2” when they’re enjoying
their beauty sleep.
So what I wanted was something that
would generate a low level tone; just
loud enough to ensure that the speaker
lines hadn’t been cut (it happens!) or
the speaker itself hadn’t developed a
mysterious case of silence (ditto!). Then
I could test each PA horn and the reels
of cable as I went.
The circuit
The circuit is dead simple; crude
even – see Fig.1. Our old friend, the
555 timer, is connected in astable mode
so it produces a square wave at about
400Hz or so. It feeds an LM386 power
amplifier IC via the volume control
pot (VR1). The LM386 gain is set at 20
due to the fact that pins 8 and 1 are left
open circuit.
Provision is made on the PC board
for components to (a) shape the output
wave somewhat – effectively in parallel
with VR1, and (b) to adjust the gain of
the LM386 if required (components
between pins 8 and 1).
A 10µF capacitor and series resistor
will set the gain, from 20 up to 200,
depending on the resistor value (open
circuit = 20, short circuit = 200).
While we made this provision, we
were happy with both the tone and
the gain, so these pads are left empty.
You can also adjust the frequency
from the 555 by varying the 150kΩ re46 Silicon Chip
+
+
+
+
OUTPUT TO
TRANSFORMER
47uF
47uF
1
K
47nF
IC1
555
IC2
LM386
V21+
A
Fig.2: the component overlay and matching photo
below. Note that the capacitors are all laid over so
there’s enough room underneath the case lid. The
empty holes in the PC board are for adjustment to
the 555 output waveform (left holes) and the LM386
gain (centre/right holes), as explained in the text.
sistor or the 10nF
capacitor. For
example,
68kΩ
and
10nF
gives about
1kHz.
Normally, the
output of the LM386
at pin 5 would drive an
8Ω speaker via the electrolytic capacitor (the Zobel
network of a 470nF capacitor
and 10Ω resistor to ground at
pin 5 helps prevent supersonic
oscillation).
But in our case, instead of driving
a speaker, we drive the primary (ie,
8Ω winding) of a 100V speaker transformer.
The secondary is taken to a pair of
binding posts which can connect to
speaker cables. But because my speakers and cables are all wired with XLR
plugs and sockets for quick connection, I’ve included a male XLR socket
as well. That makes checking fitted
leads really quick
and easy –
just plug ’em in!
To make it truly
portable, power is supplied by eight AA cells, giving a 12V rail. This connects
via a silicon diode (D1) to protect
against polarity reversal and thence
to an on-board power switch.
A LED pokes through the front panel
to show that power is applied, with a
large volume control knob alongside.
Just in case you’re wondering why
we didn’t simply connect the LM386
to oscillate and produce a square wave
(which it can do easily) we wanted to
make the level variable – and it’s just
as easy to do that with a $1 555.
Construction
With the exception of the 100V
Fig. 3: the top trace
shows the output
of the amplifier
while the lower
(green) trace shows
the (unloaded)
output from the
100V transformer.
OK, it’s not exactly
a textbook square
wave – but I find
the distortion
actually makes the
sound a little more
distinctive.
siliconchip.com.au
Parts List – 100V
Speaker/Line Checker
1 UB-1 Jiffy Box, 158 x 95 x 55mm
1 PC board, coded 04104111,
100 x 60mm
2 4 x AA cell holders
1 8Ω to 100V 5W speaker transformer (T1) (eg, Altronics M1112
or equivalent)
1 SPST slide switch (eg, Jaycar
SS-0812 [DPDT] or equivalent)
2 binding post terminals
1 chassis-mounting male XLR
socket [optional]
1 knob to suit potentiomenter
1 pack 4 rubber feet, self adhesive
4 25mm threaded pillars
4 12mm threaded pillars
8 10mm x M3 screws
4 20mm x M3 screws
4 M3 nuts
7 PC pins
Aluminium sheet (for battery clamp)
Semiconductors
1 555 timer IC (IC1)
1 LM386 Audio amplifier IC (IC2)
1 1N4004 silicon power diode (D1)
1 5mm LED (LED1)
The two 4 x AA battery holders are clamped in place by a scrap of aluminium.
In this shot you can also see the mounting pillars on which the PCB sits, along
with the output terminals and optional XLR socket.
output transformer, binding posts/XLR
socket and battery packs, everything is
mounted on one PCB, coded 04104111
and measuring 100 x 60mm.
First step, then, after checking the
PCB for defects, is to mount the components.
Start with the seven PC pins (two
for power, two for output and three for
potentiometer) then the resistors, low
profile capacitors, diode and then the
electrolytic capacitors.
Note that the electros are all mounted
“laid over” so their height does not
interfere with the front panel. When
mounting the LED, it should sit about
5mm above the PC board surface so it
can just poke through the panel.
The slide switch mounts hard down
on the PC board, which makes it just
the right height to emerge through the
panel without being too proud of it. We
deliberately selected this type of switch
so it would be harder to knock on when
bouncing around in the gear bag! Note
that it will almost certainly be a DPDT
type as SPST are not easy to find!
siliconchip.com.au
Before mounting the 10kΩ log pot,
it would be wise to cut off the excess
shaft, to the length required for the knob
you choose. The pot itself mounts flat
onto the PCB so its three terminals can
solder to the three PC pins.
You will note a couple of holes in the
board alongside the pot – these are for
a length of tinned copper wire which
goes over the top of the pot to ensure
it stays in place.
We soldered the wire to the pot body,
after scratching away some of the passivation on the body (it won’t solder
otherwise).
Finally, solder in the two ICs – making sure you get them in the right spot
and oriented the right way.
Checking
Before mounting the PCB in its box,
it should be checked. It’s so simple
it should work first off. Connect 12V
DC to the power terminals (watch the
polarity) and turn on the switch.
Ensure that the LED lights. If it
doesn’t, you either have a dead power
Capacitors
1 100µF 16V electrolytic
2 47µF 16V electrolytic
1 220nF MKT or monolithic
1 100nF MKT or monolithic
1 47nF MKT or monolithic
1 10nF MKT
Resistors
1 150kΩ
1 100kΩ
1 2.2kΩ
1 10Ω
1 10kΩ log pot, 24mm (VR1)
supply or it is connected back to front
(or perhaps you’ve put the LED in back
to front).
Wind the pot down to minimum
and connect virtually any normal (ie,
low impedance) speaker to the output
pins on the PC board. As you wind the
pot up you should be rewarded with a
raspy tone which increases in volume.
If you don’t, switch off and check
your soldering – especially for dags
between the IC pins and for dry joints
– and also your component placement
and, if applicable, polarity.
Final construction
Using the photos as a guide, drill
the nine holes required in the Jiffy
box – four for the PCB mounts, two
April 2011 47
for the transformer, one for the battery
holders (all in the base) and two for
the terminals on the end (plus, if you
wish to use an XLR socket, a larger
hole [usually 18mm]).
The PC board itself sits 37mm above
the bottom of the case on suitable
pillars. We used a combination of a
25mm and 12mm threaded pillars to
make up the distance with four 20mm
screws holding them in place from
underneath and a 10mm screw holding
the PCB onto the pillars.
If you have difficulties finding the
right length pillars, one cheap trick we
have used in the past is to use plastic
wall plugs as pillars – they’re easy to
obtain and easy to cut to the appropriate length with a sharp (hobby) knife.
The dark blue ones (10mm) make a
nice secure “platform” and you can
use small self-tapping screws.
When in place, the PC board sits
hard up against one end of the case
and actually slightly overlaps the
transformer, with a clearance of perhaps 2mm or so.
So if you wish the transformer could
be mounted further under the PC
board, as long as none of your soldered
joints under the board can short to it.
To do this, though, you will first need
to disconnect and remove the 2-way
terminal block on top (we’d done this
anyway because we needed a terminal
block for another project and this one
is redundant!).
We mounted the XLR socket between the two terminals, with pins 1
and 3 connected to the terminals (ie,
in parallel). There’s not a great deal
of room between the XLR socket and
the transformer – in fact, we had to
cut the ends off the XLR socket solder
pins to give us enough room for the
transformer and PC board.
Solder wires from the common and
5W transformer taps to the output
terminals. Most transformers have
flying leads on their primaries; solder
these to the output terminals on the PC
board (if no flying leads, look for the
“primary” or “8 ohm” labels).
Because it is not easy to buy suitable
and put on your pot knob (we didn’t
worry about a pot nut). Finally, screw
in the four lid screws and you’re done!
How to use it
Fitting the XLR socket required some
minor surgery to the back of the
pins to allow it to fit in – but there’s
still plenty of meat to solder to. Note
that the 2-way terminal block has
been removed from the top of the
transformer – it’s redundant because
the output leads solder straight to the
appropriate taps.
chassis-mounting 4 x AA holders (although Altronics has one) we used two
ordinary 4-cell “AA” battery holders
which go alongside each other under
the PC board.
These were connected in series (one
red to one black wire) with the other
red wire going to the + terminal on
the PC board and the black wire, obviously from the other battery holder, to
the – terminal.
To hold them in place we used a
scrap of aluminium as a clamp and
a single screw and nut coming up
through the bottom of the case onto
the clamp to hold the battery holders
firmly in place.
If you can find chassis-mounting 4
x AA holders, connect them the same
way but secure them to the bottom of
the case alongside each other using
suitable screws and nuts.
To avoid scratching the boss’s desk
(he’s got a thing about that), we placed
four small self-adhesive rubber feet in
the corners of the case.
You’re almost finished! Drill the
front panel for the pot shaft (10mm
hole), LED (5mm hole) and the slot
for the on/off switch (4mm wide x
~10mm long).
Place the lid on, making sure the
LED, switch and pot shaft all come
through where they are supposed to
Resistor Colour Codes
o
o
o
o
No. Value
1 150kΩ
1 100kΩ
1 2.2kΩ
1
10Ω
4-Band Code (1%)
brown green yellow brown
brown black yellow brown
red red red brown
brown black black brown
48 Silicon Chip
5-Band Code (1%)
brown green black orange brown
brown black black orange brown
red red black brown brown
brown black black gold brown
I’m sure everyone who puts together
temporary PA systems has their own
way of working – but this will give you
an idea of how I do it – especially now
I can check the installation as I go.
Usually, I erect all the horn speakers
where I want them first, then go back
and roll out the cables which connect
them together. There is a reason for
this: I know where each area of the
carnival is to be set up so provide
speaker coverage for those areas.
The speakers are “daisy chained”
one to the next – not in series but as I
mentioned earlier, all in parallel.
Each of my reels of cable has two
XLR male sockets on the reel itself and
an XLR female plug on the other end.
A short female-to-female patch lead
connects the reel to the speaker, while
the second XLR socket on the reel is
ready to accept the female plug on the
next reel, going off to the next speaker.
All are wired the same way, using
pins 1 and 3 of the XLR plugs, so I
never have a problem with phasing.
With this gadget, I don’t need an
amplifier connected (which probably
won’t even have power available at
that time of day) nor do I need a second person.
I simply go to the furthest speaker,
plug it into the checker and make
sure it’s OK. Then I plug in the patch
lead and if it tests OK, I plug it into
the still-rolled-up cable and plug the
checker into the opposite end – again,
the tone tells me if it is good.
I then roll out that cable back to the
previous speaker and repeat the procedure. So at each speaker I’m checking
it, the patch lead and the cable reel.
You’d be amazed the number of
times a reel of cable tests no-go – so I
can substitute another roll right then
and there.
It saves having to come back later to
swap it all out (and also having to roll
SC
out the cable twice).
Capacitor Codes
Value µF value IEC Code EIA Code
220nF 0.22µF
220n
224
100nF 0.1µF
100n
104
47nF 0.047µF
47n
473
10nF 0.010µF
10n
103
siliconchip.com.au
|