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Replacement modules
work well
Thank you for your help with
upgrading my Fisher amplifier
(Ask SILICON CHIP, April 1996). I
changed the input sensitivity to the
LM3876 power amplifier chip as
you suggested. The performance is
now very clean and very loud, with
excellent overall gain – the best
Fisher amplifier I have ever heard.
I was wondering if the same
modification could be made to the
25 watt chip, the LM1875T, as featured in the December 1993 issue
of SILICON CHIP. You see, I have
another amplifier with the same
preamp problem; not enough gain.
What would the modifications be?
I also want to build two sub
woofers for my Dolby Pro Logic
System. I would like to try using
some stormwater piping for the
enclosure just as you used in the
March 1995 issue.
However, that piping is too big
for my application as I want them
to fit snugly behind the two lounge
chairs for that added oomph.
I was thinking of using an 8-inch
subwoofer supplied by Jaycar (Cat.
CW-2136). For a vented enclosure,
they say that a 33-litre box with a
vent tuning of 39Hz would be a
good size. The piece of storm water
piping I have is 20cm inside dia
meter at two metres in length. It is
just big enough inside to mount an
8-inch subwoofer.
After scraping between the tracks on
this very sensitive part of the board it
burst into action. After inspecting the
so-called wire short it turned out to be
a human hair!
It sounds far fetched but that is what
stopped it from working properly. (M.
Chase, 58 Douglas St, Nowra, NSW
2541).
Electronic speedo
wanted
I have a problem with my Ducati
and that is that I have changed the
size of the front wheel. The speedo
was driven from an 18" front wheel
which is now 17". There is a commercial unit available but this is just too
94 Silicon Chip
Could you please tell me the
length of piping I would need to
get 33 litres in volume? They also
say that the vent needs to be 50mm
in diameter and 80mm long, which
I would fit at the other end of the
piping. Is this correct?
Could you please help me with
the formula for working out how
to calculate internal air volume
inside piping as I would also like to
do another version using different
size piping and woofers for my car
subwoofer?
I was also very interested in
bandpass enclosures and their
performance with subwoofers. Is
it possible to take a piece of piping
and mount the subwoofer driver
on a baffle and seal it in the middle
of the piping so that there was an
enclosure in front of the driver as
well as in back, sealed and vented
at both ends like a bass cannon?
Would this work?
I also read in another book just
recently that you can now get what
they called a “Bass Shaker”.
It’s a round device that screws to
your walls or floors and it shakes
you during heavy bass notes so
that you physically feel the action.
I would love to hear more about
these devices and how they work.
Maybe you could publish an article
on it and maybe a project on this
shaker some time in the future. It
seems to be the next logical step
towards perfection. (K. S., Morphett
Vale, SA).
small for viewing quickly in traffic
and at high speeds. There were also
other problems with this unit; the
stated maximum speed was 160km/h
but only 100km/h was read. If the
unit was left in bright sunshine, the
LCD went black then lost some of the
display. Now also at night this unit is
not backlit so it cannot be seen.
What is required is a speedo that
works off the wheel via a Hall Effect
trigger, with three or four LED sections
and a tripmeter so I can calculate the
fuel consumption. Also the circuit
board area should be as small as possible and run off 12V DC.
This of course would not be the only
use for the speedo as there are other
vehicles around the farm that it could
•
The method for increasing the
gain of the LM1875T module is
exactly the same as for the LM3876.
To double the gain, simply reduce
the 10kΩ resistor at pin to 4.7kΩ.
No other changes to the circuit are
necessary.
To obtain a volume of 33 litres
with 20cm ID tubing you would
need a length of 105cm. You should
also allow for the volume of the
loudspeaker itself and so the length
should be increased to about 108cm.
The formula to use for these calculations is the volume of a cylinder:
V = πr2h
where r is the radius and h is the
height.
We cannot give detailed answers
concerning vent design or about
bandpass systems. To design these
you need access to a speaker CAD
program, such as BassBox 5.1, reviewed in the June 1996 issue.
We have seen references to the
Bass Shaker but we can see little to
recommend the concept.
All that such a unit will do it is
to excite various panel resonances
around the room so that you will
be beset by unwanted buzzes and
rattles. It will also disturb your
neighbours.
If you are using a subwoofer with
a response down to 25Hz, it will
give you all the physical sensation
you could want. You will be able to
“feel” the bass – provided the program material does contain really
low frequencies.
be adapted to. (L. R., Glenorie, NSW).
The project which comes closest
to your requirements was the digital
speedo and fuel gauge described in
the October & November 1995 issues
of SILICON CHIP. We can supply back
issues for $7.00 each, including postage. Kits for this project are available
from CTOAN Electronics. Phone (07)
297 5421.
•
Notes & Errata
Digital Voltmeter for Cars, June 1993:
the digital readout board has a missing
track between pins 12 & 16 of IC3.
These must be linked together for the
SC
circuit to work.
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