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The budget
Senator
Loudspeaker
System . . .
. . . finishing them off
By Leo Simpson
In this second and final article on Budget Senator speakers using the
Altronics C3026 10-inch woofer, we complete the assembly details,
including the crossover network PCB and discuss hand-winding the 2.7
millihenry air-cored inductor.
L
ast month we described how to
build the cabinets, either from
scratch or based on the very
attractive Bunnings Kaboodle modules. What remains to be discussed is
mounting the drivers, assembly of the
crossover network PCB and obtaining
the 2.7mH air-cored inductors. You
will need one for each Senator speaker.
Let’s describe the inductors first. In
the original Senator loudspeaker articles described in September & October
2015, we specified a 2.7mH air-cored
inductor from Jaycar, Cat LF-1330.
These were well made but unfortunately have now been discontinued
by Jaycar.
Most constructors will want to buy
their inductors and the easiest approach is to buy them from Australian
audio company, Soundlabs, at www.
soundlabsgroup.com.au
Soundlabs have three 2.7mH aircored chokes, wound with 18 gauge,
16 gauge and 12 gauge enamelled
copper wire. Most people would be
happy with the 18-gauge model at
72 Silicon Chip
$20 each, plus packing & postage. See
www.soundlabsgroup.com.au/p/MUAC2m7-1mm/2.7mH+-+0.90+DCR+1
mm+Copper+Air+Core+Coil+18AWG
Buyers from overseas might want
to consider a similar product from
Jantzen Audio, available from Parts
Express at www.parts-express.com/
jantzen-audio-27mh-18-awg-air-coreinductor-crossover-coil--255-272
or Amazon at www.amazon.com/
Jantzen-Audio-2-7mH-InductorCrossover/dp/B0002M736A
And then there is the option to
These 2.7mH chokes from Soundlabs
are wound with different gauge wire
and on different formers, hence the
differences. The 18AWG types are $20
each +GST; the 12AWG are $65 + GST.
wind your own and save some money.
We wound prototype inductors from
18-gauge enamelled copper wire and
we have to state that it is not an easy
task.
It would be easier to wind the inductors from 20-gauge wire since it is
thinner and not so stiff but the resistance of the resulting coil would be a little higher: about 1.6Ω instead of close
to 1.0Ω measured on our prototypes.
While high fidelity purists will no
doubt argue that minimal inductor resistance is very important, the audible
difference between inductors wound
with 18-gauge and 20-gauge will be
undetectable.
You can measure the very slight
difference in bass response but you
won’t hear it.
But because many readers would
probably take the purist approach
and the cost difference between the
required 20-gauge and 18-gauge wire
is zero – you will need to buy a 1kg
reel of wire in both cases – we plunked
for the heavier gauge.
siliconchip.com.au
But that makes it harder to
wind, unless of course, you
have access to a coil winding
machine!
First make your bobbins
Our first attempt to make a
bobbin used a 25mm length of
readily available PVC electrical
conduit and two cheeks made
with a hole saw from Masonite
hardboard and then glued together.
That was OK but did not look
particularly professional and
the small diameter former made
it very difficult to wind, because
of its small radius.
In any case, when we wound
on the calculated number of turns,
the inductance was considerably
less than the required 2.7mH.
Hmm – that was annoying.
Our second attempt, pictured in
this article, used a 1-inch length of
25mm OD electrical conduit and
67mm diameter cheeks cut from
Perspex using a hole saw. We then
glued them together with Bostik PVC
Pipe Cement (Blue type N).
The larger diameter former made
winding a little easier but it was still
tricky. In fact, I aborted the second attempt which involved using a geared
manual drill clamped in a vise.
It was just too hard to maintain the
required winding tension while keeping the wire layers neat and keeping
count of the number of turns.
The method I finally used was to
stretch the required 40 metres or so of
18-gauge wire from the back end of my
garage and up the driveway and then
slowly walk “along the wire” while
I wound it onto the bobbin – while
trying to keep the layers neat, keeping count and maintaining tension. It
took about half an hour.
The finished result can be termed
“workable” but is far less neat than
an inductor produced on a coil winding machine.
And note that no matter how hard
you try to keep the layers neat, the
finished inductor will be “jumble
wound”, not “layer wound”!
By the way, if you go on-line to find
a calculator for an air-cored inductor
and feed in the parameters for the inductor we describe here, you will get
a result of 295 turns.
For example, see http://www.diyaudioandvideo.com/Calculator/Airsiliconchip.com.au
The completed
crossover to suit the
(recommended) Celestion
CDX1-1730 tweeter and Altronics
C-3026 Woofer. You can also use the
Altronics C-3004 tweeter, with a simpler
crossover (see Figs 3&4 overleaf).
CoreInductorDesigner/
As already noted, we had to use
more turns, specifically 325. Ideally,
you need to measure the inductance
although if you wind it using this
method you should get a value within
±5%, which is close enough.
Still interested in winding your
own? If so, we have produced a limited
quantity of Perspex discs which can be
glued up using a solvent-based plastic adhesive such as Sci-Grip Weldon
at Acrylics Online: www.acrylicsonline.com.au/shop-product/accesso-
The inductor we hand-wound using
325 turns of 18 gauge wire, on a
former cut from PVC conduit and
perspex cheeks (using a hole saw).
Ideally it should be “layer wound” ...
but we found this almost impossible.
ries--adhesives/scigrip-ips-weld-on16-clear-acrylic-cement
The discs can be aligned and held
in place by a 1/4-inch or M6 bolt and
nut (as pictured) but it is most important that when the finished inductor
is mounted on the PCB, it must be secured with a brass bolt and nut.
Do not use a steel bolt otherwise
the inductance will be substantially
increase and the harmonic distortion
will also increase due to the significant
non-linearity of the B-H curve of steel.
Brass is non-magnetic.
There is a trap for young players
here: some “brass” bolts and nuts are
actually brass-plated steel. If in any
doubt, check to make sure your “brass”
bolt will not be attracted to a magnet.
Just as an aside: some loudspeaker manufacturers use iron-cored inductors in their crossover networks.
This is a second-rate option. Sure, it
produces a more compact inductor
with less turns of copper wire but
the resulting inductor will be quite
non-linear and can cause significant
distortion.
Crossover network
There are two versions of the crossover network; which one you use depends on the speaker you choose.
The circuit of Fig.1 is the same as
we used for the original Senator sysJune 2016 73
3.3F
1 5W
HF PROFILE
S1
12 10W
CON3
(R1)
12 10W
(C1)
+
3.3
5W
4.7F
3.3
5W
L1 2.7mH
CON1
CON4
(R2)
CON5
INPUT
–
SC
2016
CON6
CON2
BUDGET SENATOR CROSSOVER NETWORK
+
CELESTION
CDX1–1730
TWEETER
–
+
ALTRONICS
C3026
WOOFER
–
CELESTION
TWEETER VERSION
Fig.1: the crossover required for the Celestion tweeter and Altronics woofer.
(R1)
3.3 5W
(C1)
+
CON3
CON4
4.7F
CON1
L1 2.7mH
CON5
INPUT
–
SC
2016
CON2
CON6
BUDGET SENATOR CROSSOVER NETWORK
+
ALTRONICS
C3004
TWEETER
–
+
ALTRONICS
C3026
WOOFER
–
ALTRONICS C3004
TWEETER VERSION
Fig.3: the simpler crossover, suitable for the Altronics woofer and tweeter.
Fig.2: use this PCB overlay to assemble the crossover shown
above (Fig.1) for the Celestion/Altronics combination
74 Silicon Chip
tem described in September & October
2015 and you should use this if you are
using the Altronics woofer and Celestion horn tweeter.
No changes are required, even
though the Altronics woofer is slightly less efficient than the Celestion
10-inch woofer. The accompanying
component overlay is shown in Fig.2.
On the other hand, if you take the
cheaper option and elect to use both
the Altronics woofer and tweeter, most
of the attenuation resistors are omitted
and the resulting crossover network is
shown in Fig.3, together with its component overlay in Fig.4.
The most important aspect of assembling the crossover network PCB
is to make sure you make good solder
connections to the inductor. Make
sure that you thoroughly remove the
varnish from the ends of the wires
and then tin them with solder. You
will actually need to do this anyway,
if you are going to check the inductance value.
Poke the inductor wires through the
two holes on the PCB and then secure
the inductor with a brass bolt, nut and
washer. Then solder the two connections on the PCB. The rest of the assembly is straightforward.
Mounting the crossover PCB
While the crossover PCBs in the prototype Senators were installed behind
the internal sloping panel inside the
cabinet, we do not recommend this
position as it would be virtually im-
Fig.4: the crossover for the Altronics C-3004 tweeter and
C-3026 woofer (Fig.3 above) requires fewer components.
siliconchip.com.au
Here’s an alternative method of bobbin assembly: the two larger discs form the outer cheeks, while the nine smaller discs are
stacked to form the bobbin core. Note the use of a BRASS bolt and nut – steel bolts are magnetic and will adversely affect the
performance of your crossover. The SILICON CHIP on-line shop (www.siliconchip.com.au/shop) has a limited number of these
accurately-machined bobbin assemblies (11 discs, no bolt/nut) available for $5 per pair plus p&p (currently $AU10).
possible to remove the PCB if a fault
subsequently developed.
Instead, we recommend mounting
the crossover PCB in front of the sloping panel, on the floor of the cabinet,
using four self-tapping screws. To
connect the PCB, you need to crimp
6.3mm yellow female spade connectors onto the ends of the wires from the
woofer and tweeter and plug these into
the appropriate connectors on the PCB.
You also need some 400mm-long
spade-lug to spade-lug cables using
spare speaker wire off-cuts to connect
the input terminals on the PCB to the
binding posts mounted on the rear
panel of the speaker.
If using the treble peaking switch
(only applicable with the Celestion/
Altronics combination), drill a hole
through the rear panel of the speaker
and wire the switch up to one of the
pairs of terminals marked on the PCB
(ie, the middle pin and one of the upper pins).
Alternatively, use a jumper shunt
instead, shorting out the indicated
pins to enable the treble peaking or
placing it across the lower pins to disable peaking.
except to note that you should use
black (or even later painted black!)
screws to mount the speakers.
Another important point is that it
is vital that the speakers are air-tight
when mounted.
It’s not so much of a problem with
the Celestion drivers but the Altronics
will definitely need a layer of draftexclusion tape between the case and
driver to seal them.
Draft exclusion tape is available
from hardware stores, etc. But ensure
there are no breaks in the tape to allow air in/out.
Finishing off
Your Senator speaker box(es) are
now complete and almost ready for
use. However, we do not recommend
using them “flat on the floor” as this
will tend to make the bass “boomy”.
Raising them by, say, 100mm or so
will virtually eliminate this problem
and as a bonus, will raise the tweeters
up to a level which is more in line with
a typical listening position.
Fortunately, Bunnings have an ideal
solution to the problem, again intended for kitchen cabinets.
We bought sets of their “leggz”
100mm cabinet furniture legs, as seen
below.
Each pack contains four legs so is
suitable for one speaker box. Once
fitted, they have the added advantage of being height-adjustable so can
help fix any minor discrepancies in
floor levels.
You simply screw the legs to the
outer corners of your speakers, in (say)
100mm from the sides and front.
Sit back, relax with your favourite
music . . . and enjoy!
Mounting the drivers
This is relatively straightforward,
PARTS LIST
The complete parts list for the
Budget Senator Speakers was
published in Part One, last month
siliconchip.com.au
Bunnings’ “leggz” are intended
for weighty furniture use so are
ideal for the Senator speakers.
June 2016 75
Queries on Celestion horn tweeter response
Recently, one of our readers queried the low level frequency response of the Celestion tweeter
I have been looking at the specs of the Celestion compression driver used in the Senator and Majestic speakers. Whilst the curves are excellent at high power levels,
the lower levels don’t look so good.
The compression driver is quite efficient and according to Celestion, intended for larger auditoriums where
it can be driven at optimum power levels. Lounge rooms
of the average family home being much smaller only
need a fraction of these compression driver’s capability to achieve a balanced SPL and therefore they would
appear to be working in the poorest part of their performance envelope.
Perhaps an article on compression driver technology
might be in order, as evidently their initial adoption by
the US hifi market is spreading. Their use also puts the
crossover frequency in the most psycho acoustic sensitive part of audio spectrum which is considered not best
design practice. However if the performance figures of
the Senator design using the Celestion drivers is true in
audio listening then 3-way systems are a thing of the
past. It also raises the question of driving woofer and
tweeter from separate amplifier modules. This is not
only energy efficient but with the crossover put into a
small signal network I would think cost efficient too,
especially if the two amps were on the same board. The
power supply would just be the same rating.
Kelvin Jones,
Kingston, Tas.
Comment: You raise an interesting question. Both our
frequency response curves and those by Celestion have
been done at the standard power level of one watt. However the published curves from the Celestion brochure
(reprinted above) are labelled in a confusing way and
it would be easy to misinterpret them. In each case, the
left-hand axis pertains to the upper frequency response
curve and it is labelled as “SPL (dB)”. SPL stands for
“sound pressure level”.
The right-hand axis pertains to the lower curve which
is actually the tweeter’s impedance and it is labelled
“ Z (dBo)”. We interpret this to mean the “absolute”
value of the impedance but it really should have been
scaled and labelled in Ohms.
The lower curves do not refer to a frequency response
at a lower power level.
Our measurements and listening tests have shown
that this tweeter is very smooth at all power levels and
in the case of the Majestics, performs beautifully up to
300 Watts RMS (250 Watts for the Senator), by dint of
the attenuation resistors in the crossover network.
Also, the response will vary depending on the type
of horn attached as you will note from the difference
between the plane wave tube and the exponential horn.
To obtain the most linear response and lowest harmonic distortion from any compression tweeter, it should
fitted with an exponential horn, as we have specified.
By the way, the term “compression driver” refers
76 Silicon Chip
to the fact that the diaphragm is “pressure loaded” by
the attached horn. You cannot operate a compression
driver without a horn. You can see a short description
of compression drivers at https://en.wikipedia.org/wiki/
Compression_driver
Nor does the term “compression driver” necessarily
suggest that there is significant dynamic range compression although all loudspeakers are subject to some degree
of compression due to heating of their voice coils and
the resultant increase in their resistance.
Note that for home use and especially for home movies,
the power handling ability of the Celestion horn tweeter
is very useful because many DVDs and Blu-ray discs have
program material with an enormous dynamic range, ie,
a whisper is as silent as a whisper and a cannon can be
as loud as a real cannon. This can distort and sometimes
ruin tweeters with low power handling ability.
We agree that there is no advantage in having a 3-way
system compared with the 2-way Majestic and Senator
designs.
As far as driving the tweeter from a separate amplifier
is concerned, it is true that you avoid the attenuation
losses in the crossover network but they would be more
than offset by the increased power consumption associated with having a separate Class-AB amplifier.
And of course, you would also need an active crossover network. We don’t think it is worth the extra complication.
siliconchip.com.au
4367 Studio Monitor
For those who have admired the Majestic
Loudspeaker (SILICON CHIP, June & September 2014) and the more recent Senator speakers (SILICON
CHIP, September 2015) but who don’t have the inclination
or time to build these speakers, you could always consider
a top-of-the-line commercial loudspeaker system of similar specification.
An outstanding example would be the newly released
JBL 4367 Studio Monitor. It is a 2-way system similar in
size, efficiency and power handling to the Majestic and
also uses a 15-inch woofer and a large horn.
That is where the broad similarity ends though, as both
the JBL woofer and tweeter have some exceptional features.
For example, the woofer has a large (1.5kg) neodymium
magnet and two 3-inch voice coils on a common pole piece.
Its free-air resonance is 28Hz.
And interestingly, the JBL compression driver has two
annular diaphragms and a special wave-guide horn.
Both horn tweeter and woofer are coupled via a complex
crossover circuit featuring 16 polypropylene capacitors,
nine resistors and six air-cored inductors. All are mounted
on a thick MDF board using point-to-point assembly. The
crossover frequency is 700Hz.The enclosure itself is made
from 1-inch thick MDF. Overall weight is 61.2kg.
The rated frequency response
is 30Hz to 40kHz at the -6dB points and -10dB at 26Hz.
Convoy International, the national distributor for JBL,
will be launching it at the upcoming International Hi-Fi
show which will be held at the Pulman Hotel, Albert Park
on July 1st–3rd.
Recommended retail price is a cool $25,990 per pair.
OOPS! Some of our dimensions didn’t add up!
A reader queried the dimensions of the Budget Senators published in the May issue –
according to him they didn’t quite add up. And he was right!
For some reason, Mr Murphy changed the width of the base plate from 300 x 381mm
to 320 x 381mm. “Only 20mm”, you say. But that 20mm not only threw other dimensions
out (eg, front and rear baffles) but meant that the cutting diagram also didn’t make sense.
The important diagrams are shown here. Note that the only dimension which has
changed is that
300mm width;
everything else
adds up when
this is changed.
And best of
all, the cutting
NOTE:
diagram, which
Tweeter cutout
dimensions are
we knew should
to suit
work, now reCELESTION horn;
ally does work.
for Altronics
tweeter cut
You should be
73mm diam
able to get all
hole.
the pieces (except the reflector boards, as
we mentioned
last month) out
of a single sheet
of 2400 x 1200
x 18mm MDF
with quite a bit
of “meat” left
over to account
for saw thickness etc.
SIDE A
SIDE A
FRONT A
730 x 417mm
730 x 417mm
730 x 300mm
SIDE B
SIDE B
FRONT B
730 x 417mm
730 x 417mm
730 x 300mm
REAR A
TOP A
730 x 300mm
417 x 336mm
REAR B
730 x 300mm
TOP B
BASE A
BASE B
300 x 381mm
300 x 381mm
417 x 336mm
MATERIAL: 2400 x 1200 x 18mm MDF etc.
siliconchip.com.au
June 2016 77
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