94 Silicon chip Australia’s electronics magazine siliconchip.com.au
sponses? Is it because the CDX1-1425
& 1430 are more expensive than the
CDX1-1730?
Finally, I am planning to change my
Majestic speakers over to use an ac-
tive crossover. Should I increase the
crossover frequency, say, to 2kHz? (J.
S., Melbourne, Vic)
- Loudspeaker design is a complex
process and you can’t really design
speakers based on the specifications
of the components, since there are
so many interactions (electrical and
acoustic). It’s therefore not all that
easy to answer your questions, but
we will try.
Firstly, it may be true that the Etone
1525 woofer “can handle” frequen-
cies up to 2.4kHz but our own meas-
urements of its free-air frequency re-
sponse show quite a significant dip
just above 2kHz.
We therefore decided that to get a
flat response, the crossover frequency
should be below 2kHz.
It’s generally bad practice to have a
crossover frequency too close to the
-3dB point of any driver, since the
crossover is usually designed with the
assumption that the driver’s response
is more or less flat up to the crossover
frequency; otherwise, you will get a
dip in the overall response. So you
will usually choose a crossover fre-
quency where both drivers are down
by 1dB or less.
The resonance of the CDX1-1730
tweeter is approximately 500Hz, so
our use of a 1.6kHz crossover frequen-
cy does not pose any problems for the
tweeter. The manufacturer has quoted
2.2kHz as a recommended minimum
crossover frequency but we feel that
this is very conservative!
The CDX1-1730 was chosen for its
fantastic performance and sheer pow-
er handling capability (75W RMS),
enabling the Majestics to handle up
to 300W RMS without damage while
still maintaining an excellent frequen-
cy response. We tested them at the full
300W (attracting some unwanted at-
tention from the Police!). The tweeters
handled their share with ease.
Unfortunately, the CDX1-1425 is
listed in the manufacturer’s official
specifications as having a power rat-
ing of only 25W RMS and with 2dB
less sensitivity compared to the CDX1-
1730.
Looking at the frequency response
data, there is almost no difference be-
tween the 1730 and the 1425. It all de-
pends on the type of horn used. Celes-
tion gives responses for a plane wave
tube and also for an exponential horn
(90° x 40°). The differences between
the tweeters’ performance with thosetwo horns are far greater than the dif-
ferences between the two tweeters.
Ultimately, we chose the combina-
tion of drivers not just because of their
specifications but also because we con-
ducted many listening tests with dif-
ferent woofers, tweeters and horns and
we found that this combination gave
the best overall sound quality as well
as measuring up well.
Internet specifications vary enor-
mously for these tweeters and we
are not sure where you obtained the
lower frequency limit because Celes-
tion quote the “frequency range” for
the CDX1-1730 as 1200-20,000Hz but
the CDX1-1425 response is quoted as
2,000-20,000Hz.
Because of its lower sensitivity and
power handling, we do not recom-
mend the CDX1-1425 tweeter for the
Majestic system.
One of the benefits of using an ac-
tive crossover (such as our September/
October 2017 design; siliconchip.com.
au/Series/318) is that they are usually
adjustable so you can try out different
crossover frequencies and see which
sounds best.
We suggest you stick with the
1.6kHz crossover frequency used in
our design; however, you will prob-
ably get reasonable results anywhere
between 1.5kHz and 2kHz. SCI recently built your Soft Starter
for Power Tools from the July 2012
issue (siliconchip.com.au/Arti-
cle/601). I built it from an Altronics
K6043 kit which I purchased from
Tronixlabs.
I have never seen such clear and
detailed instructions! I only found
one typo – in the parts list, it should
read “2 IN4148 small signal diodes
(D3,D4)”, not “(D4,D4)”.
It works OK but right at the very
end of the blurb it states that “if you
start the tool multiple times in quick
succession, the second and later
starts will not have as effective cur-
rent limiting due to the thermistors
heating up”.
Indeed so. I have connected it to a
Ryobi table saw. It works well on the
first start but quickly diminishes if I
use it again, even a few minutes af-
ter. This is very frustrating and kind
of defeats the purpose of the unit.Do you have any workarounds/fix-
es? A small fan in the box perhaps?
Is there any kind of heat sink, or a
bigger (metal?) box that would help?
(D. R., via email)- The heating effect you are describ-
ing is much larger than what we ex-
perienced with the prototype. That
may be because your table saw has
a much longer spin-up time than the
power tools we were testing it with
and so causes the thermistor to heat
up more.
We agree that a fan sucking air out
of the box with holes drilled in the
other end to allow fresh air to enter
would be the best solution.
You would need to fit the unit
into a larger box to accommodate
the fan. Be careful to keep the holes
small so that fingers, bits of wire and
so on can’t find their way inside the
box and contact any mains-carrying
conductors.
Mains Soft Starter not effective after multiple re-starts
The best solution would be to run
a small 230/240VAC rated fan direct-
ly off the incoming mains. However,
it would also be possible to use a 24V
DC fan connected between the +12V
and -12V rails.
To provide sufficient current to
power a DC fan in this manner, you
would need to increase the value of
the 330nF X2 capacitor substantial-
ly. Using a 1μF X2 capacitor in its
place would give you around 40mA
to run the fan (20V × 40mA = 0.8W).
We suggest also increasing the 220μF
16V capacitors to 470μF 16V.
Altronics Cat F1046 is a 50mm,
24VDC fan which draws around
60mA nominal at 24V DC. This
should drop to around 50mA at
20V; a 100W series resistor would
probably reduce its current to the
point where the circuit will operate
normally with the aforementioned
changes.